Chapter 12

Temperature

Axons propagate action potentials at lower speeds when cooled.

Amplitude (size) Depending on strength of stimulus, varies from less than 1 mV to more than 50 mV.

GRADED POTENTIALS

Conduction Decremental (not propagated); permit communication over short distances.

GRADED POTENTIALS

Origin Arise mainly in dendrites and cell body.

GRADED POTENTIALS

Polarity May be hyperpolarizing (inhibitory to generation of action potential) or depolarizing (excitatory to generation of action potential).

GRADED POTENTIALS

Refractory period Not present; summation can occur.

GRADED POTENTIALS

Types of channels Ligand-gated or mechanically-gated ion channels.

GRADED POTENTIALS

What would happen if summation of graded potentials in a neuron did not occur?

Graded potentials have different names depending on which type of stimulus causes them and where they occur. For example, when a graded potential occurs in the dendrites or cell body of a neuron in response to a neurotransmitter, it is called a postsynaptic potential (explained shortly). On the other hand, the graded potentials that occur in sensory receptors are termed receptor potentials

nerve impulse

If the total excitatory effects are greater than the total inhibitory effects and threshold is reached, one or more nerve impulses (action potentials) will be triggered. Impulses continue to be generated as long as the EPSP is at or above the threshold level.

initial segment

beginning of axon

The nervous system regulates _____________________

body activities by responding rapidly using nerve impulses

axon collaterals

branches of axon

Action Potential (AP)

buildup of electrical current in the neuron

ligand-gated channels

channel that opens when a neurotransmitter attaches

leak channels

channels that are always open

2.Opening a smaller number of channels only at the nodes, rather than many channels in each adjacent segment of membrane, represents a more energy-efficient mode of conduction. Because only small regions of the membrane ______________________, minimal inflow of Na+ and outflow of K+ occurs each time an action potential passes by. Thus, less ATP is used by sodium-potassium pumps to maintain the low intracellular concentration of Na+ and the low extracellular concentration of K+.

depolarize and repolarize

When a depolarizing graded potential or some other stimulus causes the membrane of the axon to ______________________ voltage-gated Na+ channels open rapidly. Both the electrical and the chemical gradients favor inward movement of Na+, and the resulting inrush of Na+ causes the depolarizing phase of the action potential. The inflow of Na+ changes the membrane potential from −55 mV to +30 mV. At the peak of the action potential, the inside of the membrane is 30 mV more positive than the outside.

depolarize to threshold,

is summation of postsynaptic potentials in response to stimuli that occur at the same location in the membrane of the postsynaptic cell but at different times. For example, temporal summation results from buildup of neurotransmitter released by a single presynaptic end bulb two or more times in rapid succession

temporal summation

inhibitory postsynaptic potential (IPSP)

temporary hyperpolarization of a membrane

Plasticity

the brain's ability to change, especially during childhood, by reorganizing after damage or by building new pathways based on experience

threshold

the level of stimulation required to trigger a neural impulse

repolarizing phase

the membrane potential is restored to the resting state of -70 mV

after-hyperpolarizing phase

the membrane potential temporarily becomes more negative than the resting level

Neurolemma

the outermost nucleated cytoplasmic layer of Schwann cells that surrounds the axon of the neuron. It forms the outermost layer of the nerve fiber in the peripheral nervous system.

Parts of a Neuron Most neurons have three parts:

(1) a cell body, (2) dendrites, and (3) an axon

As you have already learned,_________________________from a presynaptic neuron bind to neurotransmitter receptors in the plasma membrane of a postsynaptic cell. Each type of neurotransmitter receptor has one or more neurotransmitter binding sites where its specific neurotransmitter binds. When a neurotransmitter binds to the correct neurotransmitter receptor, an _____________________forms in the membrane of the postsynaptic cell. Neurotransmitter receptors are classified as either ionotropic receptors or metabotropic receptors based on whether the neurotransmitter binding site and the ion channel are components of the same protein or are components of different proteins.

-neurotransmitters released -ion channel opens and a postsynaptic potential (either an EPSP or IPSP)

The resting membrane potential of a neuron typically is −70 mV. What does this mean?

. In neurons, the resting membrane potential ranges from −40 to −90 mV. A typical value is −70 mV. The minus sign indicates that the inside of the cell is negative relative to the outside. A cell that exhibits a membrane potential is said to be polarized. Most body cells are polarized; the membrane potential varies from +5 mV to −100 mV in different types of cells.

The resting membrane potential arises from three major factors:

1)Unequal distribution of ions in the ECF and cytosol 2)Inability of most anions to leave the cell 3)Electrogenic nature of the Na+-K+ ATPases

Factors That Affect the Speed of Propagation The speed of propagation of an action potential is affected by three major factors: amount of myelination, axon diameter, and temperature:

1.Amount of myelination. As you have just learned, action potentials propagate more rapidly along myelinated axons than along unmyelinated axons. 2.Axon diameter. Larger diameter axons propagate action potentials faster than smaller ones due to their larger surface areas. 3.Temperature. Axons propagate action potentials at lower speeds when cooled.

Structural Classification Structurally, neurons are classified according to the number of processes extending from the cell body

1.Multipolar neurons: usually have several dendrites and one axon . Most neurons in the brain and spinal cord are of this type, as well as all motor neurons (described shortly). 2.Bipolar neurons: have one main dendrite and one axon. They are found in the retina of the eye, the inner ear, and the olfactory area of the brain. 3.Unipolar neurons: have dendrites and one axon that are fused together to form a continuous process that emerges from the cell body

Amplitude (size) All or none; typically about 100 mV.

ACTION POTENTIALS

Conduction Propagate and thus permit communication over longer distances.

ACTION POTENTIALS

Duration Shorter, ranging from 0.5 to 2 msec.

ACTION POTENTIALS

Origin Arise at trigger zones and propagate along axon.

ACTION POTENTIALS

Polarity Always consist of depolarizing phase followed by repolarizing phase and return to resting membrane potential.

ACTION POTENTIALS

Refractory period Present; summation cannot occur.

ACTION POTENTIALS

Types of channels Voltage-gated channels for Na+ and K+.

ACTION POTENTIALS

neurotransmitter receptors

Also called receptors. Large protein molecules, embedded in the cell membrane, to which neurotransmitters can bind.

Why does a stronger stimulus cause a larger graded potential than a weaker stimulus?

Although an individual graded potential undergoes decremental conduction, it can become stronger and last longer by summating with other graded potentials. Summation is the process by which graded potentials add together. If two depolarizing graded potentials summate, the net result is a larger depolarizing graded potential . If two hyperpolarizing graded potentials summate, the net result is a larger hyperpolarizing graded potential. If two equal but opposite graded potentials summate (one depolarizing and the other hyperpolarizing), then they cancel each other out and the overall graded potential disappears.

Inability of most anions to leave the cell

Another factor contributes to the negative resting membrane potential: Most anions inside the cell are not free to leave . They cannot follow the K+ out of the cell because they are attached to nondiffusible molecules such as ATP and large proteins.

amount of myelination

As you have just learned, action potentials propagate more rapidly along myelinated axons than along unmyelinated axons.

Will an action potential occur in response to a hyperpolarizing graded potential that spreads from the dendrites or cell body to the trigger zone of the axon of a neuron? Why or why not?

As you have just learned, an action potential is generated in response to a threshold stimulus but does not form when there is a subthreshold stimulus. In other words, an action potential either occurs completely or it does not occur at all. This characteristic of an action potential is known as the all-or-none principle. The all-or-none principle of the action potential is similar to pushing the first domino in a long row of standing dominoes. When the push on the first domino is strong enough (when depolarization reaches threshold), that domino falls against the second domino, and the entire row topples (an action potential occurs). Stronger pushes on the first domino produce the identical effect—toppling of the entire row. Thus, pushing on the first domino produces an all-or-none event: The dominoes all fall or none fall.

These star-shaped cells have many processes and are the largest and most numerous of the neuroglia

Astrocytes

Gliomas

Brain tumors that develop from glial cells

Neuropeptides

Chains of amino acids

axon hillock

Cone shaped region of an axon where it joins the cell body.

The same neurotransmitter can be excitatory at some synapses and inhibitory at other synapses, depending on the structure of the neurotransmitter receptor to which it binds. For example, at some excitatory synapses acetylcholine (ACh) binds to ionotropic receptors that contain cation channels that open and subsequently generate EPSPs in the postsynaptic cell . By contrast, at some inhibitory synapses ACh binds to metabotropic receptors coupled to G proteins that open K+ channels, resulting in the formation of IPSPs in the postsynaptic cell

Different Postsynaptic Effects for the Same Neurotransmitter

_______________________ have many long unbranched processes and are located mainly in white matter (also described shortly). The processes of astrocytes make contact with blood capillaries, neurons, and the pia mater (a thin membrane around the brain and spinal cord).

Fibrous astrocytes

EPSP

If the total excitatory effects are greater than the total inhibitory effects but less than the threshold level of stimulation, the result is an EPSP that does not reach threshold. Following an EPSP, subsequent stimuli can more easily generate a nerve impulse through summation because the neuron is partially depolarized.

IPSP

If the total inhibitory effects are greater than the excitatory effects, the membrane hyperpolarizes (IPSP). The result is inhibition of the postsynaptic neuron and an inability to generate a nerve impulse.

depolarizing phase

Increase in Na+ permeability and reversal of membrane potential.

spatial summation

Integration by a postsynaptic neuron of inputs (EPSPs and IPSPs) from multiple sources.

Axon diameter

Larger diameter axons propagate action potentials faster than smaller ones due to their larger surface areas.

Gated channels that randomly open and close.

Leak channels

Gated channels that open in response to binding of ligand (chemical) stimulus.

Ligand-gated channels

Gated channels that open in response to mechanical stimulus (such as touch, pressure, vibration, or tissue stretching).

Mechanically-gated channels

These neuroglia are small cells with slender processes that give off numerous spinelike projections. ______________________ function as phagocytes. Like tissue macrophages, they remove cellular debris formed during normal development of the nervous system and phagocytize microbes and damaged nervous tissue.

Microglial cells or microglia

neurosecretory cells

Neurons that secrete neurohormone rather than neurotransmitter.

Neuronal cell bodies also contain free ribosomes and prominent clusters of rough endoplasmic reticulum, termed

Nissl bodies

motor function of nervous system

Once sensory information is integrated, the nervous system may elicit an appropriate motor response by activating effectors (muscles and glands) through cranial and spinal nerves. Stimulation of the effectors causes muscles to contract and glands to secrete.

Neurolemma (Sheath of Schwann)

Outer nucleated cytoplasmic layer of the Schwann cell, which encloses the myelin sheath. Only around axons in the PNS. When an axon is injured, neurolemma aids regeneration by forming a regeneration tube that guides + stimulates regrowth of the axon. Thus CNS axons have less regeneration.

Epidermal Growth Factor (EGF)

Powerful peptide growth factor Produced by glands (salivary and duodenum) Used in laboratories to grow skin grafts

__________________ have many short branching processes and are found in gray matter (described shortly). Fibrous astrocytes have many long unbranched processes and are located mainly in white matter (also described shortly). The processes of astrocytes make contact with blood capillaries, neurons, and the pia mater (a thin membrane around the brain and spinal cord).

Protoplasmic astrocytes

Two types of neuroglia produce myelin sheaths:

Schwann cells (in the PNS) and oligodendrocytes (in the CNS)

sensory function of nervous system

Sensory receptors detect internal stimuli, such as an increase in blood pressure, or external stimuli (for example, a raindrop landing on your arm). This sensory information is then carried into the brain and spinal cord through cranial and spinal nerves.

Although an individual graded potential undergoes decremental conduction, it can become stronger and last longer by summating with other graded potentials. ________________________ is the process by which graded potentials add together. If two depolarizing graded potentials summate, the net result is a larger depolarizing graded potential . If two hyperpolarizing graded potentials summate, the net result is a larger hyperpolarizing graded potential. If two equal but opposite graded potentials summate (one depolarizing and the other hyperpolarizing), then they cancel each other out and the overall graded potential disappears.

Summation

temporal summation

Summation by a postsynaptic cell of input (EPSPs or IPSPs) from a single source over time.

Schwann cell

Supporting cells of the peripheral nervous system responsible for the formation of myelin.

reverberating circuit

The best type of neural pool for producing a prolonged output is

threshold stimulus

The minimal strength required to cause a contraction

Myelination

The sheath electrically insulates the axon of a neuron and increases the speed of nerve impulse conduction.

Gated channels that open in response to voltage stimulus (change in membrane potential).

Voltage-gated channels

nervous system

_is one of the smallest and yet the most complex of the 11 body systems.

refractory period

a period of inactivity after a neuron has fired

suprathreshold stimulus

a stimulus that is strong enough to depolarize the membrane above threshold

a cell body

also known as the perikaryon or soma, contains a nucleus surrounded by cytoplasm that includes typical cellular organelles such as lysosomes, mitochondria, and a Golgi complex.

Neuroglia

are smaller cells, but they greatly outnumber neurons—perhaps by as much as 25 times. Neuroglia support, nourish, and protect neurons, and maintain the interstitial fluid that bathes them. Unlike neurons, neuroglia continue to divide throughout an individual's lifetime.

An axon is a long, thin, cylindrical projection that often joins to the cell body at a cone-shaped elevation called the _______________.

axon hillock

trigger zone

axon hillock and initial segment

axoaxonic

axon to axon

This intricate network of billions of neurons and even more neuroglia is organized into two main subdivisions: the___________________and the _______________________.

central nervous system peripheral nervous system

Central Nervous System (CNS)

consists of the brain and spinal cord

axoplasm

cytoplasm of axon

Wallerian degeneration

degeneration of the distal portion of the axon and myelin sheath

An action potential has two main phases: a ______________________ _____________________ phase . During the depolarizing phase, the negative membrane potential becomes less negative, reaches zero, and then becomes positive.

depolarizing phase and a repolarizing phase

Sensory neurons or afferent neurons

either contain sensory receptors at their distal ends (dendrites) or are located just after sensory receptors that are separate cells. Once an appropriate stimulus activates a sensory receptor, the sensory neuron forms an action potential in its axon and the action potential is conveyed into the CNS through cranial or spinal nerves. Most sensory neurons are unipolar in structure.

At an_____________________ action potentials (impulses) conduct directly between the plasma membranes of adjacent neurons through structures called _________________________. Each gap junction contains a hundred or so tubular connexons, which act like tunnels to connect the cytosol of the two cells directly. As ions flow from one cell to the next through the connexons, the action potential spreads from cell to cell. Gap junctions are common in visceral smooth muscle, cardiac muscle, and the developing embryo. They also occur in the brain.

electrical synapse gap junctions

Axon telodendria

fine extensions of distal axon

fibrous astrocytes

found in white matter

Clusters of Neuronal Cell Bodies Recall that a ______________________refers to a cluster of neuronal cell bodies located in the PNS. As mentioned earlier, ganglia are closely associated with cranial and spinal nerves. By contrast, a ____________________ is a cluster of neuronal cell bodies located in the CNS.

ganglion (plural is ganglia) nucleus

excitatory postsynaptic potential (EPSP)

graded depolarization

parallel after-discharge circuit

incoming fiber stimulates several neurons in parallel arrays to stimulate a common output cell

Summation

increased force of contraction by a skeletal muscle fiber when a twitch occurs before the previous twitch relaxes

metabotropic receptors

is a type of neurotransmitter receptor that contains a neurotransmitter binding site but lacks an ion channel as part of its structure. However, a metabotropic receptor is coupled to a separate ion channel by a type of membrane protein called a G protein. When a neurotransmitter binds to a metabotropic receptor, the G protein either directly opens (or closes) the ion channel or it may act indirectly by activating another molecule, a "second messenger," in the cytosol, which in turn opens (or closes) the ion channel

spatial summation

is summation of postsynaptic potentials in response to stimuli that occur at different locations in the membrane of a postsynaptic cell at the same time. For example, spatial summation results from the buildup of neurotransmitter released simultaneously by several presynaptic end bulbs

ionotropic receptors

ligand-gated ion channels

The______________________division of the PNS conveys output from the CNS to effectors (muscles and glands). This division is further subdivided into a somatic nervous system and an autonomic nervous system

motor or efferent

Gaps in the myelin sheath, called _________________appear at intervals along the axon

nodes of Ranvier

diverging circuit

one input, many outputs

Axolemma

plasma membrane of axon

Norepinephrine (NE)

plays roles in arousal (awakening from deep sleep), dreaming, and regulating mood. A smaller number of neurons in the brain use epinephrine as a neurotransmitter. Both epinephrine and norepinephrine also serve as hormones. Cells of the adrenal medulla, the inner portion of the adrenal gland, release them into the blood.

converging circuit

postsynaptic neuron receives nerve impulses from several different sources

There are two types of astrocytes

protoplasmic astrocytes and fibrous astrocytes

the endocrine system _____________________

responds by releasing hormones

These flat cells surround the cell bodies of neurons of PNS ganglia . Besides providing structural support, ___________________________ regulate the exchanges of materials between neuronal cell bodies and interstitial fluid.

satellite cells

Functions of the nervous system

sensory input, integration, motor output

continuous conduction

slow conduction that occurs in nonmyelinated axons

The________________________conveys output from the CNS to skeletal muscles only. Because its motor responses can be consciously controlled, the action of this part of the PNS is voluntary. The _______________conveys output from the CNS to smooth muscle, cardiac muscle, and glands. Because its motor responses are not normally under conscious control, the action of the ANS is involuntary.

somatic nervous system (SNS) autonomic nervous system (ANS)

There are two types of summation

spatial summation and temporal summation

The brain is the part of the CNS that is located in the skull and contains about 85 billion neurons. The_______________is connected to the brain through the foramen magnum of the occipital bone and is encircled by the bones of the vertebral column. The _____________________contains about 100 million neurons. The CNS processes many different kinds of incoming sensory information. It is also the source of thoughts, emotions, and memories. Most signals that stimulate muscles to contract and glands to secrete originate in the CNS.

spinal cord spinal cord

An action potential will not occur in response to a ____________________ a weak depolarization that cannot bring the membrane potential to threshold.

subthreshold stimulus

Chromatolysis

swelling of a neuron because of injury. induced in the injured nuerons

Enteric Nervous System (ENS)

the autonomic nerves in the walls of the GI tract

neurogenesis

the formation of new neurons

saltatory conduction

the jumping of action potentials from node to node

saltatory conduction

the special mode of action potential propagation that occurs along myelinated axons, occurs because of the uneven distribution of voltage-gated channels. Few voltage-gated channels are present in regions where a myelin sheath covers the axolemma. By contrast, at the nodes of Ranvier , the axolemma has many voltage-gated channels. Hence, current carried by Na+ and K+ flows across the membrane mainly at the nodes.

Synaptic delay is caused by

the time required for calcium entry to trigger exocytosis

An action potential occurs in the membrane of the axon of a neuron when depolarization reaches a certain level termed the______________________(about −55 mV in many neurons).

threshold

postsynaptic neuron

transmits impulses away from the synapse

Axons without such a covering are said to be

unmyelinated

Continuous conduction occurs in ____________________and in muscle fibers.

unmyelinated axons

1.The action potential appears to________________________ from node to node as each nodal area depolarizes to threshold, thus the name "saltatory." Because an action potential leaps across long segments of the myelinated axolemma as current flows from one node to the next, it travels much faster than it would in an unmyelinated axon of the same diameter.

"leap"

Suppose that EPSPs summate in a postsynaptic neuron in response to simultaneous stimulation by the neurotransmitters glutamate, serotonin, and acetylcholine released by three separate presynaptic neurons. Is this an example of spatial or temporal summation?

A single postsynaptic neuron receives input from many presynaptic neurons, some of which release excitatory neurotransmitters and some of which release inhibitory neurotransmitters

A fibers

are the largest diameter axons (5-20 μm) and are myelinated. A fibers have a brief absolute refractory period and conduct nerve impulses (action potentials) at speeds of 12 to 130 m/sec (27-290 mi/hr). The axons of sensory neurons that propagate impulses associated with touch, pressure, position of joints, and some thermal and pain sensations are A fibers, as are the axons of motor neurons that conduct impulses to skeletal muscles.

Ependymal cells (CNS)

are cuboidal to columnar cells arranged in a single layer that possess microvilli and cilia. These cells line the ventricles of the brain and central canal of the spinal cord (spaces filled with cerebrospinal fluid, which protects and nourishes the brain and spinal cord). Functionally, ependymal cells produce, possibly monitor, and assist in the circulation of cerebrospinal fluid. They also form the blood-cerebrospinal fluid barrier

Axosomatic

axon to cell body

Axodendritic

axon to dendrite

presynaptic neuron

conducts impulses toward the synapse

At an electrical synapse, action potentials (impulses) conduct directly between the plasma membranes of adjacent neurons through structures called gap junctions. Each gap junction contains a hundred or so tubular connexons, which act like tunnels to connect the cytosol of the two cells directly. As ions flow from one cell to the next through the connexons, the action potential spreads from cell to cell. Gap junctions are common in visceral smooth muscle, cardiac muscle, and the developing embryo. They also occur in the brain. Electrical synapses have two main advantages:

1.Faster communication. Because action potentials conduct directly through gap junctions, electrical synapses are faster than chemical synapses. At an electrical synapse, the action potential passes directly from the presynaptic cell to the postsynaptic cell. The events that occur at a chemical synapse take some time and delay communication slightly. 2.Synchronization. Electrical synapses can synchronize (coordinate) the activity of a group of neurons or muscle fibers. In other words, a large number of neurons or muscle fibers can produce action potentials in unison if they are connected by gap junctions. The value of synchronized action potentials in the heart or in visceral smooth muscle is coordinated contraction of these fibers to produce a heartbeat or move food through the gastrointestinal tract.

Like muscle fibers, neurons are electrically excitable. They communicate with one another using two types of electrical signals:

(1) Graded potentials (described shortly) are used for short-distance communication only. (2) Action potentials (also described shortly) allow communication over long distances within the body. Recall that an action potential in a muscle fiber is called a muscle action potential. When an action potential occurs in a neuron (nerve cell), it is called a nerve action potential (nerve impulse). To understand the functions of graded potentials and action potentials, consider how the nervous system allows you to feel the smooth surface of a pen that you have picked up from a table

The nearly complete lack of neurogenesis in other regions of the brain and spinal cord seems to result from two factors:

(1) inhibitory influences from neuroglia, particularly oligodendrocytes, 2) absence of growth-stimulating cues that were present during fetal development. Axons in the CNS are myelinated by oligodendrocytes rather than Schwann cells, and this CNS myelin is one of the factors inhibiting regeneration of neurons. Perhaps this same mechanism stops axonal growth once a target region has been reached during development. Also, after axonal damage, nearby astrocytes proliferate rapidly, forming a type of scar tissue that acts as a physical barrier to regeneration. Thus, injury of the brain or spinal cord usually is permanent. Ongoing research seeks ways to improve the environment for existing spinal cord axons to bridge the injury gap. Scientists also are trying to find ways to stimulate dormant stem cells to replace neurons lost through damage or disease and to develop tissue-cultured neurons that can be used for transplantation purposes.

The electrical signals produced by neurons and muscle fibers rely on four types of ion channels: leak channels, ligand-gated channels, mechanically-gated channels, and voltage-gated channels:

1)The gates of leak channels randomly alternate between open and closed positions . Typically, plasma membranes have many more potassium ion (K+) leak channels than sodium ion (Na+) leak channels, and the potassium ion leak channels are leakier than the sodium ion leak channels. Thus, the membrane's permeability to K+ is much higher than its permeability to Na+. Leak channels are found in nearly all cells, including the dendrites, cell bodies, and axons of all types of neurons. 2.A ligand-gated channel opens and closes in response to the binding of a ligand (chemical) stimulus. A wide variety of chemical ligands—including neurotransmitters, hormones, and particular ions—can open or close ligand-gated channels. The neurotransmitter acetylcholine, for example, opens cation channels that allow Na+ and Ca2+ to diffuse inward and K+ to diffuse outward (Figure 12.11b). Ligand-gated channels are located in the dendrites of some sensory neurons, such as pain receptors, and in dendrites and cell bodies of interneurons and motor neurons. 3.A mechanically-gated channel opens or closes in response to mechanical stimulation in the form of vibration (such as sound waves), touch, pressure, or tissue stretching (Figure 12.11c). The force distorts the channel from its resting position, opening the gate. Examples of mechanically-gated channels are those found in auditory receptors in the ears, in receptors that monitor stretching of internal organs, and in touch receptors and pressure receptors in the skin. 4.A voltage-gated channel opens in response to a change in membrane potential (voltage) (Figure 12.11d). Voltage-gated channels participate in the generation and conduction of action potentials in the axons of all types of neurons.

The functions of astrocytes include the following:

1.Astrocytes contain microfilaments that give them considerable strength, which enables them to support neurons. 2.Processes of astrocytes wrapped around blood capillaries isolate neurons of the CNS from various potentially harmful substances in blood by secreting chemicals that maintain the unique selective permeability characteristics of the endothelial cells of the capillaries. 3.In the embryo, astrocytes secrete chemicals that appear to regulate the growth, migration, and interconnection among neurons in the brain. 4.Astrocytes help to maintain the appropriate chemical environment for the generation of nerve impulses. For example, they regulate the concentration of important ions such as K+; take up excess neurotransmitters; and serve as a conduit for the passage of nutrients and other substances between blood capillaries and neurons. 5.Astrocytes may also play a role in learning and memory by influencing the formation of neural synapses

Removal of the neurotransmitter from the synaptic cleft is essential for normal synaptic function. If a neurotransmitter could linger in the synaptic cleft, it would influence the postsynaptic neuron, muscle fiber, or gland cell indefinitely. Neurotransmitter is removed in three ways:

1.Diffusion. Some of the released neurotransmitter molecules diffuse away from the synaptic cleft. Once a neurotransmitter molecule is out of reach of its receptors, it can no longer exert an effect. 2.Enzymatic degradation. Certain neurotransmitters are inactivated through enzymatic degradation. For example, the enzyme acetylcholinesterase breaks down acetylcholine in the synaptic cleft. 3.Uptake by cells. Many neurotransmitters are actively transported back into the neuron that released them (reuptake). Others are transported into neighboring neuroglia (uptake). The neurons that release norepinephrine, for example, rapidly take up the norepinephrine and recycle it into new synaptic vesicles. The membrane proteins that accomplish such uptake are called neurotransmitter transporters.

The sum of all the excitatory and inhibitory effects at any given time determines the effect on the postsynaptic neuron, which may respond in the following ways:

1.EPSP. If the total excitatory effects are greater than the total inhibitory effects but less than the threshold level of stimulation, the result is an EPSP that does not reach threshold. Following an EPSP, subsequent stimuli can more easily generate a nerve impulse through summation because the neuron is partially depolarized. 2.Nerve impulse(s). If the total excitatory effects are greater than the total inhibitory effects and threshold is reached, one or more nerve impulses (action potentials) will be triggered. Impulses continue to be generated as long as the EPSP is at or above the threshold level. 3.IPSP. If the total inhibitory effects are greater than the excitatory effects, the membrane hyperpolarizes (IPSP). The result is inhibition of the postsynaptic neuron and an inability to generate a nerve impulse.

Functional Classification Functionally, neurons are classified according to the direction in which the nerve impulse (action potential) is conveyed with respect to the CNS

1.Sensory neurons or afferent neurons 2.Motor neurons or efferent neurons 3.Interneurons or association neurons

When an action potential propagates along a myelinated axon, an electric current (carried by ions) flows through the extracellular fluid surrounding the myelin sheath and through the cytosol from one node to the next. The action potential at the first node generates ionic currents in the cytosol and extracellular fluid that depolarize the membrane to threshold, opening voltage-gated Na+ channels at the second node. The resulting ionic flow through the opened channels constitutes an action potential at the second node. Then, the action potential at the second node generates an ionic current that opens voltage-gated Na+ channels at the third node, and so on. Each node repolarizes after it depolarizes. The flow of current across the membrane only at the nodes of Ranvier has two consequences:

1.The action potential appears to "leap" from node to node as each nodal area depolarizes to threshold, thus the name "saltatory." Because an action potential leaps across long segments of the myelinated axolemma as current flows from one node to the next, it travels much faster than it would in an unmyelinated axon of the same diameter. 2.Opening a smaller number of channels only at the nodes, rather than many channels in each adjacent segment of membrane, represents a more energy-efficient mode of conduction. Because only small regions of the membrane depolarize and repolarize, minimal inflow of Na+ and outflow of K+ occurs each time an action potential passes by. Thus, less ATP is used by sodium-potassium pumps to maintain the low intracellular concentration of Na+ and the low extracellular concentration of K+.

What kind of graded potential describes a change in membrane potential from −70 to −60 mV? From −70 to −80 mV?

A graded potential occurs when a stimulus causes mechanically-gated or ligand-gated channels to open or close in an excitable cell's plasma membrane. Typically, mechanically-gated channels and ligand-gated channels can be present in the dendrites of sensory neurons, and ligand-gated channels are numerous in the dendrites and cell bodies of interneurons and motor neurons. Hence, graded potentials occur mainly in the dendrites and cell body of a neuron.

Unequal distribution of ions in the ECF and cytosol

A major factor that contributes to the resting membrane potential is the unequal distribution of various ions in extracellular fluid and cytosol . Extracellular fluid is rich in Na+ and chloride ions (Cl−). In cytosol, however, the main cation is K+, and the two dominant anions are phosphates attached to molecules, such as the three phosphates in ATP, and amino acids in proteins. Because the plasma membrane typically has more K+ leak channels than Na+ leak channels, the number of potassium ions that diffuse down their concentration gradient out of the cell into the ECF is greater than the number of sodium ions that diffuse down their concentration gradient from the ECF into the cell. As more and more positive potassium ions exit, the inside of the membrane becomes increasingly negative, and the outside of the membrane becomes increasingly positive.

Which channels are open during the depolarizing phase? During the repolarizing phase?

An action potential occurs in the membrane of the axon of a neuron when depolarization reaches a certain level termed the threshold (about −55 mV in many neurons). Different neurons may have different thresholds for generation of an action potential, but the threshold in a particular neuron usually is constant. The generation of an action potential depends on whether a particular stimulus is able to bring the membrane potential to threshold . An action potential will not occur in response to a subthreshold stimulus, a weak depolarization that cannot bring the membrane potential to threshold. However, an action potential will occur in response to a threshold stimulus, a stimulus that is just strong enough to depolarize the membrane to threshold. Several action potentials will form in response to a suprathreshold stimulus, a stimulus that is strong enough to depolarize the membrane above threshold. Each of the action potentials caused by a suprathreshold stimulus has the same amplitude (size) as an action potential caused by a threshold stimulus. Therefore, once an action potential is generated, the amplitude of an action potential is always the same and does not depend on stimulus intensity. Instead, the greater the stimulus strength above threshold, the greater the frequency of the action potentials until a maximum frequency is reached as determined by the absolute refractory period (described shortly).

_________________________, which is capable of moving materials a distance of 200-400 mm per day, uses proteins that function as "motors" to move materials along the surfaces of microtubules of the neuron's cytoskeleton. Fast axonal transport moves materials in both directions—away from and toward the cell body. Fast axonal transport that occurs in an _________________direction moves organelles and synaptic vesicles from the cell body to the axon terminals. Fast axonal transport that occurs in a ________________________ direction moves membrane vesicles and other cellular materials from the axon terminals to the cell body to be degraded or recycled. Substances that enter the neuron at the axon terminals are also moved to the cell body by fast retrograde transport. These substances include trophic chemicals such as nerve growth factor and harmful agents such as tetanus toxin and the viruses that cause rabies, herpes simplex, and polio.

Fast axonal transport anterograde (forward) retrograde (backward)

Electrogenic nature of the Na+-K+ ATPases

Membrane permeability to Na+ is very low because there are only a few sodium leak channels. Nevertheless, sodium ions do slowly diffuse inward, down their concentration gradient. Left unchecked, such inward leakage of Na+ would eventually destroy the resting membrane potential. The small inward Na+ leak and outward K+ leak are offset by the Na+-K+ ATPases (sodium-potassium pumps) . These pumps help maintain the resting membrane potential by pumping out Na+ as fast as it leaks in. At the same time, the Na+-K+ ATPases bring in K+. However, the potassium ions eventually leak back out of the cell as they move down their concentration gradient. Recall that the Na+-K+ ATPases expel three Na+ for each two K+ imported

Neurogenesis in the CNS _____________________ (noo′-rō-JEN-e-sis)—the birth of new neurons from undifferentiated stem cells—occurs regularly in some animals. For example, new neurons appear and disappear every year in some songbirds. Until recently, the dogma in humans and other primates was "no new neurons" in the adult brain. Then, in 1992, Canadian researchers published their unexpected finding that the hormonelike protein ___________________________ stimulated cells taken from the brains of adult mice to proliferate into both neurons and astrocytes. Previously, EGF was known to trigger mitosis in a variety of nonneuronal cells and to promote wound healing and tissue regeneration. In 1998, scientists discovered that significant numbers of new neurons do arise in the adult human hippocampus, an area of the brain that is crucial for learning.

Neurogenesis epidermal growth factor (EGF)

s These cells encircle PNS axons. Like oligodendrocytes, they form the myelin sheath around axons. A single oligodendrocyte myelinates several axons, but each______________________ myelinates a single axon . A single Schwann cell can also enclose as many as 20 or more unmyelinated axons (axons that lack a myelin sheath) (Figure 12.7b). Schwann cells participate in axon regeneration, which is more easily accomplished in the PNS than in the CNS.

Schwann cell

Propagation of Action Potentials

Series of action potentials occurring in succession down an axon

amino acids

Several amino acids are neurotransmitters in the CNS. Glutamate (glutamic acid) and aspartate (aspartic acid) have powerful excitatory effects. Most excitatory neurons in the CNS and perhaps half of the synapses in the brain communicate via glutamate. At some glutamate synapses, binding of the neurotransmitter to ionotropic receptors opens cation channels. The consequent inflow of cations (mainly Na+ ions) produces an EPSP. Inactivation of glutamate occurs via reuptake. Glutamate transporters actively transport glutamate back into the synaptic end bulbs and neighboring neuroglia.

ATP and other purines

The characteristic ring structure of the adenosine portion of ATP is called a purine ring. Adenosine itself, as well as its triphosphate, diphosphate, and monophosphate derivatives (ATP, ADP, and AMP), is an excitatory neurotransmitter in both the CNS and the PNS. Most of the synaptic vesicles that contain ATP also contain another neurotransmitter. In the PNS, ATP and norepinephrine are released together from some sympathetic neurons; some parasympathetic neurons release ATP and acetylcholine in the same vesicles.

Spatial and Temporal Summation of Postsynaptic Potential

The greater the summation of EPSPs, the greater the chance that threshold will be reached. At threshold, one or more nerve impulses (action potentials) arise.

Integrative function

The nervous system processes sensory information by analyzing it and making decisions for appropriate responses—an activity known as integration.

Integrative function of the nervous system

The nervous system processes sensory information by analyzing it and making decisions for appropriate responses—an activity known as integration.

Which parts of a neuron contain mechanically-gated channels? Ligand-gated channels? decremental

To say that these electrical signals are graded means that they vary in amplitude (size), depending on the strength of the stimulus . They are larger or smaller depending on how many ligand-gated or mechanically-gated channels have opened (or closed) and how long each remains open. The opening or closing of these ion channels alters the flow of specific ions across the membrane, producing a flow of current that is localized, which means that it spreads to adjacent regions along the plasma membrane in either direction from the stimulus source for a short distance and then gradually dies out as the charges are lost across the membrane through leak channels. This mode of travel by which graded potentials die out as they spread along the membrane is known as ___________________conduction . Because they die out within a few millimeters of their point of origin, graded potentials are useful for short-distance communication only.

_______________________ have dendrites and one axon that are fused together to form a continuous process that emerges from the cell body .. These neurons are more appropriately called _________________________because they begin in the embryo as bipolar neurons. During development, the dendrites and axon fuse together and become a single process. The dendrites of most unipolar neurons function as sensory receptors that detect a sensory stimulus such as touch, pressure, pain, or thermal stimuli. The trigger zone for nerve impulses in a unipolar neuron is at the junction of the _______________________. The impulses then propagate toward the synaptic end bulbs. The cell bodies of most unipolar neurons are located in the ganglia of spinal and cranial nerves.

Unipolar neurons pseudounipolar neurons dendrites and axon

subthreshold stimulus

a stimulus too small to create an action potential in a neuron

relative refractory period

a stronger than usual stimulus is necessary to initiate an action potential

chemical synapse

a type of synapse at which a chemical (a neurotransmitter) is released from the axon of a neuron into the synaptic cleft, where it binds to receptors on the next structure (either another neuron or an organ)

small molecule neurotransmitters

acetylcholine, amino acids, biogenic amines, ATP and other purines, nitric oxide, and carbon monoxide.

An ___________________________ or impulse is a sequence of rapidly occurring events that decrease and reverse the membrane potential and then eventually restore it to the resting state.

action potential (AP)

An ___________________________ or impulse is a sequence of rapidly occurring events that decrease and reverse the membrane potential and then eventually restore it to the resting state. An action potential has two main phases: a ______________________ _____________________ phase . During the depolarizing phase, the negative membrane potential becomes less negative, reaches zero, and then becomes positive. During the _____________________, the membrane potential is restored to the resting state of −70 mV. Following the repolarizing phase there may be an ____________________________ during which the membrane potential temporarily becomes more negative than the resting level. Two types of voltage-gated channels open and then close during an action potential. These channels are present mainly in the axon plasma membrane and axon terminals. The first channels that open, the voltage-gated Na+ channels, allow Na+ to rush into the cell, which causes the depolarizing phase. Then voltage-gated K+ channels open, allowing K+ to flow out, which produces the repolarizing phase. The after-hyperpolarizing phase occurs when the voltage-gated K+ channels remain open after the repolarizing phase ends.

action potential (AP) depolarizing phase and a repolarizing phase repolarizing phase after-hyperpolarizing phase

Each voltage-gated Na+ channel has two separate gates, an ____________________________ In the resting state of a voltage-gated Na+ channel, the____________________________is open, but the activation gate is closed . As a result, Na+ cannot move into the cell through these channels. At threshold, voltage-gated Na+ channels are activated. In the activated state of a voltage-gated Na+ channel, both the activation and inactivation gates in the channel are open and Na+ inflow begins . As more channels open, Na+ inflow increases, the membrane depolarizes further, and more Na+ channels open.

activation gate and an inactivation gate. inactivation gate

After-Hyperpolarizing Phase While the voltage-gated K+ channels are open, outflow of K+ may be large enough to cause an ___________________phase of the action potential . During this phase, the voltage-gated K+ channels remain open and the membrane potential becomes even more negative (about −90 mV). As the voltage-gated K+ channels close, the membrane potential returns to the resting level of −70 mV. Unlike voltage-gated Na+ channels, most voltage-gated K+ channels do not exhibit an inactivated state. Instead, they alternate between closed (resting) and open (activated) states.

after-hyperpolarizing

Following the repolarizing phase there may be an ____________________________ during which the membrane potential temporarily becomes more negative than the resting level.

after-hyperpolarizing phase

As you have just learned, an action potential is generated in response to a threshold stimulus but does not form when there is a subthreshold stimulus. In other words, an action potential either occurs completely or it does not occur at all. This characteristic of an action potential is known as the _______________________ .The all-or-none principle of the action potential is similar to pushing the first domino in a long row of standing dominoes. When the push on the first domino is strong enough that domino falls against the second domino, and the entire row topples.Stronger pushes on the first domino produce the identical effect—toppling of the entire row. Thus, pushing on the first domino produces an all-or-none event: The dominoes all fall or none fall.

all-or-none principle.

Neurons

also connect all regions of the body to the brain and spinal cord. As highly specialized cells capable of reaching great lengths and making extremely intricate connections with other cells, neurons provide most of the unique functions of the nervous system, such as sensing, thinking, remembering, controlling muscle activity, and regulating glandular secretions. As a result of their specialization, most neurons have lost the ability to undergo mitotic divisions.

B fibers

are axons with diameters of 2-3 μm. Like A fibers, B fibers are myelinated and exhibit saltatory conduction at speeds up to 15 m/sec (34 mi/hr). B fibers have a somewhat longer absolute refractory period than A fibers. B fibers conduct sensory nerve impulses from the viscera to the brain and spinal cord. They also constitute all of the axons of the autonomic motor neurons that extend from the brain and spinal cord to the ANS relay stations called autonomic ganglia.

Interneurons or association neurons

are mainly located within the CNS between sensory and motor neurons .Interneurons integrate (process) incoming sensory information from sensory neurons and then elicit a motor response by activating the appropriate motor neurons. Most interneurons are multipolar in structure.

Dendrites

are the receiving or input portions of a neuron. The plasma membranes of dendrites (and cell bodies) contain numerous receptor sites for binding chemical messengers from other cells. Dendrites usually are short, tapering, and highly branched. In many neurons the dendrites form a tree-shaped array of processes extending from the cell body. Their cytoplasm contains Nissl bodies, mitochondria, and other organelles.

C fibers

are the smallest diameter axons (0.5-1.5 μm) and all are unmyelinated. Nerve impulse propagation along a C fiber ranges from 0.5 to 2 m/sec (1-4 mi/hr). C fibers exhibit the longest absolute refractory periods. These unmyelinated axons conduct some sensory impulses for pain, touch, pressure, heat, and cold from the skin, and pain impulses from the viscera. Autonomic motor fibers that extend from autonomic ganglia to stimulate the heart, smooth muscle, and glands are C fibers. Examples of motor functions of B and C fibers are constricting and dilating the pupils, increasing and decreasing the heart rate, and contracting and relaxing the urinary bladder.

Although the plasma membranes of presynaptic and postsynaptic neurons in a _________________are close, they do not touch. They are separated by the __________________a space of 20-50 nm* that is filled with interstitial fluid. Nerve impulses cannot conduct across the synaptic cleft, so an alternative, indirect form of communication occurs. In response to a nerve impulse, the presynaptic neuron releases a neurotransmitter that diffuses through the fluid in the synaptic cleft and binds to receptors in the plasma membrane of the _____________________. The postsynaptic neuron receives the chemical signal and in turn produces a postsynaptic potential, a type of graded potential. Thus, the presynaptic neuron converts an electrical signal (nerve impulse) into a chemical signal (released neurotransmitter). The postsynaptic neuron receives the chemical signal and in turn generates an electrical signal (postsynaptic potential). The time required for these processes at a chemical synapse, a _________________________ of about 0.5 msec, is the reason that chemical synapses relay signals more slowly than electrical synapses.

chemical synapse synaptic cleft, postsynaptic neuron synaptic delay

When there is damage to an axon, changes usually occur both in the cell body of the affected neuron and in the portion of the axon distal to the site of injury. Changes also may occur in the portion of the axon proximal to the site of injury. About 24 to 48 hours after injury to a process of a normal peripheral neuron , the Nissl bodies break up into fine granular masses. This alteration is called_________________. By the third to fifth day, the part of the axon distal to the damaged region becomes slightly swollen and then breaks up into fragments; the myelin sheath also deteriorates (Figure 12.29b). Even though the axon and myelin sheath degenerate, the neurolemma remains. Degeneration of the distal portion of the axon and myelin sheath is called ___________________(waw-LE-rē′-an).

chromatolysis Wallerian degeneration

Peripheral Nervous System (PNS)

consists of all nervous tissue outside the CNS

Continuous and Saltatory Conduction There are two types of propagation:_________________________. The type of action potential propagation described so far is continuous conduction, which involves step-by-step depolarization and repolarization of each adjacent segment of the plasma membrane. In continuous conduction, ions flow through their voltage-gated channels in each adjacent segment of the membrane. Note that the action potential propagates only a relatively short distance in a few milliseconds. Continuous conduction occurs in unmyelinated axons and in muscle fibers.

continuous conduction and saltatory conduction

In another arrangement, called _____________________ several presynaptic neurons synapse with a single postsynaptic neuron. This arrangement permits more effective stimulation or inhibition of the postsynaptic neuron. In a __________________(Figure 12.28b), the postsynaptic neuron receives nerve impulses from several different sources. For example, a single motor neuron that synapses with skeletal muscle fibers at neuromuscular junctions receives input from several pathways that originate in different brain regions.

convergence converging circuit

Motor neurons or efferent neurons

convey action potentials away from the CNS to effectors (muscles and glands) in the periphery (PNS) through cranial or spinal nerves Motor neurons are multipolar in structure.

slow axonal transport

conveys axoplasm in one direction only - from the cell body toward the axon terminals.

Twelve pairs of _____________________emerge from the brain and thirty-one pairs of _______________emerge from the spinal cord. Each nerve follows a defined path and serves a specific region of the body. The term _________________________refers to a structure of the nervous system that monitors changes in the external or internal environment. Examples of sensory receptors include touch receptors in the skin, photoreceptors in the eye, and olfactory (smell) receptors in the nose.

cranial nerves spinal nerves sensory receptor

Neurology

deals with normal functioning and disorders of the nervous system. A neurologist is a physician who diagnoses and treats disorders of the nervous system.

A single presynaptic neuron may synapse with several postsynaptic neurons. Such an arrangement, called __________________ permits one presynaptic neuron to influence several postsynaptic neurons (or several muscle fibers or gland cells) at the same time. In a _________________, the nerve impulse from a single presynaptic neuron causes the stimulation of increasing numbers of cells along the circuit .For example, a small number of neurons in the brain that govern a particular body movement stimulate a much larger number of neurons in the spinal cord. Sensory signals are also arranged in diverging circuits, allowing a sensory impulse to be relayed to several regions of the brain. This arrangement amplifies the signal.

divergence, diverging circuit

The postsynaptic neuron receives the chemical signal and in turn produces a postsynaptic potential, a type of graded potential. Thus, the presynaptic neuron converts an ______________________into a chemical signal (released neurotransmitter)

electrical signal (nerve impulse)

Ion Channels When ion channels are open, they allow specific ions to move across the plasma membrane, down their ___________________________—a concentration (chemical) difference plus an electrical difference. Recall that ions move from areas of higher concentration to areas of ____________________________ .Also, positively charged cations move toward a negatively charged area, and negatively charged anions move toward a positively charged area (the electrical aspect of the gradient). As ions move, they create a flow of electrical current that can change the membrane potential.

electrochemical gradient lower concentration

. A third branch of the autonomic nervous system is the_______________________, an extensive network of over 100 million neurons confined to the wall of the gastrointestinal (GI) tract. The ENS helps regulate the activity of the smooth muscle and glands of the GI tract. Although the ENS can function independently, it communicates with and is regulated by the other branches of the ANS.

enteric nervous system (ENS)

A neurotransmitter causes either an excitatory or an inhibitory graded potential. A neurotransmitter that causes depolarization of the postsynaptic membrane is excitatory because it brings the membrane closer to threshold . A depolarizing postsynaptic potential is called an ____________________________________. Although a single EPSP normally does not initiate a nerve impulse, the postsynaptic cell does become more excitable. Because it is partially depolarized, it is more likely to reach threshold when the next EPSP occurs.

excitatory postsynaptic potential (EPSP)

Brain tumors derived from glia, called __________________, tend to be highly malignant and to grow rapidly. Of the six types of neuroglia, four—astrocytes, oligodendrocytes, microglia, and ependymal cells—are found only in the CNS. The remaining two types—Schwann cells and satellite cells—are present in the PNS.

gliomas

A ______________________is a small deviation from the resting membrane potential that makes the membrane either more polarized (inside more negative) or less polarized (inside less negative). When the response makes the membrane more polarized (inside more negative), it is termed a___________________________. When the response makes the membrane less polarized (inside less negative), it is termed a ______________________________

graded potential hyperpolarizing graded potential depolarizing graded potential

We have seen that excitable cells—neurons and muscle fibers—produce two types of electrical signals:

graded potentials and action potentials

neural circuits

groups of interconnected neurons

A neurotransmitter that causes hyperpolarization of the postsynaptic membrane is inhibitory. During hyperpolarization, generation of an action potential is more difficult than usual because the membrane potential becomes inside more negative and thus even farther from threshold than in its resting state. A hyperpolarizing postsynaptic potential is termed an_____________

inhibitory postsynaptic potential (IPSP)

. The part of the axon closest to the axon hillock is the ________________. In most neurons, nerve impulses arise at the junction of the axon hillock and the initial segment, an area called the _________________ from which they travel along the axon to their destination. An axon contains mitochondria, microtubules, and neurofibrils. Because rough endoplasmic reticulum is not present, protein synthesis does not occur in the axon. The cytoplasm of an axon, called _______________ is surrounded by a plasma membrane known as the __________________ Along the length of an axon, side branches called __________________may branch off, typically at a right angle to the axon. The axon and its collaterals end by dividing into many fine processes called axon terminals or _____________________.

initial segment trigger zone axoplasm axolemma axon collaterals axon telodendria

Ionotropic Receptors: An__________________ is a type of neurotransmitter receptor that contains a neurotransmitter binding site and an ion channel. In other words, the neurotransmitter binding site and the ion channel are components of the same protein. An ionotropic receptor is a type of ligand-gated channel . In the absence of neurotransmitter (the ligand), the ion channel component of the ionotropic receptor is closed. When the correct neurotransmitter binds to the ionotropic receptor, the ion channel opens, and an EPSP or IPSP occurs in the postsynaptic cell.

ionotropic receptor

An_____________________is a type of ligand-gated channel . In the absence of neurotransmitter (the ligand), the ion channel component of the ionotropic receptor is closed. When the correct neurotransmitter binds to the ___________________, the ion channel opens, and an EPSP or IPSP occurs in the postsynaptic cell.

ionotropic receptor ionotropic receptor,

nerve

is a bundle of hundreds to thousands of axons plus associated connective tissue and blood vessels that lies outside the brain and spinal cord.

nerve fiber

is a general term for any neuronal process (extension) that emerges from the cell body of a neuron. Most neurons have two kinds of processes: multiple dendrites and a single axon

1)The gates of ________________________ randomly alternate between open and closed positions . Typically, plasma membranes have many more potassium ion (K+) leak channels than sodium ion (Na+) leak channels, and the potassium ion leak channels are leakier than the sodium ion leak channels. Thus, the membrane's permeability to K+ is much higher than its permeability to Na+. Leak channels are found in nearly all cells, including the dendrites, cell bodies, and axons of all types of neurons.

leak channels

A ______________________and closes in response to the binding of a ligand (chemical) stimulus. A wide variety of chemical ligands—including neurotransmitters, hormones, and particular ions—can open or close ligand-gated channels. The neurotransmitter acetylcholine, for example, opens cation channels that allow Na+ and Ca2+ to diffuse inward and K+ to diffuse outward (Figure 12.11b). Ligand-gated channels are located in the dendrites of some sensory neurons, such as pain receptors, and in dendrites and cell bodies of interneurons and motor neurons.

ligand-gated channel opens

CNS contains billions of neurons organized into complicated networks called __________________, functional groups of neurons that process specific types of information. In a simple series circuit, a presynaptic neuron stimulates a _____________________. The second neuron then stimulates another, and so on. However, most neural circuits are more complex.

neural circuits single postsynaptic neuron

_______________of about 0.5 msec, is the reason that chemical synapses relay signals more slowly than electrical synapses.

synaptic delay

To say that these electrical signals are graded means that they vary in amplitude (size), depending on the strength of the stimulus . They are larger or smaller depending on how many__________________________have opened (or closed) and how long each remains open. The opening or closing of these ion channels alters the flow of specific ions across the membrane, producing a flow of current that is localized, which means that it spreads to adjacent regions along the plasma membrane in either direction from the stimulus source for a short distance and then gradually dies out as the charges are lost across the membrane through leak channels. This mode of travel by which graded potentials die out as they spread along the membrane is known as ___________________conduction . Because they die out within a few millimeters of their point of origin, graded potentials are useful for short-distance communication only.

ligand-gated or mechanically-gated channels decremental

Neuroglia

make up about half the volume of the CNS. Their name derives from the idea of early histologists that they were the "glue" that held nervous tissue together. We now know that neuroglia are not merely passive bystanders but rather actively participate in the activities of nervous tissue. Generally, neuroglia are smaller than neurons, and they are 5 to 25 times more numerous. In contrast to neurons, glia do not generate or propagate action potentials, and they can multiply and divide in the mature nervous system. In cases of injury or disease, neuroglia multiply to fill in the spaces formerly occupied by neurons.

A________________________ opens or closes in response to mechanical stimulation in the form of vibration,, touch, pressure, or tissue stretching. The force distorts the channel from its resting position, opening the gate. Examples of mechanically-gated channels are those found in auditory receptors in the ears, in receptors that monitor stretching of internal organs, and in touch receptors and pressure receptors in the skin.

mechanically-gated channel

Bundles of Axons: Recall that a ______________ __________________that is located in the PNS. Cranial nerves connect the brain to the periphery, whereas spinal nerves connect the spinal cord to the periphery. A _________________is a bundle of axons that is located in the CNS. Tracts interconnect neurons in the spinal cord and brain. Gray and White Matter In a freshly dissected section of the brain or spinal cord, some regions look white and glistening, and others appear gray . ____________________is composed primarily of myelinated axons. The whitish color of myelin gives white matter its name. The _________________of the nervous system contains neuronal cell bodies, dendrites, unmyelinated axons, axon terminals, and neuroglia. It appears grayish, rather than white, because the Nissl bodies impart a gray color and there is little or no myelin in these areas. Blood vessels are present in both white and gray matter. In the spinal cord, the white matter surrounds an inner core of gray matter that, depending on how imaginative you are, is shaped like a butterfly or the letter H in transverse section; in the brain, a thin shell of gray matter covers the surface of the largest portions of the brain, the cerebrum and cerebellum

nerve is a bundle of axons tract White matter gray matter

Components of the PNS include__________________

nerves and sensory receptors

With a mass of only 2 kg (4.5 lb), about 3% of total body weight, the _________________________is one of the smallest and yet the most complex of the 11 body systems. This intricate network of billions of neurons and even more neuroglia is organized into two main subdivisions: the central nervous system and the peripheral nervous system. _________________________deals with normal functioning and disorders of the nervous system. A neurologist is a physician who diagnoses and treats disorders of the nervous system.

nervous system Neurology

The ribosomes are the sites of protein synthesis. Newly synthesized proteins produced by Nissl bodies are used to replace cellular components, as material for growth of neurons, and to regenerate damaged axons in the PNS. The cytoskeleton includes both ___________________ composed of bundles of intermediate filaments that provide the cell shape and support, and __________________________, which assist in moving materials between the cell body and axon. Aging neurons also contain_________________________ , a pigment that occurs as clumps of yellowish brown granules in the cytoplasm. Lipofuscin is a product of neuronal lysosomes that accumulates as the neuron ages, but does not seem to harm the neuron. A collection of neuron cell bodies outside the CNS is called a _______________________________.

neurofibrils microtubules lipofuscin ganglion

Damage and Repair in the PNS Axons and dendrites that are associated with a ____________________may undergo repair if the cell body is intact, if the Schwann cells are functional, and if scar tissue formation does not occur too rapidly (Figure 12.29). Most nerves in the PNS consist of processes that are covered with a neurolemma. A person who injures axons of a nerve in an upper limb, for example, has a good chance of regaining nerve function.

neurolemma

Eventually, multiple layers of glial plasma membrane surround the axon, with the Schwann cell's cytoplasm and nucleus forming the outermost layer. The inner portion, consisting of up to 100 layers of Schwann cell membrane, is the myelin sheath. The outer nucleated cytoplasmic layer of the Schwann cell, which encloses the myelin sheath, is the_______________

neurolemma

(2) Action potentials (also described shortly) allow communication over long distances within the body. Recall that an action potential in a muscle fiber is called a muscle action potential. When an action potential occurs in a________________________, it is called a ___________________________. To understand the functions of graded potentials and action potentials, consider how the nervous system allows you to feel the smooth surface of a pen that you have picked up from a table

neuron (nerve cell) nerve action potential (nerve impulse)

This chapter focuses on the organization of the nervous system and the properties of__________________ (nerve cells) and _______________ (cells that support the activities of neurons). We then examine the structure and functions of the _______________________, and of the brain and cranial nerves . The autonomic nervous system, which operates without voluntary control, will be covered in .will discuss the __________________—touch, pressure, warmth, cold, pain, and others—and their sensory and motor pathways to show how nerve impulses pass into the spinal cord and brain or from the spinal cord and brain to muscles and glands. Exploration of the nervous system concludes with a discussion of the special senses: smell, taste, vision, hearing, and equilibrium.

neurons neuroglia spinal cord and spinal nerves somatic senses

Like muscle cells, __________________possess electrical excitability the ability to respond to a stimulus and convert it into an action potential. A stimulus is any change in the environment that is strong enough to initiate an action potential. An _______________________l (nerve impulse) is an electrical signal that____________________(travels) along the surface of the membrane of a neuron. It begins and travels due to the movement of _____________________(such as sodium and potassium) between interstitial fluid and the inside of a neuron through specific ion channels in its plasma membrane. Once begun, a nerve impulse travels rapidly and at a constant strength.

neurons (nerve cells) action potential propagates ions

Neurotransmitters consisting of 3 to 40 amino acids linked by peptide bonds called __________________________ are numerous and widespread in both the CNS and PNS. Neuropeptides bind to metabotropic receptors and have excitatory or inhibitory actions, depending on the type of metabotropic receptor at the synapse. Neuropeptides are formed in the neuron cell body, packaged into vesicles, and transported to axon terminals. Besides their role as neurotransmitters, many neuropeptides serve as hormones that regulate physiological responses elsewhere in the body

neuropeptides

Many neurotransmitters are also hormones released into the bloodstream by endocrine cells in organs throughout the body. Within the brain, certain neurons, called ___________________ also secrete hormones. Neurotransmitters can be divided into two classes based on size: small-molecule neurotransmitters and neuropeptides

neurosecretory cells

single axon

of a neuron propagates nerve impulses toward another neuron, a muscle fiber, or a gland cell.

single postsynaptic neuron

often stimulated by more than one neuron

These resemble astrocytes but are smaller and contain fewer processes. Processes of ______________________ are responsible for forming and maintaining the myelin sheath around CNS axons. As you will see shortly, the myelin sheath is a multilayered lipid and protein covering around some axons that insulates them and increases the speed of nerve impulse conduction. Such axons are said to be _____________________.

oligodendrocytes myelinated

Why may electrical synapses work in two directions, but chemical synapses can transmit a signal in only one direction? At most chemical synapses, only _________________________can occur—from a presynaptic neuron to a postsynaptic neuron or an effector, such as a muscle fiber or a gland cell. For example, synaptic transmission at a neuromuscular junction (NMJ) proceeds from a somatic motor neuron to a skeletal muscle fiber (but not in the opposite direction). Only synaptic end bulbs of presynaptic neurons can release neurotransmitter, and only the postsynaptic neuron's membrane has the receptor proteins that can recognize and bind that neurotransmitter. As a result, action potentials move in one direction.

one-way information transfer

voltage gated channels

open and close in response to changes in membrane potential

mechanically gated channels

open and close in response to physical deformation of receptors

The nervous system is also responsible for our

our perceptions, behaviors, and memories, and it initiates all voluntary movements. Because this system is quite complex, we discuss its structure and function in several chapters

A fourth type of circuit is the___________________________ (Figure 12.28d). In this circuit, a single presynaptic cell stimulates a group of neurons, each of which synapses with a common postsynaptic cell. A differing number of synapses between the first and last neurons imposes varying synaptic delays, so that the last neuron exhibits multiple EPSPs or IPSPs. If the input is excitatory, the postsynaptic neuron then can send out a stream of impulses in quick succession. Parallel after-discharge circuits may be involved in precise activities such as mathematical calculations.

parallel after-discharge circuit

Throughout your life, your nervous system exhibits ____________________ (plas-TIS-i-tē), the capability to change based on experience. At the level of individual neurons, the changes that can occur include the sprouting of new dendrites, synthesis of new proteins, and changes in synaptic contacts with other neurons. Undoubtedly, both chemical and electrical signals drive the changes that occur. Despite this _________________mammalian neurons have very limited powers of regeneration, the capability to replicate or repair themselves. In the PNS, damage to dendrites and myelinated axons may be repaired if the cell body remains intact and if the Schwann cells that produce myelination remain active. In the CNS, little or no repair of damage to neurons occurs. Even when the cell body remains intact, a severed axon cannot be repaired or regrown.

plasticity, plasticity,

Graded potentials have different names depending on which type of stimulus causes them and where they occur. For example, when a graded potential occurs in the dendrites or cell body of a neuron in response to a neurotransmitter, it is called a _______________________. On the other hand, the graded potentials that occur in sensory receptors are termed receptor potentials

postsynaptic potential (explained shortly)

The PNS is divided into ____________________. The ____________________division of the PNS conveys input into the CNS from sensory receptors in the body. This division provides the CNS with sensory information about the somatic senses (tactile, thermal, pain, and proprioceptive sensations) and special senses (smell, taste, vision, hearing, and equilibrium).

sensory and motor divisions sensory or afferent

Propagation of Action Potentials To communicate information from one part of the body to another, action potentials in a neuron must travel from where they arise at the trigger zone of the axon to the axon terminals. In contrast to the graded potential, an action potential is not decremental . Instead, an action potential keeps its strength as it spreads along the membrane. This mode of conduction is called _________________ and it depends on positive feedback. As you have already learned, when sodium ions flow in, they cause_____________________channels in adjacent segments of the membrane to open. Thus, the action potential travels along the membrane rather like the activity of that long row of dominoes. In actuality, it is not the same action potential that propagates along the entire axon. Instead, the action potential regenerates over and over at adjacent regions of membrane from the _____________________to the axon terminals. In a neuron, an action potential can propagate in this direction only—it cannot propagate back toward the cell body because any region of membrane that has just undergone an action potential is temporarily in the absolute refractory period and cannot generate another action potential. Because they can travel along a membrane without dying out, action potentials function in communication over long distances.

propagation voltage-gated Na+ trigger zone

Refractory Period The period of time after an action potential begins during which an excitable cell cannot generate another action potential in response to a normal threshold stimulus is called the_____________________. During the absolute refractory period, even a very strong stimulus cannot initiate a second action potential. This period coincides with the period of Na+ channel activation and inactivation. Inactivated Na+ channels cannot reopen; they first must return to the resting state . In contrast to action potentials, graded potentials do not exhibit a refractory period. Large-diameter axons have a larger surface area and have a brief absolute refractory period of about 0.4 msec. Because a second nerve impulse can arise very quickly, up to 1000 impulses per second are possible. Small-diameter axons have absolute refractory periods as long as 4 msec, enabling them to transmit a maximum of 250 impulses per second. Under normal body conditions, the maximum frequency of nerve impulses in different axons ranges between 10 and 1000 per second. The _______________________ is the period of time during which a second action potential can be initiated, but only by a larger-than-normal stimulus. It coincides with the period when the voltage-gated K+ channels are still open after inactivated Na+ channels have returned to their resting state (see Figure 12.18).

refractory period relative refractory period

During the depolarizing phase, the negative membrane potential becomes less negative, reaches zero, and then becomes positive. During the _____________________, the membrane potential is restored to the resting state of −70 mV.

repolarizing phase

The production of graded potentials and action potentials depends on two basic features of the plasma membrane of excitable cells: the existence of a __________________________ potential and the presence of specific types of ion channels. Like most other cells in the body, the ______________________of excitable cells exhibits a membrane potential, an electrical potential difference (voltage) across the membrane. In excitable cells, this voltage is termed the resting membrane potential. The membrane potential is like voltage stored in a battery. If you connect the positive and negative terminals of a battery with a piece of wire, electrons will flow along the wire. This flow of charged particles is called____________________. In living cells, the flow of ions constitutes the electrical current.

resting membrane potencial plasma membrane current

Some circuits are organized so that stimulation of the presynaptic cell causes the postsynaptic cell to transmit a series of nerve impulses. One such circuit is called a _________________ (Figure 12.28c). In this pattern, the incoming impulse stimulates the first neuron, which stimulates the second, which stimulates the third, and so on. Branches from later neurons synapse with earlier ones. This arrangement sends impulses back through the circuit again and again. The output signal may last from a few seconds to many hours, depending on the number of synapses and the arrangement of neurons in the circuit. Inhibitory neurons may turn off a reverberating circuit after a period of time. Among the body responses thought to be the result of output signals from reverberating circuits are breathing, coordinated muscular activities, waking up, and short-term memory.

reverberating circuit

The three basic functions of the nervous system occur, for example, when you answer your cell phone after hearing it ring. The sound of the ringing cell phone stimulates sensory receptors in your ears (_____________________). This auditory information is subsequently relayed into your brain where it is processed and the decision to answer the phone is made (________________________). The brain then stimulates the contraction of specific muscles that will allow you to grab the phone and press the appropriate button to answer it (___________________).

sensory function integrative function motor function

Because some substances synthesized or recycled in the neuron cell body are needed in the axon or at the axon terminals, two types of transport systems carry materials from the cell body to the axon terminals and back. The slower system, which moves materials about 1-5 mm per day, is called ________________. It conveys axoplasm in one direction only—from the cell body toward the axon terminals. Slow axonal transport supplies new axoplasm to developing or regenerating axons and replenishes axoplasm in growing and mature axons.

slow axonal transport.

Several action potentials will form in response to a _______________________, a stimulus that is strong enough to depolarize the membrane above threshold. Each of the action potentials caused by a suprathreshold stimulus has the same amplitude (size) as an action potential caused by a threshold stimulus.

suprathreshold stimulus

The ANS is comprised of two main branches, the _____________________ With a few exceptions, effectors receive innervation from both of these branches, and usually the two branches have opposing actions. For example, neurons of the sympathetic nervous system increase heart rate, and neurons of the parasympathetic nervous system slow it down. In general, the parasympathetic nervous system takes care of "rest-and-digest" activities, and the sympathetic nervous system helps support exercise or emergency actions—the so-called "fight-or-flight" responses.

sympathetic nervous system and the parasympathetic nervous system.

that a____________________ is a region where communication occurs between two neurons or between a____________________.The term _______________________ refers to a nerve cell that carries a nerve impulse toward a synapse. It is the cell that sends a signal. A postsynaptic cell is the cell that receives a signal. It may be a nerve cell called a ______________________ that carries a nerve impulse away from a synapse or an effector cell that responds to the impulse at the synapse. Most synapses between neurons are ___________________ while others are __________________ or ____________________ In addition, synapses may be electrical or chemical, and they differ both structurally and functionally.

synapse neuron and an effector cell presynaptic neuron postsynaptic neuron axodendritic axosomatic axoaxonic

The site of communication between two neurons or between a neuron and an effector cell is called a _____________________.The tips of some axon terminals swell into bulb-shaped structures called synaptic end bulbs; others exhibit a string of swollen bumps called ______________________. Both synaptic end bulbs and varicosities contain many tiny membrane-enclosed sacs called __________________that store a chemical called a __________________ . A neurotransmitter is a molecule released from a synaptic vesicle that excites or inhibits another neuron, muscle fiber, or gland cell. Many neurons contain two or even three types of neurotransmitters, each with different effects on the postsynaptic cell.

synapse varicosities synaptic vesicles neurotransmitter

An action potential occurs in the membrane of the axon of a neuron when depolarization reaches a certain level termed the______________________(about −55 mV in many neurons). Different neurons may have different thresholds for generation of an action potential, but the threshold in a particular neuron usually is constant. The generation of an action potential depends on whether a particular stimulus is able to bring the membrane potential to threshold . An action potential will not occur in response to a ____________________ a weak depolarization that cannot bring the membrane potential to threshold. However, an action potential will occur in response to a __________________________, a stimulus that is just strong enough to depolarize the membrane to threshold. Several action potentials will form in response to a _______________________, a stimulus that is strong enough to depolarize the membrane above threshold. Each of the action potentials caused by a suprathreshold stimulus has the same amplitude (size) as an action potential caused by a threshold stimulus. Therefore, once an action potential is generated, the amplitude of an action potential is always the same and does not depend on stimulus intensity. Instead, the greater the stimulus strength above threshold, the greater the frequency of the action potentials until a maximum frequency is reached as determined by the absolute refractory period (described shortly).

threshold subthreshold stimulus threshold stimulus suprathreshold stimulus

However, an action potential will occur in response to a __________________________, a stimulus that is just strong enough to depolarize the membrane to threshold.

threshold stimulus

Both the nervous and endocrine systems have the same objective:

to keep controlled conditions within limits that maintain life

activation gate and inactivation gate

two kinds of gates in the Na+ channels:

A ______________________ opens in response to a change in membrane potential (voltage) .. Voltage-gated channels participate in the generation and conduction of action potentials in the axons of all types of neurons.

voltage-gated channel

Acetylcholine (ACh) (neurotransmitter)

which is released by many PNS neurons and by some CNS neurons. ACh is an excitatory neurotransmitter at some synapses, such as the neuromuscular junction, where the binding of ACh to ionotropic receptors opens cation channels . It is also an inhibitory neurotransmitter at other synapses, where it binds to metabotropic receptors coupled to G proteins that open K+ channels. For example, ACh slows heart rate at inhibitory synapses made by parasympathetic neurons of the vagus (X) nerve. The enzyme acetylcholinesterase (AChE) inactivates ACh by splitting it into acetate and choline fragments.