Chapter 12 - Introduction to the Nervous System

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.

*electrochemical gradients*

A ____ occurs when a stimulus causes ligand-gated or mechanically gated channels to open or close in an excitable cell's plasma membrane.

*graded potential*

Although an individual *graded potential* can die out as it spreads along the membrane, it can become stronger and last longer by summating with other ____.

*graded potentials*

Instead, an action potential keeps its strength as it spreads along the membrane. This mode of travel is called ____ and it depends on positive feedback.

*impulse propagation*

At most chemical synapses, only ____ can occur--from a presynaptic neuron to a postsynaptic neuron or to an effector, such as a muscle fiber or a gland cell. As a result, action potentials move in one direction.

*one-way information transfer*

The postsynaptic neuron receives the chemical signal and, in turn, produces a ____, a type of graded potential.

*postsynaptic potential*

A synapse is the site of communication between two neurons or between a neuron and an effector cell (muscle cell or glandular cell) through a series of events known as ____.

*synaptic transmission*

Most synapses are either:

+*Axodendritic* (from axon to dendrite) +*Axosomatic* (from axon to cell body) +*Axoaxonic* (from axon to axon)

With a few exceptions, *effectors* are stimulated by both divisions, usually with opposing actions. For example:

+*Sympathetic* neurons increase heart rate. +*Parasympathetic* neurons slow it down.

*Polarized*

+A cell that exhibits a membrane potential.

*Excitatory postsynaptic potential (EPSP)*

+A depolarizing postsynaptic potential. +A neurotransmitter that depolarizes the postsynaptic membrane is excitatory because it brings the membrane closer to threshold.

*Inhibitory postsynaptic potential (IPSP)*

+A hyperpolarizing postsynaptic potential. +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 is more negative and thus even farther from threshold than its resting state.

Stimulus intensity

+A light touch generates a low frequency of action potentials. +By contrast, a firmer grip elicits action potentials that pass down the axon at a higher frequency. +In addition to this "frequency code" for stimulus intensity, a second factor is the number of sensory neurons recruited (activated) by the stimulus. +For example, a firm grip stimulates a larger number of pressure-sensitive neurons than does a light touch.

*Mechanically gated channel*

+Opens or closes in response to mechanical stimulation such as sound waves, touch, pressure, or tissue stretching. +The force distorts the channel from its resting position, opening the gate.

*Summation*

+Process by which graded potentials add together. +A typical neuron in the CNS receives input from 1000 to 10,000 synapses. Integration of these inputs involves *summation*. +The greater the summation of EPSPs, the greater is the chance that threshold will be reached.

In the PNS, damaged dendrites and axons may be repaired if the cell body is intact, if the ____ cells are functional, and if scar tissue formation does not occur too rapidly.

Schwann

*Saltatory conduction*

+Saltat = leaping. +The special mode of action potential propagation that occurs along myelinated axons, occurs because of the uneven distribution of voltage-gated channels.

Neuroglia of the PNS completely surround the axons and cell bodies. The two types of neuroglia in the PNS are:

+Schwann cells +Satellite cells

Two types of *neuroglia* are present in the PNS.

+Schwann cells +Satellite cells

*Enteric plexuses*

are extensive networks of neurons located in the walls of organs of the gastrointestinal tract. +The neurons of these plexuses help regulate the digestive system.

*Nodes of Ranvier*

are gaps in the myelin sheath that appear at iinterverals along the axon between adjacent Schwann cells. +Each schwann cell wraps around one axon segment between two nodes of Ranvier.

*Interneurons*

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.

*Myelin sheath*

are multilayered lipids and protein coverings that surround axons. +Electrically insulates them and increases the speed of impulse conduction.

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.

Two types of voltage-gated channels open and then close during an *action potential*. These channels are present mainly in the plasma membrane of the axon and the axon terminals. The first channels to open, voltage-gated Na+ channels, allow Na+ to rush into the cell, resulting in the ____ phase. Then voltage-gated K+ channels open, allowing K+ to flow out, producing the ____ phase.

depolarizing repolarizing

*Sensory* or *afferent neurons*

either contain sensory receptors at their distal ends (dendrites) or synapse with 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.

A neurotransmitter causes either an ____ or an ____ graded potential.

excitatory inhibitory

*Unipolar neurons*

have dendrites and one axon that are fused together to form a continuous process that emerges from the cell body. +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 impulses in a *unipolar neuron* is at the junction of the dentrites and axon. +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.

*Bipolar neurons*

have one main dendrite and one axon. +They are found in the retina of the eye, in the inner ear, and in the olfactory area of the brain.

A *nerve*

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

*Action potential* or *impulse*

is a sequence of rapidly occurring events that briefly reverses the membrane potential and then eventually restores it to the resting state.

*Graded potential*

is a small deviation from the membrane potential that makes the membrane either more polarized (inside more negative) or less polarized (inside less negative).

The *synapse*

is a special junction between neurons.

The *action potential*

is an all-or-none eletrical signal.

During learning, the structure and function of particular synapses change. For example:

the changes in your synapses from studying will determine how well you do on your anatomy and physiology tests! +Some diseases and neurological disorders result from disruptions of synaptic transmission. +Synapses also are the sites of action for many therapeutic and addictive chemicals.

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.

*Neurons* carry out most of the unique functions of the nervous system, such as

sensing, thinking, remembering, controlling muscle activity, and regulating glandular secretions.

Because *graded potentials* die out within a few millimeters of their point of origin, they are useful for

short-distance communication only.

The opening or closing of these channels alters the flow of specific ions across the membrane, producing a flow of current that is localized, which means that it

spreads along the plasma membrane for a short distance and then dies out.

*Neuroglia*

support, nourish, and protect neurons, and maintain homeostasis in the interstitial fluid that bathes them.

Like muscle cells, neurons possess *electrical excitability*,

the ability to respond to a stimulus and convert it into an action potential.

*Multipolar neurons*

usually have several dendrites and one axon. +Most neurons in the brain and spinal cord are of this type.

____ cells produce myelin sheaths around axons in the PNS.

*Schwann*

____ axons propagate action potentials by continuous conduction.

*Unmyelinated*

____ axons exhibit continuous conduction; ____ axons exhibit saltatory conduction.

*Unmyelinated* *myelinated*

Although *graded potentials* may only travel a short distance, they can generate another type of membrane potential, an ____ that travels the entire length of an axon.

*action potential*

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*

Like most other cells in the body, the plasma membrane of excitable cells exhibits a ____, an electrical potential difference (voltage) across the plasma membrane.

*membrane potential*

Action potentials propagate more rapidly along ____ axons than along ____ axons.

*myelinated* *unmyelinated*

Axons with a myelin sheath are said to be ____; axons without such a covering are said to be ____.

*myelinated* *unmyelinated*

Voltage-gated channels are present primarily at the ____ (where there is no myelin sheath) rather than in regions where a myelin sheath covers the plasma membrane. Hence, current carried by Na+ and K+ flows across the membrane mainly at the nodes of Ranvier.

*nodes of Ranvier*

In excitable cells, this voltage is termed the ____.

*resting membrane potential*

*Neurons* (*nerve cells*)

+Form the complex processing networks within the brain and spinal cord and also connect all regions of the body to the brain and spinal cord.

*Ependymal cells*

+Found only in the CNS. +Are cuboidal to columnar cells arranged in a single layer that have 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). +*Ependymal cells* produce and assist in the circulation of cerebrospinal fluid. +They also participate in the formation of the *blood-cerebrospinal fluid barrier*.

*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 the neurotransmitter norepinephrine, for example, rapidly take up the norepinephrine and recycle it into new synaptic vesicles.

The diverse activities of the nervous system can be grouped into three functions:

+Sensory function +Integrative function +Motor function

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

+Sensory or afferent neurons +Motor or efferent neurons +Interneurons

*Sensory function*

+Sensory receptors detect *internal stimuli*, such as an increase in blood acidity, and *external stimuli*, such as a raindrop landing on your arm. +This *sensory* information is then carried into the brain and spinal cord through cranial and spinal nerves.

*Suprathreshold stimulus*

+Several action potentials will form in response to this, a stimulus that is strong enough to depolarize the membrane *above* threshold.

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.

*Depolarizing graded potential*

+When the response makes the membrane less polarized (inside less negative).

*Hyperpolarizing graded potential*

+When the response makes the membrane more polarized (inside more negative).

*Chromatolysis*

+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. +Following injury, the Nissl bodies break up into fine granular masses. +This altercation is called *chromatolysis*.

*Nissl bodies*

+Where protein synthesis occurs. +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.

*After-Hyperpolarizing Phase*

+While the voltage-gated K+ channels are open, outflow of K+ may be large enough to cause this phase of the action potential. +During this phase, voltage-gated K+ channels remain open and the membrane potential becomes even more negative than the resting level of -70 mV. +As the voltage-gated K+ channels close, the membrane potential returns to the resting level.

Little or no repair of damage to neurons occurs in the brain and spinal cord. Even when the cell body remains intact, a severed axon cannot be repaired or regrown.

...

Most neurons cannot divide (are *amitotic*)

because they lack the centrioles that are essential for mitosis to occur.

*Cranial nerves*

emerge from the brain. .

*Spinal nerves*

emerge from the spinal cord. +Each nerve follows a defined path and serves a specific region of the body

Excitable cells--neurons and muscle fibers--produce two types of electrical signals:

graded potentials and action potentials (impulses).

One obvious difference between graded potentials and action potentials is:

graded potentials function only in short-distance communication, but the propagation of action potentials permits communication over long distances.

*White matter*

consists primarily of the myelinated axons. +The whitish color of myelin gives white matter its name.

The *cell body*

contains a nucleus surrounded by cytoplasm that contains typical organelles such as lysosomes, mitochondria, and a Golgi complex.

Neuronal cells bodies

also contain prominent clusters of rough endoplasmic reticulum, termed *Nissl bodies*.

*Sensory receptors*

are nervous system structures that monitor changes in the external or internal environment. +Examples of *sensory receptors* include touch receptors in the skin, photoreceptors in the eye, and olfactory receptors in the nose.

*Neurons*

are responsible for most of the unique functions of the nervous system.

*Ganglia*

are small clusters of nervous tissue, consisting primarily of neuron cell bodies, that are located outside of the brain and spinal cord. +*Ganglia* are closely associated with cranial and spinal nerves.

*Neuroglia*

are smaller cells but they greatly outnumber neurons, perhaps by as much as 25 times. +Unlike neurons, neuroglia continue to divide throughout an individual's lifetime.

*Dendrites*

are the receiving or input portions of a neuron. +They usually are short, tapering, and highly branched. +In many neurons, the dendrites form a tree-shaped array of processes extending from the cell body.

*Action potential* (*impulse*)

is an electrical signal that propagates (travels) along the surface of the membrane of a neuron. +As a result of their specialization, neurons have lost the ability to undergo mitotic divisions.

*Stimulus*

is any change in the environment that is strong enough to initiate an action potential.

*Anterograde transport*

is movement from the cell body toward the synapse / axon terminals. Away from the cell body.

*Retrograde transport*

is movement from the synapse / axon terminal back toward the cell body.

*Neurolemma*

is the outer, nucleated cytoplasmic layer of the *schwann cell* enclosing the myelin sheath.

*Relative refractory period*

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.

*Cell body*

is the portion of a neuron that synthesizes new cell products or recycles old ones.

*Initial segment*

is the portion of the axon closest to the axon hillock. +In most neurons, impulses arise at the junction of the axon hillock and the initial segment.

*Summation*

is the process by which *graded potentials* add together.

*Synapse*

is the site of communication between two neurons or between a neuron and an effector cell.

*Axon terminals*

is where the axon and its collaterals end by dividing into many fine processes.

The ____ system regulates body activites by responding rapidly through impulses; the ____ system responds more slowly, though no less effectively, by releasing hormones.

nervous endocrine

Neurons display great diversity in size and shape. For example:

neuron cell bodies range in diameter from 5 micrometers (slightly smaller than a red blood cell) up to 135 micrometers (barely large enough to see with the unaided eye). +The pattern of dendritic branching is varied and distinctive for neurons in different parts of the nervous system. +A few small neurons lack an axon, and many others have very short axons. +The longest axons, however, are almost as long as a person is tall, extending from the toes to the lowest part of the brain.

Typically, ligand-gated and mechanically gated channels are present only in the dendrites and cell bodies of ____. Hence, *graded potentials* occur mainly in the dendrites and cell body of a neuron.

neurons

*Absolute refractory period*

occurs from the time the Na+ channel activation gates open to when the Na+ channel inactivation gates close. +During this period, even a very strong stimulus cannot initiate a second action potential because inactivated Na+ channels must return to the resting state before they can reopen.

*Axon*

of a neuron propagates nerve impulses toward another neuron, a muscle fiber, or a gland cell. +An axon is a long, thin, cylindrical projection that often joins the cell body at a cone-shaped elevation called the *axon hillock*.

Because impulses cannot conduct across the synaptic cleft, an alternative, indirect form of communication occurs between the presynaptic and postsynaptic neurons. In response to an impulse, the ____ neuron releases neurotransmitter molecules that diffuse through the fluid in the synaptic cleft and bind to receptors in the plasma membrane of the ____ neuron.

presynaptic postsynaptic

*Action potentials*

propagate from the trigger zone to axon terminals.

*Neuroglia*

support, nourish, and protect neurons and maintain homeostasis.

Ion Channels

Graded potentials and action potentials occur because the plasma membrane of neurons contain many different kinds of ion channels.that open or close in response to specific stimuli.

*Microtubules*

+Assist in moving materials between the cell body and axon.

*Synaptic transmission*

Neurons filter, integrate, and process information using synaptic transmission.

If two *depolarizing graded potentials* summate, the net result is:

a larger *depolarizing graded potential*.

*Neurons*

nerve cells

All you see, feel, think, and do is controlled by your ____ system.

nervous

*Gap junctions* are common in:

visceral smooth muscle, cardiac muscle, and the brain.

To communicate information from one part of the body to another, action potentials must travel from where they arise at a trigger zone to the axon terminals. In contrast, to the graded potential, an action potential is not ____ (it does not die out).

*decremental*

In an ____, impulses conduct directly between the plasma membranes of adjacent neurons through ____.

*electrical synapse* *gap junctions*

____ produce myelin sheaths around axons in the CNS.

*Oligodendrocytes*

____ cells begin to form myelin sheaths around axons during fetal development.

*Schwann*

Recall that ions move from where they are more concentrated to where they are less concentrated (the chemical part of the gradient). Also, positively charged ions (____) move toward a negatively charged area, and negatively charged ions (____) move toward a positively charged area. (the electrical part of the gradient). As ions move, they constitute a flow of electrical current that can change the membrane potential.

*cations* *anions*

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 (rather than electrons) constitutes the electrical current.

*current*

*Voltage-gated channel*

+Opens in response to a change in membrane potential (voltage), +*Voltage-gated channels* participate in the generation and conduction of action potentials.

*Unequal distribution of ions across the plasma membrane*

+A major factor that contributes to the resting membrane potential is the unequal distributions of various ions in extracellular fluid and cytosol. +Extracellular fluid is rich in Na+ and chloride ions (CI-), In cytosol, however, there are large concentrations of K+ and anions such as phosphates and amino acids. +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 is greater than the number of sodium ions that diffuse down their concentration gradient into the cell. +As more and more positive potassium ions exit, the cytosol side of the plasma membrane becomes increasingly negative and the extracellular fluid side of the plasma membrane becomes increasingly positive.

*Lower motor neuron*

+A type of motor neuron that directly supplies skeletal muscle fibers.

*Upper motor neuron*

+A type of motor neuron that synapses with a lower motor neuron farther down in the CNS in order to contract a skeletal muscle.

*Amount of myelination*

+Action potentials propagate more rapidly along myelinated axons than along unmyelinated axons.

The speed of propagation of an action potential is affected by three major factors:

+Amount of myelination +Axon diameter +Termperature

*Subthreshold stimulus*

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

*Inability of most anions to leave the cell*

+Another factor contributes to the relative negativity of the cell interior: Most anions (negatively charged ions) inside the cell are not free to leave. +They cannot follow K+ out of the cell because they are attached to nondiffusible molecules such as ATP and large proteins.

*Axon collaterals*

+Are side branches along the length of an axon that may branch off, typically at a right angle to the axon.

*Impulse propagation*

+As sodium ions flow into the neuron, voltage-gated Na+ channels in adjacent segments of the membrane open. +Thus, the action potential self-propagates along the membrane, rather like toppling that long row of dominoes by pushing over the first one in the line. +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 trigger zone to the axon terminals. +An action potential propagates in only one direction, toward the axon terminals, because any region of the axon that has just undergone an action potential must recover (experience the refractory period) before it is able to generate another action potential. +Because action potentials can travel along a membrane without dying out, they function in communication over long distances.

Four of the six types of *neuroglia* are found only in the CNS:

+Astrocytes +Oligodendrocytes +Microglia +Ependymal cells

*Temperature*

+Axons propagate action potentials at lower speeds when cooled.

*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.

Most neurons have three parts:

+Cell body +Dendrites +Axon

*Enzymatic degradation*

+Certain neurotransmitters are inactivated through enzymatic degradation. +For example, the enzyme acetylcholinesterase breaks down acetylcholine in the synaptic cleft.

*Neurofibrils*

+Composed of bundles of intermediate filaments that provide shape and support to the cell.

*Peripheral nervous system* (*PNS*)

+Consists of all nervous tissue outside the CNS. +Components of the *PNS* include nerves, ganglia, enteric plexuses, and sensory receptors.

*Enteric nervous system* (*ENS*)

+Consists of over 100 million neurons in enteric plexuses that extend most of the length of the gastrointestinal (GI) tract. +Enteric sensory neurons monitor chemical changes within the GI tract and the stretching of its walls. +Enteric motor neurons govern contraction of GI tract smooth muscle to propel food through the GI tract, secretions of the GI tract organs such as acid secretion by the stomach, and activity of GI tract endocrine cells, which secrete hormones. +The operation of the *ENS*, the "brain of the gut," is *involuntary*. +Many neurons of the enteric plexuses function independently of the CNS to some extent, although they may communicate with the CNS via sympathetic and parasympathetic neurons.

*Autonomic nervous system* (*ANS*)

+Consists of sensory neurons that convey information to the CNS from autonomic sensory receptors, located primarily in visceral organs such as the stomach and lungs. +The *ANS* also consists of motor neurons that conduct nerve impulses 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*)

+Consists of sensory neurons that convey information to the CNS from somatic receptors in the head, body wall, and limbs, and from receptors for the special senses of vision, hearing, taste, and smell. +The *SNS* also includes motor neurons that conduct inpulses from the CNS to skeletal muscles only. Because these motor responses can be consciously controlled, the action of this part of the PNS is *voluntary*.

*Central nervous system* (*CNS*)

+Consists of the brain and spinal cord. +The *brain* is the part enclosed within the skull and contains about 100 billion neurons. +The *spinal cord* is connected to the brain through the foramen magnum of the occipital bone and is encircled by the bones of the vertebral column. +The *spinal cord* contains about 100 million neurons. +The *CNS* processes many different kinds of incoming snesory 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*.

*Wallerian degeneration*

+Degeneration of the distal portion of the axon and myelin sheath.

*Depolarizing phase*

+Depolarizing graded potentials originate in the dendrites or cell body of a neuron and then travel to the trigger zone. +If the graded potential is able to depolarize the membrane to threshold, voltage-gated Na+ channels open quickly. +As voltage-gated Na+ channels open, eletrical and chemical gradients favor inward movement of Na+. +The resulting influx of Na+ produces the depolarizing phase of the action potential as the membrane potential rises from the threshold level of -55mV to +30mV. +At the peak of the action potential, the cytosol side of the membrane is 30mV more positive than the extracellular fluid side of the membrane.

An *action potential* has two main phases:

+Depolarizing phase +Repolarizing phase

*Regeneration*

+Despite plasticity, however, mammalian neurons have very limited powers of *regeneration*, the capability to replace or repair destroyed cells.

Removal of the neutrotransmitter 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:

+Diffusion +Enzymatic degradation +Uptake by cells

*Repolarizing phase*

+During this phase, the membrane potential is restored to the resting state of -70mV.

*Depolarizing phase*

+During this phase, the negative membrane potential becomes less negative, reaches zero, and then becomes positive.

*Synchronization*

+Electrical synapses can synchronize (coordinate) the activity of a group of neurons or muscle fibers. +Large numbers of neurons can produce action potentials in unison if they are connected by gap junctions. +Synchronizing action potentials in the heart or in visceral smooth muscle coordinates contractions to produce a heartbeat or move food through the gastrointestinal tract.

Electrical synapses have two main advantages:

+Faster communication +Synchronization

*Microglial cells* or *microglia*

+Found only in the CNS. +Are small cells with slender processes that give off numerous spinelike projections. +*Microglia* phagocytize microbes and damaged nervous tissue.

*Oligodendrocytes*

+Found only in the CNS. +Resemble astrocytes but are smaller and contain fewer processes. +*Oligodendrocyte* processes are responsible for forming and maintaining the myelin sheath around CNS axons.

*Astrocytes*

+Found only in the CNS. +Star-shaped cells with many processes, are the largest and most numerous of the *neuroglia*. +The processes of *astrocytes* make contact with blood capillaries, neurons, and the pia mater (a thin membrane around the brain and spinal cord). +*Astrocytes* cling to and support neurons. +They help to maintain the appropriate chemical environment for the generation of impulses by providing nutrients to neurons, removing excess neurotransmitters, and regulating the concentration of important ions. +*Astrocyte* processes wrap around blood capillaries to inhibit movement of potentially harmful substances in blood, creating a *blood-brain barrier* that restricts the movement of substances between the blood and neurons of the CNS.

*Satellite cells*

+Found only in the PNS. +Are flat cells surrounding the cell bodies of neurons of PNS ganglia (ganglia are collections of neuronal cell bodies outside the CNS). +In addition to providing structural support, *satellite cells* regulate the exchanges of materials between neuronal cell bodies and interstitial fluid.

*Schwann cells*

+Found only in the PNS. +Form the myelin sheath around axons in the PNS. +A *schwann cell* can myelinate a single axon, or enclose multiple unyelinated axons (axons that lack a myelin sheath). +*Schwann cells* participate in axon regeneration, which is more easily accomplished iin the PNS than in the CNS.

*Threshold stimulus*

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

*Trigger zone*

+In most neurons, impulses arise at the junction of the axon hillock and the initial segment, called this. +From which impulses are conducted toward the distal end of the axon.

*Axon diameter*

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

Ion channels open and close due to the presence of "gates." The gate is a part of the channel protein that can seal the channel pore shut or move aside to open the pore. The electrical signals produced by neurons and muscle fibers rely on four types of ion channels:

+Leak channels +Ligand-gated channel +Mechanically gated channel +Voltage-gated channel

*Chemical synapses*

+Most synapses are of this type. +In a chemical synapse, an impulse in a presynaptic neuron causes the release of neurotransmitter molecules that produce an impulse in a postsynaptic neuron.

Two kinds of processes (extensions) emerge from the cell body of a neuron:

+Multiple dendrites +Single axon

Both structural and functional features are used to classify the various neurons in the body. *Structurally*, neurons are classified according to the number of processes extending from the cell body:

+Multipolar neurons +Bipolar neurons +Unipolar neurons

The cytoskeleton includes both

+Neurofibrils +Microtubules

Nervous tissue consists of two types of cells:

+Neurons +Neuroglia

*Motor function*

+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.

*Ligand-gated channel*

+Opens and closes in response to a specific *ligand* (a molecule that binds to a receptor). +A wide variety of *ligands*--including neurotransmitters, hormones, and ions--can open or close *ligand-gated channels*. +The neurotransmitter, acetylcholine, for example, opens cation (positively charged ions) channels that allow Na+ and calcium ions (Ca2+) to diffuse inward and K+ to diffuse outward.

*Repolarizing phase*

+Shortly after the activation gates open, the inactivation gates close and Na+ stops entering the neuron. +Besides opening voltage-gated Na+ channels, a threshold-level depolarization also open voltage-gated K+ channels. +Because voltage-gated K+ channels open slowly, their opening occurs at about the same time the voltage-gated Na+ channels are closing. +The slower opening of voltage-gated K+ channels and the closing of previously opened Na+ channels produce the repolarizing phase of the action potential. +With K+ channels open, K+ flows out of the neuron, negative charges build up on the cytosol side of the membrane, and the membrane potential changes from +30mV to -70mV, restoring the resting membrane potential.

The PNS is divided into

+Somatic nervous system +Autonomic nervous system +Enteric nervous system

*Diffusion*

+Some of the released neurotransmitter molecules diffuse away from the synaptic cleft. +Once a neurotransmitter molecule is out of each of its receptors, it can no longer exert an effect.

The motor part of the *ANS* consists of two branches:

+Sympathetic division +Parasympathetic division

*Plasticity*

+The capability to change based on experience. +Your nervous system exhibits this throughout your life. +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.

The nervous system can be organized into two main subdivisions:

+The central nervous system +The peripheral nervous system

*Axoplasm*

+The cytoplasm of an axon. +Is surrounded by a plasma membrane.

*Leak channels*

+The gates of these channels randomly alternative between open and closed positions. +Typically, plasma membranes have many more potassium ion (K+) leak channels than sodium ion (Na+) leak channels, and K+ leak channels are leakier than the Na+ leak channels. +Thus, the membrane's permeability to K+ is much higher than its permeability to Na+.

*Threshold*

+The generation of an action potential depends on whether a stimulus is able to bring the membrane potential to a certain level termed this (about -55 mV in many neurons).

*Integrative function*

+The nervous system processes sensory information by analyzing and storing some of it, and by making decisions for appropriate responses -- an activity known as *integration*.

*Postsynaptic neuron*

+The neuron that carries an impulse away from a synapse.

*Presynaptic neuron*

+The neuron that carries an impulse toward a synapse.

*Refractory period*

+The period of time after an action potential begins, during which an excitable cell cannot generate another action potential.

*Axolemma*

+The plasma membrane that surrounds the axoplasm.

Resting membrane potential

+The resting membrane potential exists because of a small buildup of negative ions in the cytosol along the inside of the plasma membrane, and an equal buildup of positive ions in the extracellular fluid along the outside surface of the plasma membrane. +Such a separation of positive and negative electrical charges is a form of potential energy, which is measured in volts or millivolts. +The greater the difference in charge across the plasma membrane, the larger is the membrane potential (voltage).

During the depolarizing phase

+The resting membrane potential rises to threshold, when the activation gates in the Na+ channel open. +With both the activation and inactivation gates open, Na+ influx begins. +As Na+ rushes through the channels into the neuron, the buildup of positive charges on the cytosol side of the membrane increases the membrane potential to +30mV. +Although the voltage-gated Na+ channels are open for only a few ten-thousandths of a second, about 20,000 Na+ ions flow across the membrane and change the membrane potential considerably. +The sodium-potassium pumps easily bail out the 20,000 or so Na+ that enter the cell during a single action potential and maintain the low concentration of Na+ inside the cell.

*Electrogenic nature of the sodium-potassium pump*

+The small inward Na+ leak and outward K+ leak are offset by sodium-potassium pumps. +These pumps help maintain the resting membrane potential by pumping out Na+ as fast as it leaks in and, at the same time, returning K+ to the cell interior. +The sodium-potassium pumps expel three Na+ for each two K+ imported. +Since these pumps remove more positive charges from the cell than they bring into the cell, they are *electrogenic*, which means they contribute to the negativity of the resting membrane potential.

An example of the three basic functions of the nervous system when answering your phone after hearing it ring.

+The sound of the ringing cell phone stimulates sensory receptors in your ears (*sensory function*). +This auditory infromation is subsequently relayed into your brain where it is processed and the decision to answer the phone is made (*integrative function*). +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 (*motor function*).

*Synaptic end bulbs*

+The tips of some axon terminals swell into these bulb-shaped structures.

*Continuous conduction*

+The type of impulse propagation which involves step-by-step depolarization and repolarization of each adjacent segment of the plasma membrane as ions flow through each voltage-gated channel along on the membrane.

*Synaptic cleft*

+This separates neurons. +A tiny space filled with interstitial fluid.

The resting membrane potential arises from three major factors:

+Unequal distribution of ions across the plasma membrane +Inability of most anions (negatively charged ions) to leave the cell +Electrogenic nature of the sodium-potassium pump

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

1) *Graded potentials* - used for short-distance communication only 2) *Action potentials* - allow communication over long distances within the body.

A typical chemical synapse transmits a signal as follows:

1) An impulse arrives at the synaptic end bulb of a presynaptic axon. 2) The depolarizing phase of the impulse opens *voltage-gated Ca2+ channels* in the membrane of synaptic end bulbs. +Because calcium ions are more concentrated in the extracellular fluid, Ca2+ flows inward through the opened channels. 3) An increase in the concentration of Ca2+ inside the presynaptic neuron triggers exocytosis of some of the synaptic vesicles. +As vesicle membranes merge with the plasma membrane, neurotransmitter molecules within the synaptic vesicles are released into the synaptic cleft. +Each synaptic vesicle contains several thousand molecules of neurotransmitter. 4) The neurotransmitter molecules diffuse across the synaptic cleft and bind to *neurotransmitter receptors* in the postsynaptic neuron's plasma membrane. 5) Binding of neurotransmitter molecules to their receptors on ligand-gated channels opens the channels and allows particular ions to flow across the membrane. 6) As ions flow through the opened channels, the voltage across the membrane changes. +This change in membrane voltage is a postsynaptic potential. +Depending on which ions the channels admit, the postsynaptic potential may be a depolarization (excitation) or hyperpolarization (inhibition). +For example, opening of Na+ channels allows inflow of Na+, which causes depolarization. +Opening of K+ channels allows K+ to move out, producing hyperpolarization. 7) When a depolarizing postsynaptic potential reaches threshold, it triggers an action potential in the axon of the postsynaptic neuron.

The flow of current across the membrane only at the nodes of Ranvier has two conequences:

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 axon as current flows from one node to the next, action potentials travel much faster than they 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 plasma membrane, results in minimal inflow of Na+ and outflow of K+ each time an action potential occurs. +Thus, less ATP is used by sodium-potassium pumps to maintain the low intracellular concentration of Na+ and the low extracellular concentration of K+.

*Regeneration tube*

Following chromatolysis, signs of recovery in the cell body become evident. +Macrophages phagocytize the debris. +Schwann cells on either side of the injured site multiply by mitosis, grow toward each other, and may form a *regeneration tube* across the injured area. +The tube guides growth of a new axon from the proximal area, across the injured area, into the distal area previously occupied by the original axon, and eventually toward receptors and effectors previously contacted by the neuron.

____ neurons have a greater capacity for repair and regeneration than ____ neurons.

PNS CNS

If two *hyperpolarizing graded potentials* summate, the net result is:

a larger *hyperpolarizing graded potential*.

*Neuroglia*

cells that support the activities of neurons

The nervous system is organized into the ____ and ____ nervous systems.

central peripheral

*Neurons*

communicate with other cells.

*Gray matter*

contains neuronal cell bodies, dendrites, unmyelinated axons, axon terminals, and neuroglia. +It looks grayish, rather than white, because the Nissl bodies impart a gray color and there is little or no myelin in these areas.

*Motor* 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.

*Neuroglia* or simply *glia*

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. +Generally, *neuroglia* are smaller than neurons, and they are up to 5 to 50 times more numerous. +In contrast to neurons, *glia* do not generate or propagate potentials, and they can readily multiply and divide. +In cases of injury or disease, *neuroglia* multiply to fill in the spaces formerly occupied by neurons. +Brain tumors derived from *glia*, called gliomas, tend to be highly malignant and to grow rapidly.

The amount of myelin increases from birth to maturity, and its presence greatly increases the speed of impulse conduction. An infant's responses to stimuli are neither as rapid nor as coordinated as those of an older child or an adult, in part because...

myelination is still in progress during infancy.

The ____ system is one of the smallest and yet the most complex of the 11 body systems.

nervous

The ____ system is responsible for all of your perceptions, behaviors, memories, and movements.

nervous

The ____ system maintains homeostasis and integrates all body activities.

nervous

Axons in the CNS are myelinated by ____ rather than Schwann cells, and this CNS myelin is one of the factors inhibiting regeneration of neurons.

oligodendrocytes