Bio 235 ch12

ENS

"brain of the gut," Involuntary. Extend most of the length of the gastrointestinal (GI) tract. Many of the neurons of the enteric plexuses function independently of the ANS and CNS to some extent Also communicate with the CNS via sympathetic and parasympathetic neutrons. Sensory neurons of the ENS monitor chemical changes within the GI tract & stretching of its walls. Enteric motor neurons govern contractions of GI tract smooth muscle to propel food through the GI tract, secretions of GI tract organs (such as acid from the stomach), and activities of GI tract endocrine cells, which secrete hormones.

What possess electrical excitability

Neurons Some neurons are tiny and propagate impulses over a short dis- tance (less than 1 mm) within the CNS. Others are the longest cells in the body. The neurons that enable you to wiggle your toes, for example, extend from the lumbar region of your spinal cord (just above waist level) to the muscles in your foot. Some neurons are even longer. Those that allow you to feel a feather tickling your toes stretch all the way from your foot to the lower portion of your brain. Nerve impulses travel these great distances at speeds ranging from 0.5 to 130 meters per second (1 to 290 mi/hr).

Nervous tissue comprises two types of cells:

Neurons and neuroglia. Neurons and neuroglia differ structurally depending on whether they are located in the central nervous system or the peripheral nervous system.

Functional Classification

Neurons are classified according to the direction in which the nerve impulse (action potential) is conveyed with respect to the CNS. Sensory Motor Interneurons

neuropeptides

Neurotransmitters consisting of 3 to 40 amino acids linked by peptide bonds called neuropeptides 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. Scientists discovered that certain brain neurons have plasma membrane receptors for opiate drugs such as morphine and heroin. Two molecules, each a chain of five amino acids, named enkephalins. Their potent analgesic (pain-relieving) effect is 200 times stronger than morphine. Other so-called opioid pep- tides include the endorphins. It is thought that opioid peptides are the body's natural painkillers. These neuropeptides have also been linked to improved memory and learning; feelings of pleasure or euphoria; control of body temperature; regulation of hormones that affect the onset of puberty, sexual drive, and reproduction; and mental illnesses such as depression and schizophrenia.

The single axon ( axis)

Of a neuron propagates nerve impulses toward another neuron, a muscle fiber, or a gland cell. An axon is a long, thin, cylindrical projection

The resting membrane potential exists because

Of a small buildup of negative ions in the cytosol along the inside of the membrane, and an equal buildup of positive ions in the extracellular fluid (ECF) along the outside surface of the membrane. Such a separation of positive and negative electrical charges is a form of potential energy, which is measured in volts or millivolts (1 mV 0.001 V) The greater the difference in charge across the membrane, the larger the membrane potential (voltage). The cytosol or extracellular fluid elsewhere in the cell contains equal numbers of positive and negative charges and is electrically neutral.

Neuroglia of the CNS can be classified on the basis:

Of size Cytoplasmic processes Intracellular organization into four types: Astrocytes Oligodendrocytes Microglial cells Ependymal cells

Synaptic end bulbs

The tips of some axon terminals swell into bulb-shaped structures (Tortora 404) Tortora, Gerard J., Bryan Derrickson. Principles of Anatomy and Physiology, 14th Edition. Wiley, 2013-12-23. VitalBook file.

Continuous conduction

The type of action potential propagation describedwhich involves step-by- step Depolarization and repolarization of each adjacent segment of the plasma membrane

If two equal but opposite graded poten- tials summate (one depolarizing and the other hyper polarizing)

Then they cancel each other out and the overall graded potential disappears.

Polarized

A cell that exhibits a mem- brane potential. Most body cells are polarized

Nucleus

A cluster of neuronal cell bodies located in the CNS.

Why is there no protein syntheses in the Axon?

Because rough endoplasmic reticulum is not present, protein synthesis does not occur in the axon.

Synaptic vesicles

Both synaptic end bulbs and varicosities contain many tiny membrane-enclosed sacs called ... They store a chemical called a neurotransmitter

central nervous system (CNS)

Brain and spinal-cord Source of thoughts, emotions, and memories. Most signals that stimulate muscles to contract and glands to secrete originate in the CNS.

Structural classification of neurones

Breaks indicate that axons are longer than shown. Classified according to the number of processes extending from the cell body.

Tract

Bundle of axons that is located in the CNS. Interconnect neurons in the spinal cord and brain.

Spinal nerves

Thirty-one pairs emerge from the spinal cord

Action potentials propagate more rapidly along ...

Myelinated axons than along unmyelinated axons.

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.

Clusters of Neuronal Cell Bodies

Recall that a ganglion (plural is ganglia) refers to a cluster of neuronal cell bodies located in the PNS. Ganglia are closely associated with cranial and spinal nerves. By contrast, a nucleus is a cluster of neuronal cell bodies located in the CNS.

Bundles of Axons

Recall that a nerve is a bundle of axons 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 tract is a bundle of axons that is located in the CNS. Tracts interconnect neurons in the spinal cord and brain.

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 neuro- transmitter). The postsynaptic neuron receives the chemical signal and in turn generates an electrical signal (postsynaptic potential).

Schwann Cells

These cells encircle PNS axons. Like oligodendrocytes, they form the myelin sheath around axons. A single oligodendrocyte myelinates several axons, but each Schwann cell 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). Participate in axon regeneration, which is more easily accomplished in the PNS than in the CNS.

A fibers

are the largest diameter axons (5-20 m) 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, posi- tion of joints, and some thermal and pain sensations are A fibers, as are the axons of motor neurons that conduct im- pulses to skeletal muscles.

Excitatory postsynaptic potential (EPSP)

A depolarizing postsynaptic potential is called an excitatory postsynaptic potential (EPSP). Although a single EPSP normally does not initi- ate a nerve impulse, the postsynaptic cell does become more ex- citable. Because it is partially depolarized, it is more likely to reach threshold when the next EPSP occurs.

The cell body

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

Muscle action potential

Action potential in a muscle fibres

Electrical synapse

Action potentials (impulses) conduct directly between the plasma membranes of adjacent neurons through structures called gap junctions.

Axon collaterals

Along thelength of an axon, side branches called... May branch off, typically at a right angle to the axon

Summating

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

Chemical Synapses

Although the plasma membranes of presynaptic and postsynaptic neurons in a chemical synapse are close, they do not touch.

Propagation

An action potential keeps its strength as it spreads along the membrane. Depends on positive feedback.

Subthreshold stimulus

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

Axon hillock

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

The resting membrane potential is:

An electrical potential difference (voltage) that exists across the plasma membrane of an excitable cell under resting conditions.

Ependymal Cells

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: produce, possibly monitor, and assist in the circulation of cerebrospinal fluid. They also form the blood cerebrospinal fluid barrier.

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

Ganglia

Are small masses of nervous tissue, consisting primarily of neuron cell bodies, that are located outside of the brain and spinal cord. Closely associated with cranial and spinal nerves.

Dendrites

Are the receiving or input portions of a neuron. 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.

Myelin sheath

Multilayered lipid and protein covering around some axons that insulates them and increases the speed of nerve impulse conduction.

Where are most unipolar neurons located in the body?

Multipolar neuron

In the CNS, an oligodendrocyte

Myelinates parts of several axons. Each oligodendrocyte puts forth about 15 broad, flat processes that spiral around CNS axons, forming a myelin sheath. A neurolemma is not present, however, because the oligodendrocyte cell body and nucleus do not envelop the axon. Nodes of Ranvier are present, but they are fewer in number. Axons in the CNS display little regrowth after injury. This is thought to be due, in part, to the absence of a neurolemma, and in part to an inhibitory influence exerted by the oligodendrocytes on axon regrowth. The amount of myelin increases from birth to maturity, and its presence greatly increases the speed of nerve 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.

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

Satellite Cells

These flat cells surround the cell bodies of neurons of PNS ganglia. Provide structural support Regulate the exchanges of materials between neuronal cell bodies and interstitial fluid.

Microglial cells or Microglia

These neuroglia are small cells with slender processes that give off numerous spine like projections. Function as phagocytes. Like tissue macrophages, they remove cellular debris for meddling normal development of the nervous system and phagocytize microbes and damaged nervous tissue.

Oligodendrocytes

These resemble astrocytes but are smaller and contain fewer processes. Processes are responsible for forming and maintaining the myelin sheath around CNS axons.

Synaptic cleft

They are separated by the synaptic cleft, 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.

Decremental conduction

This mode of travel by which graded potentials die out as they spread along the membrane. Because they die out within a few millimeters of their point of origin, graded potentials are useful for short-distance communication only.

Signal transmission at a chemical synapse.

Through exocytosis of synaptic vesicles, a presynaptic neuron releases neurotransmitter molecules. After diffusing across the synaptic cleft, the neurotransmitter binds to receptors in the plasma membrane of the postsynaptic neuron and produces a postsynaptic potential.

An action potential has two main phases:

Depolarizing phase and a repolarizing phase

axoaxsonic

from axon to axon

axodendritic

from axon to dendrite Most synapses between neurons

Most neurons have three parts:

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

Neutrons are:

Electrically excitable.

PNS Neuroglia

Schwann cells Satellite cells

Ribosomes

Sites of protein synthesis.

The axons of neurons are usually grouped together in:

bundles

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. The membrane potential varies from +5 mV to -100 mV in different types of cells

Bipolar neuron

Has two Have one main dendrite and one axon. They are found in the retina of the eye, the inner ear, and the olfactory area (olfact to smell) of the brain.

The resting membrane potential is determined by three major factors:

(1) unequal distribution of ions in the ECF and cytosol (2) inability of most anions to leave the cell (3) the electrogenic nature of the Na-K Atlases so more + out side cell & more - inside cell There is more Na+ outside cell K+ is more inside cell

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:

(1) As you touch the pen, a graded potential develops in a sen- sory receptor in the skin of the fingers. (2) The graded potential triggers the axon of the sensory neuron to form a nerve action potential, which travels along the axon into the CNS and ultimately causes the release of neurotrans- mitter at a synapse with an interneuron. (3) The neurotransmitter stimulates the interneuron to form a graded potential in its dendrites and cell bod (4) In response to the graded potential, the axon of the inter neuron forms a nerve action potential. The nerve action potential travels along the axon, which results in neuro- transmitter release at the next synapse with another inter- neutron. (5) This process of neurotransmitter release at a synapse fol- lowed by the formation of a graded potential and then a nerve action potential occurs over and over as interneurons in higher parts of the brain (such as the thalamus and cerebral cortex) are activated. Once interneurons in the cerebral cortex, the outer part of the brain, are activated, perception occurs and you are able to feel the smooth surface of the pen touch your fingers. Perception, the conscious awareness of a sensation, is primarily a function of the cerebral cortex. Suppose that you want to use the pen to write a letter. The nervous system would respond in the following way (6) A stimulus in the brain causes a graded potential to form in the dendrites and cell body of an upper motor neuron, a type of motor neuron that synapses with a lower motor neu- ron farther down in the CNS in order to contract a skeletal muscle. The graded potential subsequently causes a nerve ac- tion potential to occur in the axon of the upper motor neuron, followed by neurotransmitter release. (7) The neurotransmitter generates a graded potential in a lower motor neuron, a type of motor neuron that directly supplies skeletal muscle fibers. The graded potential trig- gers the formation of a nerve action potential and then re- lease of the neurotransmitter at neuromuscular junctions formed with skeletal muscle fibers that control movements of the fingers. (8) The neurotransmitter stimulates the muscle fibers that control finger movements to form muscle action potentials. The mus- cle action potentials cause these muscle fibers to contract, which allows you to write with the pen.

Three factors that contribute to the resting membrane potential. P415

(1) Because the plasma membrane has more K leak channels (blue) than Na leak channels (rust), the number of K ions that leave the cell is greater than the number of Na ions that enter the cell. As more and more K ions leave the cell, the inside of the membrane becomes increasingly negative and the outside of the membrane becomes increasingly positive. (2) Trapped anions (turquoise and red) cannot follow K out of the cell because they are attached to nondiffusible molecules such as ATP and large proteins. (3) The electrogenic Na-K ATPase (purple) expels 3 Na ions for every 2 K ions imported. Uses ATP 3Na to 2K

Neurones communicatewith one another using two types of electrical signals:

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

Autonomic nervous system (ANS)

(1) sensory neurons that convey information to the CNS from autonomic sensory receptors, located primarily in visceral organs such as the stomach and lungs (2) motor neurons that conduct nerve impulses from the CNS to smooth muscle, cardiac muscle, and glands. Motor responses are not normally under conscious control, the action of the ANS is involuntary.

Somatic nervous system (SNS)

(1) 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 (2) motor neurons that conduct impulses 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.

Ion channels in the plasma membrane.

(a) Leak channels randomly open and close. (b) A chemical stimulus—here, the neurotransmitter acetylcholine—opens a ligand-gated channel. (c) A mechanical stimulus opens a mechanically-gated channel. (d) A change in membrane potential opens voltage-gated K channels during an action potential.

Organization of the nervous system

(a) Subdivisions of the nervous system. (b) Nervous system organizational chart: blue boxes: represent sensory components of the peripheral nervous system. Red boxes: represent motor components of the PNS. Green boxes: represent effectors (muscles and glands).

A typical chemical synapse transmits a signal as follows

1 A nerve impulse arrives at a synaptic end bulb (or at a vari- cosity) of a presynaptic axon. 2 The depolarizing phase of the nerve impulse opens voltage- gated Ca2 channels, which are present in the membrane of synaptic end bulbs. Because calcium ions are more concen- trated in the extracellular fluid, Ca2 flows inward through the opened channels. 3 An increase in the concentration of Ca2 inside the presyn- aptic neuron serves as a signal that triggers exocytosis of the synaptic vesicles. As vesicle membranes merge with the plasma membrane, neurotransmitter molecules within the vesicles are released into the synaptic cleft. Each synaptic ves- icle contains several thousand molecules of neurotransmitter.4 The neurotransmitter molecules diffuse across the synaptic cleft and bind to neurotransmitter receptors in the postsyn- aptic neuron's plasma membrane. The receptor shown in Fig- ure 12.23 is part of a ligand-gated channel (see Figure 12.11b); you will soon learn that this type of neurotransmitter receptor is called an ionotropic receptor. Not all neurotransmitters bind to ionotropic receptors; some bind to metabotropic re- ceptors (described shortly). 5 Binding of neurotransmitter molecules to their receptors on ligand-gated channels opens the channels and allows particu- lar 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 depo- larization (excitation) or a hyperpolarization (inhibition). For example, opening of Na channels allows inflow of Na, which causes depolarization. However, opening of Cl- or K channels causes hyperpolarization. Opening Cl- channels permits Cl- to move into the cell, while opening the K chan- nels allows K to move out—in either event, the inside of the cell becomes more negative. 7 When a depolarizing postsynaptic potential reaches thresh- old, it triggers an action potential in the axon of the postsynaptic neuron.

Factors That Affect the Speed of Propagation

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

The functions of astrocytes include the following:

1. Contain microfilaments that give them considerable strength, which enables them to support neutrons. 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. In effect, the endothelial cells create a blood-brain barrier, which restricts the movement of substances between the blood and interstitial fluid of the CNS. 3. In the embryo, secrete chemicals that appear to regulate the growth, migration, and interconnection among neurons in the brain. 4. 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 *Serve as a conduit for the passage of nutrients and other substances between blood capillaries and neutrons. 5. May also play a role in learning and memory by influencing the formation of neural synapses.

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 post- synaptic 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.

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 concentra- tion of Na and the low extracellular concentration of K.

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.

Spinal Cord

100 million neurons

Brain

85 billion neurones

Multipolar neuron

A has many processes extending from the cell body. 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).

neurotransmitter

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.

Structure of a multipolar neuron.

A multipolar neuron has a cell body, several short dendrites, and a single long axon. Direction of information flow: Dendrites Cell body Axon Axon terminals.

Presynaptic neuron

A nerve cell that carries a nerve impulse toward a synapse. It is the cell that sends a signal.

Inhibitory postsynaptic potential (IPSP)

A neurotransmitter that causes hyperpolarization of the post- synaptic membrane is inhibitory. During hy- perpolarization, 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).

All-or-none principle

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. 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.

4 Neuroglia in CNS

Astrocytes Oligodendrocytes, Microglia, Ependymal cells

6 Neuroglia

Astrocytes, Oligodendrocytes Microglia Ependymal cells Schwann cells Satellite cells

Damage and Repair in the PNS

Axons and dendrites that are associated with a neurolemma 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. 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.

Classification of Nerve Fibers

Axons can be classified into three major groups based on: Amount of myelination Diameters, Propagation speeds: In response to a nerve impulse, the presynaptic neuron

Voltage-gated channels location:

Axons of all types of neurons.

Unmyelinated

Axons without such a covering (Myelin sheath)

Schwann cells

Begin to form myelin sheaths around axons during fetal development. Each Schwann cell wraps single axon's length by spiraling many times around the axon. 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.

Axons surrounded by a myelin sheath produced either:

By Schwann cells in the PNS or by oligodendrocytes in the CNS are said to be myelinated.

Biogenic Amines

Certain amino acids are modified and decarboxylated (carboxyl group removed) to produce biogenic amines. Those that are prevalent in the nervous system include norepinephrine, epinephrine, dopamine, and serotonin. Most biogenic amines bind to metabotropic receptors; there are many different types of metabotropic receptors for each biogenic amine. Biogenic amines may cause either excitation or inhibition, depending on the type of metabotropic receptor at the synapse. 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. Brain neurons containing the neurotransmitter dopamine (DA): are active during emotional responses, addictive behaviors, and pleasurable experiences. In addition, dopamine-releasing neurons help regulate skeletal muscle tone and some aspects of movement due to contraction of skeletal muscles. The muscular stiffness that occurs in Parkinson's disease is due to degeneration of neurons that release dopamine. One form of schizophrenia is due to accumulation of excess dopamine. Norepinephrine, dopamine, and epinephrine are classified chemically as catecholamines. They all have an amino group (9NH2) and a catechol ring composed of six carbons and two adjacent hydroxyl (9OH) groups. Catecholamines are synthesized from the amino acid tyrosine. Inactivation of catecholamines occurs via reuptake into synaptic end bulbs. Then they are either recycled back into the synaptic vesicles or destroyed by enzymes. Serotonin: which is also known as 5-hydroxy- tryptamine (5-HT), is concentrated in the neurons in a part of the brain called the raphe nucleus. It is thought to be involved in sensory perception, temperature regulation, control of mood, appetite, and the induction of sleep.

Neuronal cell bodies are often grouped together in:

Clusters

Neuroglia of the PNS

Completely surround axons and cell bodies of neurons.

Neuroglia of the PNS

Completely surround axons and cell bodies. The two types of glial cells in the PNS are: Schwann cells Satellite cells

White matter

Composed primarily of myelinated axons. The whitish color of myelin gives white matter its name. In the spinal cord, the white matter surrounds an inner core of gray matter. Is shaped like a butterfly or the letter H in transverse section.

Peripheral nervous system (PNS)

Consists of all nervous tissue outside the CNS. Components of the PNS include: Nerves Ganglia Enteric plexuses Sensory receptors

Gary Matter 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. In the brain, a thin shell of gray matter covers the surface of the largest portions of the brain, thecerebrum and cerebellum

There are two types of propagation:

Continuous conduction and saltatory conduction.

presynaptic neuron

Converts an electrical signal (nerve impulse) into a chemical signal (released neuro- transmitter).

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.

Ligand-gated channels location:

Dendrites of some sensory neurons such as pain receptors and dendrites and cell bodies of interneurons and motor neurons.

Mechanically-gated channels Location:

Dendrites of some sensory neurons such as touch receptors, pressure receptors, and some pain receptors.

Sensory function

Detect internal stimuli, such as an increase in blood pressure, or external stimuli. This sensory information is then carried into the brain and spinal cord through cranial and spinal nerves.

Sensory 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.

Widespread regions of nervous tissue are grouped together as:

Either gray matter or white matter.

Absolute refractory period

Even a very strong stimulus cannot initiate a second action potential.

The production of graded potentials and action potentials depends on two basic features of the plasma membrane of excitable cells:

Existence of a resting membrane potential & Presence of specific types of ion channels.

Current

Flow of charged particles is called . In living cells, the flow of ions (rather than electrons) constitutes the electrical current.

Leak Channels Location:

Found in nearly all cells, including dendrites, cell bodies, and axons of all types of neurons.

Nodes of Ranvier

Gaps in the myelin sheath, Appear at intervals along the axon. Each Schwann cell wraps one axon segment between two nodes.

Ligand-gated channels description:

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

Mechanically-gated channels description:

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

Voltage-gated channels description:

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

Leak channels description:

Gated channels that randomly open and close.

Unipolar neuron

Has one Unipolar neurones have dendrites and one axon that are fused together to form a continuous process that emerges from the cell body. Sensory Function as sensory receptors that detect a sensory stimulus such as touch, pressure, pain, or thermal stimuli. The trigger zone is at the junction of the dendrites and axon. The impulses then propagate toward the synaptic end bulbs. The cell bodies located in the ganglia of spinal and cranial nerves.

Encoding of Stimulus Intensity

How can your sensory systems detect stimuli of differing intensities if all nerve impulses are the same size? Why does a light touch feel different from firmer pressure? The main answer to this question is the frequency of action potentials—how often they are generated at the trigger zone. A light touch generates a low frequency of action potentials. A firmer pressure elicits action potentials that pass down the axon at a higher frequency. In addition to this "frequency code," a second factor is the number of sensory neurons recruited (activated) by the stimulus. A firm pressure stimulates a larger number of pressure-sensitive neurons than does a light touch.

Hyperpolarizing graded potentials summate

If two hyperpolarizing graded potentials summate, the net result is a larger hyperpolarizing graded potential.

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. Blood vessels are present in both white and gray matter.

The trigger zone

In most neurons, nerve impulses arise at the junction of the axon hillock and the initial segment, an area called ... , from which they travel along the axon to their destination.

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 poten- tial, 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. 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.

Continuous conduction occurs

In unmyelinated axons and in muscle fibers.

Sympathetic

Increase heart rate Support exercise Emergency actions "fight- or-flight" responses

Endorphins

Inhibit pain by blocking release of substance P; may have role in memory and learning, sexual activity, control of body temperature, and mental illness. Neuropeptides

Enkephalins

Inhibit pain impulses by suppressing release of substance P; may have role in memory and learning, control of body temperature, sexual activity, and mental illness. Neuropeptides

Less polarized

Inside less negative

More polarized

Inside more negative

Graded potentials and nerve and muscle action potentials are:

Involved in the relay of sensory stimuli, integrative functions such as perception, and motor activities.

Continuous conduction

Ions flow through their voltage-gated channels in each adjacent segment of the membrane.

A 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. Each nerve follows a de- fined path and serves a specific region of the body.

A neural circuit

Is a functional group of neurons that processes a specific kind of information.

An action potential (nerve impulse)

Is an electrical signal that propagates (travels) along the surface of the membrane of a neuron. It begins and travels due to the movement of ions (such as sodium and potassium) between interstitial fluid and the inside of a neuron through specific ion channels in itsplasma membrane. Once begun, a nerve impulse travels rapidly and at a constant strength.

A stimulus

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

Neurolemma

The outer nucleated cytoplasmic layer of the Schwann cell, which encloses the myelin sheath. Found only around axons in the PNS. When an axon is injured, the neurolemma aids regeneration by forming a regeneration tube that guides and stimulates regrowth of the axon.

The electrical signals produced by neurons and muscle fibers rely on four types of ion channels:

Leak channels Ligand-gated channels Mechanically-gated channels Voltage-gated channels

The electrical signals produced by Neurons and muscle fibers rely on four types of ion channels:

Leak channels, Ligand-gated channels Mechanically-gated channels Voltage-gated channels

Membrane potential,

Like most other cells in the body, the plasma membrane 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.

Interneurons or association neurons

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.

Neuroglia or glia

Make up about half the volume of the CNS. Actively participate in the activities of nervous tissue. Neuroglia are smaller than neutrons. 5 to 25 times more numerous. In contrast to neutrons. Do not generate or propagate action potentials. 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.

Gray matter consists of

Neuron cell bodies, dendrites, unmyelinated axons, axon terminals, and neuroglia.

Nissl bodies

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

Sensory receptor

Refers to a structure of the nervous system that monitors changes in the external or internal environment. Examples: touch receptors in the skin, photoreceptors in the eye, and olfactory receptors in the nose.

Effector

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

Action potential propagates

Only a relatively short distance in a few milliseconds.

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. 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

A 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 in the axons of all types of neurons.

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. 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.

Neurons communicate with

Other neurons at synapses, which are junctions between one neuron and a second neuron or an effector cell

Varicosities

Others exhibit a string of swollen bumps

Distribution of gray matter and white matter in the spinal cord and brain.

P 410

Comparison of Graded Potentials and Action Potentials in Neurons

PIC 424

Comparison of Electrical Signals

Produced by Excitable Cells We have seen that excitable cells—neurons and muscle fibers— produce two types of electrical signals: graded potentials and ac- tion potentials (impulses). One obvious difference between them is that the propagation of action potentials permits communication over long distances, but graded potentials can function only inshort-distance communication because they are not propagated. Table 12.2 presents a summary of the differences between graded potentials and action potentials. As we discussed in Chapter 10, propagation of a muscle action potential along the sarcolemma and into the T tubule sys- tem initiates the events of muscle contraction. Although action potentials in muscle fibers and in neurons are similar, there are some notable differences. The typical resting membrane poten- tial of a neuron is 70 mV, but it is closer to 90 mV in skeletal and cardiac muscle fibers. The duration of a nerve impulse is 0.5-2 msec, but a muscle action potential is considerably lon- ger—about 1.0-5.0 msec for skeletal muscle fibers and 10-300 msec for cardiac and smooth muscle fibers. Finally, the propa- gation speed of action potentials along the largest diameter, my- elinated axons is about 18 times faster than the propagation speed along the sarcolemma of a skeletal muscle fiber.

Comparison of Electrical Signal

Produced by Excitable Cells We have seen that excitable cells—neurons and muscle fibres produce two types of electrical signals: Graded potentials and Action potentials (impulses). One obvious difference between them is that the propagation of action potentials permits communication over long distances, but graded potentials can function only inshort-distance communication because they are not propagated.

Neurons

Provide most of the unique functions of the nervous system, such as sensing, thinking, remembering, controlling muscle activity, and regulating glandular secretions. Most neurons have lost the ability to undergo mitotic divi- sions.

Initial segment

The part of the axon closest to the axon hillock

Removal of the neurotransmitter

Removal of the neurotransmitter from the synaptic cleft is essential for normal synaptic function. If a neurotransmitter could lin- ger in the synaptic cleft, it would influence the postsynaptic neu- ron, 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. Enzymaticdegradation.Certainneurotransmittersareinactivated through enzymatic degradation. For example, the enzyme acetyl- cholinesterase breaks down acetylcholine in the synaptic cleft. 3. Uptake by cells. Many neurotransmitters are actively trans- ported back into the neuron that released them (reuptake).

Two types of neuroglia produce myelin sheaths:

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

Functions of the Nervous System

Sensory function Integrative functionMotor Function

An action potential (AP) or impulse

Sequence of rapidly occurring events that decrease and reverse the membrane potential and then eventually restore it to the resting state.

Suprathreshold stimulus

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

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.

Repolarizing Phase

Shortly after the activation gates of the voltage-gated Na channels open, the inactivation gates close. Now the voltage-gated Na channel is in an inactivated state. In addition to opening voltage-gated Na channels, a threshold- level depolarization also opens voltage-gated K channels. Because the voltage-gated K channels open more 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 open voltage-gated Na channels produce the repo- larizing phase of the action potential. As the Na channels are inactivated, Na inflow slows. At the same time, the K chan- nels are opening, accelerating K outflow. Slowing of Na in- flow and acceleration of K outflow cause the membrane poten- tial to change from 30 mV to 70 mV. Repolarization also allows inactivated Na channels to revert to the resting state.

Parasympathertic

Slow down heart rate "rest-and-digest" activities

A graded potential

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 hyperpolarizing graded potential When the response makes the membrane less polarized (inside less negative), it is termed a depolarizing graded potential 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. 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. Graded potentials occur mainly in the dendrites and cell body of a neutrons. 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.

The PNS is divided into:

Somatic nervous system (SNS) Autonomic nervous system (ANS) Enteric nervous system (ENS)

Astrocytes

Star-shaped cells have many processes and are the largest and most numerous of the neuroglia. Processes of astrocytes make contact with blood capillaries, neurons, and the pia mater (a thin membrane around the brain and spinal cord).

ANS consists of two branches:

Sympathetic division Parasympathetic division

postsynaptic neuron (post- after)

That carries a nerve impulse away from a synaps

effector cell

That responds to the impulse at the synapse.

Neural circuits

The CNS contains billions of neurons organized into complicated networks called neural circuits, functional groups of neurons that process specific types of information. A single presynaptic neuron may synapse with several post- synaptic neurons. Such an arrangement, called divergence, permits one presynaptic neuron to influence several postsynaptic neurons (or several muscle fibers or gland cells) at the same time. In a diverging circuit, 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. In another arrangement, called convergence, several presynaptic neurons synapse with a single postsynaptic neuron. This arrangement permits more effective stimulation or inhibition of the post- synaptic neuron. In a converging circuit, the postsynaptic neuron receives nerve impulses from several differ- ent 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. Some circuits are organized so that stimulation of the presyn- aptic cell causes the postsynaptic cell to transmit a series of nerve impulses. One such circuit is called a reverberating circuit (Fig- ure 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 ar- rangement 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 reverber- ating circuits are breathing, coordinated muscular activities, wak- ing up, and short-term memory. A fourth type of circuit is the parallel after-discharge circuit. 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.

Electrical excitability

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

Axon terminals or axon telodendria

The axon and its collaterals end by dividing into many fine processes called...

The basic parts of a neutron are:

The basic parts of a neuron are dendrites, a cell body, and an axon.

Acetylcholine

The best-studied neurotransmitter is acetylcholine, 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 acetylcholine esterase (AChE) inactivates ACh by splitting it into acetate and choline fragments.

Encoding of Stimulus Intensity How can your sensory systems detect stimuli of differing intensities if all nerve impulses are the same size? Why does a light touch feel different from firmer pressure?

The main answer to this question is the frequency of action potentials—how often they are generated at the trigger zone. A light touch generates a low frequency of action potentials. A firmer pressure elicits action potentials that pass down the axon at a higher frequency. In addition to this "frequency code," a second factor is the number of sensory neurons recruited (activated) by the stimulus. A firm pressure stimulates a larger number of pressure sensitive neurons than does a light touch.

Graded potentials and action potentials occur because:

The membranes of neurons contain many different kinds of ion channels that open or close in response to specific stimuli. Because theipid bilayer of the plasma membrane is a good electrical insulator, the main paths for current to flow across the membrane are through the ion channels.

During the depolarizing phase

The negative membrane potential becomes less negative, reaches zero, and then becomes positive.

Integrative function Motor

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

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. 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 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.

postsynaptic potentia

The postsynaptic neuron receives the chemical signal and in turn produces a postsynaptic potential, a type of graded potential. Yhe 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 synaptic delay of about 0.5 msec, is the reason that chemical synapses relay signals more slowly than electrical synapses.

nitric oxide (NO)

The simple gas nitric oxide (NO) is an important excitatory neurotransmitter secreted in the brain, spinal cord, adrenal glands, and nerves to the penis and has widespread effects throughout the body. NO contains a single nitrogen atom, in contrast to nitrous oxide (N2O), or laughing gas, which has two nitrogen atoms. N2O is sometimes used as an anesthetic during dental procedures.

Synapse

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

Synaptic delay

The time required for these processes at a chemical synapse, a synaptic delay of about 0.5 msec, is the reason that chemical synapses relay signals more slowly than electrical synapses.

Regeneration and Repair of Nervous Tissue

Throughout your life, your nervous system exhibits placidity, 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 plasticity, 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 re- paired 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.

Cranial nerves

Twelve pairs Emerge from the brain

Depolarizing Phase

When a depolarizing graded potential or some other stimulus causes the membrane of the axon to depolarize to threshold 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. Each voltage-gated Na channel has two separate gates, an activation gate and an inactivation gate. In the resting state of a voltage-gated Na channel, the inactivation gate 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 inactiva- tion 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. This is an example of a positive feedback mechanism. During the few ten-thousandths of a second that the voltage-gated Na channel is open, about 20,000 Na flow across the membrane and change the membrane potential considerably. However, the concentration of Na hardly changes because of the millions of Na present in the extracellular fluid. 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.

Nerve actionpotential (nerve impulse).

When an action potential occurs in a neuron (nerve cell)

Electrochemical gradient

When ion channels are open, they allow specific ions to move across the plasma membrane, down their electrochemical gradient—a concentration (chemical) difference plus an electrical difference. Recall that ions move from areas of higher concentration to areas of lower concentration (the chemical part of the gradient). 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. 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.

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)

Myelinated

When they have a Myelin sheath

After-hyperpolarizing Phase

While the voltage-gated K channels are open, outflow of K may be large enough to cause an after-hyperpolarizing phase of the action potential (see Figure 12.18). During this phase, the voltage-gated K channels remain open and the membrane poten- tial becomes even more negative (about 90 mV). As the volt- age-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.

Neurosecretory cells

Within the brain, certain neurons, called neurosecretory cells, also secrete hormones. Neurotransmit- ters can be divided into two classes based on size: small-molecule neurotransmitters and neuropeptides

The three basic functions of the nervous system occur:

You answer your cell phone after hearing it ring. The sound of the ringing cell phone stimulates sensory receptors in your ears (sensory function). This auditory information 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).

An action potential consists of

a depolarizing phase and a repolarizing phase, which may be followed by an after- hyperpolarizing phase.

At a chemical synapse

a presynaptic neuron converts an electrical signal (nerve impulse) into a chemical signal (neurotransmitter release). The postsynaptic neuron then converts the chemical signal back into an electrical signal (postsynaptic potential).

Simple series circuit

a presynaptic neuron stimulates a single postsynaptic neuron. The second neuron then stimulates another, and so on. However, most neural circuits are more complex.

Threshold stimulus

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

B fibers

are axons with diameters of 2-3 m. 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 con- duct sensory nerve impulses from the viscera to the brain and spinal cord. They also constitute all of the axons of the auto- nomic motor neurons that extend from the brain and spinal cord to the ANS relay stations called autonomic ganglia.

Gamma-aminobutyricacid and glycine

are important inhibitory neurotransmitters. At many synapses, the binding of GABA to ionotropic receptors opens Cl- channels. GABA is found only in the CNS, where it is the most common inhibitory neurotransmitter. As many as one-third of all brain synapses use GABA. Antianxiety drugs such as diazepam (Valium®) enhance the action of GABA. Like GABA, the binding of glycine to tropic receptors opens Cl- channels. About half of the inhibitory synapses in the spinal cord use the amino acid glycine; the rest use GABA.

C fibers

are the smallest diameter axons (0.5-1.5 m) 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.

The plasma membranes of dendrites (and cell bodies)

contain numerous receptor sites for binding chemical messengers from other cells.

Gap junction

contains a hundred or so tubular connexons, which act like tunnels to connect the cytosol of the two cells directly. Common in visceral smooth muscle, cardiac muscle, and the developing embryo. They also occur in the brain.

After-hyperpolarizing phase

during which the membrane potential temporarily becomes more negative than the resting level.

Multiple sclerosis (MS)

is a disease that causes a progressive de- struction of myelin sheaths surrounding neurons in the CNS. It afflicts about 350,000 people in the United States and 2 million people worldwide. It usually appears between the ages of 20 and 40, affect- ing females twice as often as males. MS is most common in whites, less common in blacks, and rare in Asians. MS is an autoimmune dis- ease—the body's own immune system spearheads the attack. The condition's name describes the anatomical pathology: In multiple regions the myelin sheaths deteriorate to scleroses, which are hard- ened scars or plaques. Magnetic resonance imaging (MRI) studies re- veal numerous plaques in the white matter of the brain and spinal cord. The destruction of myelin sheaths slows and then short-circuits propagation of nerve impulses. The most common form of the condition is relapsing-remitting MS, which usually appears in early adulthood. The first symptoms may include a feeling of heaviness or weakness in the muscles, abnor- mal sensations, or double vision. An attack is followed by a period of remission during which the symptoms temporarily disappear. One attack follows another over the years, usually every year or two. The result is a progressive loss of function interspersed with remission periods, during which symptoms abate. Although the cause of MS is unclear, both genetic susceptibility and exposure to some environmental factor (perhaps a herpes virus) appear to contribute. Since 1993, many patients with relapsing-re- mitting MS have been treated with injections of beta-interferon. This treatment lengthens the time between relapses, decreases the sever- ity of relapses, and slows formation of new lesions in some cases. Unfortunately, not all MS patients can tolerate beta-interferon, and therapy becomes less effective as the disease progresses.

synapse

is a region where communication occurs between two neurons or between a neuron and an effector cell (muscle cell or glandular cell). Synapses are essential for homeostasis because they allow information to be filtered and integrated. Synapses may be electrical or chemical and they differ both structurally and functionally.

Epilepsy

is characterized by short, recurrent attacks of motor, sensory, or psychological malfunction, although it almostnever affects intelligence. The attacks, called epileptic seizures, afflict about 1% of the world's population. They are initiated by abnormal, synchronous electrical discharges from millions of neurons in the brain, perhaps resulting from abnormal reverberating circuits. The discharges stimulate many of the neurons to send nerve impulses over their conduction pathways. As a result, lights, noise, or smells may be sensed when the eyes, ears, and nose have not been stimu- lated. Moreover, the skeletal muscles of a person having a seizure may contract involuntarily. Partial seizures begin in a small area on one side of the brain and produce milder symptoms; generalized seizures involve larger areas on both sides of the brain and loss of consciousness. Epilepsy has many causes, including brain damage at birth (the most common cause); metabolic disturbances (hypoglycemia, hypo- calcemia, uremia, hypoxia); infections (encephalitis or meningitis); toxins (alcohol, tranquilizers, hallucinogens); vascular disturbances (hemorrhage, hypotension); head injuries; and tumors and abscesses of the brain. Seizures associated with fever are most common in chil- dren under the age of two. However, most epileptic seizures have no demonstrable cause. Epileptic seizures often can be eliminated or alleviated by anti- epileptic drugs, such as phenytoin, carbamazepine, and valproate sodium. An implantable device that stimulates the vagus (X) nerve has produced dramatic results in reducing seizures in some patients whose epilepsy was not well controlled by drugs. In very severe cases, surgical intervention may be an option.

Spatial summation

is summation of post- synaptic potentials in response to stimuli that occur at different locations in the membrane of a postsynaptic cell at the same time

Temporal summation

is summation of postsynaptic potentials in response to stimuli that oc- cur at the same location in the membrane of the postsynaptic cell but at different times.

A postsynaptic cell

is the cell that receives a signal. It may be a nerve cell called a postsynaptic neuron (post- after) that carries a nerve impulse away from a synapse or an effector cell that responds to the impulse at the synapse.

The 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.

An axon contains:

mitochondria, microtubules, and neurofibrils.

White matter consists primarily of:

myelinated axons of many neurons.

Inflow of sodium ions (Na) causes

the depolarizing phase

During the repolarizing phase

the membrane potential is restored to the resting state of 70 mV. Following the repolarizing phase there may be an after-hyperpolarizing phase.

neurotransmitters

neurotransmitters released 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 ion channel opens and a postsynaptic potential (either an EPSP or IPSP) 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.Many inhibitory neurotransmitters bind to ionotropic receptors that contain chloride channels. IPSPs result from opening these Cl- channels. When Cl- channels open, a larger number of chloride ions diffuse inward. The inward flow of Cl- ions causes the inside of the postsynaptic cell to become more negative (hyperpolarized).

Examples of synapses between neurons. Arrows indicate the direction of information flo

presynaptic neuron n postsynaptic neuron. Presynaptic neurons usually synapse on the axon (axoaxonic: red), a dendrite (axodendritic; blue), or the cell body (axosomatic; green).

Neuroglia

smaller cells but they greatly outnumber neurons, as much as 25 times. Support, nourish, and protect neurons, and maintain the interstitial fluid that bathes them. Continue to divide throughout an individual's lifetime.

Signal Transmission at Synapses

synapse is a region where communication occurs between two neurons or between a neuron and an effector cell (muscle cell or glandular cell). The term presynaptic neuron (pre- before) 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 sig- nal. It may be a nerve cell called a postsynaptic neuron (post- after) 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 axodendritic, while others are axoaxsonic.

Neurogenesis

the birth of new neurons from undifferentiated stem cells—occurs regularly in some animals. significant numbers of new neurons do arise in the adult hu- man hippocampus, an area of the brain that is crucial for learning

Myelinated axons in the peripheral nervous system may be repaired if

the cell body remains intact and if Schwann cells remain active.

outflow of potassium ions (K) causes

the repolarizing phase of an action potential.

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 (where there is no myelin sheath), the axe lemma has many voltage-gated channels. Hence, current carried by Na and K flows across the membrane mainly at the nodes. When an action potential propagates along a myelinated axon, an electric current (carried by ions) flows through the extra- cellular 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 chan- nels at the third node, and so on. Each node repolarizes after it depolarizes.

If the net summation of EPSPs and IPSPs is a depolarization that reaches threshold:

then an action potential will occur at the trigger zone of a postsynaptic neuron.