Objectives Functions and Divisions of the Nervous System



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11: Fundamentals of the Nervous System and Nervous Tissue

Objectives

Functions and Divisions of the Nervous System

1. List the basic functions of the nervous system.

2. Explain the structural and functional divisions of the nervous system.

Histology of Nervous Tissue

3. List the types of neuroglia and cite their functions.

4. Define neuron, describe its important structural components, and relate each to a functional role.

5. Differentiate between a nerve and a tract, and between a nucleus and a ganglion.

6. Explain the importance of the myelin sheath and describe how it is formed in the central and peripheral nervous systems.

7. Classify neurons structurally and functionally.



Membrane Potentials

8. Define resting membrane potential and describe its electrochemical basis.

9. Compare and contrast graded potentials and action potentials.

10. Explain how action potentials are generated and propagated along neurons.

11. Define absolute and relative refractory periods.

12. Define saltatory conduction and contrast it to conduction along unmyelinated fibers.



The Synapse

13. Define synapse. Distinguish between electrical and chemical synapses by structure and by the way they transmit information.

14. Distinguish between excitatory and inhibitory postsynaptic potentials.

15. Describe how synaptic events are integrated and modified.



Neurotransmitters and Their Receptors

16. Define neurotransmitter and name several classes of neurotransmitters.



Basic Concepts of Neural Integration

17. Describe common patterns of neuronal organization and processing.

18. Distinguish between serial and parallel processing.

Developmental Aspects of Neurons

19. Describe how neurons develop and form synapses.



Chapter Outline

I. Functions and Divisions of the Nervous System (pp. 386–387; Figs. 11.1–11.2)

A. The central nervous system consists of the brain and spinal cord, and is the integrating and command center of the nervous system (p. 386; Figs. 11.1–11.2).

B. The peripheral nervous system is outside the central nervous system (pp. 386–387; Fig. 11.2).

1. The sensory, or afferent, division of the peripheral nervous system carries impulses toward the central nervous system from sensory receptors located throughout the body.

2. The motor, or efferent, division of the peripheral nervous system carries impulses from the central nervous system to effector organs, which are muscles and glands.

a. The somatic nervous system consists of somatic nerve fibers that conduct impulses from the CNS to skeletal muscles, and allow conscious control of motor activities.

b. The autonomic nervous system is an involuntary system consisting of visceral motor nerve fibers that regulate the activity of smooth muscle, cardiac muscle, and glands.

II. Histology of Nervous Tissue (pp. 388–395; Figs. 11.3–11.5; Table 11.1)

A. Neuroglia, or glial cells, are closely associated with neurons, providing a protective and supportive network (pp. 388–389; Fig. 11.3).

1. Astrocytes are glial cells of the CNS that regulate the chemical environment around neurons and exchange between neurons and capillaries.

2. Microglia are glial cells of the CNS that monitor health and perform defense functions for neurons.

3. Ependymal cells are glial cells of the CNS that line the central cavities of the brain and spinal cord and help circulate cerebrospinal fluid.

4. Oligodendrocytes are glial cells of the CNS that wrap around neuron fibers, forming myelin sheaths.

5. Satellite cells are glial cells of the PNS whose function is largely unknown. They are found surrounding neuron cell bodies within ganglia.

6. Schwann cells, or neurolemmocytes, are glial cells of the PNS that surround nerve fibers, forming the myelin sheath.

B. Neurons are specialized cells that conduct messages in the form of electrical impulses throughout the body (pp. 389–395; Figs. 11.4–11.5; Table 11.1).

1. Neurons function optimally for a lifetime, are mostly amitotic, and have an exceptionally high metabolic rate requiring oxygen and glucose.

a. The neuron cell body, also called the perikaryon or soma, is the major biosynthetic center containing the usual organelles except for centrioles.

b. Dendrites are cell processes that are the receptive regions of the cell.

c. Each neuron has a single axon that generates and conducts nerve impulses away from the cell body to the axon terminals.

d. The myelin sheath is a whitish, fatty, segmented covering that protects, insulates, and increases conduction velocity of axons.

2. There are three structural classes of neurons.

a. Multipolar neurons have three or more processes.

b. Bipolar neurons have a single axon and dendrite.

c. Unipolar neurons have a single process extending from the cell body that is associated with receptors at the distal end.

3. There are three functional classes of neurons.

a. Sensory, or afferent, neurons conduct impulses toward the CNS from receptors.

b. Motor, or efferent, neurons conduct impulses from the CNS to effectors.

c. Interneurons, or association neurons, conduct impulses between sensory and motor neurons, or in CNS integration pathways.



III. Membrane Potentials (pp. 395–406; Figs. 11.6–11.15)

A. Basic Principles of Electricity (p. 395)

1. Voltage is a measure of the amount of difference in electrical charge between two points, called the potential difference.

2. The flow of electrical charge from point to point is called current, and is dependent on voltage and resistance (hindrance to current flow).

3. In the body, electrical currents are due to the movement of ions across cellular membranes.

B. The Role of Membrane Ion Channels (p. 395; Fig. 11.6)

1. The cell has many gated ion channels.

a. Chemically gated (ligand-gated) channels open when the appropriate chemical binds.

b. Voltage-gated channels open in response to a change in membrane potential.

c. Mechanically gated channels open when a membrane receptor is physically deformed.

2. When ion channels are open, ions diffuse across the membrane, creating electrical currents.

C. The Resting Membrane Potential (pp. 396–398; Figs. 11.7–11.8)

1. The neuron cell membrane is polarized, being more negatively charged inside than outside. The degree of this difference in electrical charge is the resting membrane potential.

2. The resting membrane potential is generated by differences in ionic makeup of intracellular and extracellular fluids, and differential membrane permeability to solutes.

D. Membrane Potentials That Act as Signals (pp. 398–404; Figs. 11.9–11.14)

1. Neurons use changes in membrane potential as communication signals. These can be brought on by changes in membrane permeability to any ion, or alteration of ion concentrations on the two sides of the membrane.

2. Changes in membrane potential relative to resting membrane potential can either be depolarizations, in which the interior of the cell becomes less negative, or hyperpolarizations, in which the interior of the cell becomes more negatively charged.

3. Graded potentials are short-lived, local changes in membrane potentials. They can either be depolarizations or hyperpolarizations, and are critical to the generation of action potentials.

4. Action potentials, or nerve impulses, occur on axons and are the principle way neurons communicate.

a. Generation of an action potential involves a transient increase in Na+ permeability, followed by restoration of Na+ impermeability, and then a short-lived increase in K+ permeability.

b. Propagation, or transmission, of an action potential occurs as the local currents of an area undergoing depolarization cause depolarization of the forward adjacent area.

c. Repolarization, which restores resting membrane potential, follows depolarization along the membrane.

5. A critical minimum, or threshold, depolarization is defined by the amount of influx of Na+ that at least equals the amount of efflux of K+.

6. Action potentials are all-or-none phenomena: they either happen completely, in the case of a threshold stimulus, or not at all, in the event of a subthreshold stimulus.

7. Stimulus intensity is coded in the frequency of action potentials.

8. The refractory period of an axon is related to the period of time required so that a neuron can generate another action potential.

E. Conduction Velocity (pp. 404–406; Fig. 11.15)

1. Axons with larger diameters conduct impulses faster than axons with smaller diameters.

2. Unmyelinated axons conduct impulses relatively slowly, while myelinated axons have a high conduction velocity.

IV. The Synapse (pp. 406–413; Figs. 11.16–11.19; Table 11.2)

A. A synapse is a junction that mediates information transfer between neurons or between a neuron and an effector cell (p. 406; Fig. 11.16).

B. Neurons conducting impulses toward the synapse are presynaptic cells, and neurons carrying impulses away from the synapse are postsynaptic cells (p. 406).

C. Electrical synapses have neurons that are electrically coupled via protein channels and allow direct exchange of ions from cell to cell (p. 406).

D. Chemical synapses are specialized for release and reception of chemical neurotransmitters (pp. 407–408; Fig. 11.17).

E. Neurotransmitter effects are terminated in three ways: degradation by enzymes from the postsynaptic cell or within the synaptic cleft; reuptake by astrocytes or the presynaptic cell; or diffusion away from the synapse (p. 408).

F. Synaptic delay is related to the period of time required for release and binding of neurotransmitters (p. 408).

G. Postsynaptic Potentials and Synaptic Integration (pp. 408–413; Figs. 11.18–11.19; Table 11.2)

1. Neurotransmitters mediate graded potentials on the postsynaptic cell that may be excitatory or inhibitory.

2. Summation by the postsynaptic neuron is accomplished in two ways: temporal summation, which occurs in response to several successive releases of neurotransmitter, and spatial summation, which occurs when the postsynaptic cell is stimulated at the same time by multiple terminals.

3. Synaptic potentiation results when a presynaptic cell is stimulated repeatedly or continuously, resulting in an enhanced release of neurotransmitter.

4. Presynaptic inhibition results when another neuron inhibits the release of excitatory neurotransmitter from a presynaptic cell.

5. Neuromodulation occurs when a neurotransmitter acts via slow changes in target cell metabolism, or when chemicals other than neurotransmitter modify neuronal activity.

V. Neurotransmitters and Their Receptors (pp. 413–421; Fig. 11.20; Table 11.3)

A. Neurotransmitters are one of the ways neurons communicate, and they have several chemical classes (pp. 413–419; Table 11.3).

B. Functional classifications of neurotransmitters consider whether the effects are excitatory or inhibitory, and whether the effects are direct or indirect (pp. 419–420).

C. There are two main types of neurotransmitter receptors: channel-linked receptors mediate direct transmitter action and result in brief, localized changes; and G protein–linked receptors mediate indirect transmitter action resulting in slow, persistent, and often diffuse changes (pp. 420–421; Fig. 11.20).



VI. Basic Concepts of Neural Integration (pp. 421–423; Figs. 11.21–11.23)

A. Organization of Neurons: Neuronal Pools (pp. 421–422; Fig. 11.21)

1. Neuronal pools are functional groups of neurons that integrate incoming information from receptors or other neuronal pools and relay the information to other areas.

B. Types of Circuits (p. 422; Fig. 11.22)

1. Diverging, or amplifying, circuits are common in sensory and motor pathways. They are characterized by an incoming fiber that triggers responses in ever-increasing numbers of fibers along the circuit.

2. Converging circuits are common in sensory and motor pathways. They are characterized by reception of input from many sources, and a funneling to a given circuit, resulting in strong stimulation or inhibition.

3. Reverberating, or oscillating, circuits are characterized by feedback by axon collaterals to previous points in the pathway, resulting in ongoing stimulation of the pathway.

4. Parallel after-discharge circuits may be involved in complex activities, and are characterized by stimulation of several neurons arranged in parallel arrays by the stimulating neuron.

C. Patterns of Neural Processing (pp. 422–423; Fig. 11.23)

1. Serial processing is exemplified by spinal reflexes, and involves sequential stimulation of the neurons in a circuit.

2. Parallel processing results in inputs stimulating many pathways simultaneously, and is vital to higher level mental functioning.

VII. Developmental Aspects of Neurons (pp. 423–424; Fig. 11.24)

A. The nervous system originates from a dorsal neural tube and neural crest, which begin as a layer of neuroepithelial cells that ultimately become the CNS (p. 423).

B. Differentiation of neuroepithelial cells occurs largely in the second month of development (p. 423).

C. Growth of an axon toward its target appears to be guided by older “pathfinding” neurons and glial cells, nerve growth factor and cholesterol from astrocytes, and tropic chemicals from target cells (pp. 423–424).

D. The growth cone is a growing tip of an axon. It takes up chemicals from the environment that are used by the cell to evaluate the pathway taken for further growth and synapse formation (p. 424; Fig. 11.24).

E. Unsuccessful synapse formation results in cell death, and a certain amount of apoptosis occurs before the final population of neurons is complete (p. 424).



Laboratory Correlations

1. Marieb, E. N., and S. J. Mitchell. Human Anatomy & Physiology Laboratory Manual: Main Version. Eighth Edition Update. Benjamin Cummings, 2009.

Exercise 17: Histology of Nervous Tissue

Exercise 18: Neurophysiology of Nerve Impulses



PhysioEx™ 8.0 Exercise 18B: Neurophysiology of Nerve Impulses: Computer Simulation

Online Resources for Students

myA&P™

www.myaandp.com



The following shows the organization of the Chapter Guide page in myA&P™. The Chapter Guide organizes all the chapter-specific online media resources for Chapter 11 in one convenient location, with e-book links to each section of the textbook. Students can also access A&P Flix animations, MP3 Tutor Sessions, Interactive Physiology® 10-System Suite, Practice Anatomy Lab™ 2.0, PhysioEx™ 8.0, and much more.

Objectives

Section 11.1 Functions and Divisions of the Nervous System (pp. 386–387)

Interactive Physiology® 10-System Suite: Nervous System I: Orientation

Section 11.2 Histology of Nervous Tissue (pp. 388–395)

Interactive Physiology® 10-System Suite: Nervous System I: Anatomy Review (Neurons)

Memory Game: Neural System



Section 11.3 Membrane Potentials (pp. 395–406)

MP3 Tutor Session: Generation of an Action Potential

Interactive Physiology® 10-System Suite: Nervous I: Ion Channels

Interactive Physiology® 10-System Suite: Nervous I: Membrane Potential

Interactive Physiology® 10-System Suite: Nervous I: Action Potential

Memory Game: Action Potential Propagation



PhysioEx™ 8.0: Neurophysiology of Nerve Impulses

Section 11.4 The Synapse (pp. 406–413)

A&P Flix animation: Neurophysiology of Nerve Impulses

Interactive Physiology® 10-System Suite: Nervous System II: Anatomy Review (Synapses)

Interactive Physiology® 10-System Suite: Nervous System II: Ion Channels

Interactive Physiology® 10-System Suite: Nervous System II: Synaptic Transmission

Interactive Physiology® 10-System Suite: Nervous System II: Synaptic Potentials and Cellular Integration

Section 11.5 Neurotransmitters and Their Receptors (pp. 413–421)

Section 11.6 Basic Concepts of Neural Integration (pp. 421–423)

Section 11.7 Developmental Aspects of Neurons (pp. 423–424)

Case Study: Congenital Defect



Chapter Summary

Crossword Puzzle 11.1

Crossword Puzzle 11.2

Crossword Puzzle 11.3

Crossword Puzzle 11.4

Crossword Puzzle 11.5

Web Links

Chapter Quizzes

Art Labeling Quiz

Matching Quiz

Multiple-Choice Quiz

True-False Quiz

Chapter Practice Test

Study Tools

Histology Atlas

myeBook

Flashcards



Glossary

CourseCompass™

Resources in the myA&P™ Chapter Guide are also available in the Chapter Contents section of the CourseCompass™. Students can also access A&P Flix animations, MP3 Tutor Sessions, Interactive Physiology® 10-System Suite, Practice Anatomy Lab™ 2.0, PhysioEx™ 8.0, and much more.

Answers to End-of-Chapter Questions

Multiple-Choice and Matching Question answers appear in Appendix G of the main text.

Short Answer Essay Questions

13. Anatomical division includes the CNS (brain and spinal cord) and the PNS (nerves and ganglia). Functional division includes the somatic and autonomic motor divisions of the PNS. The autonomic division is divided into sympathetic and parasympathetic subdivisions. (pp. 386–387)

14. a. The cell body is the biosynthetic and metabolic center of a neuron. It contains the usual organelles, but lacks centrioles. (pp. 389–390)

b. Dendrites and axons both function to carry electrical current. Dendrites differ in that they are short, transmit toward the cell body, and function as receptor sites. Axons are typically long, are myelinated, and transmit away from the cell body. Only axons can generate action potentials, the long-distance signals. (pp. 390–391)

15. a. Myelin is a whitish, fatty, phospholipid-insulating material (essentially the wrapped plasma membranes of oligodendrocytes or Schwann cells).

b. CNS myelin sheaths are formed by flaplike extensions of oligodendrocytes and lack a neurilemma. Each oligodendrocyte can help to myelinate several fibers. PNS myelin is formed by Schwann cells; the wrapping of each Schwann cell forms the internode region. The sheaths have a neurilemma, and the fibers they protect are capable of regeneration. (pp. 391–392)

16. Multipolar neurons have many dendrites, one axon, and are found in the CNS (and autonomic ganglia). Bipolar neurons have one axon and one dendrite, and are found in receptor end organs of the special senses such as the retina of the eye and olfactory mucosa. Unipolar neurons have one process that divides into an axon and a dendrite and is a sensory neuron with the cell body found in a dorsal root ganglion or cranial nerve ganglion. (pp. 392–393; Table 11.1)

17. A polarized membrane possesses a net positive charge outside, and a net negative charge inside, with the voltage across the membrane being at –70 mv. Diffusion of Na+ and K+ across the membrane establishes the resting potential because the membrane is slightly more permeable to K+. The Na+-K+ pump, an active transport mechanism, maintains this polarized state by maintaining the diffusion gradient for Na+ and K+. (p. 396)

18. a. The generation of an action potential involves: (1) an increase in sodium permeability and reversal of the membrane potential; (2) a decrease in sodium permeability; and (3) an increase in potassium permeability and repolarization. (pp. 399–402)

b. The ionic gates are controlled by changes in the membrane potential and activated by local currents.

c. The all-or-none phenomenon means that the local depolarizing current must reach a critical “firing” or threshold point before it will respond, and when it responds, it will respond completely by conducting the action potential along the entire length of its axon. (pp. 403–404)

19. The CNS “knows” a stimulus is strong when the frequency or rate of action potential generation is high. (p. 404)

20. a. An EPSP is an excitatory (depolarizing) postsynaptic potential that increases the chance of a depolarization event. An IPSP is an inhibitory (hyperpolarizing) postsynaptic potential that decreases the chance of a depolarization event. (pp. 411–412)

b. It is determined by the type and amount of neurotransmitter that binds at the postsynaptic neuron and the specific receptor subtype it binds to. (pp. 411–413)

21. Each neuron’s axon hillock keeps a “running account” of all signals it receives via temporal and spatial summation. (p. 412)

22. The neurotransmitter is quickly removed by enzymatic degradation or reuptake into the presynaptic axon. This ensures discrete limited responses. (p. 408)

23. a. A fibers have the largest diameter and thick myelin sheaths and conduct impulses quickly; B fibers are lightly myelinated, have intermediate diameters, and are slower conductors. (p. 406)

b. Absolute refractory period is when the neuron is incapable of responding to another stimulus because the sodium gates are still open or are inactivated. (p. 404)

c. A node of Ranvier is an interruption of the myelin sheath between the wrappings of individual Schwann cells or of the oligodendrocyte processes. (p. 391)

24. In serial processing, the pathway is constant and occurs through a definite sequence of neurons; the response is predictable and stereotyped. In parallel processing, impulses reach the final CNS target by multiple pathways. Parallel processing allows for a variety of responses. (p. 423)

25. First, they proliferate; second, they migrate to proper position; third, they differentiate. (pp. 423–424)

26. Development of the axon is due to the chemical signals neurotropin and NGF that interact with receptors on the developing axon to support and direct its growth. (p. 424)



Critical Thinking and Clinical Application Questions

1. The resting potential would decrease, that is, become less negative, because the concentration gradient causing net diffusion of K+ out of the cell would be smaller. Action potentials would be fired more easily, that is, in response to smaller stimuli, because the resting potential would be closer to threshold. Repolarization would occur more slowly because repolarization depends on net K+ diffusion from the cell and the concentration gradient driving this diffusion is lower. Also, the subsequent hyperpolarization would be smaller. (pp. 396–398)

2. Local anesthetics such as novocaine and sedatives affect the neural processes usually at the nodes of Ranvier, by reducing the membrane permeability to sodium ions. (p. 402)

3. The bacteria remain in the wound; however, the toxin produced travels via axonal transport to reach the cell body. (p. 391)



4. In MS, the myelin sheaths are destroyed. Loss of this insulating sheath results in shunting of current and eventual cessation of neurotransmission. (pp. 405–406)

5. Glycine is an inhibitory neurotransmitter that is used to modulate spinal cord transmission. Strychnine blocks glycine receptors in the spinal cord, leading to unregulated stimulation of muscles, and spastic contraction to the point where the muscles cannot relax. (p. 417; Table 11.3)

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