11: Fundamentals of the Nervous
System and Nervous Tissue
Objectives
Neurons
Define neuron, describe its important structural components, and relate each to a functional role.
Differentiate between a nerve and a tract, and between a nucleus and a ganglion.
Explain the importance of the myelin sheath and describe how it is formed in the central and
peripheral nervous systems.
Membrane Potentials
Define resting membrane potential and describe its electrochemical basis.
Compare and contrast graded potentials and action potentials.
Explain how action potentials are generated and propagated along neurons.
Define saltatory conduction and contrast it to conduction along unmyelinated fibers.
Neurons
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.
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.
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.
Cross References
Additional information on topics covered in Chapter 11 can be found in the chapters listed below.
1. Chapter 2: Enzymes and enzyme function
2. Chapter 3: Passive and active membrane transport processes; membrane potential;
cytoskeletal elements; cell cycle
3. Chapter 4: Nervous tissue
4. Chapter 9: Synapse (neuromuscular junction)
5. Chapter 13: Membrane potentials; neural integration
6. Chapter 14: Cholinergic and adrenergic receptors and other neurotransmitter effects;
autonomic synapses
7. Chapter 15: Receptors for the special senses; synapses involved in the special senses;
neurotransmitters in the special senses
8. Chapter 16: Nervous system modulation of endocrine function
9. Chapter 18: Membrane potential and the electrical activity of the heart
10. Chapter 19: Baroreceptors and chemoreceptors in blood pressure and flow regulation
11. Chapter 22: Chemoreceptors and stretch receptors related to respiratory function
12. Chapter 23: Sensory receptors and control of digestive processes
13. Chapter 24: Examples of receptors
Multimedia in the Classroom and Lab
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 ebook 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
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)