An Overview of the Nervous System

Objectives

When you have completed this section, you should be able to

• describe the subdivisions and organization of the nervous system;
• explain the general functions of the nervous system;
• distinguish between afferent neurons, interneurons, and efferent neurons;
• discuss the general physiological properties of neurons; and
• compare the nervous and endocrine systems.

Think About It

What basic physiological properties do a nerve cell and a muscle cell have in common? What physiological properties does each one have that the other one lacks?

Key Point Review

1. List the three basic functions of the nervous system and name the three basic classes of neurons that correspond to these functions.

2. What three physiological properties of neurons enable them to perform their communicative function?

3. Contrast the physiological properties of the nervous and endocrine systems.

Cells of the Nervous System

Objectives

When you have completed this section, you should be able to

• list the types of neuroglia and describe their locations and functions;
• define terms pertaining to the general structure of a neuron and classify neurons on the basis of structure;
• explain how a myelin sheath is formed;
• describe the various modes of axonal transport; and
• explain how a nerve fiber regenerates.

Think About It

Damaged nerve fibers in the CNS cannot regenerate. Why?

Key Point Review

4. How is a glial cell different from a neuron? Name the six types of glial cells and state the functions of each.

5. Sketch a multipolar neuron and label its soma, dendrites, axon, telodendria, synaptic knobs, myelin sheath, and nodes of Ranvier.

6. How is myelin produced? How does myelin production in the CNS differ from that in the PNS?

7. How are materials synthesized in the soma delivered to the axon terminals?

8. How can a severed peripheral nerve fiber regenerate and find its way back to the original target cells?

Electrophysiology of Neurons

Objectives

When you have completed this section, you should be able to

• describe the properties of the resting membrane potential;
• explain how a local potential results from stimulation of a neuron;
• explain how a neuron generates an action potential; and
• explain how a nerve impulse is propagated along an axon.

Key Point Review

9. What is a current sink? Define current in relation to the movement of cations.

10. How does the plasma membrane at the trigger zone differ from that on the soma? How does it resemble the membrane at a node of Ranvier?

11. What makes an action potential stop rising at +35 mV and start falling back toward the RMP?

12. Explain why myelinated fibers can transmit signals much faster than unmyelinated fibers.

Synapses

Objectives

When you have completed this section, you should be able to

• explain how messages are transmitted from one neuron to another;
• explain how stimulation of a postsynaptic cell is stopped; and
• give examples of different types of neurotransmitters and describe their actions.

Think About It

As described, does the previous experiment conclusively prove that the second frog's heart slowed as a result of something released by the vagus nerves? If you were Loewi, what control experiment would you do to rule out alternative explanations?

Think About It

Small neurotransmitters such as ACh and monoamines can be synthesized in the synaptic knobs, but neuropeptides can be synthesized only in the soma and must be transported to the synaptic knobs from there. Why is the synthesis of neuropeptides limited to the soma?

Key Point Review

13. Concisely describe four steps that occur in the interval between the arrival of an action potential at the telodendria and the beginning of a new action potential in the postsynaptic neuron.

14. Distinguish between ionotropic and metabotropic effects.

15. Describe three mechanisms that cause synaptic transmission to stop.

16. What is the function of neuromodulators? Name two of them.

Neural Integration

Objectives

When you have completed this section, you should be able to

• explain how postsynaptic potentials determine whether or not a neuron fires;
• explain how complex information about the environment is converted into a meaningful code by the nervous system;
• describe how learning and memory are related to changes in synaptic physiology; and
• explain how neurons work together in groups to process information and produce effective output.

Think About It

How does neuronal recruitment resemble the process of multiple motor unit summation described in chapter 12?

Key Point Review

17. Explain how EPSPs and IPSPs differ. How do they affect the probability that a postsynaptic neuron will fire?

18. Contrast the two types of summation at a synapse.

19. Describe how the nervous system communicates quantitative and qualitative information about stimuli.

20. How does long-term potentiation enhance the transmission of nerve signals along certain pathways?

21. List the four types of neuronal circuits and describe their similarities and differences. Discuss the unity of form and function in these four types–that is, explain why each type would not perform as it does if its neurons were connected differently.

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