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Chapter 7

The Nervous System: Neurons and Synapses

Chapter Scope

This chapter begins a four‑chapter unit (chapters 7 through 10) on the basic structure and function of neurons and synapses in the nervous system. The electrical membrane potential of a neuron at rest that was introduced in the last chapter, now “comes to life” as appropriate stimuli alter the permeability of the plasma membrane to ions. The carefully synchronized opening and closing of Na+ and K+ gates or “channels” result in the movement of electrical charges that generates a nerve impulse, or action potential.

Action potentials reach the end of each neuron where these electrical signals are either transmitted directly to the next cell in the sequence via electrical synapses or gap junctions, or indirectly are responsible for activating the release of specialized neurotransmitter chemicals. Released from vesicles into the synaptic space, these neurotransmitters diffuse a short distance, bind to specialized receptors integrated in the membrane of the next effector cell in the conduction pathway and promote the formation of new action potentials. Effector cells such as another neuron, a muscle fiber (chapters 12, 13), or a gland cell (chapters 11, 20), will then respond.

The release and action of specific neurotransmitters, especially acetylcholine (ACh), is carefully detailed in this chapter. Others, such as the catecholamines (dopamine, norepinephrine, and epinephrine) and a growing number of less well‑known neurochemicals (amino acids, polypeptides, nitric oxide, or endocannabinoid) are particularly active in the CNS.

It is important to have a solid understanding of the nervous system’s structure and function presented in these four chapters for a successful (and enjoyable) comprehension of the organ system chapters that follow. The nervous system forms the basic communication network linking all tissues of the body to the brain and to each other. As will be featured in chapter 11, the nervous system’s fast electrical signals (action potentials) often work together with slower‑responding chemical messengers (hormones). Both messenger systems, however, must cooperate effectively in the maintenance of overall body homeostasis.

I. Neurons and Supporting Cells

The nervous system is composed of neurons, which produce and conduct electrochemical impulses, and supporting cells, which assist the functions of neurons. Neurons are classified functionally and structurally; the various types of supporting cells perform specialized functions.

A. Multiple Choice

  1. Which of the following is not a function of neurons?

a. respond to physical and chemical stimuli

b. conduct electrical impulses

c. release specific chemical regulators

d. All of these are neuron functions.

  1. Nissl bodies located only in the cell body are composed of

a. mitochondria.

b. rough endoplasmic reticulum.

c. Golgi apparatus.

d. lysosomes.

  1. A grouping of cell bodies located within the CNS is known as a

a. tract.

b. nerve.

c. nucleus.

d. ganglion.

  1. Involuntary effectors (glands, smooth or cardiac muscle) are innervated (stimulated) by

a. autonomic nerves.

b. efferent nerves.

c. motor nerves.

d. All of these nerves innervate involuntary effectors.

  1. The most common type of neuron (motor neuron, for example) is

a. bipolar.

b. multipolar.

c. pseudounipolar.

  1. Myelin sheaths around axons within the CNS are formed by

a. Schwann cells.

b. microglia.

c. astrocytes.

d. oligodendrocytes.

  1. The most abundant supporting (glial) cell in the CNS, which forms end-feet around capillaries associated with the blood-brain barrier, is the

a. astrocyte.

b. oligodendrocyte.

c. satellite cell.

d. microglia.

___ 8. The supporting cells of the nervous system that line the ventricles (cavities) of the brain, that form choroid plexuses producing cerebrospinal fluid (CSF); and more recently that seem to function as stem cells (able to divide and differentiate into new neurons and neuroglial cells), best describes the

  1. Astrocyte.

  2. Oligodendrocyte.

  3. satellite cell.

  4. Microglia.

  5. ependymal cell.

___ 9. Which statement about Schwann cells is not true?

a. They remain alive as their cytoplasm is forced to the outside of the myelin sheath.

b. They have extensions, like tentacles of an octopus that form myelin sheaths around several axons simultaneously.

c. Adjacent cells form gaps exposing nodes of Ranvier along an axon.

  1. They are only found in the peripheral nervous system (PNS).

  2. They can form a regeneration tube; helping to reconnect and reestablish nerve function after an axon has been cut.

___ 10. Which part of neurons is progressively destroyed in those people with the chronic disease, multiple sclerosis (MS)?

a. cell body

b. axons

c. dendrites

d. axon hillock

e. myelin sheath

___ 11. Neurotrophins, are important chemicals secreted by neurons that

a. help make the blood-brain barrier.

b. promote neuron growth, especially in the developing fetal brain.

c. make myelin for neuron axons.

d. keep the CNS tissue clear of debris and foreign particles.

  1. relay impulses from one neuron to the next.

___ 12. Which of the following is not a function of glial cells known as astrocytes?

  1. absorb released K+ from the extracellular fluid

  2. absorb certain neurotransmitters such as glutamate for reuse

  3. absorb energy molecules such as glucose for production of ATP

  4. regulate the differentiation (specialization) of glial cells and neurons in the adult brain from stem cells

  5. All of these are functions of astrocytes.

B. True or False/Edit

___ 13. The nervous system is composed of two principal types of cells — neurons and supporting cells (neuroglia or glial cells).

___ 14. Neurons cannot divide by mitosis, although some neurons can regenerate severed portions or sprout new branches under some conditions.

___ 15. In the brain, neurons outnumber glial cells five to one.

___ 16. Orthograde (forward flow) and retrograde (reverse flow) transport in neurons is characteristic of rapid axonal transport.

___ 17. Association neurons (interneurons) are located entirely within the central nervous system (CNS).

___ 18. A continuous, living sheath of Schwann cells surrounds all axons in the central nervous system (CNS) but not in the peripheral nervous system (PNS).

___ 19. The myelin sheaths surrounding CNS axons are formed by glial cells known as oligodendrocytes after birth.

___ 20. The myelin sheaths around axons of the CNS give this tissue a gray color and thus form gray matter.

___ 21. Myelinated axons conduct impulses more rapidly than those that are unmyelinated..

___ 22. Regeneration of CNS axons is inhibited by many factors including growth-inhibiting proteins in the membranes of myelin sheaths and from oligodendrocytes as well as glial scars formed from astrocytes.

___ 23. Astrocytes are glial cells that surround capillaries of the CNS using their end-feet to uptake glucose molecules from the blood; and can also take up such substances as K+ and glutamate neurotransmitters from the extracellular fluid.

___ 24. Spaces (pores) are found between endothelial cells lining the capillaries of the brain, and thus form the blood‑brain barrier.

C. Label the Figure — Neuron Structure

Study figure 7.1 and notice the differences in structure between sensory neurons and motor neurons. Then correctly label each neuron type using the term “sensory” or “motor.” Complete the exercise by labeling the various parts of each neuron in the spaces provided. (When finished, check your work with figure 7.1 in your textbook.)

Figure 7.1 The structure of two kinds of neurons.

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