Concepts of Human Anatomy & Physiology   5/e   Van De Graaff/Fox
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I. Neurons and Supporting Cells

Concept: 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 according to structure or function; the various types of supporting cells perform specialized functions.

II. Electrical Activity in Axons

Concept: The permeability of the axon membrane to Na+ and K+ is regulated by gates, which open in response to stimulation. Net diffusion of these ions occurs in two stages: first Na+ moves into the axon, then K+ moves out. This flow of ions, and the changes in the membrane potential that result constitute an event known as an action potential, or a nerve impulse.

III. The Synapse

Concept: Axons end close to, or in some cases at the point of contact with, another cell. Once action potentials reach the end of an axon, they directly or indirectly stimulate (or inhibit) the other cell. In specialized cases, action potentials can directly pass from one cell to another. In most cases, however, the action potentials stop at the axon ending, where they stimulate the release of a chemical neurotransmitter that affects the next cell.

IV. Acetylcholine as a Neurotransmitter

Concept: When acetylcholine (ACh) binds to its receptor it directly or indirectly causes the opening of chemically regulated gates. In most cases, this produces a depolarization called an excitatory postsynaptic potential, or EPSP. In some cases, however, ACh causes a hyperpolarization known as an inhibitory postsynaptic potential, or IPSP.

V. Monamines as Neurotransmitters

Concept: A variety of chemicals in the CNS function as neurotransmitters. Among these are the monamines, a chemical family that includes dopamine, norepinephrine, and serotonin. Although these molecules have similar mechanisms of action, they are used by different neurons for different functions.

VI. Other Neurotransmitters

Concept: A surprisingly large number of diverse molecules appear to function as neurotransmitters. These include some amino acids and their derivatives, many polypeptides, and even the gas nitric oxide.

VII. Synaptic Integration

Concept: The summation of numerous EPSPs may be needed to produce a depolarization of sufficient magnitude to stimulate the postsynaptic cell. The net effect of EPSPs on the postsynaptic neuron is reduced by hyperpolarizations (IPSPs), which is produced by inhibitory neurotransmitters. The activity of neurons within the central nervous system is thus the net result of both excitatory and inhibitory effects.

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