Acetylcholine in the CNS

There are many cholinergic neurons (those that use ACh as a neurotransmitter) in the CNS, where the axon terminals of one neuron typically synapse with the dendrites or cell body of another. The dendrites and cell body thus serve as the receptive area of the neuron, and it is in these regions that receptor proteins for neurotransmitters and chemically regulated gated channels are located. The first voltage-regulated gated channels are located at the axon hillock, a cone-shaped elevation on the cell body from which the axon arises. The initial segment of the axon, which is the unmyelinated region of the axon around the axon hillock, has a high concentration of voltage-regulated gated channels. It is here that action potentials are first produced (see fig. 7.22).

Depolarizations—EPSPs—in the dendrites and cell body spread by cable properties to the initial segment of the axon in order to stimulate action potentials. If the depolarization is at or above threshold by the time it reaches the initial segment of the axon, the EPSP will stimulate the production of action potentials, which can then regenerate themselves along the axon. If, however, the EPSP is below threshold at the initial segment, no action potentials will be produced in the postsynaptic cell (fig. 7.26). Gradations in the strength of the EPSP above threshold determine the frequency with which action potentials will be produced at the axon hillock, and at each point in the axon where the impulse is conducted. The action potentials that begin at the initial segment of the axon are conducted without loss of amplitude toward the axon terminals.

Earlier in this chapter, the action potential was introduced by describing the events that occurred when a depolarization stimulus was artificially produced by stimulating electrodes. Now it is apparent that EPSPs, conducted from the dendrites and cell body, serve as the normal stimuli for the production of action potentials at the axon hillock, and that the action potentials at this point serve as the depolarization stimuli for the next region, and so on. This chain of events ends at the terminal boutons of the axon, where neurotransmitter is released.

Cell bodies ■ and dendrites

Axon-

Cell bodies ■ and dendrites

Axon-

Relative amounts of excitatory neurotransmitter

Relative amounts of excitatory neurotransmitter

■ Figure 7.26 The graded nature of excitatory postsynaptic potentials (EPSPs). Stimuli of increasing strength produce increasing amounts of depolarization. When a threshold level of depolarization is produced, action potentials are generated in the axon.

Alzheimer's disease, the most common cause of senile dementia, often begins in middle age and produces progressive mental deterioration. Brain lesions develop that consist of dense extracellular deposits of an insoluble protein called amyloid beta protein, and degenerating nerve fibers. Twisted fibrils, called neurofibrillar tangles, form within the dead or dying neurons. Alzheimer's is associated with a loss of cholinergic neurons that terminate in the hippocampus and cerebral cortex of the brain (areas concerned with memory storage). Treatments for Alzheimer's disease currently include the use of cholinesterase (AChE) inhibitors to augment cholinergic transmission in the brain, and the use of vitamin E and other antioxidants to limit the oxidative stress produced by free radicals (see chapter 5), which may contribute to neural damage.

Test Yourself Before You Continue

1. Distinguish between the two types of chemically regulated channels and explain how ACh opens each type.

2. State a location at which ACh has stimulatory effects. Where does it exert inhibitory effects? How are stimulation and inhibition accomplished?

3. Describe the function of acetylcholinesterase and discuss its physiological significance.

4. Compare the properties of EPSPs and action potentials and state where these events occur in a postsynaptic neuron.

5. Explain how EPSPs produce action potentials in the postsynaptic

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