McKenna's Pharmacology for Nursing, 2e

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P A R T 5  Drugs acting on the autonomic nervous system

Acetyl CoA + Choline (from diet)

1

Enzyme

Nerve terminal

2

ACh

AP

Synaptic vesicle

Ca+

3

Synaptic cleft

ACh E

A

4

Acetic acid + Choline to blood vessel

ACh E

Acetylcholinesterase

5

FIGURE 29.5  Sequence of events at a cholinergic synapse. 1. Synthesis of acetylcholine (ACh) from choline (a substance in the diet) and a cofactor (the enzyme is choline acetyltransferase, CoA). 2. Uptake of neurotransmitter into storage (synaptic) vesicle. 3. Release of neurotransmitter by an action potential (AP) in the presynaptic nerve. 4. Diffusion of neurotransmitter across the synaptic cleft. 5. Combination of neurotransmitter with receptor. The events resulting from ACh’s occupation of receptor sites depend on the nature of the postsynaptic cell. ACh excites some cells and inhibits others. An enzyme, acetylcholinesterase (AChE), found in the tissues and on the postsynaptic cell, inactivates ACh (A) . Some of the products diffuse into the circulation, but most of the choline formed is taken up and reused by the cholinergic neuron.

Cholinergic receptor

Neuron or effector cell

Termination of response Once the effector cell has been stimulated by ACh, stimulation of the receptor site must be terminated and destruction of any ACh must occur. The destruction of ACh is carried out by the enzyme a cetylcholinesterase . This enzyme reacts with the ACh to form a chemically inactive compound. The breakdown of the released ACh is accomplished in 1/1000 second and the receptor is vacated, allowing the effector membrane to repolarise and be ready for the next stimulation.

digestion, absorption and metabolism of nutrients and slows metabolism and function to save energy. ■■ The parasympathetic system comprises CNS cells that arise in the cranium and sacral region of the spinal cord, long preganglionic axons that secrete ACh, ganglia located very close to or within the effector tissue and short postganglionic axons that also secrete ACh. ■■ ACh is made by choline from the diet and packaged into storage vesicles to be released by the cholinergic nerve into the synaptic cleft. ACh is broken down to an inactive form almost immediately by acetylcholinesterase. ■■ Parasympathetic system receptors are classified as muscarinic or nicotinic, depending on what response they have to these plant alkaloids.

KEY POINTS

■■ The parasympathetic system, when stimulated, acts as a rest-and-digest response. It increases the