Termination of neurotransmitter activity

For any substance to serve effectively as a neurotransmitter, it must be rapidly removed or inactivated from the synapse or, in this case, the neuro-effector junction. This is necessary in order to allow new signals to get through and influence effector tissue function. Neurotransmitter activity may be terminated by three mechanisms:

• Diffusion out of the synapse

• Enzymatic degradation

• Reuptake into the neuron

The primary mechanism used by cholinergic synapses is enzymatic degradation. Acetylcholinesterase hydrolyzes acetylcholine to its components cho-line and acetate; it is one of the fastest acting enzymes in the body and acetylcholine removal occurs in less than 1 msec. The most important mechanism for removal of norepinephrine from the neuroeffector junction is the reuptake of this neurotransmitter into the sympathetic neuron that released it. Norepinephrine may then be metabolized intraneuronally by monoamine oxidase (MAO). The circulating catecholamines — epinephrine and norepi-nephrine — are inactivated by catechol-O-methyltransferase (COMT) in the liver.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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