Neurotransmitters of autonomic nervous system

The two most common neurotransmitters released by neurons of the ANS are acetylcholine (Ach) and norepinephrine (NE). Several distinguishing features of these neurotransmitters are summarized in Table 9.3. Nerve fibers that release acetylcholine are referred to as cholinergic fibers and include all preganglionic fibers of the ANS — sympathetic and parasympathetic systems; all postgan-glionic fibers of the parasympathetic system; and sympathetic postganglionic

Table 9.3 Distinguishing Features of Neurotransmitters of Autonomie Nervous System

Feature

Acetylcholine

Norepinephrin

Epinephrine3

Site of

All preganglionic neurons of

Most sympathetic postganglionic neurons;

Adrenal medulla (80% of

Release

autonomic nervous system; all postganglionic neurons of parasympathetic system; some sympatheticpostganglionicneurons innervating sweat glands; (alpha motor neurons innervating skeletal muscle)b

adrenal medulla (20% of secretion)

secretion)

Receptor

Nicotinic, muscarinic (cholinergic)

oty a2, ßj (adrenergic)

a1; a2, px, |32 (adrenergic)

Termination

Enzymatic degradation by

Reuptake into nerve terminals; diffusion

Metabolic transformation by

of Activity

acetylcholinesterase

out of synaptic cleft and uptake at extraneuronal sites; metabolic transformation by monoamine oxidase (within nerve terminal) or catechol-O-methyl-transferase within liver

catechol-O-methyltransferase within liver

a Although epinephrine is not a direct neurotransmitter for the autonomic nervous system, its release from the adrenal medulla supplements the effects of a mass sympathetic discharge. b Alpha motor neurons, a component of the somatic nervous system, also release acetylcholine as a neurotransmitter.

a Although epinephrine is not a direct neurotransmitter for the autonomic nervous system, its release from the adrenal medulla supplements the effects of a mass sympathetic discharge. b Alpha motor neurons, a component of the somatic nervous system, also release acetylcholine as a neurotransmitter.

fibers innervating sweat glands (see Figure 9.2). Nerve fibers that release norepinephrine are referred to as adrenergic fibers. Most sympathetic postgan-glionic fibers release norepinephrine.

As previously mentioned, the cells of the adrenal medulla are considered modified sympathetic postganglionic neurons. Instead of a neurotransmitter, these cells release hormones into the blood. Approximately 20% of the hormonal output of the adrenal medulla is norepinephrine. The remaining 80% is epinephrine (EPI). Unlike true postganglionic neurons in the sympathetic system, the adrenal medulla contains an enzyme that methylates norepi-nephrine to form epinephrine. The synthesis of epinephrine, also known as adrenalin, is enhanced under conditions of stress. These two hormones released by the adrenal medulla are collectively referred to as the catechola-mines.

Essentials of Human Physiology

Essentials of Human Physiology

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