The major ANS neurotransmitters are ACh and NE. ACh is secreted by somatic motor neurons as well, but in the ANS, it is released by all preganglionic axons and all parasympathetic postganglionic axons at synapses with their effectors.
Major ANS
Neurotransmitters
The major ANS neurotransmitters
are ACh and NE. ACh is secreted by somatic motor neurons as well, but in the
ANS, it is released by all preganglionic axons and all parasympathetic
postganglionic axons at synapses with their effectors. The cholinergic fibers release ACh. Most sympathetic postganglionic
axons or adrenergic fibers release
NE. One exception is sympathetic postgangli-onic fibers, which secrete ACh onto
the sweat glands.
There are no consistent
excitatory or inhibitory effects of ACh or NE upon their effectors. This is
because the action of each neurotransmitter is based on the receptor to which
it binds. Each ANS neu-rotransmitter binds with two or more types of
recep-tors. In this way, the neurotransmitter can activate or inhibit actions
at various body targets.
Autonomic neurotransmitters exert
their actions by binding to protein receptors in the effector cell mem-branes,
such as by stimulation at neuromuscular junctions and synapses. The effector
cell membranes are altered by receptor binding. Permeability to cer-tain ions
can increase. In smooth muscle cells, action potentials may occur, followed by
muscular contrac-tion. Also, gland cells may respond to membrane changes by
secreting various products.
Acetylcholinesterase (AChE) is
the enzyme that quickly decomposes ACh released by cholinergic fibers. This
decomposition also occurs at neuromus-cular junctions in the skeletal muscle.
Therefore, ACh commonly affects postsynaptic membranes for only fractions of a
second. NE released from adrener-gic fibers is mostly removed from synapses by
active transport back into nerve endings. Monoamine oxi-dase is the
mitochondrial enzyme that inactivates NE, usually within a few seconds. Some NE
molecules can diffuse to nearby tissues or the bloodstream to be decomposed by
other enzymes. Other NE molecules stay active for some time. Therefore, NE is
able to produce a longer effect than ACh. When the adrenal medulla releases NE
and epinephrine into the blood circulation, triggered by sympathetic
stimulation, the substances can cause sympathetic responses in organs that may
last up to 20 minutes.
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