6-7. PHYSIOLOGY OF THE SYMPATHETIC NERVOUS SYSTEM
a. The sympathetic nervous system is stimulated by the hypothalamus. The
nerves of the sympathetic nervous system arise from the thoracolumbar section of the
spinal cord. These nerves have short postganglionic fibers. These fibers synapse in
the sympathetic chain ganglia that lie near the spinal cord. A ganglion is a joining of
nerve fibers. Following synapse, the impulses travel down long postganglionic fibers
and synapse at the effector organ.
b. The neurotransmitter at the preganglionic synapse is acetylcholine, while the
neurotransmitters at the effector organ are norepinephrine and epinephrine.
Norepinephrine and epinephrine are released by the adrenal medulla and circulate in
the blood. Norepinephrine is also released by the postganglionic adrenergic neuron.
The enzymes, catechol-o-methyltransferase (COMT) and monoamine oxidase (MAO)
c. Circulating epinephrine and norepinephrine are destroyed by COMT. The
norepinephrine, which is released by the neuron, is either reabsorbed by the neuron or
destroyed in the synapse by MAO.
Figure 6-4. Sympathetic nervous system.