Page 416 - Clinical Anatomy

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The autonomic nervous system 401

The sacral outflow
The anterior primary rami of S2, 3 and occasionally 4 give off nerve fibres
termed the pelvic splanchnic nerves or nervi erigentes, which join the sympa-
thetic pelvic plexus for distribution to the pelvic organs. Tiny ganglia in the
walls of the viscera then relay postganglionic fibres.
The sacral parasympathetic system has been termed by Cannon ‘the
mechanism for emptying’. It supplies visceromotor fibres to the muscles of
the rectum and inhibitor fibres to the internal anal sphincter, motor fibres to
the bladder wall and inhibitor fibres to the internal vesical sphincter. In
addition, vasodilator fibres supply the erectile cavernous sinuses of the
penis and the clitoris.

Afferent parasympathetic fibres

Visceral afferent fibres from the heart, lung and the alimentary tract are con-
veyed in the vagus nerve. Sacral afferents are conveyed in the pelvic
splanchnic nerves and are responsible for visceral pain experienced in the
bladder, prostate, rectum and uterus. The reference of pain from these
structures to the sacral area, buttocks and posterior aspect of the thighs is
explained by the similar segmental supply of the sacral dermatomes.
Note that although afferent fibres are conveyed in both sympathetic and
parasympathetic nerves, they are completely independent of the auto-
nomic system. They do not relay in the autonomic ganglia and have their
cell stations, just like somatic sensory fibres, in the dorsal ganglia of the
spinal and cranial nerves. They simply use the autonomic nerves as a con-
venient anatomical conveyor system from the periphery to the brain.