Sympathetic and parasympathetic neuron chains from the middle and lower levels of the spinal cord control smooth muscle in the walls and sphincters (muscular rings that close the openings) of the urinary bladder and rectum, as well as controlling sexual glands and smooth muscle in blood vessels of the genitals. Sympathetic activity prevents urination or defecation by causing relaxation of the urinary bladder and rectum while also causing contraction of their sphincters. Parasympathetic activity leads to urination or defecation by causing contraction of the urinary bladder or rectum while also causing relaxation of their sphincters.
Axons from the brain descend in the spinal cord to control these autonomic neurons, but this anatomy is complex and poorly understood. There appear to be areas in both the cerebrum and the brainstem from which axons descend through the spinal cord, probably mainly in the lateral columns, to synapse, probably ipsilaterally, at levels of the spinal cord with lateral gray horns containing autonomic neurons. Dysfunction of these descending axons may cause sympathetic or parasympathetic abnormalities, such as loss of control of urination, defecation, or genital function, depending on the level of the lesion. This occurs commonly with spinal cord lesions, and uncommonly with brain lesions, possibly because these axons are more spread out or redundant in the brain than they are in the spinal cord. There is more conscious control over these autonomic functions than most others. There are also minor components of the sphincters involved in urination and defecation composed of skeletal muscle, innervated by lower motor neurons, that are under voluntary control, but these appear to be insufficient by themselves for full control.
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