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Purves D, Augustine GJ, Fitzpatrick D, et al., editors. Neuroscience. 2nd edition. Sunderland (MA): Sinauer Associates; 2001.

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Neuroscience. 2nd edition.

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Autonomic Regulation of the Bladder

The autonomic regulation of the bladder provides a good example of the interplay between the voluntary motor system (obviously, we have voluntary control over urination), and the sympathetic and parasympathetic divisions of the visceral motor system, which operate involuntarily.

The arrangement of afferent and efferent innervation of the bladder is shown in Figure 21.8. The parasympathetic control of the bladder musculature, the contraction of which causes bladder emptying, originates with neurons in the sacral spinal cord segments (S2–S4) that innervate visceral motor neurons in parasympathetic ganglia in or near the bladder wall. Mechanoreceptors in the bladder wall supply visceral afferent information to the spinal cord and to higher autonomic centers in the brainstem (primarily the nucleus of the solitary tract), which in turn project to the various central coordinating centers for bladder function in the brainstem tegmentum and elsewhere.

Figure 21.8. Autonomic control of bladder function.

Figure 21.8

Autonomic control of bladder function.

The sympathetic innervation of the bladder originates in the lower thoracic and upper lumbar spinal cord segments (T10-L2), the preganglionic axons running to sympathetic neurons in the inferior mesenteric ganglion and the ganglia of the pelvic plexus. The postganglionic fibers from these ganglia travel in the hypogastric and pelvic nerves to the bladder, where sympathetic activity causes the internal urethral sphincter to close (postganglionic sympathetic fibers also innervate the blood vessels of the bladder, and in males the smooth muscle fibers of the prostate gland). Stimulation of this pathway in response to a modest increase in bladder pressure from the accumulation of urine thus closes the internal sphincter and inhibits the contraction of the bladder wall musculature, allowing the bladder to fill. At the same time, moderate distension of the bladder inhibits parasympathetic activity (which would otherwise contract the bladder and allow the internal sphincter to open). When the bladder is full, afferent activity conveying this information centrally increases parasympathetic tone and decreases sympathetic activity, causing the internal sphincter muscle to relax and the bladder to contract. In this circumstance, the urine is held in check by the voluntary (somatic) motor innervation of the external urethral sphincter muscle (see Figure 21.8).

The voluntary control of the external sphincter is mediated by α-motor neurons of the ventral horn in the sacral spinal cord segments (S2–S4), which cause the striated muscle fibers of the sphincter to contract. During bladder filling (and subsequently, until circumstances permit urination) these neurons are active, keeping the external sphincter closed and preventing bladder emptying. During urination (or “voiding,” as clinicians often call this process), this tonic activity is temporarily inhibited, leading to relaxation in the external sphincter muscle. Thus, urination results from the coordinated activity of sacral parasympathetic neurons and temporary inactivity of the α-motor neurons of the voluntary motor system.

The central governance of these events stems from the rostral pons, the relevant pontine circuitry being referred to as the micturition center (micturition is also “medicalese” for urination). This phrase implies more knowledge about the central control of bladder function than is actually available. As many as five other central regions have been implicated in the coordination of urinary functions, including the locus coeruleus, the hypothalamus, the septal nuclei, and several cortical regions. The cortical regions primarily concerned with the voluntary control of bladder function include the paracentral lobule, the cingulate gyrus, and the frontal lobes. This functional distribution accords the motor representation of perineal musculature in the medial part of the primary motor cortex (see Chapter 17), and the planning functions of the frontal lobes (see Chapter 26), which are equally pertinent to bodily functions (remembering to stop by the bathroom before going on a long trip, for instance).

Importantly, paraplegic patients, or patients who have otherwise lost descending control of the sacral spinal cord, continue to exhibit autonomic regulation of bladder function, since urination is eventually stimulated reflexively at the level of the sacral cord by sufficient bladder distension. Unfortunately, this reflex is not efficient in the absence of descending motor control, resulting in a variety of problems in paraplegics and others with diminished or absent central control of bladder function. The major difficulty is incomplete bladder emptying, which often leads to chronic urinary tract infections from the culture medium provided by retained urine, and thus the need for an indwelling catheter to ensure adequate drainage.

By agreement with the publisher, this book is accessible by the search feature, but cannot be browsed.

Copyright © 2001, Sinauer Associates, Inc.
Bookshelf ID: NBK10886

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