Porth's Essentials of Pathophysiology, 4e

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Kidney and Urinary Tract Function

U N I T 7

Pontine Micturition Center The immediate coordination of the normal micturi- tion reflex occurs in the micturition center in the pons, facilitated by descending input from the forebrain and ascending input from the reflex centers in the spinal cord 5 (Fig. 27-3). This center is thought to coordinate the activity of the detrusor muscle and the external sphincter. As bladder filling occurs, ascending spinal afferents relay this information to the micturition cen- ter, which also receives important descending informa- tion from the forebrain concerning behavioral cues for bladder emptying and urine storage. Descending pathways from the pontine micturition center produce coordinated inhibition or relaxation of the external sphincter. Disruption of pontine control of micturi- tion, as in spinal cord injury, results in uninhibited spinal reflex–controlled contraction of the bladder without relaxation of the external sphincter, a condi- tion known as detrusor–sphincter dyssynergia (to be discussed). Cortical and Subcortical Centers Cortical brain centers enable inhibition of the micturi- tion center in the pons and conscious control of urina- tion. Neural influences from the subcortical centers in the basal ganglia modulate the contractile response. They modify and delay the detrusor contractile response during filling and then modulate the expulsive activity of the bladder to facilitate complete emptying. In infants and young children the pathways to the cortical and subcortical centers are not fully developed and micturition occurs whenever the bladder is suffi- ciently distended. At about age 2 1/2 years it begins to come under cortical control; in most children, complete control is achieved by 3 years of age. The neuromediators for the ANS play a central role in micturition and maintenance of continence. 6,7 Parasympathetic innervation of the bladder is medi- ated by the neurotransmitter acetylcholine. Two types of cholinergic receptors, nicotinic and muscarinic, affect various aspects of micturition. Nicotinic (N) receptors are found in the synapses between the preganglionic and postganglionic neurons of the sympathetic and para- sympathetic nervous systems, as well as motor endplates of the striated muscle fibers of the external sphincter and pelvic muscles. Muscarinic (M) receptors are found in the postganglionic parasympathetic endings of the detrusor muscle. Several subtypes of M receptors have been identified. Both M 2 and M 3 receptors appear to mediate detrusor muscle activity, with the M 3 subtype mediating direct activation of detrusor muscle con- traction. The M 2 subtype appears to act indirectly by inhibiting sympathetically mediated detrusor muscle relaxation. 5–7 The identification of muscarinic receptor subtypes has facilitated the development of medications Neuromediator Control of Bladder Function

sphincter and the pelvic floor muscles that together con- trol the outflow of urine. These functions are controlled by three neurologic centers: the spinal cord reflex cen- ters, the micturition center in the pons, and cortical and subcortical centers. 5–7 Spinal Cord Centers The centers for reflex control of bladder function are located in the sacral (S1 through S4) and thoracolumbar (T11 through L2) segments of the spinal cord (Fig. 27-2). The parasympathetic neurons for the detrusor muscle of the bladder are located in the sacral segments of the spi- nal cord; their axons travel to the bladder by way of the pelvic nerve . The lower motor neurons for the external sphincter also are located in the sacral segments of the spinal cord. These neurons receive their control from the motor cortex through the corticospinal tract and send impulses to the external sphincter through the pudendal nerve . The bladder neck and trigone area of the blad- der, because of their different embryonic origins, receive sympathetic outflow from the thoracolumbar (T11 to L2) segments of the spinal cord. In the male, the seminal vesicles, ampulla, and vas deferens also receive sympa- thetic innervation from the thoracolumbar segments of the cord. The afferent input from the bladder and urethra is carried to the central nervous system (CNS) by fibers that travel with the parasympathetic (pelvic), somatic (pudendal), and sympathetic (hypogastric) nerves. The pelvic nerve carries sensory fibers from the stretch receptors in the bladder wall, the pudendal nerve car- ries sensory fibers from the external sphincter and pelvic muscles, and the hypogastric nerve carries sensory fibers from the trigone area.

Sympathetic neurons

T11

Afferent fibers Efferent motor Efferent inhibitory

T12 L1

L2

Sympathetic hypoglossal nerve

Parasympathetic pelvic nerve

S1

S2

S2

S3

S3 S4

Internal sphincter External sphincter Detrusor muscle

Somatic pudendal nerve

FIGURE 27-2. Nerve supply to the bladder and the urethra.