Innervation of Urinary Bladder and Lower Ureter

Most of the nerve fasciculi in the urinary bladder wall contain unmyelinated or finely myelinated fibers. A small proportion of larger myelinated and, presumably, sensory fibers are connected with terminal arborizations regarded as stretch receptors. Many other putative sensory endings have been described in the submucosa and mucous membrane. The parasympathetic nerves may transmit many or most of the afferent fibers from the trigonal area of the urinary bladder and from the lowest parts of the ureters, including those conveying painful impulses. However, some afferents from the neck of the bladder and prostatic urethra may reach the spinal cord via the pudendal nerves.

Sensations associated with vesical distention may be mediated through sympathetic pathways because vague discomfort may still be experienced by patients with transverse lesions of the cord below the level of the uppermost lumbar segments. This suggests that there is an afferent inflow from the bladder through the upper lumbar or lowest thoracic posterior spinal nerve roots. Alternatively, such sensations may be produced by the stimulation of nerve endings in the peritoneum over a distended bladder. However, “presacral neurectomy” (removal of the superior hypogastric plexus) rarely completely alleviates discomfort in patients with painful and intractable cystitis because only a proportion of the vesical afferent fibers traverse the hypogastric nerves and superior hypogastric plexus. Other afferent fibers traveling in the perivascular plexuses of the vesical and iliac arteries may also reach the superior hypogastric plexus. Beyond the plexus, the fibers run in lumbar splanchnic nerves to the sympathetic trunks, pass through rami communicantes to the upper lumbar and lowest thoracic spinal nerves, and enter the spinal cord through the posterior roots of these nerves.

The parasympathetic supply to the bladder produces contraction of the walls and relaxation of the sphincteric mechanism and is thus actively involved in micturition. Many credit the sympathetic supply with opposing effects, such as relaxation of the detrusor muscles of the vesical wall by activation of betaadrenergic receptors and contraction of the internal sphincter by activation of alpha-adrenergic receptors. However, the sympathetic nervous system may play a minor role in bladder function, and the preponderance of evidence suggests that human bladder function depends on the integrity of the parasympathetic and somatic motor innervation of the bladder.

There are multiple interactive reflex arcs that are important in the volitional control of the bladder. The first is a connection between the posteromedial frontal lobe to the pontine nuclei via the basal ganglia. Lesions of this loop result in detrusor hyperreflexia and failure of volitional suppression of the detrusor reflex. The second reflex arc extends from the pontine nuclei to the motor neurons of the sacral region that innervate the bladder; again interruption results in detrusor hyper-reflexia. The third reflex arc includes afferents from the detrusor muscle to the motor neurons of the bladder and the fourth involves afferents from the external urethral sphincter to the motor nuclei; loss of these reflex arcs results in distention of the bladder with failure to empty.

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