The Bladder The bladder is a bag of smooth muscle that stores urine and contracts to expel urine under voluntary control. It is a low-pressure system that expands to accommodate increasing volumes of urine without an appreciable rise in pressure. This function appears to be mediated primarily by the sympathetic nervous system. During bladder filling, there is an accompanying increase in outlet resistance. The bladder muscle (the detrusor) should remain inactive during bladder filling, without involuntary contractions. When the bladder has filled to a certain volume, fullness is registered by tension-stretch receptors, which signal the brain to initiate a micturition reﬂex. This reﬂex is controlled by cortical control mechanisms, depending on the social circumstances and the state of the patient’s nervous system. Normal voiding is accomplished by voluntary relaxation of the pelvic ﬂoor and urethra, accompanied by sustained contraction of the detrusor muscle, leading to complete bladder emptying.
Innervation The lower urinary tract receives its innervation from three sources: (i) the sympathetic and (ii) parasympathetic divisions of the autonomic nervous system, and (iii) the neurons of the somatic nervous system (external urethral sphincter). The autonomic nervous system consists of all efferent pathways with ganglionic synapses that lie outside the central nervous system. The sympathetic system primarily controls bladder storage, and the parasympathetic nervous system controls bladder emptying. The somatic nervous system plays only a peripheral role in neurologic control of the lower urinary tract through its innervation of the pelvic ﬂoor and external urethral sphincter.
The sympathetic nervous system originates in the thoracolumbar spinal cord, principally T11 through L2 or L3 (see Chapter 6). The ganglia of the sympathetic nervous system are located close to the spinal cord and use acetylcholine as the preganglionic neurotransmitter. The postganglionic neurotransmitter in the sympathetic nervous system is norepinephrine, and it acts on two types of receptors: α-receptors, located principally in the urethra and bladder neck, and βreceptors, located principally in the bladder body. Stimulation of α-receptors increases urethral tone and thus promotes closure, whereas α-adrenergic receptor blockers have the opposite effect. Stimulation of β-receptors decreases tone in the bladder body.
The parasympathetic nervous system controls bladder motor function—bladder contraction and bladder emptying. The parasympathetic nervous system originates in the sacral spinal cord, primarily in S2 to S4, as does the somatic innervation of the pelvic ﬂoor, urethra, and external anal sphincter. Sensation in the perineum is also controlled by sensory fibers that connect with the spinal cord at this level. For this reason, examination of perineal sensation, pelvic muscle reﬂexes, and pelvic muscle or anal sphincter tone is relevant to clinical evaluation of the lower urinary tract. The parasympathetic neurons have long preganglionic neurons and short postganglionic neurons, which are located in the end organ. Both the preganglionic and postganglionic synapses use acetylcholine as their neurotransmitter, acting on muscarinic receptors. Because acetylcholine is the main neurotransmitter used in bladder muscle contraction, virtually all drugs used to control detrusor muscle overactivity have anticholinergic properties.