Two muscular sphincters surround the urethra. The upper sphincter, composed of smooth muscle, is called the internal urethral sphincter; the lower sphincter, composed of voluntary skeletal muscle, is called the external urethral sphincter. The actions of these sphincters are regulated in the process of urination, which is also known as micturition.
■ Figure 17.3 A pseudocolor radiograph of the urinary system. In this photograph, shades of gray are assigned colors. The calyces of the kidneys, the renal pelvises, the ureters, and the urinary bladder are visible.
Micturition is controlled by a reflex center located in the second, third, and fourth sacral levels of the spinal cord. Filling of the urinary bladder activates stretch receptors that send impulses to this micturition center. As a result, parasympathetic neurons are activated, causing rhythmic contractions of the detrusor muscle of the urinary bladder and relaxation of the internal urethral sphincter. At this point, a sense of urgency is perceived by the brain, but there is still voluntary control over the external urethral sphincter. When urination is consciously allowed to occur, descending motor tracts to the micturition center inhibit somatic motor fibers to the external urethral sphincter. This muscle then relaxes, and urine is expelled. The ability to voluntarily inhibit micturition generally develops between the ages of 2 and 3.
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