Fig. 11.3 ). Mechanoreceptors in the bladder wall are excited by both stretch and contraction of the muscles in the bladder wall. Thus as urine accumulates and distends the bladder, the mechanoreceptor afferents begin to discharge. The pressure in the urinary bladder is low during filling (5 to 10 cm H2O), but it increases abruptly when micturition begins. Micturition can be triggered either reflexively or voluntarily. In reflex micturition, bladder afferent fibers excite neurons that project to the brainstem and activate the micturition center in the rostral pons (Barrington’s nucleus). The descending projections also inhibit sympathetic preganglionic neurons that prevent voiding. When a sufficient level of activity occurs in this ascending pathway, micturition is triggered by the micturition center. Commands reach the sacral spinal cord through a reticulospinal pathway. Activity in the sympathetic projection to the bladder is inhibited, and the parasympathetic projections to the bladder are activated. Contraction of muscle in the wall of the bladder causes a vigorous discharge of the mechanoreceptors that supply the bladder wall and thereby further activates the supraspinal loop. The result is complete emptying of the bladder.
A spinal reflex pathway also exists for micturition. This pathway is operational in newborn infants. However, with maturation, the supraspinal control pathways take on a dominant role in triggering micturition. After spinal cord injury, human adults lose bladder control during the period of spinal shock (urinary incontinence). As the spinal cord recovers from spinal shock, some degree of bladder function is recovered because of enhancement of the spinal cord micturition reflex. However, the bladder has increased muscle tone and fails to empty completely. These circumstances frequently lead to urinary infections.
Autonomic Centers in the Brain
Influence over autonomic output is maintained by autonomic centers, which consist of local networks of neurons, in a variety of brain regions. The micturition center in the pons, which was just discussed, is one example. Many other autonomic centers with diverse functions exist. Vasomotor and vasodilator centers are in the medulla, and respiratory centers are in the medulla and pons. Perhaps the greatest concentration of autonomic centers is found in the hypothalamus.
The Hypothalamus and Preoptic Area
The hypothalamus is part of the diencephalon. Some of the nuclei of the hypothalamus are shown in
Fig. 11.4 . Located anteriorly from the hypothalamus are telencephalic structures: the preoptic region and septum, both of which help regulate autonomic function. Important fiber tracts that course through the hypothalamus are the fornix, the medial forebrain bundle, and the mammillothalamic tract. The fornix is used as a landmark to divide the hypothalamus into medial and lateral zones.
The hypothalamus has many functions; see for a discussion of hypothalamic control of endocrine function. Its control of autonomic function is emphasized here. In its control of autonomic function, the hypothalamus functions much like a control system that is termed, in •Fig. 11.4 Illustration of Main Nuclei of the Hypothalamus, Viewed From the Third Ventricle. Anterioristotheright.(RedrawnfromNautaWJH,HaymakerW.The Hypothalamus. Springfield,IL:CharlesCThomas;1969.) engineering, a servomechanism: that is, a system in which a particular physiological parameter is controlled through the use of negative feedback loops to maintain the parameter at a particular set point or value. The following examples illustrate this principle for body temperature, body weight and adiposity, and water intake.
Homeothermic animals maintain a relatively constant core body temperature in situations of fluctuating environmental temperatures and differing levels of bodily activity that cause endogenous heat production. This ability rests on information from three main groups of thermoreceptors located in the skin, CNS, and viscera.