 In this video, I will describe the micturition reflex, describe voluntary and involuntary neural control of micturition. Micturition is a technical term for what we commonly refer to as urination, the process of emptying the urinary bladder. Urine flows into the urinary bladder through the ureters. The ureters open at the ureteral openings, and then urine flows out of the urinary bladder through the urethra. There is a triangular region between the ureteral openings and the urethra that I'm highlighting in the yellow color here. This is known as the trigon. Let's use a different color so that we can see my righty, trigon. So the trigon contains stretch receptors, sensory receptors that detect filling of the urinary bladder and relay this information in through afferent fibers, sensory neurons, that carry the signal into the spinal cord in order to provide the input for the micturition reflex. We can also see in this illustration the major muscles of the urinary bladder and urethra that are involved in regulating the process of micturition. The detrusor muscle is the smooth muscle found in the wall of the urinary bladder that contracts during micturition to help force urine out through the urethra. The internal urethral sphincter is a ring of smooth muscle at the opening of the urethra from the urinary bladder that provides involuntary control over opening of the urethra to help regulate micturition. The external urethral sphincter is skeletal muscle that enables voluntary control over micturition. During micturition, the detrusor muscle will contract while the internal urethral sphincter and external urethral sphincters will relax. The control of micturition involves control centers in the central nervous system. The micturition control centers are located both in the spinal cord as well as in the brain. We have stretch receptors in the trigon that detect filling of the urinary bladder and relay this information into the central nervous system through afferent fibers of the pelvic nerve. The parasympathetic division of the autonomic nervous system sends commands through the pelvic nerve that stimulate contraction of the detrusor and inhibit contraction of the internal urethral sphincter. The parasympathetic efferent fibers release acetylcholine as a stimulus for micturition. In contrast, sympathetic efferent fibers that travel through the hypogastric nerve to reach the urinary bladder release the neurotransmitter norepinephrine in order to inhibit contraction of the detrusor and stimulate contraction of the internal urethral sphincter. While the parasympathetic division of the autonomic nervous system will stimulate micturition, the sympathetic division of the autonomic nervous system will inhibit micturition. There are also somatic efferent fibers that are involved in regulating micturition by stimulating contraction of the external urethral sphincter. The somatic efferent fibers of the pudendal nerve prevent micturition. Therefore to enable micturition, the somatic efferent fibers will release less acetylcholine causing relaxation of the external urethral sphincter. The micturition reflex starts with stretch receptors in the trigon detecting the filling of the urinary bladder and relaying that information in through efferent fibers of the pelvic nerve to the control centers in the central nervous system. So the micturition control centers in the central nervous system will generate commands to stimulate micturition. This will include stimulating the parasympathetic efferent fibers of the pelvic nerve that releases acetylcholine to activate the detrusor and inhibit the internal urethral sphincter. There is also decreased activity of the sympathetic efferent fibers in the hypogastric nerve enabling micturition to proceed as well as decreased activity of somatic efferent fibers in the pudendal nerve that enable relaxation of the external urethral sphincter. So as the detrusor contracts and the internal urethral sphincter and external urethral sphincter relax, urine is able to flow out through the urethra in the process of micturition. The micturition reflex enables an involuntary mechanism to stimulate micturition, the emptying of the urinary bladder and the micturition reflexes the primary mechanism driving micturition in infants. However, as we mature, we develop voluntary control of micturition. The brain provides this voluntary control by sending commands to stimulate the external urethral sphincter. The descending commands from the brain will stimulate the lower motor neurons in the spinal cord that will stimulate somatic efferent fibers of the pudendal nerve. The somatic efferent fibers of the pudendal nerve stimulate contraction of the external urethral sphincter preventing micturition. Or we can activate this pathway voluntarily in order to hold our bladder and prevent micturition until we get an opportunity to go to the bathroom.