Micturition PDF

Micturition  Micturition Micturition is the process by which the urinary bladder empties when it becomes filled. This process involves two main steps: a) The bladder fills progressively until the tension in its walls rises above a threshold level. b) This tension elicits a nervous...

Micturition  Micturition Micturition is the process by which the urinary bladder empties when it becomes filled. This process involves two main steps: a) The bladder fills progressively until the tension in its walls rises above a threshold level. b) This tension elicits a nervous reflex called the micturition reflex that empties the bladder or, if this fails, at least causes a conscious desire to urinate. Although the micturition reflex is an autonomic spinal cord reflex, it can also be inhibited or facilitated by centers in the cerebral cortex or brain stem. Physiological Anatomy of the Bladder A smooth muscle chamber composed of two main parts: (1) The body, the major part in which urine collects. (2) The neck, a funnel-shaped extension connecting with the urethra. The smooth muscle is called the detrusor muscle. Its cells fuse with one another and therefore, an action potential can spread throughout the whole muscle cells, to cause contraction of the entire bladder at once. The ureters enter obliquely through the detrusor muscle The internal urethral sphincter, formed by smooth muscle bundles; under involuntary control. The external urethral sphincter; of skeletal muscle and it is under voluntary control  Innervation of the Bladder Principal nerve supply of the bladder: 1. Parasympathetic innervation: Both sensory and motor nerve fibers transmitted through the pelvic nerve from the sacral plexus, (mainly S2 and S3). The sensory fibers detect the degree of stretch in the bladder wall. The motor nerves fibers innervate the detrusor muscle → contraction. 2. Somatic innervation: Pudendal nerve transmits skeletal motor fibers to the external bladder sphincter. 3. Sympathetic innervation Originate in the upper lumbar segment of spinal cord L1 – L3 (mainly L2), transmitted through the hypogastric nerves. They mainly a) stimulate the blood vessels but also b) inhibits micturition through: Inhibition of the bladder smooth muscle (detrusor muscle). Excite the smooth muscle of the internal urethral sphincter. Innervation of the Bladder Guyton The Micturition Reflex  Components of the micturition reflex ❑Micturition is fundamentally a spinal reflex facilitated and inhibited by higher brain centers and subject to voluntary facilitation and inhibition. ❑Components of micturition reflex 1. Receptors: stretch receptors bladders smooth muscle. Stimulated by stretch when urine volume reaches 400 ml. NB: the 1st urge to void is felt at a volume of 150 ml. 2. Afferent: Sensory neurons in the parasympathetic nerves travel in the pelvic nerves. 3. The center: the reflex is integrated in the sacral segments (S2, S3 & S4). 4. Efferent: motor neurons in the parasympathetic nerves (pelvic nerves) 5. Effectors: Smooth muscle in the wall of the bladder “detrusor muscle” (which contracts). Smooth muscle of the internal sphincter (which relaxes).  Control of Urinary Reflex The reflex is adjusted by the activity of facilitatory and inhibitory centers in the brainstem (mainly pons). There is another facilitatory area in the posterior hypothalamus. The higher centers normally exert final control of micturition as follows: (1) Can prevent micturition until a convenient time presents itself. (2) Help initiate micturition and inhibit the external urinary sphincter when it is time to urinate. The Micturition Reflex  Cystometrogram A curve represent the relation between intravesical pressure and volume. When there is no urine in the bladder, the intravesicular pressure is about 0. By the time the urine have collected, the pressure rises slightly. This constant level of pressure is caused by intrinsic tone of the bladder wall. Beyond 300 to 400 milliliters, collection of more urine in the bladder causes the pressure to rise rapidly and the micturition reflex is elicited.  Micturition abnormalities A. Autonomic Bladder: Occurs due loss of the voluntary control (inhibition or facilitation) from higher centers as a result of spinal cord damage above S2. The voiding reflex is intact; bladder filling is followed (automatically) by reflex emptying B. Atonic Baldder: Caused by destruction of the parasympathetic nerves or the sacral segments. The micturition reflex is lost. Therefore filling is not followed by emptying The full bladder does not contract (atonic), becomes distended, thin-walled, and hypotonic. The high pressure within the bladder may force some droplets of urine to pass out. This is called overflow incontinence. References Ganong’s Review of Medical Physiology - 25th (2016) Hall Guyton and Hall Textbook of Medical Physiology (2016) Berne-Levy-Physiology 7th edition. Ameman Human Anatomy and Physiology. The core of medical physiology, volume 2.