PHSI 208 Lecture Notes - Lecture 45: Internal Sphincter Muscle Of Urethra, Detrusor Urinae Muscle, Spinal Cord
Document Summary
Filling of the bladder activates stretch receptors initiating the micturition reflex. The internal sphincter is also made of smooth muscle that is under sympathetic control. External sphincter is made of skeletal muscle; under resting conditions there are tonic active motor neurons that keeps it contracted at all times. As the bladder fills up with urine, the bladder wall stretches and activates the mechanically activated sensory neurons. This reflex only proceeds when the person wants it to proceed (descending inputs from the cerebral cortex can either allow or inhibit this reflex) Once exceeding 500ml, the internal sphincter is forced open and leads to the reflexive opening of external sphincter and loss of voluntary opposition. Incontinence = the ability to control urination voluntarily. Infants: corticospinal connections necessary for voluntary control have yet to be established. Aging: loss of muscle tone, stroke, alzheimer"s disease, or other cns problems affecting the cerebral cortex or hypothalamus, prostate growth in males.
