lost The internal sphincter. therefore, s purely involuntary, as it Is under complete control of the automatic But in addition,

, there is an external sphincter of the urethra, which surrounds Ihe membranous urethra. It is, his to logically made up of striated muscles and supplied by somatic nerves (via pudenda! nerve), that is, II should be purely voluntary One can voluntarily cause contraction of this (extemaF) sphincter, but voluntary relaxation of it cannot be made Thus,in the middle of an act of micturition one, if he desires, can stop the micturition temporarily by causing the external sphincter to contract voluntarily. But- unless there is some accumulation of urine in the bladder with a consequent rise of hydrostatic pressure within the bladder, relaxation of external sphincter tiy pure voluntary effort is not possible Mechanism of micturition The micturition reflexes Although, to some extent, this rs still a bil speculative, the essentials are as follows 1 When the bladder is sufficiently tilled, the hydrostatic pressure wtthin the bladder nses the rise Of pressure causes simulation of afferent nerves (parasympatheHc) spinal center is stmulated -* efferent parasympathetic fibers (from the spinal center) are now simulated contraction of the detrusor and relaxation of the menial sphincter evacuation This is the fundamental micturition reflex. 2 Durring the abovementioned reflex action, sympathetic efferent are inhibited the contraction of the detrusor and relaxation of tie internal sphincter are facili­tated 3 However, filling of the bladder to a certan degree by itself is not enough From the spinal tenter,