Micturition

Micturition 

Voiding or expulsion of urine stored in the urinary bladder is called micturition.

Urinary bladder gets gradually filled up with the urine.

Though the capacity of urinary bladder is above 800 ml, but as the volume is around 500 ml, its wall gets stretched.

Urine formed by nephrons is ultimately carried to the urinary bladder where it is stored till a voluntary signal which is initiated by the stretching of urinary bladder as it gets fill with urine.

Stretch receptors generate nerve impulse that is carried by sensory neuron to brain producing the sensation of fullness.

This initiates the autonomic reflex (parasympathetic involving sacral spinal nerves) resulting in contraction of detrusor muscles of urinary bladder and inhibition of motor impulse of voluntary, striated external sphincter, making it relaxed; and urine comes out.

Micturition can be initiated voluntarily also by contracting the abdominal muscle which applies pressure over urinary bladder, activating the stretch receptors.