The sphincter urethrae in the perineum is a striated muscle under somatic control. It can impede the flow of urine independently of the visceral sphincter at the base of the bladder. Furthermore, descending signals within the spinal cord can override the autonomic reflex. The signals from the stretch reflex are experienced as an urge to urinate, but the urge frequently is lost if the reflex is inhibited. Thus micturition is a voluntary act in which the cerebrum permits the operation of the reflex and voluntarily relaxes the sphincter urethrae.
Micturition is a learned behavior.
Toilet training of human infants represents the assertion of control by somatic commands over the autonomic pathways. Initially, control is not possible because the descending spinal pathways have not matured. Later, children learn when and where micturition is appropriate, using culturally specific cues. Similar controls are present in other species. Pets can be trained to recognize appropriate places for urination.