The bladder is surrounded by smooth muscle called the detrusor muscle . The muscle layer at the bladder neck forms the internal sphincter, which is not under voluntary control . A layer of skeletal muscle that forms the external sphincter and is under voluntary control surrounds the urethra. The bladder receives both ANS innervation. Sympathetic system innervate the bladder neck and urethra to cause contraction and facilitate storage .
Parasympathetic system innervates the fundus and stimulation causes sustained contraction .
Sensory fibers innervate the bladder to detect fullness, which are part of the micturition reflex . When stretch receptors of the wall of the bladder trigger the reflex, parasympathetic fibers stimulate the detrusor muscle that contracts the bladder and opens the internal sphincter. A voluntary relaxation of the skeletal muscles of the external sphincter is then required. Thus the net filtration pressure is only about 10 mmHg, but this small pressure difference on the high glomerulus permeability produces nearly 125 ml per min of ultrafiltrate . The afferent arteriole compensates for any increase in blood pressure not by expansion, which would increase capillary flow, but by contraction that increases resistance to flow .