Also known as active neuropathic incontinence. The detrusor muscle is the muscle of urinary bladder wall. It is primarily controlled by local reflex arcs, though there is cortical inhibitory control as well. The reflex arcs include visceral sensory (afferent) neurons that detect bladder wall stretch and visceral motor neurons that stimulate detrusor contraction and expulsion of urine. At rest, this reflex is suppressed by upper motor neurons projecting from supraspinal centers in the pons and cortex, allowing voluntary control of micturition. Spinal cord injury above the conus medullaris can damage these cortical regulatory, causing hyperreflexia in response to stretch (following a brief period of bladder areflesxia).
Parasympathetic neurons from the pelvic plexus (S2-S4) provide excitatory motor innervation to the detrusor muscle and inhibitory innervation to the internal urethral sphincter. Sympathetic neurons from spinal levels T11-L2 inhibit the detrusor muscle.
Damage to upper motor neurons that control the urinary bladder stretch-contraction reflex arc causes neurogenic detrusor instability. Damage at any level above the conus medullaris can cause this problem.
Source: USMLE World 2009