Bladder neuroanatomy

Neuroanatomy of the bladder is complex, described here is a summary of the co-ordination of micturition.

The bladder acts as a reservoir normally storing 400-500 mL of urine under low pressure (<15 cmH₂O) before voluntary voiding can occur at a socially-convenient time. Bladder filling and emptying involves the detrusor and the bladder outlet (bladder neck, proximal urethra and striated muscles of the pelvic floor) acting reciprocally ¹.

During storage of urine, the detrusor is relaxed, and the bladder neck and proximal urethra are closed to maintain continence. During voiding initial relaxation of the pelvic floor with the opening of the bladder neck is followed by detrusor contraction until the bladder is emptied.

Co-ordination of micturition involves control by six centers in the CNS:

1. sacral micturition center (located in the intermediolateral grey of the sacral cord from S2-S4) provides reflex control of micturition. Afferent impulses transmitted via the pelvic splanchnic nerves provide the sensation of bladder fullness and efferent parasympathetic impulses via the same nerves cause reflex detrusor contraction ¹
2. pudendal nucleus (located in the anterior horn of S2-4), innervated by the detrusor motor center which in turn, controls the skeletal muscle of the external sphincter via the perineal branch of the pudendal nerve ²
3. sympathetic efferent nerve supply to the bladder (located in the intermediolateral grey column from T11-L2) facilitates bladder storage by relaxing the detrusor and increasing the resistance of the bladder outlet and prostatic urethra ²
4. detrusor motor center (located in the medial part of the pontine reticular formation) of the brainstem is responsible for co-ordinating relaxation of the external sphincter with contraction of the detrusor via reticulospinal fibers which innervate the sacral segments ²
5. anterior vermis of the cerebellum is thought to maintain tone in the pelvic floor musculature and influence co-ordination between periurethral striated muscle relaxation and bladder emptying ^1,3
6. cortical inhibitory center (located in the middle frontal gyrus, on the medial surface of the cerebral hemisphere) inhibits the detrusor motor center, hence inhibiting voiding until appropriate ²

Spinal cord injury disrupts the above ascending and descending pathways, preventing normal control of micturition.