"Skin-CNS-bladder" reflex pathway for micturition after spinal cord injuryand its underlying mechanisms

Purpose: A "skin-CNS-bladder" reflex pathway for inducing micturition after spinal cord injury has been established in cat. This reflex pathway which is basically a somatic reflex are with a modified efferent limb that passes somatic motor impulses to the bladder, has been designed to allow spinal c ord injured patients to initiate voiding by scratching the skin. Materials and Methods: The skin-CNS-bladder reflex was established in the c at by intradural microanastomosis of the left L7 ventral root (VR) to the S 1 VR while leaving the L7 dorsal root (DR) intact to conduct cutaneous affe rent signals that can trigger the new micturition reflex are. After allowin g 11 weeks for axonal regeneration, urodynamic, pharmacological and electro physiological studies were conducted in pentobarbital or chloralose anesthe tized animals. Results: A detrusor contraction was initiated at short latency by scratchin g the skin or by percutaneous electrical stimulation in the L7 dermatome. M aximal bladder pressures during this stimulation were similar to those acti vated by bladder distension in control animals. Electrophysiological record ing revealed that single stimuli (0.3 to 3 mA, 0.02 to 0.2 msec duration) t o the left L7 spinal nerve in which the efferent axons had degenerated evok ed action potentials (0.5 to 1 mV) in the left S1 spinal nerve distal to th e anastomosis. In addition, increases in bladder pressure were elicited by trains of the stimuli (5 to 20 Hz, 5 seconds) applied to the L7 spinal nerv e. Urodynamic studies including external sphincter EMG recording demonstrat ed that the new reflex pathway could initiate voiding without detrusor-exte rnal urethral sphincter dyssynergia. Atropine (0.05 mg./kg., i.v.) or trime thaphan (5 mg./kg., i.v.), a ganglionic blocking agent, depressed the bladd er contractions elicited by skin stimulation. The skin-CNS-bladder reflex c ould also be elicited after transecting the spinal cord at the L2-L3 or L7- S1 levels. Conclusion: The cross-wired somato-autonomic bladder reflex is effective in initiating bladder contractions and coordinated voiding in cats with an in tact neuraxis and can also induce bladder contractions after acute transect ion of the lumbar spinal cord. The new pathway is mediated by cholinergic t ransmission involving both nicotinic and muscarinic receptors. It is conclu ded that somatic motor axons can innervate bladder parasympathetic ganglion cells and thereby transfer somatic reflex activity to the bladder smooth m uscle.