Coordination of the bladder detrusor and the external urethral sphincter in a rat model of spinal cord injury: Effect of injury severity

Recovery of urinary tract function after spinal cord injury (SCI) is import ant in its own right and may also serve as a model for studying mechanisms of functional recovery after injury in the CNS. Normal micturition requires coordinated activation of smooth muscle of the bladder (detrusor) and stri ated muscle of the external urethral sphincter (EUS) that is controlled by spinal and supraspinal circuitry. We used a clinically relevant rat model o f thoracic spinal cord contusion injury to examine the effect of varying th e degree of residual supraspinal connections on chronic detrusor-EUS coordi nation. Urodynamic evaluation at 8 weeks after SCI showed that detrusor con tractions of the bladder recovered similarly in groups of rats injured with a 10 gm weight dropped 12.5, 25, or 50 mm onto the spinal cord. In contras t, the degree of coordinated activation of the EUS varied with the severity of initial injury and the degree of preservation of white matter at the in jury site. The 12.5 mm SCI resulted in the sparing of 20% of the white matt er at the injury site and complete recovery of detrusor-EUS coordination. I n more severely injured rats, the chronic recovery of detrusor-EUS coordina tion was very incomplete and correlated to decreased innervation of lower m otoneurons by descending control pathways and their increased levels of mRN A for glutamate receptor subunits NR2A and GluR2. These results show that t he extent of recovery of detrusor-EUS coordination depends on injury severi ty and the degree of residual connections with brainstem control centers.