Further Characterization of the External Urethral Sphincter in Spinal Cord Injury: Study during Spinal Shock and Evolution of Responsiveness to Alpha-Adrenergic Stimulation

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Abstract
To shed further insight into its ever evolving concepts, the activity of the external urethral sphincter was studied in patients with spinal cord injury. Study during the phase of acute spinal shock revealed persistent electromyographic activity in the external urethral sphincter and no activity in the external anal sphincter, suggesting the presence of functional dissociation between the 2 sphincters from the onset of acute spinal injury. The genesis of dissociated activity was discussed by reference to recent experiments on the individual differences in motor neuron and muscle subtypes. Only the external urethral sphincter of chronic paraplegics demonstrated a uniformly increased electromyographic activity after .alpha.-adrenergic stimulation, while the response in the external anal sphincter was variable. The mechanism of increased response to .alpha.-adrenergic stimulation in the denervated external urethral sphincter appears to reside either in direct postsynaptic stimulation or recruitment of motor neurons other than the somatic pudendal system, since pretreatment with competitive muscle relaxant failed to block this increased response in the external urethral sphincter.