Reduction of perineal evoked excitatory postsynaptic potentials in cat lumbar and sacral motoneurons during micturition.

These experiments were undertaken to examine whether both premotoneuronal mechanisms and direct actions on motoneurons could contribute to suppression of excitatory perineal reflex pathways during micturition. Intracellular recordings were obtained from motoneurons innervating the external urethral sphincter (EUS), external anal sphincter (EAS), and selected hindlimb muscles in decerebrate male cats. The peak amplitudes of EPSPs evoked by electrical stimulation of peripheral cutaneous afferents were measured during micturition. In the EUS, EAS, and hindlimb motoneurons examined, EPSPs produced by stimulation of perineal afferents (superficial perineal or sensory pudendal nerves) were reduced in amplitude during micturition. The sample of PSPs evoked by stimulation of hindlimb cutaneous nerves recorded in hindlimb motoneurons revealed that these PSPs could also be reduced. In contrast, no changes were seen in monosynaptic EPSPs evoked by muscle afferent stimulation. The present study demonstrates that during micturition there is a strong suppression of perineal reflexes to both sphincter and hindlimb motoneurons. Since reduced EUS activity is required for efficient micturition, suppression of the strong excitatory perineal input to EUS motoneurons likely contributes to decreased EUS activity during the bladder contraction. It appears that the micturition circuitry utilizes both premotoneuronal mechanisms and direct motoneuronal inhibition to achieve this reflex suppression. The function of the micturition-related reduction of perineal reflexes to hindlimb or EAS motoneurons is not known at this time and further investigations are required to elucidate the interaction between micturition circuitry and hindlimb cutaneous pathways.