Monosynaptic reflex depression in cats with organophosphorus neuropathy: effects of tri-o-cresyl phosphate.

The electrophysiology of organophosphorus induced delayed neurotoxicity was studied to determine the extent peripheral nerve changes affect monosynaptic reflex responses of the spinal cord. Cats were given a single dermal dose of 1500 mg/kg or two dermal doses of 500 mg/kg of tri-o-cresyl phosphate. Animals were observed for 60 days after which monosynaptic reflex (MSR) responses were recorded from L7 and S1 ventral roots after stimulation of the tibial or common peroneal nerves. Post-tetanic potentiated responses as well as ventral and dorsal root compound action potentials were also recorded. There was a significant decrease from control in the unconditioned MSR. The post-tetanic potentiated response was significantly decreased in the S1 ventral root, however, when expressed as a factor of potentiation of the unconditioned MSR, it was found to be increased, although not significantly. The dorsal and ventral root compound action potentials were also significantly decreased from control with their conduction velocities being no different from control The decrease in the unconditioned MSR as well as the dorsal and ventral root compound action potentials were attributed to peripheral nerve damage. The absence of any significant change in the post-tetanic potentiated response expressed as a factor of potentiation of the unconditioned MSR was attributed to a decrease in both the discharge zone as well as the subliminal fringe. The absence of any change in the conduction velocity indicates at least some of the large myelinated peripheral fibers were spared from significant damage.