Connexions between hair-plate afferents and motoneurones in the cockroach leg

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Abstract
The trochanteral hair-plate afferents in the metathoracic leg of the cockroach, Periplaneta americana, were stimulated electrically and at the same time intracellular recordings were made from either motoneurones, interneurones or afferent terminals within the metathoracic ganglion. Activity in the hair-plate afferents evoked short latency excitatory postsynaptic potentials (EPSPs) in femur extensor motoneurones and inhibitory postsynaptic potentials (IPSPs) in femur flexor motoneurones. The latency of the IPSPs was on average 1·8 ms longer than the latency of the EPSPs. Intracellular recordings from terminal branches of the hair-plate afferents showed that the delay between the peak of the afferent terminal spike and the beginning of the EPSPs is about 0·4 ms. This finding, together with the observations that the amplitude of the EPSPs is increased by the passage of hyperpolarizing current and decreased following high-frequency stimulation, indicates that the EPSPs are evoked via monosynaptic chemical synaptic junctions. The observations of the long latency of the IPSPs, the need for a number of afferents to be simultaneously active for them to be evoked and the occasional variability in latency, all indicate that the IPSPs are evoked via a disynaptic pathway. These findings provide information on the synaptic mechanisms underlying the reflexes from the trochanteral hair plate to femur flexor and extensor motoneurones identified in a previous investigation (Wong & Pearson, 1976).