Postsynaptic potentials in the hypoglossal motoneurons set up by hypoglossal nerve stimulation

In nembutalized cats intracellular potentials were recorded from hypoglossal motoneurons innervating either protruder or retractor muscles of the tonge (protruder and retractor motoneurons: P-Mns and R-Mns). Responses to stimulation of the hypoglossal nerve were explored and found to consist of an antidromic spike followed by an afterhyperpolarization (AHP) and a postsynaptic potential (PSP). When hypoglossal nerve stimulation was made with an intensity three times as large as the threshold for the hypoglossal motor fibers, the PSPs became evident under blockage of soma-dendritic invasion of the antidromic spike. In most of P-Mns or R-Mns, the PSPs were IPSPs, independent of the side of peripheral stimulation. The latencies were about 12 msec. Even when the cell membrane was hyperpolarized by injecting a hyperpolarizing current of up to 16 nA, the reversal point of the IPSP was difficult to find. In a small fraction of hypoglossal motoneurons the PSPs to hypoglossal nerve stimulation were EPSPs with latencies of 10 to 12 msec.