[EPSP's of the motor neurons of cat masticatory muscles induced by stimulation of low-threshold infraorbital nerve fibers].

The effects of the infraorbital Aalpha afferents stimulation on the masseter motoneurons were investigated in cats under chloralose-nembutal anesthesia. It is found that stimuli 1.4-2.5 times threshold (T) evoke complex EPSPs in 69% of the investigated motoneurons, their latency being 2.1 +/- 0.2 ms (1.5-3.0 ms, n = 36), amplitude up to 30 mV and duration 9-15 ms. These EPSPs consist of two simpler responses produced by activation of two separate groups of afferent fibres. The short-latent EPSPs arise after activation of the infraorbital nerve by 1.1-1.5 T stimuli and have the same latency as the complex EPSP (1.5-3.0 ms) but a smaller amplitude (up to 2.0 mV) and shorter duration (up to 6 ms). The stability of these EPSPs during high frequency stimulation (120/c) and the development of facilitation and inhibition similar to those which appear in monosynaptic EPSPs in masseter motoneurons during stimulation of the 1a muscle afferents give reason to suggest that these EPSPs are monosynaptic. The slow rising rate indicates that they appear on distal dendrites of the motoneurons. The long-latent EPSPs arise with latency of 7-9 ms after activation of the infraorbital nerve by 1.1-1.5 T stimuli. Their amplitude reaches 1.5-2.0 mV and duration 7-9 ms. The long latency of these EPSPs in combination with low ability to repeat high frequency stimulation are consistent with their polysynaptic origin. It is suggested that the excitatory input from the lowest threshold Aalpha afferents of the infraorbital nerve to masseter motoneurons creates conditions for the development of a transient jaw closing reflex in response to light tactile stimulation of the cat's perioral region.