INTEGRATION IN TRIGEMINAL PREMOTOR INTERNEURONS IN THE CAT .3. INPUT CHARACTERISTICS AND SYNAPTIC ACTIONS OF NEURONS IN SUBNUCLEUS-GAMMA OFTHE ORAL NUCLEUS OF THE SPINAL TRIGEMINAL TRACT WITH A PROJECTION TO THE MASSETERIC MOTONEURON SUBNUCLEUS

A population of last-order interneurones within the rostrodorsal part of the oral nucleus of the spinal trigeminal tract (NVspo-gamma) has b een investigated in 21 chloralose anaesthetised cats. The neurones wer e identified by their antidromic (AD) response to microstimulation (me dian current 9 mu A, range 3-39 mu A) of the ipsior the contralateral masseteric subnucleus of the trigeminal motor nucleus. Fifty-one of 11 3 interneurones tested were discharged from the ipsilateral and eight from the contralateral motor nucleus. The average conduction time was 0.50 ms from the ipsilateral and 0.74 ms from the contralateral motone urone pool. Conduction velocities of the axons ranged from 2.0 to 14.0 ms. The pattern of primary afferent input onto the selected neurones was analysed by graded electrical stimulation of dissected trigeminal nerves. Low-threshold afferents innervating the intraoral mucosa inclu ding the tongue and the perioral skin of the lower lip were the most e ffective inputs, as judged from both the frequency of occurrence and f rom the latencies of the evoked spike discharges. Ninety-six percent o f the neurones responded to stimulation of the inferior alveolar nerve (Alv inf) and 83% responded to stimulation of the lingual nerve (Ling ). The median threshold strength required to evoke the Alv inf and the Ling responses was 1.7 T (range 1.0-3.6 T) and 1.3 T (range 1.0-5.0 T ), respectively. The median latency to spike discharges evoked by the Alv inf was 2.0 ms (range 1.3-4.8 ms) and to the Ling it was 2.5 ms (r ange 1.4-7.0 ms). Action potentials elicited by stimulation of the mas seteric and digastric nerves were observed in 40% and 10% of the neuro nes, respectively. These responses, which had median latencies of more than 8 ms (range 4.7-16.0 ms), were only seen at stimulation intensit ies above 2 T (range 2.5-25 T). An input from the maxillary whisker ne rve was seen in only one case. Post-spike averages of the extracellula r field potentials within the trigeminal motoneurone subnuclei evoked by interneuronal spikes were made in a subsample of 51 NVspo-gamma neu rones activated by iontophoresis of L-glutamic acid. Excitatory synapt ic effects within the masseteric subnucleus were observed in eight cas es. An inhibitory effect was seen in one case. One specific neurone ga ve an excitatory extracellular field potential within the digastric mo toneurone subnucleus. This interneurone was AD activated from the diga stric, but not from the masseteric subnucleus. The physiological prope rties of the NVspo-gamma-mass interneurones are discussed in relation to their suggested roles in the phase-dependent control of the trigemi nal motoneurones during ore-facial masticatory behaviours.