Integration in trigeminal premotor interneurones in the cat. 3. Input characteristics and synaptic actions of neurones in subnucleus-gamma of the oral nucleus of the spinal trigeminal tract with a projection to the masseteric motoneurone subnucleus.

A population of last-order interneurones within the rostrodorsal part of the oral nucleus of the spinal trigeminal tract (NVspo-gamma) has been investigated in 21 chloralose anaesthetised cats. The neurones were identified by their antidromic (AD) response to microstimulation (median current 9 microA, range 3-39 microA) of the ipsior the contralateral masseteric subnucleus of the trigeminal motor nucleus. Fifty-one of 113 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 motoneurone 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 including the tongue and the perioral skin of the lower lip were the most effective inputs, as judged from both the frequency of occurrence and from the latencies of the evoked spike discharges. Ninety-six percent of 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 (range 1.4-7.0 ms). Action potentials elicited by stimulation of the masseteric and digastric nerves were observed in 40% and 10% of the neurones, respectively. These responses, which had median latencies of more than 8 ms (range 4.7-16.0 ms), were only seen at stimulation intensities above 2 T (range 2.5-25 T). An input from the maxillary whisker nerve was seen in only one case. Postspike averages of the extracellular field potentials within the trigeminal motoneurone subnuclei evoked by interneuronal spikes were made in a subsample of 51 NVspo-gamma neurones activated by iontophoresis of L-glutamic acid. Excitatory synaptic effects within the masseteric subnucleus were observed in eight cases. An inhibitory effect was seen in one case. One specific neurone gave an excitatory extracellular field potential within the digastric motoneurone subnucleus. This interneurone was AD activated from the digastric, but not from the masseteric subnucleus.(ABSTRACT TRUNCATED AT 400 WORDS)