Non-reciprocal facilitation of trigeminal motoneurons innervating jaw-closing and jaw-opening muscles induced by iontophoretic application of serotonin in the guinea pig.

Effects of iontophoretically applied serotonin (5-HT) and its antagonist, methysergide (MS), on masseter (a jaw-closer, MA.MNs) and anterior digastric motoneurons (a jaw-opener, AD.MNs) were studied in paralyzed guinea pigs, chloralose-anesthetized or decerebrate. Unitary activity was recorded with multibarrel capillary electrodes from MA.MNs and AD.MNs identified by antidromic spikes evoked by stimulation of the masseter and anterior digastric nerves, respectively. Under chloralose anesthesia, both MA.MNs and AD.MNs were almost quiescent, and application of 5-HT alone induced no changes in discharge of either of them. However, iontophoretically applied 5-HT increased the frequency of discharge induced by iontophoretic application of glutamate in 26 of 34 MA.MNs (76%) and 17 of 30 AD.MNs (59%) tested. MS depressed the glutamate-induced activity in 17 MA.MNs and 3 AD.MNs, respectively, in which 5-HT exerted a facilitatory effect on the glutamate-induced activity. In decerebrate preparations, the firing index of spikes of MA.MNs monosynaptically evoked by stimulation of the trigeminal mesencephalic nucleus was increased by 5-HT and decreased by MS. 5-HT also enhanced the discharge of MA.MNs induced by a ramp-and-hold stretch of the masseter muscle. We conclude that 5-HT alone does not excite either MA.MNs or AD.MNs, but potentiates the effect of excitatory inputs to them: 5-HT exerts a modulatory facilitatory action on trigeminal motoneurons.

Intracellular response properties of neurons in the spinal trigeminal nucleus to peripheral and cortical stimulation in the cat.

The responses of the secondary neurons in the spinal trigeminal nucleus oralis (STNo) were recorded intracellularly to peripheral and cortical stimulation in chloralose-anesthetized cats. Electrical stimulation of the trigeminal sensory nerves (the frontal, infraorbital and inferior alveolar nerves) evoked an EPSP superimposed by one or a few spikes followed by a biphasic IPSP in one group of STNo neurons (Type I), and a prolonged EPSP superimposed by a burst of spikes in the other group of STNo neurons (Type II). Nearly half of Type I neurons were trigeminothalamic neurons projecting to the contralateral ventral posteromedial nucleus, while the remaining Type I and all the Type II neurons were non-projection neurons. A majority of Type I neurons responded with spike potentials to stimulation of only one sensory nerve, while most Type II neurons responded to stimulation of more than one nerve. Stimulation of the contralateral primary somatosensory cortex evoked IPSPs in most Type I projection neurons, and EPSPs in all Type II as well as most Type I non-projection neurons. In Type I neurons touch or pressure applied to a circumscribed area in the facial skin evoked an EPSP superimposed by one or a few spikes followed by a biphasic IPSP, and IPSPs were evoked from a wide surrounding area in the face by the same mechanical stimulation. In Type II neurons innocuous mechanical stimulation within a wide area evoked an EPSP, while IPSPs could not be induced from anywhere. The results indicate that postsynaptic inhibition is involved in the surround inhibition as well as corticofugal descending inhibition of sensory transmission in the trigeminal sensory nucleus.

[Adaptation of acrylic resin dentures polymerized using various activation modes].

The purpose of this in vitro study was to compare the dimensional accuracy of maxillary dentures made using a conventional heat-activated PMMA resin, a pour resin, a visible light-activated resin, and a microwave-activated acrylic resin. Two simple methods for measuring dimensional accuracy were used: (1) weight of impression material entrapped between the base and master die and (2) measurement of the posterior border gap at five locations. The volume of space between the denture base and the master die was determined by (1) computation and (2) estimation. Statistical analysis (Bartlett, ANOVA and Tukey's Tests) supported the following conclusions: (1) all groups showed a processing contraction, most apparent from buccal flange to buccal flange, (2) the poorest fitting group was processed in a brass flask and a water bath at a temperature which rose from 70 to 100 degrees C, using a heat activated resin (Acron), (3) the visible light activated resin (Triad) produced dentures of intermediate accuracy, as did Acupac 20 when either heat or microwave activated, (4) the two best fitting groups were prepared from a chemically activated resin system using pressure at low heat (PER form), and the resin developed for microwave activation (Acron MC).

Phase-linked modulation of excitability of presynaptic terminals of low-threshold afferent fibers in the inferior alveolar nerve during cortically induced fictive mastication in the guinea pig.

Excitability of presynaptic terminals of low-threshold primary afferent fibers in the inferior alveolar nerve was tested in the trigeminal spinal nucleus of the ketamine-anesthetized, paralyzed guinea pig, by Wall's method. Fictive mastication was induced by repetitive stimulation of the cortical masticatory area, and was monitored by rhythmical burst activity in the jaw-opening anterior digastric motoneuron pool. The excitability was rhythmically modulated in a phase-linked manner during the masticatory cycle: it was decreased coincidentally with the digastric burst activity (jaw-opening phase) and increased during the middle and late periods of the interburst phase (jaw-closing phase) of the masticatory cycle. The results imply that presynaptic modulation of synaptic transmission of peripheral inputs from primary afferents to interneurons in the jaw-opening reflex pathway may contribute to the rhythmical modulation of the jaw-opening reflex evoked by innocuous stimulation of the intraoral structures during mastication; presynaptic inhibition contributing to the depression of the jaw-opening reflex during the jaw-closing phase and presynaptic facilitation to its enhancement during the jaw-opening phase.