Effects of hypoglossal afferent stimulation on masseteric motoneurons in cats

EXPERIhfEP;TAL

61, 1-14

NETROLOGY

Effects

of Hypoglossal

on Masseteric Y.

L. J.

GOLDBERG,

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NARAMURA,

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(1978)

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Afferent

Stimulation

Motoneurons N.

in Cats

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C. D.

CLEMENTE

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30, 1977;

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Hypoglossal nerve stimulation in cats induced a prolonged hyperpolarizing potential in both ipsilateral and contralateral masseteric motoneurons, with a depolarizing phase following the peak of hyperpolarization. The time course of the potential corresponded to the previously reported hypoglossally induced suppression and facilitation of the masseteric reflex. The early phase of the hyperpolarizing potential changed into a depolarizing potential on intracellular Cl- injection, lvhereas the late hyporpolarizing phase, following the depolarizing potential, did not. Transverse section of the reticular formation between the trigeminal motor and hypoglossal nuclei selectively blocked the late phase of the hypoglossally induced inhibition of the masseteric reflex, whereas section of the trigeminal spinal nucleus selectively abolished the early phase of the inhibition evoked by hypoglossal stimulation on the side of section. Stimulation of the hypoglossal nerve produced a spike train in supratrigeminal inhibitory neurons for masseteric motoneurons ; the latency of initial spikes and the number of spikes in the spike train corresponded, respectively, to the onset and the amplitude of the early phase of hyperpolarization of masseteric motoneurons. We concluded that the afferent impulses in the hypoglossal nerve, which induced the sequence of early inhibition-facilitation-late inhibition of the masseteric reflex, do so primarily postsynaptically on masseteric motoneurons, the early and late inhibition being mediated, respectively, via the intranuclear ascending pathway in the spinal nucleus and supratrigeminal inhibitory neurons on the stimulated side and via the reticular formation. Abbreviation

: IPSP-inhibitory

postsynaptic

potential.

1 This research was supported by a grant from the United States Public Health Service (MH 10083). The present address of Dr. Kakamura is Section of Physiology, Institute of Stomatognathic Science, Tokyo Medical and Dental University, Tokyo, Japan, and that of Dr. Mizuno is Department of Anatomy, Faculty of Medicine. Kyoto University, Kyoto, Japan.

0014~4886/78/0611-0001$02.00j0 All

Copyright 0 1978 rights of reproduction

by Academic Press, Inc. in any form reserved.

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ET

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INTRODUCTION Recent neurophysiological (5, 6, 10, 13, 14, 17) and anatomical (2) experiments clearly demonstrated the presence of afferent fibers in the hypoglossal nerve of the cat. It was also shown that these afferents have reflex connections with hypoglossal (6), vagal (10, 14), facial (6, lo), and trigeminal (13) motor nuclei. In a previous report, ipsilateral and contralateral hypoglossal afferents, which entered the brain stem via the hypoglossal roots, were shown to have both inhibitory and facilitatory influences on the masseteric monosynaptic reflex (13). In the present study the nature of these influences, as elucidated by intracellular recording from masseteric motoneurons, is described together with the central routes involved in the hypoglossotrigeminal influences. METHODS Thirty-one cats with the cerebellum removed were used. Basic surgic+rl and experimental procedures were described in a previous report (13), as were the techniques used for recording field potentials in the trigeminal motor nucleus and for recording intracellular potentials in masseteric motoneurons (12). For stimulation of the inferior alveolar nerve, a pair of fine screws was implanted about 8 mm apart through holes made from the ventral surface of the mandible to the mandibular canal. The exposed portion of the screws was covered with dental resin for insulation from surrounding tissues. Partial transection of the brain stem was carried out in three cats at levels between the trigeminal motor and the hypoglossal nuclei, and the effects of hypoglossal nerve stimulation were checked on the masseteric reflex before and after the partial transections. This operation was done with a thin knife made from a razor blade. The brain stems of these animals were embedded in celloidin to check the extent of each lesion on serial Nissl- or Weil-stained sections. RESULTS Efect of Hypoglossal Nerve Stimulation on the Antidromic Field Potential in the Masseteric Motor Nucleus. The masseteric motor nucleus was

located by the negative field potential evoked antidromically by stimulation of the ipsilateral masseteric nerve (Fig. 1E). With the recording electrode in the sameposition within the nucleus, single shocks to the ipsilateral and contralateral hypoglossal nerve trunk induced a positive field potential (Fig. 1A). Upon low-frequency stimulation (0.5 to l/s), this positive potential

RYPOGLOSSAL

EFFECTS

ON

MASSETERIC

IiEURONS

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FIG. 1. Field potentials recorded in the masseteric motor nucleus and evoked by hypoglossal nerve stimulation. A-The field potential in the right masseteric motor nucleus evoked by shocks (0.03 ms in duration, 4 V) delivered to the right hypoglossal nerve at various frequencies (1 to IO/s). Each record displays about 10 sweeps. E-The antidromic field potential also recorded in the right masseteric motor nucleus evoked by single shocks to the right masseteric nerve (0.02 ms, 4 V) at I/s. B-The interactions between these two field potentials evoked at the rate of l/s while the interval between the two stimulations was changed. C-A record similar to B, except that the sweep speed was increased and the interactions at five different intervals between hypoglossal and masseteric nerve stimulations were superimposed. D-The interval between the two different stimulations was constant, but the frequencies of stimulation were changed. Negative deflection is downward ; AC recording (time constant, 0.002 s),

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appeared after a latency of 8 to 11 ms, reached its peak at 13 to 1.5 ms, and lasted 10 to 20 ms. This potential was easily depressed by increasing the frequency of stimulation of the hypoglossal nerve (Fig. IA). The antidromic field potential in the masseteric motor nucleus was depressed when the positive field potential was evoked by hypoglossal nerve stimulation (Figs. lB, C). High-frequency hypoglossal nerve stimulation led to a depression of the positive field potential and was no longer effective in suppressing the antidromic field potential evoked by stimulation of the masseteric nerve (Fig. 1D). These effects were observed in five different cats, and the results indicate that the depression in excitability of masseteric motoneurons induced by hypoglossal nerve stimulation corresponds in time to the hypoglossally induced positive potential within the masseteric motor nucleus. Hypoglossal Effects Recorded Intracellularly from Masseteric Motoneurons. Forty-nine masseteric motoneurons were identified by antidromic spike potentials evoked by stimulation of the ipsilateral masseteric nerve. The effects of hypoglossal stimulation were tested on 36 ipsilateral and 9 contralateral masseteric motoneurons. In the remaining four neurons, the effects of both ipsilateral and contralateral hypoglossal nerve stimulation were tested. Hypoglossal nerve stimulation parameters were usually set at an intensity capable of inducing complete suppression of the masseteric monosynaptic reflex. At this intensity, prolonged hyperpolarizing potentials were evoked in masseteric motoneurons (Figs. 2A-E). Some characteristics of these hyperpolarizing potentials are shown in Table 1. The hyperpolarizing potential usually started between 8 and 14 ms after hypoglossal nerve stimulation and reached a peak between 13 and 21 ms, usually with an amplitude of 3 to 6 mV (maximum, 11.7 mV). They ordinarily lasted 100 to 200 ms. No overall difference could be found with respect to the latency of onset and the peak of the hyperpolarizing potential between ipsilateral and contralateral nerve stimulation. In three of four neurons for which both right and’left stimulations were tested, ipsilateral stimulation showed an apparent shorter latency [ ipsilateral, 9.7 + 0.4 ms ; contralateral, 11.0 + 0.8 ms (mean f SD)], but statistically this difference was not nor was there any significant differhighly significant (0.05 > P > 0.02), ence in peak latencies. Hyperpolarizing potentials induced in 12 masseteric motoneurons (6 ipsilateral and 6 contralateral) were compared when one or three pulses (250 to 500/s) were delivered to the hypoglossal nerve. The overall mean difference in onset latency was a significant decrease (P < 0.01) of 0.8 ms (maximum, 2.5 ms). The latency to the peak of hyperpolarization did not show any definite tendency to shift in either direction, although a decrease of 0.7 111s was obtained as the mean value of the difference (P > 0.1). The

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FIG. 2. Intracellular potentials recorded in masseteric motoneurons induced by hypoglossal nerve stimulation. A, B, and C were obtained from the same neuron, stimulating the ipsilateral hypoglossal nerve with trains of three pulses delivered at 300/s (12.5 V ; 2 ms in duration in A and B, and 0.1 ms in C). These trains were applied at l/s in A and C, but in B the frequency was increased successively from 1 to 10/s. D was obtained from another neuron by stimulation of the ipsilateral hypoglossal nerve with single pulses (0.3 ms in duration, 2 V) at various frequencies from 0.5 to 15/s. E and F were obtained from another neuron with a KC1 electrode. The responses were obtained by single shocks delivered to the ipsilateral hypoglossal nerve (l/s, 0.2 ms in duration, 12.5 V). Note in F. after Cl- injection, the reversal of the early phase of the hyperpolarizing potential to a depolarizing one, followed by a more prolonged late phase of hyperpolarization. All records consist of about 10 superimposed sweeps. The horizontal bars in the top records of A and B and the points in the middle record of C show the times of pulse application.

peak amplitude either increased (seven neurons) or showed no changes (five neurons). The mean increase in amplitude was 50% (P < 0.01 ). The effect of changing stimulation intensities was studied on the hyperpolarizing potential in two neurons. As the stimulus intensity increased, a shortened latency of onset of hyperpolarization with an increase in the peak latency and amplitude resulted. Increasing the duration of the stimulus pulses induced similar changes (Figs. 2X, C ). When the frequency of hypoglossal nerve Stild~~tkJll was increasetl, the h!perpolarizing potential was easily depressed and (lit1 not follow at frequencies of more than 20/s (Figs. 213, U),

NAKAMURA

ET

TABLE Valves for Some Hyperpolarizing

Characteristics Potentials Single

Ipsilateral

AL.

1 of the Hypoglossally Induced in Masseteric Motoneurons5

shocks Contralateral

Three Ipsilateral

shocks Contralateral

Onset latency

(ms)

10.8 f 2.2 (7.6-14.2)

10.7 f 1.9 (7.6-13.5)

9.6 f 1.5 (7.0-12.0)

10.7 z!z 1.6 (8.0-12.5)

Peak latency

(ms)

16.2 i 2.8 (10.0-21.0)

15.9 f 1.7 (13.5-18.5)

14.7 f 3.1 (10.0-19.6)

16.0 f 0.9 (15.5-18.0)

4.8 f 2.1 (2.0-11.7)

4.0 f 1.5 ([email protected])

5.8 f 2.7 (2.7-11.0)

4.3 f 1.3 (2.8-6.6)

Peak amplitude

(mV)

“Average values with standard deviations were obtained as responses to single shocks (ipsilateral hypoglossal stimulation, 34 neurons; contralateral, 10 neurons) and three shocks (ipsilateral, 8 neurons; contralateral, 6 neurons) to the hypoglossal nerve. Ranges are in parentheses.

In a number of cases, the hypoglossally induced hyperpolarizing potential was interrupted in its course by a depolarizing potential or a hump in the depolarizing direction (Figs. ZA, C, E, arrows) when supramaximal shock for complete suppression of the masseteric reflex was applied. This depolarization (or depolarization trend) corresponded in time to the facilitatory phase of the masseteric reflex (13) and had a peak latency of 25 to 40 ms (mean, 32 ms) appearing after the peak of the hyperpolarizing potential. It was found to occur in 16 of 48 neurons tested by single shocks (Fig. ZE) and 10 of 14 neurons tested by a train of three pulses (Figs. 2A-C). In 14 of the 26 neurons which displayed a depolarization phase, the peak of the depolarization reached a level more depolarized than the resting potential. The maximum depolarization was 5.0 mV with respect to the resting potential, and in some neurons, a spike potential was induced during the depolarization phase. In certain cells, a depolarizing potential was induced by increasing the number of pulses applied to the hypoglossal nerve, even though this potential was not seen after a single-pulse stimulation. Increasing the duration of the stimulus pulse or increasing the intensity of the pulses augmented this depolarization phase. After the depolarizing phase, the membrane potential moved again in the hyperpolarizing direction (Figs. 2A, C, E) and reached a peak, usually at 20 to 60% of the amplitude of the early hyperpolarizing phase. From this point, the membrane potential gradually returned to the resting level 100 to 300 ms after the stimulation. Thus, the hypoglossally induced hyperpolarizing potential in masseteric motoneurons could be divided into an early and a late phase by the depolarizing potential,

HYI’OGLOSSAL

EFFECTS

ON

hlASSETERIC

NEURONS

/

The late hyperpolarizing phase was much more variable from one neuron to another and at times was not even present. In these instances, the early hyperpolarizing potential appeared to have been cut short by the depolarizing potential. Spike potentials monosynaptically evoked in masseteric motoneurons by stimulation of the trigeminal mesencephalic nucleus were inhibited during the hyperpolarization and facilitated during the depolarization induced hy hypoglossal nerve stimulation. An example of this inhibition is shown in Figs. 3A-F (upper traces ) with the simultaneously recorded suppression of the masseteric monosynaptic reflex (Figs. 3A-F, lower traces ) . Spikes antidromically evoked in inasseteric niotoneurons by masseteric nerve stimulation were inhibited during the early hyperpolarization. In 15 neurons, this early phase of the hyperpolarizing potential reversed to a depolarizing potential with a similar time course when Cl- was injected intracellularly through a KCl-filled recording electrode (Fig. 2F j. Inhihition of antidromic and monosynaptic spikes was still obtained after the reversal. In contrast to the early phase, the late phase of the hyperpolarizing potential was not reversed by intracellular Cl- injection (Fig. ZF). Ccntval Patlm~a~v Iwokvd irl flzc H?lpoylossorrlrrssctrric Irhihitoq Efwts. Partial transection of the hraiti stein was made between the tri-

FIG. 3. Inhibition of the monosynaptic spikes of masseteric motoneurons (upper traces) and suppression of the masseteric reflex (lower traces) induced by hypoglossal stimulation. Each pair of traces in this figure was recorded simultaneously in a cat. G-Control record of the monosynaptic spike and the masseteric reflex evoked by ipsilateral mesencephalic nucleus stimulation (0.5/s, 0.1 ms in duration, 5 V). H-The hyperpolarizing potential in the masseteric motoneuron evoked by ispilateral hypoglossal stimulation (0.5 ms in duration, 10 V). A to F-The effects of hypoglossal stimulation (same as H) on the monosynaptic spike and the masseteric reflex (same as G) at various intervals betureen the two stimuli.

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...

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FIG. 4. Effects of partial transections at different levels of the brain stem on hypoglossally induced changes of the masseteric reflex. Test masseteric reflexes were recorded from the right masseteric nerve and evoked by stimulation of the right trigeminal mesencephalic nucleus (0.3 ms in duration, 10 V) every 2 s. Conditioning stimuli to the right (F-J) and left (K-N) hypoglossal nerves consisted of three pulses (250/s, 0.1 ms in duration, 12.5 V). A-The level, mediolateral extent, and sequence of the sections as viewed on the floor of the fourth ventricle. F and KObtained prior to any section. G and L, H and M, I and N, and J-Obtained after the animal sustained the first, second, third and fourth sections, respectively. The extent of each section is shown by the black areas in B, C, D. and E. The abscissae in the graphs are the conditioning-test intervals, and the ordinates are the amplitudes of the reflex. Abbreviations in A to E: C-cuneate nucleus; IC-inferior

HYPOGLOSSAL

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ON

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9

geminal motor nucleus and the hypoglossal nucleus in three cats. Supra-

maximalstimulationfor completesuppressionof the massetericreflex was delivered to both the ipsilateral and the contralateral hypoglossal nerves before and after transections. When partial transections were made at the caudal level (near the hypoglossal nucleus), eve11 extensive lesions, such as a virtual hemisection of the reticular tegmentum, hardly affected the early phase of inhibition if the spinal tract and nucleus of the trigeminal nerve were almost intact. In these instances, there was a slight increase in the onset latency of the early inhibition, a depression of the late inhibition, as well as a decrease in the peak height of the facilitation. These effects were observed when the lesion was made on either side of the brain stem. When a circ&scribed section was n1ade at the level of the facial nucleus or more rostrally, destroying the spinal nucleus of the trigeminal nerve and its vicinities, the effectiveness of stimulation of the hypoglossal nerve ipsilateral to the lesion was clearly reduced. This occurred most markedly when the spinal tract of the trigeminal nerve was involved in the section. Examples of these results of partial transections in a cat are shown in Fig. 4. Right or left hypoglossal stimulation in the intact cat induced a complete suppression of the right masseteric reflex (Figs. 4F, K). An initial lesion was made on the right side at the n1iddle level of the medulla oblongata (Fig. 4A, 1st). This lesion widely involved the reticular tegmentum and partially the trigeminal spinal nucleus, but the spinal tract was left intact (Fig. 4B). The latency and the degree of the early suppression did not change following this lesion, but the late phase of suppression was virtually abolished (Figs. -CG, L). In addition, a second lesion was n1ade on the left side at the level of the facial nucleus (Fig. 4‘4, 2nd). The lateral edge of the lesion was adjacent to the medial border of the trigeminal spinal tract and involved the spinal nucleus and the lateral portion of the reticular tegmentum (Fig. 4C). The right hypoglossal effects were not altered (Fig. 4H), but the early suppression induced by the left hypoglossal stimulation was remarkably reduced (Fig. 431) even though the spinal tract of the trigeminal nerve was almost intact. A third lesion was made on the left side at the level of the rostra1 pole of the hypoglossal nucleus (Fig. 4LA, 3rd). The brain stem was almost completely hemisected (Fig. 4D). Following this section, the left llypoglossal nerve stimulation becalne ineffective (Fig. 4N), whereas the effect of right hypoglossal nerve stimulation was still evident (Fig. 41). A fourth lesion was then lnatle in the colliculus ; ICP-inferior cerebellar cuneate nucleus (Monakow’s nucleus) tract: SO-superior olivary nucleus tract ; V-trigeminal spinal nucleus ; nucleus ; X-dorsal nucleus of vagus

peduncle ; L-medial lemniscus ; M-accessory ; N VIII-statoacoustic nerve ; P-pyramidal ; TB-trapezoid body ; TV-trigeminal spinal VII-facial nucleus ; VIII IA-lateral vestibular nerve ; X II--hylmglossal nucleus.

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dorsomedial part of the right half of the brain stem at the level of the rostra1 pole of the facial nucleus (Fig. 4A, 4th)) extending cranially to the caudal vicinity of the supratrigeminal nuclear region (Fig. 4E). This restricted lesion completely abolished the effects of right hypoglossal stimulation (Fig. 4J). The effects of hypoglossal nerve stimulation were tested on eight supratrigeminal neurons, which were regarded as inhibitory neurons for masseteric motoneurons, based on their location as well as the characteristic spike burst pattern evoked by stimulation of either the lingual or the inferior alveolar nerve on the ipsilateral side (4, 8, 9). In all these supratrigeminal neurons, a spike train was induced by stimulation of the ipsilateral hypoglossal nerve. Figure 5 illustrates a typical example of hypoglossally evoked discharges of supratrigeminal neurons in relation to the early hyperpolarizing potential in masseteric motoneurons. As shown in Fig. SA, stimulation of the right inferior alveolar nerve evoked a burst of eight or nine spikes in a supratrigeminal neuron on the right side. The first spike of the burst appeared with a latency of 1.3 ms. In the same cat, an inhibitory postsynaptic potential (IPSP) with a latency of 1.9 ms was induced in a right masseteric motoneuron, which was identified by antidromically evoked spike potentials from the right masseteric nerve (Fig. SG), by stimulation of the right inferior alveolar nerve (Fig. 5B) at the same stimulation parameters as used for the production of supratrigeminal spike bursts (Fig. 5A). The latency difference between the first spike of supratrigeminal burst discharges and the onset of the IPSP in the masseteric motoneuron was 0.6 ms, which was in the monosynaptic range. Thus, this supratrigeminal neuron was assumed to be an inhibitory neuron for masseteric motoneurons. Stimulation of the right hypoglossal nerve induced a train of three or four spikes in this supratrigeminal inhibitory neuron (Fig. SD). The initial spike appeared about 13.9 ms after application of single shocks to the hypoglossal nerve, which evoked an IPSP in the same right masseteric motoneuron, as shown in Fig. 5B. The onset of the hypoglossally induced 1PSP was estimated to be about 14.5 ms (Fig. SF). Thus, the latency difference between the initial spike of the supratrigeminal neuron and the IPSP in the masseteric motoneuron was about 0.6 ms, i.e., monosynaptic. Note also that there is a parallelism between the number of spike potentials in a spike train in the supratrigeminal neuron and the amplitude of the IPSP of the masseteric motoneuron. The amplitude of the IPSP evoked by inferior alveolar nerve stimulation (Fig. SC) was about twice as large as that of the hypoglossally evoked IPSP (Fig. SF) ; correspondingly, the number of spike potentials evoked by stimulation of the inferior alveolar nerve was about twice that evoked by hypoglossal-nerve stimulation.

I-IYF’OGLOSSAL

EFFECTS

CT, J”

-3

ON

MASSETKRIC

NEURONS

11

F ~15m” G l;, lomsec J

150illV '

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2 msec

FIG. 5. Effects of stimulation of the inferior alveolar and hypoglossal nerves on a supratrigeminal neuron and a masseteric motoneuron. The entire record was obtained from the same cat, superimposing several traces. A and D-Extracellular records from the same right supratrigeminal neuron; positive deflection shown upward. B, C, and E-G-Intracellular records from the same right masseteric motoneuron. A-CStimulation of the right inferior alveolar nerve (l/s, 0.03 ms in duration, 8 V). D-F -Stimulation of the right hypoglossal nerve (l/s, 0.03 ms in duration, 12 V). GStimulation of the right masseteric nerve (l/s, 0.03 ms in duration, 2.8 V). Arrows indicate the times of stimulus application. The time base in E applies to A, B. D, and E; that in F, to C and F; that in G. to G. The voltage calibration in F and G applies, respectively, to A-F and G.

DISCUSSION from masseteric motoneurons revealed that hypoglossal stimulation induced a long-latency, prolonged hyperpolarizing potential, the time course of which closely resembled that previously reported for suppression of the masseteric reflex (13 ). The latencies of onset and peak and the amplitude of the peak of this hyperpolarizing potential were affected in a manner similar to the suppression of the masseteric reflex when the parameters of hypoglossal stimulation were changed. The spike potential of the masseteric motoneuron monosynaptically evoked from the mesencephalic nucleus was inhibited during hypoglossally induced hyperpolarization. In a number of neurons, supramaximal stimulation of the hypoglossal nerve induced a depolarizing potential or a phase approaching the resting membrane potential in the prolonged hyperpolarizing potential, corresponding in time to the hypoglossally induced facilitatory phase of the masseteric reflex. Previous results indicated that this depolarizing potential might be evoked by fiber groups diferent from those that induce suppression at weak intensity (13). By this depolarizing potential, the hyperpolarizing potential was divided into two phases, an early and a late phase. These results indicate that the hypoglossally induced suppression and facilitation of the masseteric reflex is primarily due to postsynaptic Intracellular

recordings

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depression and facilitation, respectively, of the excitability of masseteric motoneurons. The early phase of the hyperpolarizing potential could be reversed to a depolarizing one by intracellular Cl- injection. Initial segment-somadendritic block of the antidromic spike potential in masseteric motoneurons was observed during the early phase of hyperpolarization and even after the reversal. The results indicate that the early phase is composed of IPSPS. A positive field potential was evoked in the masseteric motor nucleus by hypoglossal nerve stimulation. The latency of onset and the peak of this potential corresponded in time to those previously reported for the early phase of suppression of the masseteric reflex, as well as to those of the early hyperpolarizing potential evoked by hypoglossal stimulation. Thus, it stands to reason that the positive field potential is due to the current generating the early phase of the hypoglossally induced hyperpolarizing potential. In contrast to the early phase, the late phase of the hypoglossally induced hyperpolarizing potential in masseteric motoneurons was not reversed to a depolarizing potential by intracellular Cl- injection. It is interesting to note the similarity of the responses of masseteric motoneurons to lingual (4)) inferior dental (7)) and hypoglossal nerve stimulation. In all cases, there is an initial phase of hyperpolarizing with all the characteristics of somatic IPSPs. This is followed by a long-lasting phase of hyperpolarizing which cannot be reversed to depolarization by intracellular Cl- injection. Evidence was presented that inhibitory interneurons in the supratrigeminal nucleus mediate this initial hyperpolarizing phase of lingual (4) and inferior dental origin (8). On the other hand, it was proposed that the late phase was mediated by the bulbar reticular formation (16). As reported previously (13)) transection of the brain stem at the precollicular level or at the medullospinal junction did not affect the hypoglossal influences on the masseteric reflex. Thus, neither cortical, diencephalic, nor spinal structures are necessary for the observed hypoglossal influences on the masseteric reflex. The results of the analysis of the pathways responsible for the hypoglossomasseteric effects by means of partial transection of the brain stem are characterized as follows. (i) Transection of the reticular formation selectively depressed the late phase of inhibition if the trigeminal spinal nucleus and tract were left intact, even when the lesion transected virtually the whole extent of the reticular formation. (ii) The early phase of the inhibition was selectively depressed or totally abolished by transection of the spinal nucleus of the trigeminal nerve on the stimulated side. These results suggest that the early phase of inhibition is mediated by a compact

HYPOGLOSSAL

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route in and around the trigeminal spinal nucleus, including the intranuclear ascending pathway on the stimulated side ( 1, 3, 15), whereas the late phase is effected through a diffuse pathway in the reticular formation. The early inhibition evoked by contralateral hypoglossal stimulation would be mediated through some crossing pathway at the level of rostra1 pons. Stimulation of the hypoglossal nerve was found to induce a train of spikes in supratrigeminal inhibitory neurons on the stimulated side, which were identified as such by their location, by a characteristic pattern of spike bursts evoked by either lingual or inferior alveolar nerve stimulation, as well as by the latency of the initial spikes preceding by one synaptic delay the onset of the early hyperpolarizing potential evoked in masseteric motoneurons by the same stimulation of these nerves (3, 8, 9). Actually, the latency of the initial spikes and the number of spikes in a spike train in supratrigeminal neurons correlated with the latency of onset and the amplitude of the early phase of hyperpolarizing potential in masseteric motoneurons evoked by the same stimulation of the hypoglossal nerve. Thus, we concluded that supratrigeminal inhibitory neurons on the side of hypoglossal nerve stimulation are the final inhibitory neurons in the polysynaptic neuronal chain responsible for the hypoglossally evoked early phase of hyperpolarizing potential in masseteric motoneurons. The early inhibition of masseteric motoneurons evoked by contralateral hypoglossal nerve stimulation would be mediated directly to masseteric motoneurons by supratrigeminal inhibitory neurons on the stimulated side through their axons crossing the brain stem at the level of the trigeminal motor nucleus (11). REFERENCES 1. CARPENTER, M. B., AND G. R. HANNA. 1961. Fiber projections from the spinal trigeminal nucleus in the cat. J. Cowp. Ncnrol. 117 : 117-125. 2. DAULT, S. H., AND D. R. SMITH. 1969. A quantitative study of the nucleus of the mesencephalic tract of the trigeminal nerve of the cat. Bwt. Rec. 165: 79-88. 3. GOBEL, S., AND M. B. PI-RVIS. 1972. Anatomical studies of the organization of the spinal V nucleus; The deep bundles ,nd the spinal V tract. Rroirl Rrs. 48: 27-44. 4. GOLDBERG, L. J., AND Y. ~\TAKA~~ITRA. 1968. Lingually induced inhibition of masseteric motoneurones. Exprricutin 24 : 371- 373. 5. GREEN, J. D., AND K. XEXSIII. 1963. Membrane potentials in hypoglossal motoneurons. J. Nruropllysiol. 2~6 : 835-856. 6. HANSON, J., AND L. WIDEN. 1970. Afferent fibers in the hypoglossal nerve of cat. L-lcta Physiol. Scud. 79 : 24-36. 7. KIDOKORO, Y., K. KUBOTA, S. SHUTO, AND R. SI’MINO. 1968. Reflex organization of cat masticatory muscles. J. Ncnrophysiol. 31 : 695-708. 8. KIDOKORO, Y., K. KUBOTA, S. SHUTO, AND R. SUMINO. 1968. Possible interneurons responsible for reflex inhibition of motoneurons of ja\v-closing muscles from the inferior dental nerve. J. Nrrrrophysiol. 31 : 709-716.

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S., AND K. I% OLSON. 1976. Localization of evoked potentials in the digastric, supra- and intertrigeminal subnuclei of the cat. Exp. Brairz Rrs. 26: 299-3 18. 10. LINDQUIST, C., AND A. MARTENSSON. 1969. Reflex responses induced by stimulation of hypoglossal afferents. Acta Physiol. Scud. 77 : 234-240. 11. MIZUNO, N. 1970. Projection fibers from the main sensory trigeminal nucleus and supratrigeminal region. J. Conzp. Newel. 159 : 457-472. 12. NAKAMURA, Y., L. J. GOLDBERG, AND C. D. CLEMENTE. 1967. Nature of suppression of the masseteric monosynaptic reflex induced by stimulation of the orbital gyrus of the cat. Brain Rcs. 6: 184-198. 13. NAKAMURA, Y., L. J. GOLDBERG, N. MIZUNO, AND C. D. CLEMENTE. 1970. Masseteric reflex inhibition induced by afferent impulses in the hypoglossal nerve. Bruin Rcs. 18: 241-256. 14. SAUERLAND, E. K., AND N. MIZUNO. 1968. Hypoglossal afferents: Elicitation of a polysynaptic hypoglosso-laryngel reflex. Brain. Rrs. 10 : 256-258. 1.5. STEWART, W. A., AND R. B. KING. 1963. Fiber projections from the nucleus caudalis of the spinal trigeminal nucleus. J. COMERS. Neural. 121 : 271-286. 16. SUMINO, R. 1971. Central neural pathways involved in the jaw-opening reflex in the cat. Pages 315-331 in R. DUBNER AND Y. KAWAMURA, Eds., Oral-Facial Sensory and Motor Mechmiswzs. Appleton-Century-Crofts, New York. 17. ZAPATA, P., AND G. TORREALBA. 1971. Mechanosensory units in the hypoglossal nerve of the cat. Brain Res. 32: 349-367. LANDGREN,