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Functional differentiation of hypoglossal motoneurons during the amygdaloid or cortically induced rhythmical jaw and tongue movements in the rat
Bruin Rrsrctrch Bulletin, Vol. 13, pp. 147-154, 1984. 0 Ankho International
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Functional Differentiation of Hypoglossal Motoneurons During the Amygdaloid or Corticallv Induced Rhvthmical Jaw and Tongie Movements in the Rat TOSHIO KAKU’ Depurtment
of Physiology,
Faculty of Dentistry,
Received
Kyushu University 61, Fukuoka,
17 January
Jupan 812
1984
KAKU, T. Func~tionul di@rentiation of hypoglossal motonrunms during thr amygduloid or cortically induwd rhythmical ju,l, and tongue movrmcnts in the rat. BRAIN RES BULL 13(l) 147-154, 1984.-Activity of hypoglossal motoneurons was
studied in relation to the amygdaloid or cortically induced rhythmical jaw and tongue movements in the rat. Tongue protrudor and retrusor motoneurons were antidromically identified by stimulation of the medial and the lateral hypoglossal nerve branches, respectively. About 80% of protrudor motoneurons excited during the opening phase of the rhythmical jaw movements with or without prior excitation. Most of them did not respond to amygdaloid or cortical stimulation. Fiftythree percent of retrusor motoneurons were excited during the closing phase and 3 1% discharged around the peak opening. The majority of them and a few protrudor motoneurons responded to amygdaloid or cortical stimulation with similar latencies. Activities of the medial and the lateral hypoglossal branches corresponded with the discharge patterns of respective motoneurons. Tongue protrusion and retrusion appeared to slightly precede jaw opening and closing, respectively during the rhythmical jaw and tongue movements. This was probably due to activity of the brain stem pattern generator. Hypoglossal motoneuron
Cortex
Rhythmical jaw movements
Central amygdaloid nucleus
from the beginning of the jaw closing to the middle of the jaw opening phase and the tongue retrusion did from the middle of the jaw opening phase to the beginning of the jaw closing. Moreover, amygdaloid control of hypoglossal motoneurons has not been reported except the author’s preliminary papers [_5,6]. The present study reports the differential activities of protrudor and retrusor motoneurons in relation to the rhythmical jaw movements induced by stimulation of the frontal cortex or the ipsilateral or the contralatera1 central amygdaloid nuclei. curred
FERRIER [2] initially found that repetitive electrical stimulation of the lateral frontal cortex induced the rhythmical jaw movements like mastication in rabbits. Kaada et ul. [4] re-
ported the stimulation of the anteromedial nuclei of amygdala also induced the chewing movement in cats. Rioch [ 171proposed a hypothesis of peripheral patterning of the rhythmical jaw movements by interaction of cortical jaw opening influence and closing reflex activity. Sumi [ 191 initially suggested that chewing rhythm was generated by the chewing center in the brain stem, which was activated by the higher brain structures and/or orofacial afferents in rabbits. Soon after him, Dellow and Lund [I] proposed a hypothesis of the brain stem pattern generator explaining the rhythmical jaw movements and this has been now generally accepted. Cortical control of the cat’s hypoglossal motoneurons was initially found by Porter [ 161. Cortical excitation of the rabbit’s hypoglossal nerve fibers was reported by Sumi [ 191, who demonstrated the single hypoglossal fiber activity during the rhythmical jaw movement and found that the majority discharged in the jaw closing phase of the rhythmical jaw movement. Morimoto and Kawamura [12] observed the rhythmical tongue movement during the rhythmical jaw movement in cats, and found that the tongue protrusion oc-
METHOD
Forty-five adult rats (Wistar King A, 200-300 g) were used under urethane anesthesia (0.9-I. 1 g/kg, IP). Bipolar silver wire electrodes with the interpolar distance of 1.5 mm insulated with acrylic resin except for the tips, were attached to the right medial and lateral branches of the hypoglossal nerve to identify each motoneuron as well as to record the nerve action potentials. In some cases, a similar electrode was also attached to the mylohyoid nerve for recording its action potentials. The rat’s head was fixed on a stereotaxic apparatus
‘Requests for reprints should be addressed to T. Kaku at the present address: Department of Prosthetic Dentistry II, Faculty of Dentistry, Kyushu University 61. Fukuoka. Japan 812.
147
14X
P-Mtn
R-Mtn
Rec.
0.3mm c-Am
i - Am
-~
lmm
FIG. I. Recording site (Rec.) in hypoglossal nucleus and site of central amygdaloid stimulation (Stim.). Upper left and right show the recording site of P-Mtn and R-Mtn, respectively (arrow), stained with pontamine sky blue. The former is situated in the ventral half of hypoglossal nucleus and the latter, in the dorsal half. Lower left and right show the site of contralateral and ipsilateral central amygdaloid (c-Am and i-Am. respectively) stimulation (arrow) by passing current (100 PA, IO set) after the experiment.
(Narishige, model SN-2). Bilateral central amygdaloid nuclei and the frontal cortex were electrically stimulated through concentric bipolar stainless steel electrodes which had tip separations of 0.5 mm for the amygdaloid nuclei and of 1.0 mm for the cortex. Cortical stimulation was applied to the most effective site for jaw movements according to Ohta and Sasamoto [ 151 and the amygdaloid electrodes were stereotaxically oriented (A 6.2, L 4.1, H -2.7, slightty modified from the map by Kiinig and Klippel [9]). The electrode positions were histologically verified after each experiment and the data from inaccurate etectrode positions were discarded (Fig. 1 tower). Stimulus parameters were square waves of 0.1 msec duration and 0.5 to 1.5 mA intensity which was adjusted to slightly exceed the threshold of rhythmical jaw and tongue movements at the most effective frequency of 25 to 40 Hz. The lowest threshold for the jaw movements was observed when the electrode position was found at the center of the central amygdaloid nucleus and the stronger stimulation was necessary for the threshold of jaw movements when the electrode position was the more distant from the nucleus. Activity of single hypoglossal motoneurons was recorded extracellularly through glass microelectrodes filled with 2 M NaCl or 0.5 M Na acetate containing pontamine sky blue (DC resistance of about 5 to 10 MR). The latter was used to verify the recording site by passing current through the re-
cording microelectrode (Fig. I upper). Jaw movements uerc monitored with a displacement transducer which was connected to the steel bar tightly glued to mandibular incisors. One mm opening of the rat’s jaw was made by 0.0 g force of depressing the mandibular incisors. Some of the rhythmical jaw movements and the nerve action potentials were stored in a data recorder (TEAC. model R-61) and averaged or intcgrated through an on-tine computer (Signal processor. San-ei model 7TO8). The body temperature was kept around 36OC during the experimen1. RESUI
1S
Protrudor and retrusor motoneurons were antidromicalty identified by stimulation of the medial and lateral branches of the hypoglossal nerve. respectively, since the medial and lateral branches were preliminarily confirmed to innervate the rat’s ipsilateral protrudor muscles and retrusor ones. respectively [ 131. Criteria for the antidromic identification of these neurons were as follows: “all or none” type appearance of discharge with a constant short latency. and the capability of action potentials following high frequency repetitive stimulation exceeding 400 Hz. Both protrudor and
HYPOGLOSSAL
C
c-Am.
ACTIVITY
AND CHEWING
c-Am.
ctx.
i-Am.
ctx.
RHYTHM
149
spike with an antidromic latency of 1.03 msec in response to the stimulation of the medial branch. The antidromic spike, however, was reduced by the superimposed antidromic field potential of the other motoneurons. This motoneuron excited following to triple shocks of the contralateral central amygdaloid nucleus with a latency of 13.9 msec (Fig. 2A middle) and also responded to triple shocks of the cortex with 11.3 msec latency (Fig. 2A right) but did not respond to stimulation of the ipsilateral central amygdaloid nucleus which was not shown in the figure. Figure 2B shows similar responses of a retrusor motoneuron. The antidromic action potential was evoked by the stimulation of the lateral branch with a latency of 0.9 msec. This motoneuron responded to double shocks of the contralateral central amygdaloid nucleus (Fig. 2B middle) and the frontal cortex (Fig. 2B right) with latencies of 7.7 msec and 13.1 msec, respectively. This cell did not respond to the stimulation of the ipsilateral central amygdaloid nucleus. Figure 2C shows another retrusor motoneuron. This motoneuron responded to the stimulation delivered to all three. Double shocks stimulation of the contralateral central amygdaloid nucleus, the ipsilateral one and the cortex induced action potentials with the latencies of 10.4 msec, 10.5 msec and 12.7 msec, respectively. Effect of double or triple pulse stimulation of the cortex, the ipsilateral or the contralateral central amygdaloid nucleus were summarized in Table 1, in which the upper table (A) shows the number of protrudor motoneurons and the lower one (B) that of retrusor motoneurons. The upper row shows the number of motoneurons which were excited by cortical stimulation and the lower row shows the number of motoneurons that did not respond to the cortical stimulation. Thirty-seven protrudor motoneurons (7%) out of 47 did not respond to any stimulation (Ctx, i-Am and c-Am). Among the remaining 10 protrudor motoneurons, 7 motoneurons responded to cortical stimulation and eight did to contralateral amygdaloid stimulation, and four did to ipsilateral amygdaloid stimulation, although some of them responded to stimulation of two or all three sites. On the other hand,
positive
5 nl**c
FIG. 2. An example of the electrical activity of protrudor and retrusor motoneurons. A. Left, antidromic discharge of a protrudor motoneuron appeared at fixed short latency after stimulation of medial branch (Med). Middle and right, driven discharge by stimulation of contralateral central amygdaloid nucleus (c-Am, 3 msec interval, arrow) and contralateral cortex (Ctx, 2 msec interval. arrow), respectively. B. Left, antidromic response of retrusor motoneurons to stimulation of lateral branch (Lat). Middle and right show driven discharge after amygdaloid (c-Am, arrow) and cortical (Ctx, arrows) stimulation, respectively. C. Retrusor motoneuron. Left, middle and right show driven discharge after stimulation of contralateral (cAm), ipsilateral amygdaloid nuclei (i-Am) and contralateral cortex (Ctx). Downward square wave pulse shows calibration of 1 mV. Note faster sweep speed in A left and B left, shown in A left, and slower sweep for others shown in C right. Upward negative in this and following figures.
retrusor motoneurons scarcely showed spontaneous discharge. Effect of double or triple pulses stimulation of the contralateral frontal cortex, the ipsilateral or the contralatera1 central amygdaloid nucleus was examined on every motoneuron. The contralateral cortical influence was exclusively investigated in this study since the cortical excitation of hypoglossal nerve was preliminarily confirmed to be contralaterally dominant. Figure 2A shows an example of the electrical activity of a protrudor motoneuron. This motoneuron showed a large
TABLE
1
NUMBER OF HYPOGLOSSAL MOTONEURONS EXCITED OR NOT
BY TRAIN
PULSE STIMULATION OF THE FRONTAL CORTEX OR CENTRAL AMYGDALOID NUCLEUS
c-Am: Ex
i-Am: Ex c & i-Am: Ex Am: No Ex
A P-Mtn (Total: 47) Ctx: Ex Ctx: No Ex
4 (8.5%) 2
I 1
2 0
0 37 (79%)
6 (11%) 2
20 (37%)
B R-Mtn (Total: 54) Ctx: Ex Ctx: No Ex
I4 (26%) IO (18.5%)
0
I
1
A. Protrudor motoneurons (P-Mtn). Upper row shows number of motoneurons excited by cortical stimulation, which responded or did not (No Ex) to stimulation of the contralateral amygdaloid nucleus (c-Am), the ipsilateral one (i-Am) or both (c & i-Am). Lower row shows number of motoneurons not excited by cortical stimulation which responded or did not to amygdaloid stimulation. B. Retrusor motoneurons (R-Mtn). Same tabulation as in A.
TABLE 2 MEAN LATENCY OF INITIAL DISCHARGE OF P-Mm (LEFT) R-Mtn (RIGHT) AFTER INITIATION OF CORTICAL OR AMYGDALOID STIMULATION
P-Mm Ctx
R-f&l
I1 .s ?. 0.4
I I.5 + 0.3 12.2 _+ I.0 10.3 t 1.0
i-Am c-Am
AND
11.4 It 1.0 10.4 rt 0.4
(mean i S.E.: msec.) Abbreviations are the same as Table I. Mean and its SE fmscc).
after initiation of train pulse stimulation of the cortex. the ipsilateral or the contralateral central amygdabid nuclei. As seen in the table, no statistically significant difference war; detected between the respective response latencies of the protrudor and the retrusor motoneurons to stimulation of any regions. However, both protrudor and retrusor motoneurons showed shorter response latencies to stimulation of contralateral central amygdaloid nucleus than to stimulation of the other.
Spontane~~us movements of the jaw or the tongue were observed unless the rat wa\ waking up from the anesthesia. When repetitive stimulation was applied to the cortex or the central amygdaloid nucleus, the rhythmical jaw and tongue movements were observed. The jaw was situated at the jaw resting position before the amygdatoid or cortical stimulation was applied. Slightly after the beginning of the stimulation the jaw opened to the peak and then closed to the initial jaw resting position. These open-close movements were repeated during and slightly after the stimulation. On the other hand, the tongue moved forth and back fairly synchronous with jaw opening and closing. respectively. To observe the relationship between the rhythmical jaw movements and the activity of the hypoglossal motoneur(~ns. the discharge patterns of both motoneurons were classified in relation to rhythmical jaw movements as shown in Figs. 3. ? not
thirty-four retrusor motoneurons (63%) out of 54 responded to stimulation of at least one of the three sites. Twenty-one motoneurons responded to cortical stimulation, and 32 did to contralateral amygdaloid stimulation and nine did to ipsilatera1 amygdaloid stimulation, although some of them responded to stimulation of two or three. Comparing the excitatory influence of bilateral central amygdaloid nuclei on hypoglossal motoneurons, both protrudor and retrusor motoneurons received stronger excitatory inputs with slightly shorter latencies from the contralate~l central amygdaloid nucleus than from the ipsilateral nucleus. Table 2 shows the mean latency of the initial response of protrudor motoneurons (left) or retrusor motoneurons (right)
and Table 3.
TABLE 3 TYPE OF DISCHARGE
-
PATTERN CLASSiFfED
IN RELATION
--
‘I-0 RHYTHMICAL
JAW
MOVEMENIS (RJM)
P-Mtn St.-site Discharge Pattern
Ctx
~-Ant c-Am --P-
Closing
Opening non driven
non driven
driven
driven opening-i Opening-peak
Pre- & Opening
16
non driven driven
Pre-opening
Opening-I
Other
Other
RJM unrelated
RJM unrelated
Total
‘l”otal
-p iI
I1
t4
51
5I
HYPOGLOSSAL
ACTIVITY
AND CHEWING
151
RHYTHM
P -Mtn
+ i-Am. 6.
*
,I
,I
‘1,
+ ctx. C.
Ill\
-
_ O.Z
FIG. 3. An example of discharge pattern of protrudor motoneuron (P-Mtn) in relation to rhythmical jaw movement (RJM). A. Discharge during opening phase of RJM induced by stimulation of contralateral amygdaloid nucleus (c-Am, 25 Hz). This discharge pattern was classified as Opening-nondriven type. Lower trace indicates jaw position and downward deflection corresponds to jaw opening in this and subsequent figures. B. Discharge driven by cortical stimulation (Ctx. 25 Hz) and additional discharge during opening phase of RJM. This was classified as Opening-driven type. C. Occasional driven discharge after c-Am stimulation (20 Hz) and facilitation of discharge before and during opening phase of RJM. This pattern was classified as Pre & Opening type. D. Discharge before opening phase of RJM and this classified as Preopening type. Note exclusive discharge shortly before jaw opening during and after i-Am stimulation (25 Hz). Middle trace shows mylohyoid nerve discharge during RJM. Downward arrow indicates the beginning of the repetitive stimulation and upward arrow does the end of the stimulation in this and subsequent figures.
Discharge patterns of protrudor motoneurons were classified into the Opening type, the Pre & Opening type, the Pre-opening type and the rhythmical jaw movements (RJM) unrelated type. The Opening type is characterized by burst discharges during the jaw opening phase of the rhythmical jaw movements. The Opening type is further divided into following two subtypes; the Opening-nondriven type (Fig. 3A) and the Opening-driven type (Fig. 3B). The Opening-nondriven type motoneurons discharged only during the jaw opening phase (Fig. 3A). Neurons of the Opening-driven type responded mostly to each of repetitive stimuli and showed additional burst discharges in the jaw opening phase (Fig. 3B). Discharge pattern of the Pre & Opening type resembled that of the Opening-driven type, but no discharge was observed during the jaw closing phase (Fig. 3C). In other words, motoneurons of this type mostly responded to each of repetitive stimuli during the jaw resting period before initiation of jaw movement. When the rhythmical jaw movements were induced, they discharged in bursts during the opening phase, became silent during the closing phase and responded mostly to each stimulus in the pre-opening phase. Motoneurons of the Pre-opening type mostly responded to each of repetitive stimuli during the jaw resting period before initiation of the jaw movement and when the rhythmical jaw movements were induced, they discharged only before each opening phase (Fig. 3D). In Fig. 3D, action potentials of the mylohyoid nerve were also recorded and the mylohyoid nerve activity coincided with the amplitude of rhythmical jaw opening. This is quite reasonable since the mylohyoid nerve innervates two jaw opening muscles. the mylohyoid and the anterior digastric ones. The
FIG. 4. An example of discharge pattern of retrusor motoneuron
(R-Mtn) in relation to RJM. A. Discharge during opening phase of RJM induced by c-Am stimulation (33 Hz). This pattern was classified as Closing-nondriven type. B. Discharge mostly driven by i-Am stimulation (33 Hz) and additional discharge during closing
phase of RIM. This was classified as Closing-drive type. C. Similar discharge pattern as in B except driven discharge appeared to be inhibited during opening phase of RJM induced by Ctx stimulation (25 Hz). This was classified as Closing-opening-f type. D. Discharge around peak opening of RJM induced by c-Am stimulation (33 Hz). This was classified as Opening-peak type. E. Discharge mostly driven by i-Am stimulation (25 Hz) and appeared to be inhibited during opening phase of RJM. This classified as Opening-l type.
rhythmical jaw movements unrelated type showed no discharge during the rhythmical jaw movements and did not respond to any stimulation (Ctx, i-Am and c-Am). The example of this type was not shown in the figure. of retrusor motoneurons were Discharge patterns classified into the Closing type, the Opening-peak type, the Opening-I type and the rhythmical jaw movements unrelated type. The Closing type is characterized by burst discharges during the jaw closing phase of the rhythmical jaw movements. This type is further divided into three subtypes: the Closing-nondriven type, the Closing-driven type, and the Closing-opening-I type. Motoneurons of the Closingnondriven type discharged only during the jaw closing phase (Fig. 4A). However, motoneurons of the Closing-driven type mostly responded to each of repetitive stimuli and showed additional burst discharges in the jaw closing phase (Fig. 4B). Motoneurons of the Closing-opening-1 type mostly responded to each of the repetitive stimulation, and showed additional burst discharge during the jaw closing phase, but they were silent during the jaw opening phase. Their driven discharges appeared to be inhibited during the jaw opening phase (Fig. 4C). The discharge pattern of the Opening-peak type is characterized by the burst discharges around the opening peak of the rhythmical jaw movements and this type is further divided into two subtypes: the Opening-peaknondriven and the Opening-peak-driven types. Motoneurons in the Opening-peak-nondriven type do not respond to each of repetitive stimulation (Fig. 4D) and motoneurons in the opening-peak-driven type respond to each stimulation. The discharge pattern of the Opening-I type resembles that of the Closing-opening-I type, but no burst discharges were seen in this type neurons. They mostly responded to each of repetitive stimuli during the jaw closing phase and jaw resting period. However, they are silent and appeared to be inhibited during the jaw opening phase (Fig. 4E). Motoneurons of the rhythmical jaw movements unrelated
152
KAKll c-Am.
(20 HZ.~
A. lO.bmV 2mm t open 0.2
lrmv asmm
I
/
*PC”
0.1 sec.
FIG. 5. Hypoglossal nerve action potential in relation to RJM induced by Ctx stimulation. A. Upper trace recorded from medial branch (Med) innervating protrudor muscle. Middle, from lateral branch (Lat) innervating retrusor. Lower, jaw position and downward opening as in Figs. 2 and 3. B. Averaged recording from A during 10 cycle of RJM. Upper from medial branch, middle from lateral branch and lower from jaw movement. Calibration of sweep speed and jaw movement are shown in C. C. Nerve action potential was integrated with time constant of 10 msec and added in sum of 10 cycle of RJM. Jaw movement averaged in same way as in B. Maximum activity of medial branch occurred in mid-opening phase and that of lateral branch near end of closing phase.
type showed no discharge during the jaw movements and did not respond to any stimulation (Ctx, i-Am and c-Am). Table 3 summarized classification of 53 protrudor and 5 1 retrusor motoneurons of which discharge patterns were examined during the rhythmical jaw movements induced by stimulation of all three sites, bilateral central amygdaloid nuclei and the cortex. Most of them were classified into a single type although some were classified into two different types induced by stimulation of different regions. Forty-three (about Sl%) of 53 protrudor motoneorons showed jaw movements related discharges although most of them were not excited by short train pulse stimulation of any stimulus sites (Ctx, i-Am and c-Am). Twenty-two (about 42%) protrudor motoneurons were classified into the Opening type, and 16 {about 30%) were into the Pre & Opening type. Only four motoneurons were in the Pre-opening type. Some protrudor motoneurons showed jaw movements related discharges but could not be classified into above three types. A few discharged only at the end of jaw closing, a few did at the peak of jaw opening and after the end of jaw closing and others discharged during the jaw opening phase and at the end of jaw closing. Forty-seven (about 9%) retrusor motoneurons showed the jaw movements related discharges. A majority of them (27 of 51) were classified into the Closing type. Sixteen (about 31%) were in the Opening-peak type. A few were in the Opening-I type. Some retrusor motoneurons showed the jaw movements related discharges but could not be classified into three types. The discharge pattern of two of them resembled that of the Opening type of protrudor motoneurons. Another neuron discharged both at the beginning of the jaw opening and during the jaw closing phase. No statistically significant difference in antidromic latencies was found among any of these groups of hypoglossal motoneurons.
FIG. 6. Nerve action potential in relation to c-Am induced RJM in same way as in Fig. 5. A. Upper from medial branch (Med). middle from lateral (Lat) and lower show jaw movement. B. Averaged recording from A. Upper from medial branch, middle from lateral one and lower from jaw movement. C. Integrated response as in Fig. 5C. Maximum action potential of medial branch in mid-opening phase and that of lateral branch in mid-closing phase.
The effects of the repetitive stimulation were also examined for relationship between action potentials of the hypoglossal nerve branches and the rhythmical jaw movements as shown in Figs. 5 and 6. In these figures, the upper traces showed recordings from the medial branches, the middle did those from the lateral branches and the lower did recordings of jaw movements. Figure 5 shows the rhythmical jaw movements (about 3.5 Hz) elicited by repetitive stimulation of the frontal cortex (25 Hz) and the action potentials of the medial and the lateral branches well synchronized with the jaw movement (Fig. SA). The action potentials from both branches during 10 cycles of the rhythmical jaw movements were averaged through the on-tine computer which was triggered by the beginning of jaw opening (Fig. 5B). The medial branch discharged mostiy during the opening phase and the lateral did during the closing phase. Then the positive components of the action potentials were integrated with a time constant of 10 msec and added in sum of lO_cycles of the jaw movements (Fig. 5C). The maximum integrated activity of the medial branch was found in the middle of the opening phase and that of the lateral one was near the end of the closing phase. Figure 6 shows the rhythmical jaw movements (about 3 Hz) induced by repetitive stimulation of the contralateral central amygdaloid nucleus (c-Am, 20 HZ) and the nerve action potentials in another rat as in Fig. 5. In this case, the maximum activity of the medial branch was found in the middle of the opening phase and that of the lateral branch was in the middle of the closing phase, These activities of the medial and the lateral branches corresponded very well with the discharge patterns of protrudor and retrusor motoneurons. DISCUSSK)N
Cortical control of the hypoglossal motoneurons in cats was initially reported by Porter [ 163 who demonstrated EPSPs with onset latency of 6 to 12 msec after stimulation of the contratateral cortex. He proposed contribution of a
HYPOGLOSSAL
ACTIVITY
AND CHEWING
153
RHYTHM
common internuncial cell in and near the spinal trigeminal nucleus for the cortical excitatory pathway and the reflex pathway from the lingual nerve. Sumi [19] observed cortical excitation of hypoglossal nerve fibers with similar latencies in rabbits. However, they did not differentiate protrudor and retrusor motoneurons. In the present study, more retrusor motoneurons (21154) than the protrudor ones (7/47) were excited by stimulation of the contralateral cortex although both motoneurons showed a mean latency of 11.5 msec (7. I-16.9 msec) with no statistical difference. In most cases, the contralateral corticohypoglossal projection was investigated since cortical stimulation induced faster and stronger excitation of the contralatera1 hypoglossal nerve than that of ipsilateral one. Ohta and his co-workers [ lO,lS] reported a possibility of monosynaptic cortical projection to the contralateral trigeminal motoneurons with an antidromical conduction time of 4.0 msec in the rat’s cortico-motoneuronal pathway. If the rats’ cortico-hypoglossal excitation were induced by a monosynaptic pathway of the same conduction velocity as the cortico-trigeminal pathway, the conduction time for the cortico-hypoglossal pathway might be 5.0 msec, because the conduction distances of the former and the latter were estimated to be 25 mm and 20 mm, respectively. The mean onset latencies of the cortically induced EPSPs and action potentials of contralateral jaw opening trigeminal motoneurons were 4.6kO.15 msec [IO] and 8.9kO.4 msec (Ohta, personal communication), respectively. The mean onset latency of cortical excitation of hypoglossal motoneurons was 11.5 msec and 2.6 msec longer than that of the cortico-trigeminal excitation after subtraction of both latencies. This value was 1.6 msec longer than the estimated difference in conduction times. Therefore, internuncial cells were thought to be necessary for cortical excitation of hypoglossal motoneurons in the rat as suggested by Porter [ 161 and Sumi [ 191. Amygdaloid control of hypoglossal motoneurons has not yet been reported except the author’s preliminary papers [5,6], although amygdaloid control of trigeminal motoneurons has been reported 17, 10, 14, 181. Hopkins and Holstege [3] found that the central amygdaloid nucleus sent the most dense descending terminals in the supratrigeminal region including the parabrachial nucleus and many terminals in and near the trigeminal main sensory and spinal tract nuclei, primarily in the ipsilateral side. In the present study, stimulation of the contralateral central amygdaloid nucleus excited more hypoglossal motoneurons with the shorter mean latency than that of the ipsilateral nucleus as in the case of jaw-opening trigeminal motoneurons [ 181. The supratrigeminal region as well as the trigeminal main sensory nucleus were found to send commissural fibers to the contralateral trigeminal motor nucleus [I I]. These regions as well as the trigeminal spinal tract nucleus may have the initial relay cell to contralateral hypoglossal motoneurons and the ipsilateral pathway is thought to need more relay cells than the contralateral side because of the longer onset latency. More retrusor motoneurons than pJotJudoJ ones were excited by amygdaloid stimulation and both motoneurons showed similar mean onset latencies, as in the case of cortical stimulation. Therefore, the central amygdaloid nucleus and the frontal cortex send contralaterally dominant and stronger excitatory projections to the retJUSOJ motoneurons than to the pJOtJUdOJ ones.
The relationship between the rhythmical jaw movements and the activity of hypoglossal motor fibers was initially investigated by Sumi [ 191. He found that the majority of hypoglossal fibers showed burst discharges in the closing phase, some discharged in the opening phase and others were inactive or active but unrelated to the rhythmical jaw movements. However, he did not differentiate the protrudor fibers and the retrusor ones. In the present study, the rhythmical jaw and tongue movements were induced by stimulation of the central amygdaloid nucleus or the frontal cortex and the tongue was observed to move forth and back fairly synchronous with jaw-opening and closing, respectively. The maximum activity of the medial branch and that of the lateral branch were thought to correspond with the maximum protrusion and the maximum retrusion, respectively, since the magnitude of mylohyoid nerve activity coincided with that of jaw-opening during the rhythmical movements. The maximum protrudor activity was found in the middle of the jaw opening phase and the maximum retrusor activity was in the middle or near the end of the jaw closing phase, induced by amygdaloid or cortical stimulation, respectively. Therefore, the rhythmical tongue movements are thought to temporally lead the rhythmical jaw movements in 7 to 9% of a period. This finding fairly corresponded with a study of tongue and jaw movements in the cat made by Morimoto and Rawamura [12]. They found that the maximum protrusion occurred at the middle of the jaw opening phase and the maximum retrusion occurred at the beginning ofjaw closing. The phenomena found in nerve branch recording were confirmed by single unit recording from the hypoglossal motoneurons. The majority of protrudor motoneurons maximally discharged in the opening phase with or without preopening activities and the majority of retrusor ones did maximally from the beginning to the middle of the closing phase. This earlier activation of protrudor motoneurons than retrusor ones was observed exclusively during the rhythmical jaw and tongue movements and the frontal cortex or the central amygdaloid nucleus exerts stronger excitation of the retrusor motoneurons than the protrudor ones with no difference in the onset latencies. Therefore, the earlier activity of the protrudor motoneurons than the retrusor ones is thought to be induced by the so-called “brain stem pattern generator” [l] other than the excitatory pathway from the frontal cortex or the central amygdaloid nucleus. Repetitive stimulation of the frontal cortex or the central amygdaloid nucleus possibly activates the brain stem pattern generator which may induce the earliest activation of the PJOtJUdOJ motoneurons and then sequential activation of the jawopening motoneurons, the retrusor and finally the jaw closing ones. The author thinks that the stimulus induced (driven) discharges of hypoglossal motoneurons were due to excitatory projections from the frontal cortex or the central amygdaloid nucleus and the rhythmical jaw movement Jelated discharges were due to activity of the “brain stem pattern generator.”
ACKNOWLEDGEMENTS The author wishes to thank Professor Masahiro Ohta for his encouragement and advice during this study. This investigation was partly supported by a special Grant-in-Aid for the author’s laboratory from the Ministry of Education, Science and Culture of Japan.
REFERENCES I. Dellow. P. G. and J. P. Lund. Evidence for central timing of rhythmical mastication. J Physid 215: l-13, 1971. 2. Ferrier, D. The Functio~7 off/w Brain. vol 23. London: Smith Elder and Co., 1886, pp. 260-262. 3. Hopkins, D. A. and C. Holstege. Amygdaloid projection to the mesencephalon pons and medulla oblongata in the cats. /<.rp Bruin Rrs 32: 52%547, 1978. 4. Kaada, B. R., P. Andersen and J. Jansen Jr. Stimulation of the amygdaloid nuclear complex in unanesthetized cats. Neuroloy?: 4: 48-64. 1954. 5. Kaku, T. The rhythmical jaw and tongue movements and activities of the hypoglossal motoneurons. .I Phvsiol Sot. Jpn 44: 172, 1982. 6. Kaku, T. and M. Ohta. Control of the rhythmical jaw and tongue movements by the central nervous system. .fn~r .I Ortrl Bid 24: Suppl 116, 1982. 7.Kawamura, Y. and S. Tsukamoto. Analysis of jaw movements from the cortical jaw motor area and amygdala. Jpn .I Phvyird lo: 471-488, 1960. 8. Kawamura, Y. and S. Tsukamoto. Neural descending pathways from the cortical jaw motor area and amygdaloid nucleus to jaw muscles. ./pn .I Physiol IO: 489-498, 1960. 9. Konig, J. F. P. and R. A. Klippel. The Rat Bruin. A Srcwotaxit
Atlas
of’ the Forehrairr
and Lnu,c’r Parts
of the Brclitl .\‘tc,u.
Baltimore: Williams and Wilkins, 1963. IO. Mishima, K., K. Sasamoto and M. Ohta. Amygdaloid or cortical facilitation of antidromic activity of trigeminal motoneurons in the rat. Camp Biochrm Plry.sio/ 73A: 355-3.59. 1982.
I I. Mizuno, nucleus
N. Projection fibers from the main sensory trigrminal and the supratrigeminal region. .I C‘ort7p NtT7rrrd 139:
457-472.
1970.
12. Morimoto. T. and Y. Kawamura. movements elicited by stimulation
Properties of tongue and JEW ofthe orbital gyrus in the cat.
Arch Oral Bid 18: 361-372. 1973. 13. Moriyama, Y.. T. Kaku and K. Sasamoro.
The innervation ot the tongue muscles by the medial and the lateral branches of the hypoglossal nerve in the rat. .I Ph.mio/ Sm Jp 44: 172, 1983. 14. Nakamura. Y. and Y. Kubo. Masticatory rhythm in intracellular potential of trigeminal motoneurons induced by stimulation or orbital cortex and amygdala in cats. Hrtrirr HIV 148: W-509.
197X. I.5 Ohta. M. motoneurons
and K. Sasamoto. in the rat. Crm7p
C‘ortrcaJ control
Bi(e
l7ctn
Physicd
of trigeminal 66A: 99-106.
1980.
I h.
Porter, R. Cortical actions on hypoglossal motoneurons in cats: .4 proposed role for a common internuncial cell. .I PhFGjl 193: 295-308. 1967. 17. Rioch. J. M. The neural mechanism of masticatton. J/r/ .I Phy.vio/ 10% 161176. 1934. IX. Sasamoto. K. and M. Ohta. Amygdaloid-induced jaw opening and facilitation or inhibition of the trigeminal motoneurons in the rat. C’omp Bioc,hc,m Physiol 73.4: 349-354, 1982. IV. Sumi. T. Activity in single hypoglo>sal fibers during cortically induced swallowing and chewing in rabbits. yflcarrv Arch 314: 329-346. 1970.
0361-9230184 $3.00 + .OO
Inc. Printed in the U.S.A.
Functional Differentiation of Hypoglossal Motoneurons During the Amygdaloid or Corticallv Induced Rhvthmical Jaw and Tongie Movements in the Rat TOSHIO KAKU’ Depurtment
of Physiology,
Faculty of Dentistry,
Received
Kyushu University 61, Fukuoka,
17 January
Jupan 812
1984
KAKU, T. Func~tionul di@rentiation of hypoglossal motonrunms during thr amygduloid or cortically induwd rhythmical ju,l, and tongue movrmcnts in the rat. BRAIN RES BULL 13(l) 147-154, 1984.-Activity of hypoglossal motoneurons was
studied in relation to the amygdaloid or cortically induced rhythmical jaw and tongue movements in the rat. Tongue protrudor and retrusor motoneurons were antidromically identified by stimulation of the medial and the lateral hypoglossal nerve branches, respectively. About 80% of protrudor motoneurons excited during the opening phase of the rhythmical jaw movements with or without prior excitation. Most of them did not respond to amygdaloid or cortical stimulation. Fiftythree percent of retrusor motoneurons were excited during the closing phase and 3 1% discharged around the peak opening. The majority of them and a few protrudor motoneurons responded to amygdaloid or cortical stimulation with similar latencies. Activities of the medial and the lateral hypoglossal branches corresponded with the discharge patterns of respective motoneurons. Tongue protrusion and retrusion appeared to slightly precede jaw opening and closing, respectively during the rhythmical jaw and tongue movements. This was probably due to activity of the brain stem pattern generator. Hypoglossal motoneuron
Cortex
Rhythmical jaw movements
Central amygdaloid nucleus
from the beginning of the jaw closing to the middle of the jaw opening phase and the tongue retrusion did from the middle of the jaw opening phase to the beginning of the jaw closing. Moreover, amygdaloid control of hypoglossal motoneurons has not been reported except the author’s preliminary papers [_5,6]. The present study reports the differential activities of protrudor and retrusor motoneurons in relation to the rhythmical jaw movements induced by stimulation of the frontal cortex or the ipsilateral or the contralatera1 central amygdaloid nuclei. curred
FERRIER [2] initially found that repetitive electrical stimulation of the lateral frontal cortex induced the rhythmical jaw movements like mastication in rabbits. Kaada et ul. [4] re-
ported the stimulation of the anteromedial nuclei of amygdala also induced the chewing movement in cats. Rioch [ 171proposed a hypothesis of peripheral patterning of the rhythmical jaw movements by interaction of cortical jaw opening influence and closing reflex activity. Sumi [ 191 initially suggested that chewing rhythm was generated by the chewing center in the brain stem, which was activated by the higher brain structures and/or orofacial afferents in rabbits. Soon after him, Dellow and Lund [I] proposed a hypothesis of the brain stem pattern generator explaining the rhythmical jaw movements and this has been now generally accepted. Cortical control of the cat’s hypoglossal motoneurons was initially found by Porter [ 161. Cortical excitation of the rabbit’s hypoglossal nerve fibers was reported by Sumi [ 191, who demonstrated the single hypoglossal fiber activity during the rhythmical jaw movement and found that the majority discharged in the jaw closing phase of the rhythmical jaw movement. Morimoto and Kawamura [12] observed the rhythmical tongue movement during the rhythmical jaw movement in cats, and found that the tongue protrusion oc-
METHOD
Forty-five adult rats (Wistar King A, 200-300 g) were used under urethane anesthesia (0.9-I. 1 g/kg, IP). Bipolar silver wire electrodes with the interpolar distance of 1.5 mm insulated with acrylic resin except for the tips, were attached to the right medial and lateral branches of the hypoglossal nerve to identify each motoneuron as well as to record the nerve action potentials. In some cases, a similar electrode was also attached to the mylohyoid nerve for recording its action potentials. The rat’s head was fixed on a stereotaxic apparatus
‘Requests for reprints should be addressed to T. Kaku at the present address: Department of Prosthetic Dentistry II, Faculty of Dentistry, Kyushu University 61. Fukuoka. Japan 812.
147
14X
P-Mtn
R-Mtn
Rec.
0.3mm c-Am
i - Am
-~
lmm
FIG. I. Recording site (Rec.) in hypoglossal nucleus and site of central amygdaloid stimulation (Stim.). Upper left and right show the recording site of P-Mtn and R-Mtn, respectively (arrow), stained with pontamine sky blue. The former is situated in the ventral half of hypoglossal nucleus and the latter, in the dorsal half. Lower left and right show the site of contralateral and ipsilateral central amygdaloid (c-Am and i-Am. respectively) stimulation (arrow) by passing current (100 PA, IO set) after the experiment.
(Narishige, model SN-2). Bilateral central amygdaloid nuclei and the frontal cortex were electrically stimulated through concentric bipolar stainless steel electrodes which had tip separations of 0.5 mm for the amygdaloid nuclei and of 1.0 mm for the cortex. Cortical stimulation was applied to the most effective site for jaw movements according to Ohta and Sasamoto [ 151 and the amygdaloid electrodes were stereotaxically oriented (A 6.2, L 4.1, H -2.7, slightty modified from the map by Kiinig and Klippel [9]). The electrode positions were histologically verified after each experiment and the data from inaccurate etectrode positions were discarded (Fig. 1 tower). Stimulus parameters were square waves of 0.1 msec duration and 0.5 to 1.5 mA intensity which was adjusted to slightly exceed the threshold of rhythmical jaw and tongue movements at the most effective frequency of 25 to 40 Hz. The lowest threshold for the jaw movements was observed when the electrode position was found at the center of the central amygdaloid nucleus and the stronger stimulation was necessary for the threshold of jaw movements when the electrode position was the more distant from the nucleus. Activity of single hypoglossal motoneurons was recorded extracellularly through glass microelectrodes filled with 2 M NaCl or 0.5 M Na acetate containing pontamine sky blue (DC resistance of about 5 to 10 MR). The latter was used to verify the recording site by passing current through the re-
cording microelectrode (Fig. I upper). Jaw movements uerc monitored with a displacement transducer which was connected to the steel bar tightly glued to mandibular incisors. One mm opening of the rat’s jaw was made by 0.0 g force of depressing the mandibular incisors. Some of the rhythmical jaw movements and the nerve action potentials were stored in a data recorder (TEAC. model R-61) and averaged or intcgrated through an on-tine computer (Signal processor. San-ei model 7TO8). The body temperature was kept around 36OC during the experimen1. RESUI
1S
Protrudor and retrusor motoneurons were antidromicalty identified by stimulation of the medial and lateral branches of the hypoglossal nerve. respectively, since the medial and lateral branches were preliminarily confirmed to innervate the rat’s ipsilateral protrudor muscles and retrusor ones. respectively [ 131. Criteria for the antidromic identification of these neurons were as follows: “all or none” type appearance of discharge with a constant short latency. and the capability of action potentials following high frequency repetitive stimulation exceeding 400 Hz. Both protrudor and
HYPOGLOSSAL
C
c-Am.
ACTIVITY
AND CHEWING
c-Am.
ctx.
i-Am.
ctx.
RHYTHM
149
spike with an antidromic latency of 1.03 msec in response to the stimulation of the medial branch. The antidromic spike, however, was reduced by the superimposed antidromic field potential of the other motoneurons. This motoneuron excited following to triple shocks of the contralateral central amygdaloid nucleus with a latency of 13.9 msec (Fig. 2A middle) and also responded to triple shocks of the cortex with 11.3 msec latency (Fig. 2A right) but did not respond to stimulation of the ipsilateral central amygdaloid nucleus which was not shown in the figure. Figure 2B shows similar responses of a retrusor motoneuron. The antidromic action potential was evoked by the stimulation of the lateral branch with a latency of 0.9 msec. This motoneuron responded to double shocks of the contralateral central amygdaloid nucleus (Fig. 2B middle) and the frontal cortex (Fig. 2B right) with latencies of 7.7 msec and 13.1 msec, respectively. This cell did not respond to the stimulation of the ipsilateral central amygdaloid nucleus. Figure 2C shows another retrusor motoneuron. This motoneuron responded to the stimulation delivered to all three. Double shocks stimulation of the contralateral central amygdaloid nucleus, the ipsilateral one and the cortex induced action potentials with the latencies of 10.4 msec, 10.5 msec and 12.7 msec, respectively. Effect of double or triple pulse stimulation of the cortex, the ipsilateral or the contralateral central amygdaloid nucleus were summarized in Table 1, in which the upper table (A) shows the number of protrudor motoneurons and the lower one (B) that of retrusor motoneurons. The upper row shows the number of motoneurons which were excited by cortical stimulation and the lower row shows the number of motoneurons that did not respond to the cortical stimulation. Thirty-seven protrudor motoneurons (7%) out of 47 did not respond to any stimulation (Ctx, i-Am and c-Am). Among the remaining 10 protrudor motoneurons, 7 motoneurons responded to cortical stimulation and eight did to contralateral amygdaloid stimulation, and four did to ipsilateral amygdaloid stimulation, although some of them responded to stimulation of two or all three sites. On the other hand,
positive
5 nl**c
FIG. 2. An example of the electrical activity of protrudor and retrusor motoneurons. A. Left, antidromic discharge of a protrudor motoneuron appeared at fixed short latency after stimulation of medial branch (Med). Middle and right, driven discharge by stimulation of contralateral central amygdaloid nucleus (c-Am, 3 msec interval, arrow) and contralateral cortex (Ctx, 2 msec interval. arrow), respectively. B. Left, antidromic response of retrusor motoneurons to stimulation of lateral branch (Lat). Middle and right show driven discharge after amygdaloid (c-Am, arrow) and cortical (Ctx, arrows) stimulation, respectively. C. Retrusor motoneuron. Left, middle and right show driven discharge after stimulation of contralateral (cAm), ipsilateral amygdaloid nuclei (i-Am) and contralateral cortex (Ctx). Downward square wave pulse shows calibration of 1 mV. Note faster sweep speed in A left and B left, shown in A left, and slower sweep for others shown in C right. Upward negative in this and following figures.
retrusor motoneurons scarcely showed spontaneous discharge. Effect of double or triple pulses stimulation of the contralateral frontal cortex, the ipsilateral or the contralatera1 central amygdaloid nucleus was examined on every motoneuron. The contralateral cortical influence was exclusively investigated in this study since the cortical excitation of hypoglossal nerve was preliminarily confirmed to be contralaterally dominant. Figure 2A shows an example of the electrical activity of a protrudor motoneuron. This motoneuron showed a large
TABLE
1
NUMBER OF HYPOGLOSSAL MOTONEURONS EXCITED OR NOT
BY TRAIN
PULSE STIMULATION OF THE FRONTAL CORTEX OR CENTRAL AMYGDALOID NUCLEUS
c-Am: Ex
i-Am: Ex c & i-Am: Ex Am: No Ex
A P-Mtn (Total: 47) Ctx: Ex Ctx: No Ex
4 (8.5%) 2
I 1
2 0
0 37 (79%)
6 (11%) 2
20 (37%)
B R-Mtn (Total: 54) Ctx: Ex Ctx: No Ex
I4 (26%) IO (18.5%)
0
I
1
A. Protrudor motoneurons (P-Mtn). Upper row shows number of motoneurons excited by cortical stimulation, which responded or did not (No Ex) to stimulation of the contralateral amygdaloid nucleus (c-Am), the ipsilateral one (i-Am) or both (c & i-Am). Lower row shows number of motoneurons not excited by cortical stimulation which responded or did not to amygdaloid stimulation. B. Retrusor motoneurons (R-Mtn). Same tabulation as in A.
TABLE 2 MEAN LATENCY OF INITIAL DISCHARGE OF P-Mm (LEFT) R-Mtn (RIGHT) AFTER INITIATION OF CORTICAL OR AMYGDALOID STIMULATION
P-Mm Ctx
R-f&l
I1 .s ?. 0.4
I I.5 + 0.3 12.2 _+ I.0 10.3 t 1.0
i-Am c-Am
AND
11.4 It 1.0 10.4 rt 0.4
(mean i S.E.: msec.) Abbreviations are the same as Table I. Mean and its SE fmscc).
after initiation of train pulse stimulation of the cortex. the ipsilateral or the contralateral central amygdabid nuclei. As seen in the table, no statistically significant difference war; detected between the respective response latencies of the protrudor and the retrusor motoneurons to stimulation of any regions. However, both protrudor and retrusor motoneurons showed shorter response latencies to stimulation of contralateral central amygdaloid nucleus than to stimulation of the other.
Spontane~~us movements of the jaw or the tongue were observed unless the rat wa\ waking up from the anesthesia. When repetitive stimulation was applied to the cortex or the central amygdaloid nucleus, the rhythmical jaw and tongue movements were observed. The jaw was situated at the jaw resting position before the amygdatoid or cortical stimulation was applied. Slightly after the beginning of the stimulation the jaw opened to the peak and then closed to the initial jaw resting position. These open-close movements were repeated during and slightly after the stimulation. On the other hand, the tongue moved forth and back fairly synchronous with jaw opening and closing. respectively. To observe the relationship between the rhythmical jaw movements and the activity of the hypoglossal motoneur(~ns. the discharge patterns of both motoneurons were classified in relation to rhythmical jaw movements as shown in Figs. 3. ? not
thirty-four retrusor motoneurons (63%) out of 54 responded to stimulation of at least one of the three sites. Twenty-one motoneurons responded to cortical stimulation, and 32 did to contralateral amygdaloid stimulation and nine did to ipsilatera1 amygdaloid stimulation, although some of them responded to stimulation of two or three. Comparing the excitatory influence of bilateral central amygdaloid nuclei on hypoglossal motoneurons, both protrudor and retrusor motoneurons received stronger excitatory inputs with slightly shorter latencies from the contralate~l central amygdaloid nucleus than from the ipsilateral nucleus. Table 2 shows the mean latency of the initial response of protrudor motoneurons (left) or retrusor motoneurons (right)
and Table 3.
TABLE 3 TYPE OF DISCHARGE
-
PATTERN CLASSiFfED
IN RELATION
--
‘I-0 RHYTHMICAL
JAW
MOVEMENIS (RJM)
P-Mtn St.-site Discharge Pattern
Ctx
~-Ant c-Am --P-
Closing
Opening non driven
non driven
driven
driven opening-i Opening-peak
Pre- & Opening
16
non driven driven
Pre-opening
Opening-I
Other
Other
RJM unrelated
RJM unrelated
Total
‘l”otal
-p iI
I1
t4
51
5I
HYPOGLOSSAL
ACTIVITY
AND CHEWING
151
RHYTHM
P -Mtn
+ i-Am. 6.
*
,I
,I
‘1,
+ ctx. C.
Ill\
-
_ O.Z
FIG. 3. An example of discharge pattern of protrudor motoneuron (P-Mtn) in relation to rhythmical jaw movement (RJM). A. Discharge during opening phase of RJM induced by stimulation of contralateral amygdaloid nucleus (c-Am, 25 Hz). This discharge pattern was classified as Opening-nondriven type. Lower trace indicates jaw position and downward deflection corresponds to jaw opening in this and subsequent figures. B. Discharge driven by cortical stimulation (Ctx. 25 Hz) and additional discharge during opening phase of RJM. This was classified as Opening-driven type. C. Occasional driven discharge after c-Am stimulation (20 Hz) and facilitation of discharge before and during opening phase of RJM. This pattern was classified as Pre & Opening type. D. Discharge before opening phase of RJM and this classified as Preopening type. Note exclusive discharge shortly before jaw opening during and after i-Am stimulation (25 Hz). Middle trace shows mylohyoid nerve discharge during RJM. Downward arrow indicates the beginning of the repetitive stimulation and upward arrow does the end of the stimulation in this and subsequent figures.
Discharge patterns of protrudor motoneurons were classified into the Opening type, the Pre & Opening type, the Pre-opening type and the rhythmical jaw movements (RJM) unrelated type. The Opening type is characterized by burst discharges during the jaw opening phase of the rhythmical jaw movements. The Opening type is further divided into following two subtypes; the Opening-nondriven type (Fig. 3A) and the Opening-driven type (Fig. 3B). The Opening-nondriven type motoneurons discharged only during the jaw opening phase (Fig. 3A). Neurons of the Opening-driven type responded mostly to each of repetitive stimuli and showed additional burst discharges in the jaw opening phase (Fig. 3B). Discharge pattern of the Pre & Opening type resembled that of the Opening-driven type, but no discharge was observed during the jaw closing phase (Fig. 3C). In other words, motoneurons of this type mostly responded to each of repetitive stimuli during the jaw resting period before initiation of jaw movement. When the rhythmical jaw movements were induced, they discharged in bursts during the opening phase, became silent during the closing phase and responded mostly to each stimulus in the pre-opening phase. Motoneurons of the Pre-opening type mostly responded to each of repetitive stimuli during the jaw resting period before initiation of the jaw movement and when the rhythmical jaw movements were induced, they discharged only before each opening phase (Fig. 3D). In Fig. 3D, action potentials of the mylohyoid nerve were also recorded and the mylohyoid nerve activity coincided with the amplitude of rhythmical jaw opening. This is quite reasonable since the mylohyoid nerve innervates two jaw opening muscles. the mylohyoid and the anterior digastric ones. The
FIG. 4. An example of discharge pattern of retrusor motoneuron
(R-Mtn) in relation to RJM. A. Discharge during opening phase of RJM induced by c-Am stimulation (33 Hz). This pattern was classified as Closing-nondriven type. B. Discharge mostly driven by i-Am stimulation (33 Hz) and additional discharge during closing
phase of RIM. This was classified as Closing-drive type. C. Similar discharge pattern as in B except driven discharge appeared to be inhibited during opening phase of RJM induced by Ctx stimulation (25 Hz). This was classified as Closing-opening-f type. D. Discharge around peak opening of RJM induced by c-Am stimulation (33 Hz). This was classified as Opening-peak type. E. Discharge mostly driven by i-Am stimulation (25 Hz) and appeared to be inhibited during opening phase of RJM. This classified as Opening-l type.
rhythmical jaw movements unrelated type showed no discharge during the rhythmical jaw movements and did not respond to any stimulation (Ctx, i-Am and c-Am). The example of this type was not shown in the figure. of retrusor motoneurons were Discharge patterns classified into the Closing type, the Opening-peak type, the Opening-I type and the rhythmical jaw movements unrelated type. The Closing type is characterized by burst discharges during the jaw closing phase of the rhythmical jaw movements. This type is further divided into three subtypes: the Closing-nondriven type, the Closing-driven type, and the Closing-opening-I type. Motoneurons of the Closingnondriven type discharged only during the jaw closing phase (Fig. 4A). However, motoneurons of the Closing-driven type mostly responded to each of repetitive stimuli and showed additional burst discharges in the jaw closing phase (Fig. 4B). Motoneurons of the Closing-opening-1 type mostly responded to each of the repetitive stimulation, and showed additional burst discharge during the jaw closing phase, but they were silent during the jaw opening phase. Their driven discharges appeared to be inhibited during the jaw opening phase (Fig. 4C). The discharge pattern of the Opening-peak type is characterized by the burst discharges around the opening peak of the rhythmical jaw movements and this type is further divided into two subtypes: the Opening-peaknondriven and the Opening-peak-driven types. Motoneurons in the Opening-peak-nondriven type do not respond to each of repetitive stimulation (Fig. 4D) and motoneurons in the opening-peak-driven type respond to each stimulation. The discharge pattern of the Opening-I type resembles that of the Closing-opening-I type, but no burst discharges were seen in this type neurons. They mostly responded to each of repetitive stimuli during the jaw closing phase and jaw resting period. However, they are silent and appeared to be inhibited during the jaw opening phase (Fig. 4E). Motoneurons of the rhythmical jaw movements unrelated
152
KAKll c-Am.
(20 HZ.~
A. lO.bmV 2mm t open 0.2
lrmv asmm
I
/
*PC”
0.1 sec.
FIG. 5. Hypoglossal nerve action potential in relation to RJM induced by Ctx stimulation. A. Upper trace recorded from medial branch (Med) innervating protrudor muscle. Middle, from lateral branch (Lat) innervating retrusor. Lower, jaw position and downward opening as in Figs. 2 and 3. B. Averaged recording from A during 10 cycle of RJM. Upper from medial branch, middle from lateral branch and lower from jaw movement. Calibration of sweep speed and jaw movement are shown in C. C. Nerve action potential was integrated with time constant of 10 msec and added in sum of 10 cycle of RJM. Jaw movement averaged in same way as in B. Maximum activity of medial branch occurred in mid-opening phase and that of lateral branch near end of closing phase.
type showed no discharge during the jaw movements and did not respond to any stimulation (Ctx, i-Am and c-Am). Table 3 summarized classification of 53 protrudor and 5 1 retrusor motoneurons of which discharge patterns were examined during the rhythmical jaw movements induced by stimulation of all three sites, bilateral central amygdaloid nuclei and the cortex. Most of them were classified into a single type although some were classified into two different types induced by stimulation of different regions. Forty-three (about Sl%) of 53 protrudor motoneorons showed jaw movements related discharges although most of them were not excited by short train pulse stimulation of any stimulus sites (Ctx, i-Am and c-Am). Twenty-two (about 42%) protrudor motoneurons were classified into the Opening type, and 16 {about 30%) were into the Pre & Opening type. Only four motoneurons were in the Pre-opening type. Some protrudor motoneurons showed jaw movements related discharges but could not be classified into above three types. A few discharged only at the end of jaw closing, a few did at the peak of jaw opening and after the end of jaw closing and others discharged during the jaw opening phase and at the end of jaw closing. Forty-seven (about 9%) retrusor motoneurons showed the jaw movements related discharges. A majority of them (27 of 51) were classified into the Closing type. Sixteen (about 31%) were in the Opening-peak type. A few were in the Opening-I type. Some retrusor motoneurons showed the jaw movements related discharges but could not be classified into three types. The discharge pattern of two of them resembled that of the Opening type of protrudor motoneurons. Another neuron discharged both at the beginning of the jaw opening and during the jaw closing phase. No statistically significant difference in antidromic latencies was found among any of these groups of hypoglossal motoneurons.
FIG. 6. Nerve action potential in relation to c-Am induced RJM in same way as in Fig. 5. A. Upper from medial branch (Med). middle from lateral (Lat) and lower show jaw movement. B. Averaged recording from A. Upper from medial branch, middle from lateral one and lower from jaw movement. C. Integrated response as in Fig. 5C. Maximum action potential of medial branch in mid-opening phase and that of lateral branch in mid-closing phase.
The effects of the repetitive stimulation were also examined for relationship between action potentials of the hypoglossal nerve branches and the rhythmical jaw movements as shown in Figs. 5 and 6. In these figures, the upper traces showed recordings from the medial branches, the middle did those from the lateral branches and the lower did recordings of jaw movements. Figure 5 shows the rhythmical jaw movements (about 3.5 Hz) elicited by repetitive stimulation of the frontal cortex (25 Hz) and the action potentials of the medial and the lateral branches well synchronized with the jaw movement (Fig. SA). The action potentials from both branches during 10 cycles of the rhythmical jaw movements were averaged through the on-tine computer which was triggered by the beginning of jaw opening (Fig. 5B). The medial branch discharged mostiy during the opening phase and the lateral did during the closing phase. Then the positive components of the action potentials were integrated with a time constant of 10 msec and added in sum of lO_cycles of the jaw movements (Fig. 5C). The maximum integrated activity of the medial branch was found in the middle of the opening phase and that of the lateral one was near the end of the closing phase. Figure 6 shows the rhythmical jaw movements (about 3 Hz) induced by repetitive stimulation of the contralateral central amygdaloid nucleus (c-Am, 20 HZ) and the nerve action potentials in another rat as in Fig. 5. In this case, the maximum activity of the medial branch was found in the middle of the opening phase and that of the lateral branch was in the middle of the closing phase, These activities of the medial and the lateral branches corresponded very well with the discharge patterns of protrudor and retrusor motoneurons. DISCUSSK)N
Cortical control of the hypoglossal motoneurons in cats was initially reported by Porter [ 163 who demonstrated EPSPs with onset latency of 6 to 12 msec after stimulation of the contratateral cortex. He proposed contribution of a
HYPOGLOSSAL
ACTIVITY
AND CHEWING
153
RHYTHM
common internuncial cell in and near the spinal trigeminal nucleus for the cortical excitatory pathway and the reflex pathway from the lingual nerve. Sumi [19] observed cortical excitation of hypoglossal nerve fibers with similar latencies in rabbits. However, they did not differentiate protrudor and retrusor motoneurons. In the present study, more retrusor motoneurons (21154) than the protrudor ones (7/47) were excited by stimulation of the contralateral cortex although both motoneurons showed a mean latency of 11.5 msec (7. I-16.9 msec) with no statistical difference. In most cases, the contralateral corticohypoglossal projection was investigated since cortical stimulation induced faster and stronger excitation of the contralatera1 hypoglossal nerve than that of ipsilateral one. Ohta and his co-workers [ lO,lS] reported a possibility of monosynaptic cortical projection to the contralateral trigeminal motoneurons with an antidromical conduction time of 4.0 msec in the rat’s cortico-motoneuronal pathway. If the rats’ cortico-hypoglossal excitation were induced by a monosynaptic pathway of the same conduction velocity as the cortico-trigeminal pathway, the conduction time for the cortico-hypoglossal pathway might be 5.0 msec, because the conduction distances of the former and the latter were estimated to be 25 mm and 20 mm, respectively. The mean onset latencies of the cortically induced EPSPs and action potentials of contralateral jaw opening trigeminal motoneurons were 4.6kO.15 msec [IO] and 8.9kO.4 msec (Ohta, personal communication), respectively. The mean onset latency of cortical excitation of hypoglossal motoneurons was 11.5 msec and 2.6 msec longer than that of the cortico-trigeminal excitation after subtraction of both latencies. This value was 1.6 msec longer than the estimated difference in conduction times. Therefore, internuncial cells were thought to be necessary for cortical excitation of hypoglossal motoneurons in the rat as suggested by Porter [ 161 and Sumi [ 191. Amygdaloid control of hypoglossal motoneurons has not yet been reported except the author’s preliminary papers [5,6], although amygdaloid control of trigeminal motoneurons has been reported 17, 10, 14, 181. Hopkins and Holstege [3] found that the central amygdaloid nucleus sent the most dense descending terminals in the supratrigeminal region including the parabrachial nucleus and many terminals in and near the trigeminal main sensory and spinal tract nuclei, primarily in the ipsilateral side. In the present study, stimulation of the contralateral central amygdaloid nucleus excited more hypoglossal motoneurons with the shorter mean latency than that of the ipsilateral nucleus as in the case of jaw-opening trigeminal motoneurons [ 181. The supratrigeminal region as well as the trigeminal main sensory nucleus were found to send commissural fibers to the contralateral trigeminal motor nucleus [I I]. These regions as well as the trigeminal spinal tract nucleus may have the initial relay cell to contralateral hypoglossal motoneurons and the ipsilateral pathway is thought to need more relay cells than the contralateral side because of the longer onset latency. More retrusor motoneurons than pJotJudoJ ones were excited by amygdaloid stimulation and both motoneurons showed similar mean onset latencies, as in the case of cortical stimulation. Therefore, the central amygdaloid nucleus and the frontal cortex send contralaterally dominant and stronger excitatory projections to the retJUSOJ motoneurons than to the pJOtJUdOJ ones.
The relationship between the rhythmical jaw movements and the activity of hypoglossal motor fibers was initially investigated by Sumi [ 191. He found that the majority of hypoglossal fibers showed burst discharges in the closing phase, some discharged in the opening phase and others were inactive or active but unrelated to the rhythmical jaw movements. However, he did not differentiate the protrudor fibers and the retrusor ones. In the present study, the rhythmical jaw and tongue movements were induced by stimulation of the central amygdaloid nucleus or the frontal cortex and the tongue was observed to move forth and back fairly synchronous with jaw-opening and closing, respectively. The maximum activity of the medial branch and that of the lateral branch were thought to correspond with the maximum protrusion and the maximum retrusion, respectively, since the magnitude of mylohyoid nerve activity coincided with that of jaw-opening during the rhythmical movements. The maximum protrudor activity was found in the middle of the jaw opening phase and the maximum retrusor activity was in the middle or near the end of the jaw closing phase, induced by amygdaloid or cortical stimulation, respectively. Therefore, the rhythmical tongue movements are thought to temporally lead the rhythmical jaw movements in 7 to 9% of a period. This finding fairly corresponded with a study of tongue and jaw movements in the cat made by Morimoto and Rawamura [12]. They found that the maximum protrusion occurred at the middle of the jaw opening phase and the maximum retrusion occurred at the beginning ofjaw closing. The phenomena found in nerve branch recording were confirmed by single unit recording from the hypoglossal motoneurons. The majority of protrudor motoneurons maximally discharged in the opening phase with or without preopening activities and the majority of retrusor ones did maximally from the beginning to the middle of the closing phase. This earlier activation of protrudor motoneurons than retrusor ones was observed exclusively during the rhythmical jaw and tongue movements and the frontal cortex or the central amygdaloid nucleus exerts stronger excitation of the retrusor motoneurons than the protrudor ones with no difference in the onset latencies. Therefore, the earlier activity of the protrudor motoneurons than the retrusor ones is thought to be induced by the so-called “brain stem pattern generator” [l] other than the excitatory pathway from the frontal cortex or the central amygdaloid nucleus. Repetitive stimulation of the frontal cortex or the central amygdaloid nucleus possibly activates the brain stem pattern generator which may induce the earliest activation of the PJOtJUdOJ motoneurons and then sequential activation of the jawopening motoneurons, the retrusor and finally the jaw closing ones. The author thinks that the stimulus induced (driven) discharges of hypoglossal motoneurons were due to excitatory projections from the frontal cortex or the central amygdaloid nucleus and the rhythmical jaw movement Jelated discharges were due to activity of the “brain stem pattern generator.”
ACKNOWLEDGEMENTS The author wishes to thank Professor Masahiro Ohta for his encouragement and advice during this study. This investigation was partly supported by a special Grant-in-Aid for the author’s laboratory from the Ministry of Education, Science and Culture of Japan.
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