FXPER~MENTAL

NEUROLOGY

Excitatory

and

and

62, 30-47 (1978)

Inhibitory

Adjacent

Inputs

Reticular

to Hypoglossal

Formation

Neurons

Motoneurons in Cats

ALAN A. LOWER Division

of Biological

Sciences, Faculty of Dentistry, University Ontario, Canada, M5G lG6

of

Toronto,

Toronto,

Received May

23,1978

To elucidate some of the brain stem mechanisms involved in tongue motility, extracellular microelectrode recordings were made from single neurons in the region of the hypoglossal nucleus in 10 decerebrate and 23 anesthetized (chloralose) adult cats. The antidromic response characteristics and the synaptically evoked responses of 71 motoneurons that supplied tongue protrusive (P) or retrusive (R) muscles were documented. Protrusive motoneurons could be synaptically excited by temporomandibular joint (TMJ), glossopharyngeal (IX), and/or superior laryngeal (SLN) nerve stimuli, whereas R motoneurons could be activated by lingual and/or IX nerve stimulation. Conditioning effects revealed that the inhibition of the antidromic responses was shorter in duration than the inhibitory effects noted when synaptically evoked responses were conditioned. Conditioning stimuli delivered to the lingual, TMJ, IX, and SLN nerves were most effective in inhibiting the synaptically evoked responses of P and R motoneurons for conditioning-test intervals of as much as 400 ms. Those conditioning stimuli which also could synaptically activate a motoneuron tended to facilitate the cell’s synaptically evoked responses at conditioning-test intervals of about 10 ms, whereas conditioning stimuli which did not synaptically activate the cell resulted in only the long-lasting inhibition. Abbreviations : P-protrusive ; R-retrusive ; EPSP (IPSP)-excitatory (inhibitory) postsynaptic potential ; TM J-temporomandibular joint ; SLN-superior laryngeal ; IX-glossopharyngeal ; XII-hypoglossal. 1 The author wishes to thank Dr. B. J. Sessle, whose expertise and guidance were essential in this study. Financial support was obtained from a Canadian Medical Research Council Dental Training Grant awarded to the Faculty of Dentistry, University of Toronto. The author’s present address is Department of Orthodontics, Faculty of Dentistry, University of British Columbia, Vancouver, British Columbia, Canada, V6T lW5. 30 0014~4886/78/0621-0030$02.00/O Copyright All righta

0 1978 by Academic Press, Inc. of reproduction in any form reserved.

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INTRODUCTION Although the tongue is capable of highly organized and delicate movements, present neurophysiologic knowledge does not allow one to describe the mechanisms involved in the precisely coordinated events which occur. Previous studies characterized some of the neural events occurring within single hypoglossal (XII) motoneurons as a result of lingual (10, 13), glossopharyngeal (IX) (2)) and superior laryngeal (SLN) (26) nerve stimuli, and complex patterns of excitatory and inhibitory postsynaptic potentials (EPSP, IPSP) contributing to the excitatory and inhibitory effects of these stimuli were reported. With the exception of inferior alveolar (27), lingual (IO), and masseter (11, 27) nerve stimuli, there is no information available as to the effects of specific peripheral stimuli on identified P and R motoneurons, yet recent studies (6, 7, 16) indicated profound influences on tongue electromyographic activity in response to TMJ, IX, SLN, and dental nerve stimuli. This lack of differentiation of different functional types of XII motoneurons in past studies confuses the results obtained. If the effects of other specific stimuli on P and R motoneurons were known, our knowledge of the neural integration of the XII nucleus would be considerably enhanced. Thus, the purpose of this study was to delineate some of the differences in the responses of P and R motoneurons to various peripheral stimuli. The effects of stimulating lingual, TMJ, tooth pulp, infraorbital, IX, and SLN nerves, and forepaw (as a nonspecific stimulus) on P and R motoneurons were investigated. METHODS Thirty-three cats (2.2 to 5.0 kg) were initially anesthetized with sodium thiamylal to permit tracheal and vessel cannulations and were either maintained on a-chloralose (23 animals) or decerebrated electrolytically at the midcollicular level (10 animals). The infraonbital, lingual, IX, SLN, and XII nerves as well as the small afferents supplying the TMJ (17) were dissected out for subsequent stimulation. In addition, electrodes were placed on the medial and lateral branches of the XII nerve. The medial branch supplies primarily the genioglossus and intrinsic tongue muscles, whereas the lateral branch of the XII nerve innervates the retrusive muscles of the tongue musculature (10). In 15 animals, two deep cavities were cut into the dentine of the canine teeth, and an electrical stimulus was delivered by abipolar electrodes cemmented into the cavities (16). To investigate the possible role of nociceptive inputs, serrated forceps were used on the forep.aw and orofacial regions. Mechanical (innocuous) stimuii were delivered by a mechanical stimulator (18) to various orofacial regions. In some instances, a hand-held probe was used for mechanical stimulation of these regions. The caudal part of the brain stem was exposed

32

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by an occipital craniotomy and the caudal (5 mm) part of the cerebellum was carefully removed by suction to expose the medullary region for electrode penetration. When the dissection of the animal was completed, muscular relaxation was produced and maintained with gallamine triethiodide (Flaxedil, Pulenc, Ltd., Montreal, Quebec) and the animal was artificially res,pired for the remainder of the experiment. In all animals, blood pressure, expired percentage COZ, and rectal temperature were continuously monitored and maintained within normal physiological limits. Tungsten microelectrodes were used to record extracellularly in the XII nucleus from the cell bodies of P and R motoneurons innervating the tongue musculature. Neuronal responses were amplified, displayed on an oscilloscope, stored, and photographed as required. Motoneurons within the XII nucleus were identified by their antidromic response to low-intensity stimulation (0.1 to 5.0 mA, 0.1 ms) of the XII nerve and/or its medial or lateral branch. The threshold stimulus intensity for antidromic activation of a motoneuron was considered to be that intensity necessary to activate the motoneuron 10 times in 20 trials. The antidromic response was identified as a consistent, “all-or-none” response with a short latency (less than 3.0 ms) to stimulation rates of 100 to 200/s. In contrast, motor axons followed .stimulation rates of 330 to 500/s, and had a positive polarity. In seven cats, electrolytic lesions were placed for histological identification of approximate XII motoneuron sites within the nucleus, The brain stem was perfused with 10% formalin in saline and subsequently blocked in paraffin and lo-pm sections were cut prior to staining with gallocyanin. At the conclusion of nine experiments, the distance from the P and R stimulating electrode sites to the approximate recording site in the brain stem was measured so that the conduction velocity of XII motoneurons could be determined. The intracranial course of the XII nerve was estimated from measurements (corrected for shrinkage effects) taken from the histologic sections. For a small number (10) of P motoneurons, activity during natural (water-induced) swallowing was examined to determine if they participated in the synergy. Direct visual observation, together with nerve recordings from the motor component of the SLN nerve (24) and the P branch of the XII nerve, were used to monitor the swallow synergy. Swallows could be differentiated from respiratory, cough, or elementary reflexes on the basis of duration and sequential pattern of the efferent discharges recorded in the SLN and P nerves. For an additional 15 P motoneurons tested, the jaw was opened by pulling on a cord attached to the mandibular cuspid teeth in order to investigate the activity of these motoneurons during jaw opening.

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The effects of various conditioning stimuli could be tested on both the antidromically and synaptically evoked responses recorded extracellularly from XII motoneurons. An approximate 50% control level of firing to 20 P or R sti’muli (l/s) was obtained and a conditioning stimulus was then applied at various intervals (ms) preceding the test stimulus that activated the motoneuron. The time course of any effect was o’btained by comparing, at various conditioning-test intervals, the control number of spikes evoked by 20 test stimuli with the total elicited by a similar number of stimuli in the presence of a conditioning stimulus. Changes in excitability were reflected as an increase or decrease in the frequency of occurrence of the antidromically or synaptically evoked response of the XII motoneuron. All conditioning stimuli were delivered at a suprathreshold level (i.e., greater than that required to evoke an orofacial reflex response before the animal was paralyzed). Each trial consisted of an analysis of the conditioning effect of one peripheral stimulus on either the antidromically or synaptically evoked response of a P or R motoneuron. RESULTS The XII nucleus was identified on the basis of obtaining a primarily negative field potential (14, 25) in response to stimulation of the XII nerve or its medial (P) or lateral (R) branches. The XII nucleus extended between P 10 and P 14.5 and was found only in the penetrations made between 0.5 and 1.5 mm lateral to the midline. The nucleus was identified at a depth of approximately 1500 pm from the brain stem surface, and it extended 1000 pm vertically. Protrusive motoneurons were located in penetrations made 0.5 mm lateral to the midline, and in ,the deeper levels of the more lateral penetrations (e.g., 1.5 mm, Fig. 3) ; retrusive motoneurons were generally located in the dorsolateral section of the nucleus. Both P and R motoneurons appeared to be distributed along the entire rostrocaudal course of the nucleus. Protrusive

Motmeurons

Characteristics. Fifty-three cells were identified as P motoneurons on the basis of their antidromic response to stimulation of the P branch of the XII nerve (Fig. 1A). Hypoglossal nerve stimulation in the same animal often also excited the P motoneuron, but R (lateral branch) nerve stimulation did not. Histograms of the distribution of the conduction velocities an’d antidromic response latencies of the 53 P motoneurons are shown in Fig. 1B. The mean conduction velocity was 36.5 +- 8.7 m/s based on a mean conduction path of 39.4 mm (measured in nine cats). The mean antidromic latency was 1.2 * 0.3 ms, and the mean threshold for

34

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12 -

A

IO8 -

. . . . . . . . . . . . . . . TM,

-SLN ------.

,x

I

10 Latency 1 msec 1 2( I-

,

)’

0 10

20 30 Conduction

40 Velocity

so ( mlsec )

M)

0.4

0.6

0.8 1.0 1.2 1.4 1.6 1.8 Antidromlc latency ( msec b

2.0

FIG. 1. A-Histograms of the latency distribution of 30 protrusive (P) motoneurons’ responses to temporomandibular joint (TMJ), superior laryngeal (SLN), and glossopharyngeal (IX) nerve stimuli. The records reveal the antidromic responses of two P motoneurons to two successive threshold P nerve stimuli. The motoneuron on the left had an antidromic latency of 1.4 ms, an antidromic threshold of 0.4 mA, and a response to SLN nerve stimulation with a latency of 14.0 ms. In contrast, the motoneuron on the right had an antidromic latency of 1.5 ms, a threshold of 0.5 mA, and a SLN nerve-evoked response with a latency of 32.0 ms. Voltage calibration, 0.4 mV; time calibration, 8.0 ms for the cell on the left and 16.0 ms for the cell on the right.

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antidromic activation of the 53 P cells was 0.8 * 0.4 mA. Thirty of the P motoneurons tested revealed a synaptically evoked response to one or more of the peri’pheral stimuli. Except for tooth tap stimuli which activated two motoneurons, only TMJ, IX, and SLN nerve stimuli were effective in eliciting a response. Glossopharyngeal and TM J nerve-evoked responses were of a short latency, but SLN nerve-evoked responses were either of a short or long latency (Fig. 1A). Figure 1A also shows the distribution of these response latencies in P motoneurons with a predominant grouping of the latencies between 5.0 and 15.0 ms. However, six longer-latency responses were found between 25.0 and 35.0 ms with SLN nerve stimulation. The mean latency of the respouses of P motoneurons to TMJ nerve stimuli was 7.9 * 2.9 ms and that to IX nerve stimuli was 9.8 -+ 3.1 ms. The early response to SLN nerve stimulation had a mean latency of 10.5 -+ 2.1 ms, and the late response had a significantly longer (P < 0.01) mean latency of 26.7 * 2.6 ms. The six P motoneurons which had responses to SLN nerve stimuli with long latencies could be activated only by high-intensity stimulation. No motoneurons were found which fired at both long and short latencies in response to SLN nerve stimulation. Although not systematically tested, P motoneurons were also found to be active during respiration, jaw rotation, and swallowing. Eight of the motoneurons discharged in a burst at a rate of 15 to ZO/min even when the artificial respirator was briefly turned off in the paralyzed animal. This rate was similar to the respiratory rate seen before paralysis of the animal, and suggested that the activity of these P motoneurons may be related to respiration. Of 15 P motoneurons tested, 5 fired when the jaw was opened as little as 4.0 mm and continued to fire as long as the jaw remained open. It was also demonstrated that P motoneurons participated in the swallow synergy (water-induced). This was tested in 10 P motoneurons in decerebrate cats prior to administration of the muscle relaxant or after the muscle paralysis had been allowed to wear off. By initially identifying individual P motoneurons and then looking for their distinctive waveforms during a swallow, it was possible to confirm that at least 4 of the 10 P motoneurons participated in the swallow synergy. Conditioning Effects. A synaptically evoked response to any of the peripheral stimuli could not be demonstrated in 23 of the P motoneurons. However, conditioning effects on the antidromic responses of six of these cells were investigated. The antidromic responses of P motoneurons were inhibited by lingual conditioning stimuli at conditioning-test intervals of 10 to 60 ms. However, TMJ, IX, and SLN nerve conditioning stimuli

B-Histograms of the distribution of conduction latencies of 53 protrusive motoneurons.

velocities and antidromic

response

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. . . . . . . . . . . . . . . . TM,

-SLN .-----,x --mm-m-.Ling ---m-m-.Ling ------Control

Control VW

Ling(lOl w Ling(lOl w

Conditioning-Test

Interval

( msec 1

FIG. 2. The graph on the left portrays the time course of the changes in excitability of the antidromic firing of a protrusive (P) motoneuron in response to suprathreshold temporomandibular joint (TMJ), superior laryngeal (SLN), glossopharyngeal (IX), and lingual (Ling) nerve conditioning stimuli at various time intervals preceding the test stimulus to the P branch of the XII nerve that antidromically activated the motoneuron. All four conditioning stimuli inhibited the antidromic response at conditioning-test intervals to 60 ms, but TMJ and SLN nerve stimuli also initially facilitated the response. Five superimposed records of the threshold antidromic (Anti) response are shown on the right. A control level was obtained by setting the test stimulus level to the P nerve just below that required to produce antidromic activation of the motoneuron to show that suprathreshold SLN and TMJ nerve conditioning stimuli delivered 10 ms prior to the test stimulus facilitated the antidromic response, but Ling stimulation had no effect. All records are five superimposed traces. Voltage calibration, 0.4 mV; time calibration, 2.0 ms.

caused an initial facilitation that was followed by a short period of inhibition lasting from 20 to 60 ms (Fig. 2). The conditioning effects on a P motoneuron’s response to TM J, IX, and SLN nerve test stimuli were also investigated. The conditioning effects on P motoneurons’ responsesto SLN nerve stimuli were recorded in 34 trials. Conditioning stilmuli were delivered to the lingual (8 trials), TMJ (6 trials), and IX (11 trials) nerves. In addition, tooth pulp, tooth tap, infraorbital nerve, and forepaw conditioning stimuli were studied in nine trials. Similar consistent time courses of facilitatory and

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inhibitory responses were seen in all tests. Lingual nerve stimuli inhibited the SLN nerve-evoked response from conditioning-test intervals of 20 to 500 ms (see Figs. 3 and 4). However, in two of the eight trials with lingual nerve conditioning stimuli, a return toward the control firing level was seen (Fig. 4) at conditioning-test intervals of about 40 ms ; this return was temporary, as it was followed by a long-lasting inhibition (to 500 ms). stimuli inhibited the Temporomandibular joint nerve conditioning response of P motoneurons to SLN nerve stimulation in six trials ; in two of the trials, a facilitation preceded the inhibition. Similarly, IX nerve conditioning stimuli initially facilitated the SLN nerve-evoked response of P cells in 2 of 11 trials, but then inhibited the response for conditioningtest intervals of as much as 500 ms in all 11 trials. Forepaw stimuli were ineffective, but infraorbital nerve, tooth pulp, and tooth tap stimuli inhibited the SLN-evoked responses for as much as 200 ms. An example of the effects of various conditioning stimuli on a P motoneuron’s response to SLN nerve stimulation is shown in Fig. 3. Inhibition followed TMJ, lingual, and SLN nerve stimulation and persisted for 300 to 400 ms (Fig. 4). Similar conditioning effects on P motoneurons’ responses to TMJ and IX nerve stimuli were documented. A SLN or IX nerve conditioning stimulus produced a prolonged inhi,bition of a P motoneuron’s response to a TMJ nerve stimulus with an indication of a return toward the control level around conditioning-test intervals of 40 and 80 ms; on occasion, these conditioning stimuli also initially facilitated the responses. Lingual nerve conditioning stimuli inhibited both the TMJ and IX nerve-evoked responses of P motoneurons for as much as 400 ms ; tooth tap co,nditioning stimuli initially facilitated the responses tested, but this was followed by an inhibition lasting as much as 200 ms. Forepow stimulation had no effect. Retrusive

Motoneurons

Characteristics. The antidromic ch’aracteristics and synaptically evoked responses of 18 R motoneurons were examined. The mean conduction velocity was calculated to be 36.5 k 11.6 m/s based on a mean conduction path of 44.0 mm. The mean antidromic l.atency was 1.3 * 0.3 ms, and the mean threshold for antidromic activation of the R motoneurons was 1.7 * 0.9 mA. Of the 18 motoneurons identified, 12 could be synaptically activated and only by lingual and/or IX nerve stimuli. The mean latency of the responses to lingual nerve stimulation was 7.0 + 2.4 ms, and that to IX nerve stimulation was 8.6 +- 1.1 ms. Cojzditiouiry Effects. Although a synaptically evoked response to any of the peripheral stimuli could not be demonstrated in six of the R moto-

Lingl80)

V

TMJBO) --d&-mI

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MOTONEURONS

neurons, conditioning effects on the antidromic responses of two of these motoneurons were investigated. Lingual nerve conditioning stimuli initially facilitated the antidromic response at conditioning-test intervals of about

10 ms and then produced an inhibition lasting 60 ms. A similar early facilitation was also noted with IX nerve stimulation, and a subsequent inhibition was recorded at conditioning-test intervals of 10 to 60 ms. However, conditioning stimuli delivered to TMJ or SLN nerves resulted only in a period of inhibition of the antidromically elicited response of R motoneurons, and this occurred at conditioning-test intervals of 20 to 40 111s. None of the conditioning stimuli had an apparent effect at conditioning-test intervals larger than 60 111s. The conditioning effects on R motoneurons’ responses to IX or lingual nerve test stimuli were recorded in 27 trials. Temporomandibular joint,

IX,

and SLN nerve conditioning stimuli inhibited the response evoked

by lingual

nerve stimulation

at conditioning-test

intervals

of 10 to 400 ms.

However, in two trials, IX nerve conditioning stimuli also produced an initial facilitation at conditioning-test intervals of about 10 ms. On one occasion, the SLN nerve conditioning stimulus produced a return toward the control rate of firing at conditioning-test intervals of about 40 ms.

Temporomandibular joint, lingual, and SLN nerve conditioning stimuii inhibited

the IX

nerve-evoked

responses

at conditioning-test

intervals

of

10 to 400 ms. However, lingual nerve conditioning stimuli caused an initial facilitation (during conditioning-test intervals of less than 10 ms) in two trials. Forepaw stimulation appeared to have no effect whereas infraorbital responses

nerve and tooth pulp at conditioning-test intervals

conditioning stimuli inhibited the of 10 to 200 ms. Mechanical stimn-

FIG. 3. The effects of various conditioning stimuli on the response of a protrusive (P) motoneuron to superior laryngeal nerve stimulation. The motoneuron was 1.0 mm posterior to the obex, 1.5 mm lateral to the midline, at a depth of 3000 pm below the surface. The arrow on the histological section on the left approximates the motoneuron’s position within the hypoglossal (XII) nucleus. The XII nucleus is also shown in the histological section (X 5) on the right. The horizontal bar represents 1500 pm for the section on the left. The upper left record shows the P motoneuron’s threshold antidromic (Anti) response to two successive P nerve stimuli. The motoneuron was activated by superior laryngeal (SLN) nerve stimulation and fired spontaneously (record A). On a longer time base, bursts of activity were seeu in the paralyzed animal regardless of whether the respiratory pump was operating (B) or not (C) The records on the right indicate that the control level of firing of the SLN nerve-evoked response (five superimposed) was facilitated (decrease in latency) by the delivery stimulus. (IAg), ing-test

of a temporomandibular joint (TMJ) nerve stimulus 6.0 ms prior to the test However, a marked inhibition of the response was seen with TM J, lingual and glossopharyngeal (IX) nerve conditioning stimuli at an 80-ms conditioninterval. Voltage calibration, 0.2 mV; time calibration, 1.0 s for records B and

C, 4.0 ms for all other records; V, nucleus of the spillal tract of the trigcmiual

nerve.

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... ... ... .. .. ... TM, ._-____, x .-m-m-..Ling -------Control

--.I 100

-5.

.'

/'

200 Conditioning-Test

I

I

I

300 Interval ( msec I

4x

5ol

FIG. 4. Time course of the conditioning effects on a protrusive motoneuron’s response to superior laryngeal (SLN) nerve stimulation. The graph shows the time course of the conditioning effects on the SLN nerve-evoked response (control) of the protrusive motoneuron described in Fig. 3. Temporomandibular joint (TM J) nerve conditioning stimuli caused an initial facilitation of the response followed by an inhibition lasting to 400 ms. Lingual (Ling) nerve conditioning stimuli caused an initial inhibition followed by a slight return toward the control level at 40 ms and then a longer inhibition lasting more than 400 ms. Glossopharyngeal (IX) nerve conditioning stimuli also produced a marked inhibition of the response.

lation of the maxillary canine tooth inhibited the synaptically evoked responsesof R motoneurons at conditioning-test intervals of 20 to 400 ms. Adjacent Reticular Formation Neurons Many cells (68) at positions immediately lateral to and above and below the XII nucleus could not be identified as P or R motoneurons on the basis of antidromic response characteristics. However, these cells could be excited by several other stimuli ; an example of such a neuron is shown in Fig. 5. Th e most frequently occurring excitatory responses (and their mean latencies) were evoked by lingual (8.3 -t- 4.3 ms), infraorbital (8.9 -C4.8 ms), TMJ (11.6 + 4.4 ms), IX (9.1 * 4.0 ms), and SLN (11.3 2 5.6 ms) nerve stimulation. Other stimuli which activated these neurons included forepaw (15.5 + 3.3 ms), maxillary tooth pulp (13.3 f 5.7 ms), and XII (13.2 -+ 4.0 ms) nerves. Eighteen neurons dis-

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charged in rhythmic bursts at 15 to 2O/min. An additional 11 neurons fired spontaneouslyand did not appear to have an inherent rhythmicity. DISCUSSION The extent of the XII nucleus in the brain stem noted in this study generally agreed with previous anatomic (22) and physiologic (3) findings. The cellular sites of P and R motoneurons were in accordance with the earlier field potential investigations of XII nucleus organization (10). Previous investigators (18, 27) recorded from neurons with reticular formation characteristics adjacent to the XII nucleus which had V, IX, and SLN nerve inputs, and histological confirmation of projections from the dorsolateral reticular formation to XII motoneurons has been presented (28). The latencies (8 to 16 ms) of the neurons which could not be identified as P or R cells in this report suggest that these cells were activated by afferent pathways which involved several synapses. Their widespread input from sites which were shown in many instances to be effective in modulating XII motoneurons, plus their proximity to the XII nucleus, indicates their possible role as interneurons which might function in modulating XII nucleus activity. However, many of these cells may instead have a functional relationship with other brain stem nuclei or centers (e.g., solitary tract nucleus, V spinal nucleus, respiratory center, swallowing center). A previous study (27) did not report any difference in the mean antidromic latency of P and R motoneurons (1.05 ms) which is in agreement with the present report. The findings of conduction velocity are also compatibIe with those reported for XII motoneurons (14, 25) and with anatomical studies of the diameter of XII nerve fibers (1, 2.5). In light of previous investigations of tongue reflex and XII motoneuron activity in response to SLN nerve stimulation (16, 26)) it is not surprising that SLN nerve stimuli activated P motoneurons. An interesting observation in this report was that two different latencies were observed for the SLN nerve-evoked responses of P cells-one with a mean latency of 10.5 ms and a later one with a mean of 26.7 ms. Previous studies (16, 24) demonstrated a short latency response, but the longer-latency effects have not been previously reported, although late SLN nerve-induced EPSPs with an average latency of 30 ms in XII motoneurons have been described (26). The long-latency input may be related to the period of facilitation seen between the two periods of inhibition recorded in the genioglossus muscle in response to suprathreshold stimulation of the SLN nerve (6). It is also possible that high-threshold afferents (in contrast to low-threshold afferents discussed below) with different central pathways were activated and that they resulted in a longer-latency response

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in the XII nucleus. The early-latency SLN nerve response may be mediated by brain stem pathways (17), whereas the longer-latency SLN nerve response may involve the participation of higher centers or brain stem mechanisms initiated by slower conducting, high-threshold afferents. Low-threshold stimuli delivered to the IX nerve also elicited synaptically evoked responses in P and R motoneurons in agreement with reports of IX nerve-induced tongue reflex responses (1, 15), XII nerve activity (16, ZO), and XII motoneuron activation (2). The latency of the motoneuron responses suggests the involvement of a multisynaptic pathway, probably involving the solitary tract and reticular formation interneurons in a manner similar to that described above for SLN nerve responses. The activation of both P and R motoneurons by IX nerve stimuli may be related to the nerve’s innervation of the posterior tongue as well as the pharynx (21) with afferent information from each region having a different effect on P and R motoneurons. Because the entire IX nerve was stimulated in this study, it was not possible to distinguish the effects of stimulation of these two regions. The activation of P motoneurons by TMJ nerve stimulation provides central neuronal data in support of the protrusive tongue activity related to jaw opening in man and experimental animals (5, 7, 8). A multisynaptic pathway involving low-threshold TM J afferents and interneurons in the V brain stem sensory nuclei is suggested by evidence that these afferents activate neurons in the V spinal nucleus as a result of rotating the isolated TMJ (4). Furthermore, there are histological reports of V spinal to XII nucleus projections (23j and physiological studies have FIG. 5. Activation by various peripheral stimuli of a neuron 2.0 mm lateral to the midline at the level of the obex and at a depth of 2500 pm. When the medial branch (P) of the XII nerve was stimulated at 0.4 mA (not shown), a positive field potential was evoked, but the neuron was not excited. The response of this neuron to P stimulation had a threshold of 0.8 mA and a latency of 8.0 ms and followed stimulation rates of as much as 10/s. Five superimposed traces after threshold P nerve stimulation (upper left record) reveal that single spikes occurred in three of the five trials. However, at 5.0 mA, a burst of repetitive spikes in response to a single P nerve stimulus was noted. Consistent patterns of neuronal responses in relation to stimulus intensity were also seen with other peripheral stimuli which activated the neuron including hypoglossal (XII), retrusive branch (R), temporomandibular joint (TM J), superior laryngeal (SLN), and lingual (Ling) nerve stimulation. Opening the jaw, pinching the ipsilateral ear and forepaw, and mechanical stimulation of the cornea also were effective in activating the neuron. These four stimuli were applied after the oscilloscope had started its sweep, so the records shown do not give an accurate indication of the response latency of these stimuli. The top left record is of five superimposed responses to five successive P nerve stimuli; all the other records are single responses. Voltage calibration, 0.2 mV; time calibration, 5.0 ms for the left and middle columus, 2.5 s for right column.

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also postulated that the V spinal nucleus may be involved as a source of internuncial cells which may modulate activity in the XII nucleus (13). Other neurons near the XII nucleus which were activated by jaw rotations and TMJ nerve stimuli (see Fig. 5) could also be involved in the excitation of P motoneurons in response to jaw rotations. The demonstration that P motoneurons were active during inspiration, jaw rotation, and swallowing confirmed the data reported earlier for the genioglossus muscle (1, 7-9). Earlier studies (24, 26) documented the response characteristics of XII motoneurons during swallowing and respiration, but the specific involvement of P motoneurons during these events had not been reported previously. In contrast to P motoneurons which were activated by TMJ, IX, and SLN nerve stimuli, R motoneurons were activated by lingual and IX nerve stimuli ; the V spinal nucleus (12, 23)) the reticular formation (28), and the solitary tract nucleus (19) may be implicated as interneuronal sites. Genioglossus reflex activity was reported with lingual nerve stimulation (15) and tactile stimulation of the anterior part of the tongue (29), but the present findings support the report that lingual or IX nerve stimulation elicits reflex activity predominantly related to retrusive movements of the tongue (16). A central nervous system study (10) documented that P motoneurons are suppressed‘by lingual nerve stimuli, but R motoneurons as reported in this study were initially facilitated and then inhibited at later conditioning-test intervals. It appears that lingual nerve stimuli activate R motoneurons only, although they have profound inhimbitory effects on both P and R motoneurons. Because TMJ, IX, and SLN nerve stismulican activate P motoneurons, sensory information carried in these nerves may contribute to those responsesin which the genioglossusmuscle is known to participate (speech, tongue thrusting, mastication, suckling, crying, retching, vomiting, gagging, coughing, snoring, licking, etc.). The activation of P motoneurons by TMJ nerve stimulation is compatible with the findings of genioglossus muscle activity elicited ‘by jaw rotations (5, 7, 8). One might also postulate that the protrusive tongue activity seen in response to jaw opening may be associated with an inhibition of tongue retrusion, because stimuli delivered to TMJ nerves activated P motoneurons and inhibited the synaptically evoked responses of R motoneurons. Lingual afferents activated during mastication and other orofacial functions may, on the basis of the present findings, result in a tongue retrusion in order to protect the anterior portion of the tongue from occlusal trauma. It appears that stimuli applied to the tongue itself (lingual and IX nerves) result in activity related predominantly to a retrusive tongue movement. However, stimuli delivered to the posterior part of the oral cavity (IX and SLN

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nerves) and the TMJ activate P motoneurons and may result in a tongue protrusion. This may also serve as a protective reflex in that it aids in the maintenance of a patent airway. Thus, the stimulation of different perioral sites may evoke tongue movenients related to specific protective functions. Conditioning effects revealed that the inhibition of the antidromic responses was of a shorter duration (20 to 60 ms) than the inhibitory effects noted when synaptically evoked responses were conditioned (20 to 400 ms). This is not surprising in view of the fact that the former would involve postsynaptic mechanisms (with the possible exception of some remote dendritic inhibition) whereas the latter could involve mechanisms of both presynaptic inhibition (2, 18, 20) and postsynaptic inhisbition (10, 26, 27). Those conditioning stimuli which also could synaptically activate a motoneuron tended to facilitate the cell’s synaptically evoked responses at conditioning-test intervals of less than 10 ms, whereas conditioning stimuli which did not synaptically activate the cell resulted in only the long-lasting inhibition. Previous investigators (2, 12, 13) reported intracellular EPSP, EPSP-IPSP, IPSP, as well as more complex responses in XII motoneurons, in response to lingual and IX nerve stimulation. One report (26) documented IPSP sequences in XII motoneurons in response to SLN nerve stimulation with a duration of 20 to 150 ms which is considerably shorter than the inhibitory effects it was also shown that SLN noted in the present findings. However, nerve-induced protrusive movements of the tongue can be inhibited for more than 400 ms by conditioning stimuli delivered to the lingual, IX, and SLN nerves; a short period of facilitation often preceded the inhibitory effects (16). In the present findings, early facilitation could not be found with lingual nerve conditioning of the synaptically evoked responses of P motoneurons, but was documented with R motoneurons. The inhibition (lasting until 400 ms) of both P and R motoneurons reported here confirms the earlier data of inhibitory effects in the genioglossus muscle of conditioning stimuli applied to the infraorbital nerve, maxillary tooth pulp, and canine tooth (16). The effects on XII motoneurons with tooth tapping has not been reported previously, and emphasizes the possible role of dental occlusion in modulating XII nucleus activity. For example, the long-lasting i’nhibition from tooth tap and tooth pulp stimuli on P motoneuron activity may be related to a tongue protective reflex during mastication. The nonspecific stimulus delivered to the forepaw had no effect on the synaptically evoked responses of XII motoneurons which demonstrates the important regional contritbutions of perioral stimuli on tongue activity and further suggests that the effects described are not part of some generalized phenomenon such as the startle reaction (6).

46

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A. LOWE

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