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Glossopharyngeal influences on hypoglossal motoneurones in the cat
Brain Research, 74 (1974) 161-166
© Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
161
Glossopharyngeal influences on hypoglossal motoneurones in the cat
I. W. HUNTER AND R. PORTER Department of Physiology, Monash University, Clayton, Victoria 3168 (Australia)
(Accepted March 21st, 1974)
Cajal 4 described convergent pathways from trigeminal and glossopharyngeal afferents on to hypoglossal motoneurones. His diagram (p. 711) indicates separate di-synaptic routes with the second order neurone for lingual nerve afferents in the main sensory nucleus o f the trigeminal nerve and that for glossopharyngeal afferents in the nucleus of tractus solitarius. Miller and Sherrington 6 investigated the different reflex effects produced by stimulation of these separate pathways in the decrebrate cat. Stimulation of the lingual nerve, or activation of mechanoreceptors on the surface of the tongue supplied by this nerve, caused a hollowing of the tongue, while activation of the glossopharyngeal afferents caused bunching up of the tongue and the initiation of swallowing. Electrophysiological studies of these reflex effects have been carried out by Blom and Skoglund 3, Blom 2, PorterS, 9 and Morimoto et al. 7. With low intensity lingual nerve stimulation, reflex activation o f hypoglossal motoneurones supplying the intrinsic musculature of the tongue was produced readily and the synaptic basis of the response was a complex excitatory postsynaptic potential (EPSP) probably caused by repetitive firing of internuncial cells in or near the trigeminal nucleusS, 10. In the present study the lingual-hypoglossal reflex has been used as a test response for the examination of the convergent influences produced on hypoglossal motoneurones by stimulation o f glossopharyngeal afferents. The results indicate that motoneurones caused to discharge by lingual nerve afferents may be inhibited by glossopharyngeal afferent influences. In this respect, the findings in the cat are in accord with the observations of Duggan et al. 5 which showed inhibitory effects on hypoglossal motoneurones o f the rat when glossopharyngeal afferents were stimulated. Moreover, they demonstrate that the effects seen in individual motoneurones sampled with a microelectrode in the rat are also revealed when a large population o f motoneurones is examined. Thus the individual responses are representative. Sixteen experiments were performed on cats which weighed between 2.2 and 3.8 kg. Anaesthesia was induced with an intramuscular injection of ketamine hydrochloride ('Ketalar', Parke-Davis, 30 mg/kg) and maintained by intravenous injection of 60 mg/kg chloralose (British Drug Houses).
162
SHORT COMMUNICATIONS
The larynx and pharynx were reflected and a number of nerves involved in the supply of the tongue were carefully dissected free from surrounding tissues. The hypoglossal nerve branch supplying intrinsic musculature of the anterior part of the tongue, the lingual nerve and the glossopharyngeal nerve on each side were divided distally and prepared for stimulation or recording. In some experiments, the internal laryngeal branches of the superior laryngeal nerve were treated similarly. The nerves were lifted into a pool of warm paraffin oil contained within the elevated skin edges of the dissection. Stimuli were delivered through bipolar chlorided silver electrodes from a Grass $8 stimulator. Monophasic recordings were made from one or other hypoglossal nerve branch using a Tektronix 122 preamplifier and a differential amplifier in a Tektronix 565 oscilloscope. These potentials were also recorded on a magnetic tape recorder for later averaging in a Biomac computer (Data Laboratories). A test reflex response was generated in the hypoglossal nerve by single pulse stimulation o f the ipsilateral lingual nerve and the magnitude of this response was estimated by measuring the area under the compound action potential. Stimulus intensity was adjusted to give approximately a half-maximal response and these stimuli were then applied at a frequency of once every 2 sec, a rate which did not cause significant attenuation of the response. In series of 10 trials alternating with 10 control observations, the test stimulus was paired with a subliminal conditioning shock to one of the other nerves, adjusted to be below threshold for producing any reflex discharge in the hypoglossal axons. This conditioning stimulus could be applied to an ipsilateral or contralateral glossopharyngeal nerve and its effect was assessed by measuring the average change in area of the evoked lingual-hypoglossal reflex as a percentage of the average magnitude o f the 10 reflex responses preceding and following the conditioned series. In all cases, the mean blood pressure of the animal was maintained above 100 m m Hg throughout the experiment; rectal temperature was maintained between 36 °C and 38 °C using a controlled heating blanket and it was found that stable lingualhypoglossal reflex responses could be obtained over many hours without systematic shifts in the control response during the experimental period. The lingual-hypoglossal reflex response recorded from the hypoglossal nerve as a compound action potential has been described by Blom z and by Porter 9. In the present experiments, the response was very similar to that found in decerebrate cats. The latency o f the initial deflexion was between 3.5 and 4.0 msec. The shape and amplitude of the responses were also similar to those already published. An initial, synchronous volley of hypoglossal discharges with a duration of about 2 msec was regularly observed. In some preparations, an ipsilateral glossopharyngeal-hypoglossal reflex could also be produced. This was much more variable than the lingual-hypoglossal reflex response in the same preparations. Larger stimuli were required to produce a minimal response and this showed less synchronous discharge of hypoglossal axons. The latency was characteristically longer (by about 1 msec) than the latency of the ipsilateral lingual-hypoglossal reflex and the duration of the reflex discharge, which
163
SHORT COMMUNICATIONS
STIM,
I •
•
•
e
e
e
o
6
e
t
e
e
i
•
•
•
msec
Fig. 1. Responses recorded in a filament of the hypoglossal nerve following single pulse stimulation of the ipsilateral glossopharyngeal nerve. The upper trace shows the compound action potential in hypoglossal axons produced by near threshold stimulation of the glossopharyngeal nerve (4.0 V, 0.5 msec duration square wave stimulus) delivered at the time indicated by the arrow. The second and third traces show the effect of increasing strengths of stimulation. A maximal response was produced with a stimulus of 10 V and then the latency of the first part of the compound action potential was of the order of 4.5 msec. Voltage calibration 0.5 mV for all traces. Time in msec.
showed a number of variable peaks in the compound action potential was often more than 5 msec (Fig. 1). With a subliminal conditioning stimulus facilitation of the lingual-hypoglossal reflex was regularly observed. This facilitation sometimes reached its peak in two steps. The maximum facilitation was reached when the test stimulus followed about 2 msec after the conditioning stimulus. It was not as pronounced as the facilitation produced by weak, subthreshold stimulation of the contralateral lingual nerve in the same experiments. But, in all experiments, some early facilitation of the lingual-hypoglossal reflex was produced by glossopharyngeal afferents. Fig. 2 illustrates the changes in magnitude of a test lingual-hypoglossal reflex which occurred as a result o f subliminal stimulation of ipsilateral glossopharyngeal afferents at a series o f different times before and after the test stimulus. The upper 3 curves refer to individual, representative experiments. In both the second and third
164
SHORT COMMUNICATIONS
curves, it is clear t h a t the early facilitation o f the reflex was a b r u p t l y t e r m i n a t e d when the c o n d i t i o n i n g stimulus was delivered 5 or m o r e msec before the test stimulus. W i t h longer intervals between the c o n d i t i o n i n g a n d the test stimuli, the l i n g u a l - h y p o g l o s s a l reflex was always depressed by s t i m u l a t i o n o f g l o s s o p h a r y n g e a l afferents and this depression was p r o n o u n c e d a n d p r o l o n g e d (lasting f r o m 5 to 1130 msec after the g l o s s o p h a r y n g e a l stimulus). The lowest curve o f Fig. 2 s u m m a r i z e s the results o f 8 experiments for all o f which the time course o f the c o n d i t i o n i n g effects o f g l o s s o p h a r y n g e a l stimulation was similar to the 3 shown in the u p p e r curves. It m a y be c o n t r a s t e d with the s u m m a r y o f the p o o l e d results o f 6 e x p e r i m e n t s c o n d u c t e d in the same a n i m a l s using weak, subliminal s t i m u l a t i o n o f the c o n t r a l a t e r a l lingual nerve (Fig. 3). L o w t h r e s h o l d lingual afferents p r o d u c e d a p r e d o m i n a n t l y facilitating effect on the c o n t r a l a t e r a l test l i n g u a l -
"l A
LU (/)
z
o o. u') 100
1001 60"-' r - ,0 ; ~
- 5
~ 1
W
m: . . .20. .
30
DELAY
1o' 50i (msec)
J
I
60
I
r$~alllll!
70
100
150
Fig. 2. Examples of the effect of a single, subliminal stimulus delivered to glossopharyngeal afferents in modifying the magnitude of an ipsilateral lingual-hypoglossal reflex. The upper 3 curves are the results of single experiments in 3 cats and are representative. The lowest curve summarizes the results averaged for 8 experiments in 8 cats. Each point on the upper 3 curves is the average of 10 observations expressed as a percentage of the 10 preceding and succeeding control observations. In two cases the mean plus standard error (vertical bars) are represented. The abscissa is the time (in msec) by which the application of the conditioning, glossopharyngeal stimulus preceded (positive values) or followed (negative values) the application of the test stimulus to the lingual nerve.
165
SHORT COMMUNICATIONS 280
CONTRALATERAL LINGUAL NERVE z
O
200
w
10o
---....
[ I -5
I 0
I 5
I 10
I
I 20
I
I i I I 30 40 DELAY (msec)
I 50
I
~ 60
I
I 70
I
= 80
I
I// 90
I I 100 120
Fig. 3. The effects on the magnitude of an ipsilateral lingual-hypoglossal reflex produced by subliminal stimulation of contralateral lingual afferents. The graph plots the average results obtained in 6 experiments in 6 cats.
hypoglossal reflex. The early facilitation was eroded by a depression between 5 msec and 30 msec after the conditioning stimulus but this erosion rarely reduced the size o f the test reflex below control levels. Under these anaesthetic conditions, the major effect of ipsilateral and contralateral lingual afferents was thus facilitation (see also ref. 9). Contralateral glossopharyngeal nerve stimulation was also capable of modifying the size o f a test lingual-hypoglossal reflex. The facilitation demonstrated when the conditioning and test stimuli were closely associated in time was similar to that produced by stimulation of ipsilateral glossopharyngeal afferents, but the depression o f the reflex response which followed tended to be much less marked and of much shorter duration. It was not possible in these experiments to record the afferent volleys in glossopharyngeal nerves stimulated with weak shocks. So it is not known which afferent fibres produced the results reported here. But since the effects were obtained with weak stimuli, larger myelinated afferents, some of which supply mechanoreceptors are most likely to be involved 11. The probable site for convergence o f the influences of lingual and hypoglossal afferents is the pool o f hypoglossal motoneurones supplying the intrinsic musculature of the tongue 4. But it is theoretically possible that some of the influences observed could have resulted from convergence onto interneurones in the vicinity o f the trigeminal nucleus or tractus solitarius. No anatomical evidence for such convergence exists. The convergence of facilitatory influences f r o m afferent fibres in both lingual nerves has been described previously and complex depolarizing synaptic potentials have been recorded in individual hypoglossal motoneurones following single shocks
166
SHORT COMMUNICATIONS
to either lingual nerve 9. Recently, in the rat, h y p e r p o l a r i z i n g responses with a d u r a t i o n o f 20-60 msec have been recorded in h y p o g l o s s a l m o t o n e u r o n e s when stimuli were a p p l i e d to the g l o s s o p h a r y n g e a l nerve 1. O n l y a few o f these hyperp o l a r i z i n g responses were p r e c e d e d by d e p o l a r i z i n g potentials. I t is possible that, in the cat also, the p r e d o m i n a n t effect o f g l o s s o p h a r y n g e a l stimulation is to cause hyperp o l a r i z a t i o n o f h y p o g l o s s a l m o t o n e u r o n e s a n d this c o u l d result in the depression o f responsiveness seen in the p r e s e n t experiments. In the rat, s t i m u l a t i o n o f the g l o s s o p h a r y n g e a l nerve caused a pause in the a m i n o acid i n d u c e d firing o f h y p o g l o s s a l m o t o n e u r o n e s which was a n t a g o n i z e d by strychnineL O n l y a b o u t 1 0 ~ o f cells showed a n y excitation following glossop h a r y n g e a l nerve stimulation. The pause in a m i n o acid i n d u c e d firing o f the m a j o r i t y o f h y p o g l o s s a l neurones h a d a l a t e n c y o f 5-10 msec and a d u r a t i o n o f 25-100 msec.
1 B/SCOE, T. J., DUGGAN, A. W., AND LODGE, D., The inhibition of hypoglossal motoneurones by
glossopharyngeal nerve stimulation in the rat, J. Physiol. (Lond.), 226 (1972) 71P. 2 BLOM,S., Afferent influences on tongue muscle activity, Acta physiol, scand., 49, Suppl. 170 (1960) 1-97.
3 BLolvl,S., AND SKOGLUND, S., Some observations on the control of the tongue muscles, Experientia (Basel), 15 (1959) 12. 4 CAJAL, S. RAMONY, Histologie du Systdme Nerveux del'Homme et des Vertdbrds, Vol. 1, Maloine, Paris, 1909. 5 DUGGAN,A. W., LODGE, D., AND BISCOE, T. J., The inhibition of hypoglossal motoneurones by impulses in the glossopharyngeal nerve of the rat, Exp. Brain Res., 17 (1973) 261-270. 6 MILLER, F. R., AND SHERRINGTON,C. S., Some observations on the bucco-pharyngeal stage of reflex deglutition in the cat, Quart. J. exp. Physiol., 9 (1916) 147-186. 7 MORIMOTO,T., TAKATA,M., AND KAWAMURA,Y., Effect of lingual nerve stimulation on hypoglossal motoneurons, Exp. NeuroL, 22 (1968) 174-190. 8 PORTER, R., Synaptic potentials in hypoglossal motoneurones, J. Physiol. (Lond.), 180 (1965) 209-224. 9 PORTER, R., The synaptic basis of a bilateral lingual-hypoglossal reflex in cats, J. Physiol. (Lond.), 190 (1967) 611-627. 10 SUMr,T., Synaptic potentials of hypoglossal motoneurons and their relation to reflex deglutition, ,lap. J. PhysioL, 19 (1969) 68-79. 11 ZOTTERMAN,Y., Action potentials in glossopharyngeal nerve and in chorda tympani, Skand. Arch. Physiol.. 72 (1935) 73-77.
© Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
161
Glossopharyngeal influences on hypoglossal motoneurones in the cat
I. W. HUNTER AND R. PORTER Department of Physiology, Monash University, Clayton, Victoria 3168 (Australia)
(Accepted March 21st, 1974)
Cajal 4 described convergent pathways from trigeminal and glossopharyngeal afferents on to hypoglossal motoneurones. His diagram (p. 711) indicates separate di-synaptic routes with the second order neurone for lingual nerve afferents in the main sensory nucleus o f the trigeminal nerve and that for glossopharyngeal afferents in the nucleus of tractus solitarius. Miller and Sherrington 6 investigated the different reflex effects produced by stimulation of these separate pathways in the decrebrate cat. Stimulation of the lingual nerve, or activation of mechanoreceptors on the surface of the tongue supplied by this nerve, caused a hollowing of the tongue, while activation of the glossopharyngeal afferents caused bunching up of the tongue and the initiation of swallowing. Electrophysiological studies of these reflex effects have been carried out by Blom and Skoglund 3, Blom 2, PorterS, 9 and Morimoto et al. 7. With low intensity lingual nerve stimulation, reflex activation o f hypoglossal motoneurones supplying the intrinsic musculature of the tongue was produced readily and the synaptic basis of the response was a complex excitatory postsynaptic potential (EPSP) probably caused by repetitive firing of internuncial cells in or near the trigeminal nucleusS, 10. In the present study the lingual-hypoglossal reflex has been used as a test response for the examination of the convergent influences produced on hypoglossal motoneurones by stimulation o f glossopharyngeal afferents. The results indicate that motoneurones caused to discharge by lingual nerve afferents may be inhibited by glossopharyngeal afferent influences. In this respect, the findings in the cat are in accord with the observations of Duggan et al. 5 which showed inhibitory effects on hypoglossal motoneurones o f the rat when glossopharyngeal afferents were stimulated. Moreover, they demonstrate that the effects seen in individual motoneurones sampled with a microelectrode in the rat are also revealed when a large population o f motoneurones is examined. Thus the individual responses are representative. Sixteen experiments were performed on cats which weighed between 2.2 and 3.8 kg. Anaesthesia was induced with an intramuscular injection of ketamine hydrochloride ('Ketalar', Parke-Davis, 30 mg/kg) and maintained by intravenous injection of 60 mg/kg chloralose (British Drug Houses).
162
SHORT COMMUNICATIONS
The larynx and pharynx were reflected and a number of nerves involved in the supply of the tongue were carefully dissected free from surrounding tissues. The hypoglossal nerve branch supplying intrinsic musculature of the anterior part of the tongue, the lingual nerve and the glossopharyngeal nerve on each side were divided distally and prepared for stimulation or recording. In some experiments, the internal laryngeal branches of the superior laryngeal nerve were treated similarly. The nerves were lifted into a pool of warm paraffin oil contained within the elevated skin edges of the dissection. Stimuli were delivered through bipolar chlorided silver electrodes from a Grass $8 stimulator. Monophasic recordings were made from one or other hypoglossal nerve branch using a Tektronix 122 preamplifier and a differential amplifier in a Tektronix 565 oscilloscope. These potentials were also recorded on a magnetic tape recorder for later averaging in a Biomac computer (Data Laboratories). A test reflex response was generated in the hypoglossal nerve by single pulse stimulation o f the ipsilateral lingual nerve and the magnitude of this response was estimated by measuring the area under the compound action potential. Stimulus intensity was adjusted to give approximately a half-maximal response and these stimuli were then applied at a frequency of once every 2 sec, a rate which did not cause significant attenuation of the response. In series of 10 trials alternating with 10 control observations, the test stimulus was paired with a subliminal conditioning shock to one of the other nerves, adjusted to be below threshold for producing any reflex discharge in the hypoglossal axons. This conditioning stimulus could be applied to an ipsilateral or contralateral glossopharyngeal nerve and its effect was assessed by measuring the average change in area of the evoked lingual-hypoglossal reflex as a percentage of the average magnitude o f the 10 reflex responses preceding and following the conditioned series. In all cases, the mean blood pressure of the animal was maintained above 100 m m Hg throughout the experiment; rectal temperature was maintained between 36 °C and 38 °C using a controlled heating blanket and it was found that stable lingualhypoglossal reflex responses could be obtained over many hours without systematic shifts in the control response during the experimental period. The lingual-hypoglossal reflex response recorded from the hypoglossal nerve as a compound action potential has been described by Blom z and by Porter 9. In the present experiments, the response was very similar to that found in decerebrate cats. The latency o f the initial deflexion was between 3.5 and 4.0 msec. The shape and amplitude of the responses were also similar to those already published. An initial, synchronous volley of hypoglossal discharges with a duration of about 2 msec was regularly observed. In some preparations, an ipsilateral glossopharyngeal-hypoglossal reflex could also be produced. This was much more variable than the lingual-hypoglossal reflex response in the same preparations. Larger stimuli were required to produce a minimal response and this showed less synchronous discharge of hypoglossal axons. The latency was characteristically longer (by about 1 msec) than the latency of the ipsilateral lingual-hypoglossal reflex and the duration of the reflex discharge, which
163
SHORT COMMUNICATIONS
STIM,
I •
•
•
e
e
e
o
6
e
t
e
e
i
•
•
•
msec
Fig. 1. Responses recorded in a filament of the hypoglossal nerve following single pulse stimulation of the ipsilateral glossopharyngeal nerve. The upper trace shows the compound action potential in hypoglossal axons produced by near threshold stimulation of the glossopharyngeal nerve (4.0 V, 0.5 msec duration square wave stimulus) delivered at the time indicated by the arrow. The second and third traces show the effect of increasing strengths of stimulation. A maximal response was produced with a stimulus of 10 V and then the latency of the first part of the compound action potential was of the order of 4.5 msec. Voltage calibration 0.5 mV for all traces. Time in msec.
showed a number of variable peaks in the compound action potential was often more than 5 msec (Fig. 1). With a subliminal conditioning stimulus facilitation of the lingual-hypoglossal reflex was regularly observed. This facilitation sometimes reached its peak in two steps. The maximum facilitation was reached when the test stimulus followed about 2 msec after the conditioning stimulus. It was not as pronounced as the facilitation produced by weak, subthreshold stimulation of the contralateral lingual nerve in the same experiments. But, in all experiments, some early facilitation of the lingual-hypoglossal reflex was produced by glossopharyngeal afferents. Fig. 2 illustrates the changes in magnitude of a test lingual-hypoglossal reflex which occurred as a result o f subliminal stimulation of ipsilateral glossopharyngeal afferents at a series o f different times before and after the test stimulus. The upper 3 curves refer to individual, representative experiments. In both the second and third
164
SHORT COMMUNICATIONS
curves, it is clear t h a t the early facilitation o f the reflex was a b r u p t l y t e r m i n a t e d when the c o n d i t i o n i n g stimulus was delivered 5 or m o r e msec before the test stimulus. W i t h longer intervals between the c o n d i t i o n i n g a n d the test stimuli, the l i n g u a l - h y p o g l o s s a l reflex was always depressed by s t i m u l a t i o n o f g l o s s o p h a r y n g e a l afferents and this depression was p r o n o u n c e d a n d p r o l o n g e d (lasting f r o m 5 to 1130 msec after the g l o s s o p h a r y n g e a l stimulus). The lowest curve o f Fig. 2 s u m m a r i z e s the results o f 8 experiments for all o f which the time course o f the c o n d i t i o n i n g effects o f g l o s s o p h a r y n g e a l stimulation was similar to the 3 shown in the u p p e r curves. It m a y be c o n t r a s t e d with the s u m m a r y o f the p o o l e d results o f 6 e x p e r i m e n t s c o n d u c t e d in the same a n i m a l s using weak, subliminal s t i m u l a t i o n o f the c o n t r a l a t e r a l lingual nerve (Fig. 3). L o w t h r e s h o l d lingual afferents p r o d u c e d a p r e d o m i n a n t l y facilitating effect on the c o n t r a l a t e r a l test l i n g u a l -
"l A
LU (/)
z
o o. u') 100
1001 60"-' r - ,0 ; ~
- 5
~ 1
W
m: . . .20. .
30
DELAY
1o' 50i (msec)
J
I
60
I
r$~alllll!
70
100
150
Fig. 2. Examples of the effect of a single, subliminal stimulus delivered to glossopharyngeal afferents in modifying the magnitude of an ipsilateral lingual-hypoglossal reflex. The upper 3 curves are the results of single experiments in 3 cats and are representative. The lowest curve summarizes the results averaged for 8 experiments in 8 cats. Each point on the upper 3 curves is the average of 10 observations expressed as a percentage of the 10 preceding and succeeding control observations. In two cases the mean plus standard error (vertical bars) are represented. The abscissa is the time (in msec) by which the application of the conditioning, glossopharyngeal stimulus preceded (positive values) or followed (negative values) the application of the test stimulus to the lingual nerve.
165
SHORT COMMUNICATIONS 280
CONTRALATERAL LINGUAL NERVE z
O
200
w
10o
---....
[ I -5
I 0
I 5
I 10
I
I 20
I
I i I I 30 40 DELAY (msec)
I 50
I
~ 60
I
I 70
I
= 80
I
I// 90
I I 100 120
Fig. 3. The effects on the magnitude of an ipsilateral lingual-hypoglossal reflex produced by subliminal stimulation of contralateral lingual afferents. The graph plots the average results obtained in 6 experiments in 6 cats.
hypoglossal reflex. The early facilitation was eroded by a depression between 5 msec and 30 msec after the conditioning stimulus but this erosion rarely reduced the size o f the test reflex below control levels. Under these anaesthetic conditions, the major effect of ipsilateral and contralateral lingual afferents was thus facilitation (see also ref. 9). Contralateral glossopharyngeal nerve stimulation was also capable of modifying the size o f a test lingual-hypoglossal reflex. The facilitation demonstrated when the conditioning and test stimuli were closely associated in time was similar to that produced by stimulation of ipsilateral glossopharyngeal afferents, but the depression o f the reflex response which followed tended to be much less marked and of much shorter duration. It was not possible in these experiments to record the afferent volleys in glossopharyngeal nerves stimulated with weak shocks. So it is not known which afferent fibres produced the results reported here. But since the effects were obtained with weak stimuli, larger myelinated afferents, some of which supply mechanoreceptors are most likely to be involved 11. The probable site for convergence o f the influences of lingual and hypoglossal afferents is the pool o f hypoglossal motoneurones supplying the intrinsic musculature of the tongue 4. But it is theoretically possible that some of the influences observed could have resulted from convergence onto interneurones in the vicinity o f the trigeminal nucleus or tractus solitarius. No anatomical evidence for such convergence exists. The convergence of facilitatory influences f r o m afferent fibres in both lingual nerves has been described previously and complex depolarizing synaptic potentials have been recorded in individual hypoglossal motoneurones following single shocks
166
SHORT COMMUNICATIONS
to either lingual nerve 9. Recently, in the rat, h y p e r p o l a r i z i n g responses with a d u r a t i o n o f 20-60 msec have been recorded in h y p o g l o s s a l m o t o n e u r o n e s when stimuli were a p p l i e d to the g l o s s o p h a r y n g e a l nerve 1. O n l y a few o f these hyperp o l a r i z i n g responses were p r e c e d e d by d e p o l a r i z i n g potentials. I t is possible that, in the cat also, the p r e d o m i n a n t effect o f g l o s s o p h a r y n g e a l stimulation is to cause hyperp o l a r i z a t i o n o f h y p o g l o s s a l m o t o n e u r o n e s a n d this c o u l d result in the depression o f responsiveness seen in the p r e s e n t experiments. In the rat, s t i m u l a t i o n o f the g l o s s o p h a r y n g e a l nerve caused a pause in the a m i n o acid i n d u c e d firing o f h y p o g l o s s a l m o t o n e u r o n e s which was a n t a g o n i z e d by strychnineL O n l y a b o u t 1 0 ~ o f cells showed a n y excitation following glossop h a r y n g e a l nerve stimulation. The pause in a m i n o acid i n d u c e d firing o f the m a j o r i t y o f h y p o g l o s s a l neurones h a d a l a t e n c y o f 5-10 msec and a d u r a t i o n o f 25-100 msec.
1 B/SCOE, T. J., DUGGAN, A. W., AND LODGE, D., The inhibition of hypoglossal motoneurones by
glossopharyngeal nerve stimulation in the rat, J. Physiol. (Lond.), 226 (1972) 71P. 2 BLOM,S., Afferent influences on tongue muscle activity, Acta physiol, scand., 49, Suppl. 170 (1960) 1-97.
3 BLolvl,S., AND SKOGLUND, S., Some observations on the control of the tongue muscles, Experientia (Basel), 15 (1959) 12. 4 CAJAL, S. RAMONY, Histologie du Systdme Nerveux del'Homme et des Vertdbrds, Vol. 1, Maloine, Paris, 1909. 5 DUGGAN,A. W., LODGE, D., AND BISCOE, T. J., The inhibition of hypoglossal motoneurones by impulses in the glossopharyngeal nerve of the rat, Exp. Brain Res., 17 (1973) 261-270. 6 MILLER, F. R., AND SHERRINGTON,C. S., Some observations on the bucco-pharyngeal stage of reflex deglutition in the cat, Quart. J. exp. Physiol., 9 (1916) 147-186. 7 MORIMOTO,T., TAKATA,M., AND KAWAMURA,Y., Effect of lingual nerve stimulation on hypoglossal motoneurons, Exp. NeuroL, 22 (1968) 174-190. 8 PORTER, R., Synaptic potentials in hypoglossal motoneurones, J. Physiol. (Lond.), 180 (1965) 209-224. 9 PORTER, R., The synaptic basis of a bilateral lingual-hypoglossal reflex in cats, J. Physiol. (Lond.), 190 (1967) 611-627. 10 SUMr,T., Synaptic potentials of hypoglossal motoneurons and their relation to reflex deglutition, ,lap. J. PhysioL, 19 (1969) 68-79. 11 ZOTTERMAN,Y., Action potentials in glossopharyngeal nerve and in chorda tympani, Skand. Arch. Physiol.. 72 (1935) 73-77.