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Afferent projections in the hypoglossal nerve to the facial neurons of the cat
140
Brabt Research, 99 (1975) t 4 ( ) 1 4 ; ()~ Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
Afferent projections in the hypogiossai nerve to the facial neurons of the cat
T. T A N A K A
Department of Physiology, School of Medicine, Mie University, Tsu, Mie-ken (Japan) (Accepted July 22nd, 1975)
The existence of sensory fibers in the hypoglossal nerve has been claimed by several authors. Cooper s and Blom 1 recorded afferent impulses in the isolated filaments of the hypoglossal nerve on stretching the tongue, but the structure of the muscle receptors which were stimulated has not been clarified. On the other hand, responses of autonomic afferent fibers in the hypoglossal nerve were reported ~,11. Tarkhan and Abou-E1-Naga 12 suggested that afferent fibers passing through the peripheral hypoglossal trunk enter the brain stem mostly along the intracranial run of the vagal nerve. In a recent study, Sauerland and Mizuno 9 presented a hypoglosso-laryngeal reflex mediated by the afferent fibers which join the vagal nerve at the level of the nodosa ganglion. In another recent work 5, it has been shown that afferent fibers involved in a hypoglosso-facial reflex have the same course as those described by Sauerland and Mizuno. This report deals with an investigation of the hypoglosso-facial reflex by means of intracellular recording of synaptic potentials in facial neurons on stimulation of the hypogtossal nerve. Adult cats weighing 2.0-4.0 kg were anesthetized with pentobarbital sodium (35 mg/kg, i.p.). After surgical procedures the animals were paralyzed by gallamine triethiodide with artificial ventilation. Branches of hypoglossal nerves on both sides were severed at the point of their entrance into the extrinsic and intrinsic tongue musculature. The cut end of each nerve was stimulated in an electrode assembly which consisted of a pair of chloridized silver rings covered with a plastic sleeve. The entire assembly was secured onto adjacent musculature and was covered with warm liquid paraffin. With such an arrangement, independent stimulation of respective branches was possible and current spread to the adjacent nerve branches was minimized. In some experiments, lingual nerves on both sides were severed and prepared for stimulation. Stimuli were brief current pulses of 0.05q3.1 msec in duration. After the cerebellum had been removed, the facial nucleus (FN) was approached with microelectrodes inserted through the floor of the fourth ventricle in the dorsoventral direction at an angle of 30° from the frontal plane. Glass microetectrodes filled with 3 M KCI (5-15 M ~ , DC resistance) were selected. The F N neurons were identified by antidromic activation due to stimulation of the ipsilateral peripheral facial nerve.
141
+
10msec I t
J +
50mV
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1 H~
iI
i
20reset-
20msec J
B +
50mV1 I
lOmsec
t
i f , l 1o'0 I
3.0 3.5
I
I
'1
I
4.0 415 5.0 5.5 m$ec
6.0
Fig. 1. A: an example of field potentials in the facial nucleus (FN) following stimulation of the ipsilateral hypoglossal nerve. B and C: record of responses of a FN neuron by stimulation of the ipsilateral nerve and by the contralateral hypoglossal nerve respectively. Lower trace of each record is an extracellular record under similar conditions. D - F show the effects of increase in stimulus intensity to the ipsilateral hypoglossal nerve. G : small depolarization and subsequent hyperpolarization by ipsilateral hypoglossal nerve stimulation. H : depolarization develops into action potential with increasing strengths of stimulation. I: latency histogram of EPSPs evoked in 40 FN neurons by stimulation of the ipsilateral hypoglossal nerve. The abscissa shows latency in msec and the ordinate shows the number of cells.
Fig. I A shows a representative e x a m p l e o f the field p o t e n t i a l s elicited by single volley stimuli delivered to the ipsilateral h y p o g l o s s a l nerve a n d r e c o r d e d in F N at a d e p t h o f a r o u n d 5.6 m m f r o m the floor o f the f o u r t h ventricle where the field p o t e n t i a l s were m a x i m a l l y developed. By d e p t h analyses o f the field potentials, it was noted t h a t the h y p o g l o s s a l n e r v e - e v o k e d field p o t e n t i a l s were usually m a x i m a l in size in the ventral p a r t o f F N . L a t e n c y o f the negative deflection was a b o u t 5 msec and d u r a t i o n of the reflex discharges, which consisted o f a n u m b e r o f small peaks, was often over 10 msec (Fig. 1A). The a m p l i t u d e o f the p o t e n t i a l s varied with different intensities o f stimuli a p p l i e d to the h y p o g l o s s a l nerve. I n the present study, the reflex responses in F N c o u l d n o t always be i n d u c e d bilaterally by s t i m u l a t i o n o f hypoglossal afferents. The significant field p o t e n t i a l s e v o k e d by 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 h y p o g l o s s a l nerve were observed only in a few p r e p a r a t i o n s . I n t r a c e l l u l a r recordings f r o m F N neurons on h y p o g l o s s a l nerve s t i m u l a t i o n are s h o w n in Fig. 1 B - H . Fig. 1B shows repetitive discharges o f a F N n e u r o n induced by ipsilateral h y p o g l o s s a l nerve stimulation. T h e onset o f the first a c t i o n p o t e n t i a l was 3.8 msec after the stimuli a n d the firing rate o f the b u r s t was a b o u t 350/sec. The d e p o -
142 larizing potential in Fig. IC was obtained from the same FN neuron as in Fig. I B in response to contralateral hypoglossal nerve stimulation. With supramaximal stimuli (about 10 times the threshold), the EPSPs were still small and failed to evoke a full action potential in the neuron. The latency of the EPSPs was about 6 msec (Fig. 1C) and usually longer than that of the EPSPs evoked by ipsilateral hypoglossal nerve stimulation. In about half of the cases, synaptic potentials were evoked by stimulation of bilateral hypoglossal nerves. The EPSPs produced by contralateral hypoglossal nerve stimulation could not always initiate action potentials. This finding may explain in part the reason why potential changes due to contralateral hypoglossal nerve stimulation were difficult to detect by extracellular exploration. Fig. 1D-F shows intracellular potential changes in the same FN neuron evoked by stimuli of different intensities to the ipsilateral hypoglossal nerve. An irregular depolarization with a slow rising phase appeared to begin at 5.4 msec after the stimulus (Fig. 1D). The duration of the EPSPs was approximately 25 msec. With an increase in stimulus intensity (Fig. 1D-F), there were increases in both amplitude and duration of the EPSPs but decreases in latency from 5.4 msec to 3.8 msec (see Fig. 1D-F and also Fig. 1B). As seen in Fig. 1F, repetitive firing of action potentials was induced by the strongest stimulus. A latency histogram for the EPSPs evoked by stimulation of the ipsilateral hypoglossal nerve was made from 40 FN units, excluding those which had latencies longer than 6 msec (Fig. lI). The latencies ranging from 3.0 to 6.0 msec in this histogram are obviously longer (by about 1 msec) than those of disynaptic EPSPs evoked in the FN neurons by trigeminal nerve stimulation 1°. In an investigation of synaptic potentials produced in the hypoglossal neurons by stimulation of the hypoglossat nerve, Porter s suggested that synaptic potentials might derive from spread of stimulating current to low-threshold afferent fibers in the lingual nerve. However, the latency of the synaptic effects induced in the FN neurons from lingual stimulation was about 2 msec in the present experiments and always shorter than that of EPSPs evoked by single volleys in the hypoglossal afferents. The lingual nerveevoked field potentials were recorded mostly in the dorsolateral part of the FN, while the reflex responses to hypoglossal nerve stimulation were observed most predominantly in the ventral part of FN. Moreover, in preparations with the vagal nerve cut, no synaptic potentials were produced in the FN neurons by stimulation of the hypoglossal nerve, as described below (see Fig. 2C). These results would argue against the hypothesis of direct stimulus spread. The synaptic potentials evoked in the FN neurons by stimulation of the hypoglossal nerve were not all depolarizing, but some consisted of an initial small depolarization and subsequent hyperpolarization (Fig. I G). The initial depolarization provoked a spike potential, when it was large, on strong stimulation (Fig. 1H). In a few preparations, hypoglossal nerve stimulation evoked only hyperpolarization without a conspicuous depolarization preceding it. In several experiments, attempts were made to identify the peripheral afferent pathway for the hypoglosso-facial reflex. The hypoglossal nerve-evoked field potentials could be recorded in the preparations in which the dorsal roots of the first and second cervical nerves were interrupted bilaterally (Fig. 2A). Intracellular recording
143
5mV
25mV1
+
,
D
I {
I
-
--
~1"-
-I
÷
10msec
~
20os,0
E
-f,
I
Fig. 2. Records of field potential (A) and intracellular recording from a FN neuron (B) on stimulation of the ipsilateral hypoglossal nerve after cutting bilateral dorsal roots of C1 and C2. C-E: responses of a FN neuron after severance of the vagal nerve. Intracrania[ section of the rootlets of the vaga[ nerve eliminated EPSPs on stimulation of the ipsilateral hypoglossal nerve (C), but EPSPs produced by stimulation of the contralateral hypoglossal nerve remained unchanged (D) and an increase in amplitude of EPSPs due to strong stimuli led to firing of action potential (E). Lower traces of C and D, extracellular control.
from the F N neurons in these animals revealed that the configuration of synaptic potentials is exactly similar to that of synaptic potentials obtained from animals with intact cervical nerves (Fig. 2B). The potentials of Fig. 2B began at 4.5 msec after the stimulus and duration of the potentials corresponded to a negative phase of the field potential. These responses remained unchanged when the hypoglossal or glossopharyngeal nerve was severed at its entrance into the brain stem. However, the reflex responses were almost completely abolished when the vagal nerve was cut intracranially, proximal to the nodosa ganglion. The record of Fig. 2C demonstrates that ipsilateral hypoglossal nerve stimulation produced no synaptic potentials when the vagal nerve was severed. In contrast, polysynaptic EPSPs could be evoked in this neuron by stimulation of the contralateral hypoglossal nerve (Fig. 2D) and increase of stimulus intensity added a spike potential to EPSPs (Fig. 2E). From these results it would be very likely that afferent fibers of a bypoglosso-facial reflex travel along the hypoglossal trunk and enter the brain stem mainly through the rootlets of the vagal nerve, presumably joining them at the level of the nodosa ganglion. This conclusion is consistent with the findings of Sauerland and Mizuno 9, and Hanson and Wid6nL There is increasing evidence that activation of the hypoglossal afferents produces facilitatory and/or inhibitory influences on the motor nuclei of the brain stem 4-v. The location of the cell bodies of hypoglossal afferents is still unknown, but it may be assumed that hypoglossal afferents are involved in a coordinate activity of the brain stem motor system.
144 The a u t h o r expresses his thanks to Prof. K. Sasaki for reading the m a n u s c r i p t a n d p r o v i d i n g valuable suggestions. T h a n k s are also due to Mr. T. A s a h a r a for his technical assistance.
1 BLOM, S., Afferent influences on tongue muscle activity, Acta physioL stand., 49, Suppl. 170 (1960) 1-97. 2 COOPER,S., Afferent impulses in the hypoglossal nerve on stretching the cat's tongue, J. Physiol. (Lond.), 126 (1954) 32P. 3 DOWNMAN,C. B. B., Afferent fibers of the hypoglossal nerve, J. Anat. (Lond.), 73 (1939) 387-395. 4 GREEN, J. D., AND NEGISm, K., Membrane potentials in hypoglossal motoneurons, J. NeurophysioL, 26 (1963) 835-856. 5 HANSON,J., AND WIDEN, L., Afferent fibers in the hypoglossal nerve of cat, Acta physiol, scand., 79 (1970) 24-36. 6 LINDQUIST,C., AND M~.RTENSSON,A., Reflex responses induced by stimulation of hypoglossal afferents, Acta physiol, scand., 77 (1969) 234-240. 7 NAKAMURA,Y., Possible afferent components in the hypoglossal nerve influencing the trigeminal monosynaptic reflex of the cat, Anat. Rec., 160 (1968) 399. 8 PORTER, R., Synaptic potentials in hypoglossal motoneurons, J. Physiol. (Lond.), 180 (1965) 209-224. 9 SAUERLAND,E. K., AND MZZtJNO,N., Hypoglossal nerve afferents: elicitation of a polysynaptic hypoglossolaryngeal reflex, Brain Research, 10 (1968) 256-258. 10 TANAKA,T., YU, H., AND KITAI,S. T., Trigeminal and spinal inputs to the facial nucleus, Brain Research, 33 0971) 504-508. 11 TARKHAN,A. A.~ Uber das Vorhandensein afferenter Fasern im Nervus hypoglossus, Arch. Psychiat. Nervenkr., 105 (1936) 475-483. 12 TARKHAN,A. A., AND ABou-EL-NAGA,T., Sensory fibers in the hypoglossal nerve, J, Anat. (Lond.), 81 (1947) 23-32.
Brabt Research, 99 (1975) t 4 ( ) 1 4 ; ()~ Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
Afferent projections in the hypogiossai nerve to the facial neurons of the cat
T. T A N A K A
Department of Physiology, School of Medicine, Mie University, Tsu, Mie-ken (Japan) (Accepted July 22nd, 1975)
The existence of sensory fibers in the hypoglossal nerve has been claimed by several authors. Cooper s and Blom 1 recorded afferent impulses in the isolated filaments of the hypoglossal nerve on stretching the tongue, but the structure of the muscle receptors which were stimulated has not been clarified. On the other hand, responses of autonomic afferent fibers in the hypoglossal nerve were reported ~,11. Tarkhan and Abou-E1-Naga 12 suggested that afferent fibers passing through the peripheral hypoglossal trunk enter the brain stem mostly along the intracranial run of the vagal nerve. In a recent study, Sauerland and Mizuno 9 presented a hypoglosso-laryngeal reflex mediated by the afferent fibers which join the vagal nerve at the level of the nodosa ganglion. In another recent work 5, it has been shown that afferent fibers involved in a hypoglosso-facial reflex have the same course as those described by Sauerland and Mizuno. This report deals with an investigation of the hypoglosso-facial reflex by means of intracellular recording of synaptic potentials in facial neurons on stimulation of the hypogtossal nerve. Adult cats weighing 2.0-4.0 kg were anesthetized with pentobarbital sodium (35 mg/kg, i.p.). After surgical procedures the animals were paralyzed by gallamine triethiodide with artificial ventilation. Branches of hypoglossal nerves on both sides were severed at the point of their entrance into the extrinsic and intrinsic tongue musculature. The cut end of each nerve was stimulated in an electrode assembly which consisted of a pair of chloridized silver rings covered with a plastic sleeve. The entire assembly was secured onto adjacent musculature and was covered with warm liquid paraffin. With such an arrangement, independent stimulation of respective branches was possible and current spread to the adjacent nerve branches was minimized. In some experiments, lingual nerves on both sides were severed and prepared for stimulation. Stimuli were brief current pulses of 0.05q3.1 msec in duration. After the cerebellum had been removed, the facial nucleus (FN) was approached with microelectrodes inserted through the floor of the fourth ventricle in the dorsoventral direction at an angle of 30° from the frontal plane. Glass microetectrodes filled with 3 M KCI (5-15 M ~ , DC resistance) were selected. The F N neurons were identified by antidromic activation due to stimulation of the ipsilateral peripheral facial nerve.
141
+
10msec I t
J +
50mV
II!' I
1 H~
iI
i
20reset-
20msec J
B +
50mV1 I
lOmsec
t
i f , l 1o'0 I
3.0 3.5
I
I
'1
I
4.0 415 5.0 5.5 m$ec
6.0
Fig. 1. A: an example of field potentials in the facial nucleus (FN) following stimulation of the ipsilateral hypoglossal nerve. B and C: record of responses of a FN neuron by stimulation of the ipsilateral nerve and by the contralateral hypoglossal nerve respectively. Lower trace of each record is an extracellular record under similar conditions. D - F show the effects of increase in stimulus intensity to the ipsilateral hypoglossal nerve. G : small depolarization and subsequent hyperpolarization by ipsilateral hypoglossal nerve stimulation. H : depolarization develops into action potential with increasing strengths of stimulation. I: latency histogram of EPSPs evoked in 40 FN neurons by stimulation of the ipsilateral hypoglossal nerve. The abscissa shows latency in msec and the ordinate shows the number of cells.
Fig. I A shows a representative e x a m p l e o f the field p o t e n t i a l s elicited by single volley stimuli delivered to the ipsilateral h y p o g l o s s a l nerve a n d r e c o r d e d in F N at a d e p t h o f a r o u n d 5.6 m m f r o m the floor o f the f o u r t h ventricle where the field p o t e n t i a l s were m a x i m a l l y developed. By d e p t h analyses o f the field potentials, it was noted t h a t the h y p o g l o s s a l n e r v e - e v o k e d field p o t e n t i a l s were usually m a x i m a l in size in the ventral p a r t o f F N . L a t e n c y o f the negative deflection was a b o u t 5 msec and d u r a t i o n of the reflex discharges, which consisted o f a n u m b e r o f small peaks, was often over 10 msec (Fig. 1A). The a m p l i t u d e o f the p o t e n t i a l s varied with different intensities o f stimuli a p p l i e d to the h y p o g l o s s a l nerve. I n the present study, the reflex responses in F N c o u l d n o t always be i n d u c e d bilaterally by s t i m u l a t i o n o f hypoglossal afferents. The significant field p o t e n t i a l s e v o k e d by 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 h y p o g l o s s a l nerve were observed only in a few p r e p a r a t i o n s . I n t r a c e l l u l a r recordings f r o m F N neurons on h y p o g l o s s a l nerve s t i m u l a t i o n are s h o w n in Fig. 1 B - H . Fig. 1B shows repetitive discharges o f a F N n e u r o n induced by ipsilateral h y p o g l o s s a l nerve stimulation. T h e onset o f the first a c t i o n p o t e n t i a l was 3.8 msec after the stimuli a n d the firing rate o f the b u r s t was a b o u t 350/sec. The d e p o -
142 larizing potential in Fig. IC was obtained from the same FN neuron as in Fig. I B in response to contralateral hypoglossal nerve stimulation. With supramaximal stimuli (about 10 times the threshold), the EPSPs were still small and failed to evoke a full action potential in the neuron. The latency of the EPSPs was about 6 msec (Fig. 1C) and usually longer than that of the EPSPs evoked by ipsilateral hypoglossal nerve stimulation. In about half of the cases, synaptic potentials were evoked by stimulation of bilateral hypoglossal nerves. The EPSPs produced by contralateral hypoglossal nerve stimulation could not always initiate action potentials. This finding may explain in part the reason why potential changes due to contralateral hypoglossal nerve stimulation were difficult to detect by extracellular exploration. Fig. 1D-F shows intracellular potential changes in the same FN neuron evoked by stimuli of different intensities to the ipsilateral hypoglossal nerve. An irregular depolarization with a slow rising phase appeared to begin at 5.4 msec after the stimulus (Fig. 1D). The duration of the EPSPs was approximately 25 msec. With an increase in stimulus intensity (Fig. 1D-F), there were increases in both amplitude and duration of the EPSPs but decreases in latency from 5.4 msec to 3.8 msec (see Fig. 1D-F and also Fig. 1B). As seen in Fig. 1F, repetitive firing of action potentials was induced by the strongest stimulus. A latency histogram for the EPSPs evoked by stimulation of the ipsilateral hypoglossal nerve was made from 40 FN units, excluding those which had latencies longer than 6 msec (Fig. lI). The latencies ranging from 3.0 to 6.0 msec in this histogram are obviously longer (by about 1 msec) than those of disynaptic EPSPs evoked in the FN neurons by trigeminal nerve stimulation 1°. In an investigation of synaptic potentials produced in the hypoglossal neurons by stimulation of the hypoglossat nerve, Porter s suggested that synaptic potentials might derive from spread of stimulating current to low-threshold afferent fibers in the lingual nerve. However, the latency of the synaptic effects induced in the FN neurons from lingual stimulation was about 2 msec in the present experiments and always shorter than that of EPSPs evoked by single volleys in the hypoglossal afferents. The lingual nerveevoked field potentials were recorded mostly in the dorsolateral part of the FN, while the reflex responses to hypoglossal nerve stimulation were observed most predominantly in the ventral part of FN. Moreover, in preparations with the vagal nerve cut, no synaptic potentials were produced in the FN neurons by stimulation of the hypoglossal nerve, as described below (see Fig. 2C). These results would argue against the hypothesis of direct stimulus spread. The synaptic potentials evoked in the FN neurons by stimulation of the hypoglossal nerve were not all depolarizing, but some consisted of an initial small depolarization and subsequent hyperpolarization (Fig. I G). The initial depolarization provoked a spike potential, when it was large, on strong stimulation (Fig. 1H). In a few preparations, hypoglossal nerve stimulation evoked only hyperpolarization without a conspicuous depolarization preceding it. In several experiments, attempts were made to identify the peripheral afferent pathway for the hypoglosso-facial reflex. The hypoglossal nerve-evoked field potentials could be recorded in the preparations in which the dorsal roots of the first and second cervical nerves were interrupted bilaterally (Fig. 2A). Intracellular recording
143
5mV
25mV1
+
,
D
I {
I
-
--
~1"-
-I
÷
10msec
~
20os,0
E
-f,
I
Fig. 2. Records of field potential (A) and intracellular recording from a FN neuron (B) on stimulation of the ipsilateral hypoglossal nerve after cutting bilateral dorsal roots of C1 and C2. C-E: responses of a FN neuron after severance of the vagal nerve. Intracrania[ section of the rootlets of the vaga[ nerve eliminated EPSPs on stimulation of the ipsilateral hypoglossal nerve (C), but EPSPs produced by stimulation of the contralateral hypoglossal nerve remained unchanged (D) and an increase in amplitude of EPSPs due to strong stimuli led to firing of action potential (E). Lower traces of C and D, extracellular control.
from the F N neurons in these animals revealed that the configuration of synaptic potentials is exactly similar to that of synaptic potentials obtained from animals with intact cervical nerves (Fig. 2B). The potentials of Fig. 2B began at 4.5 msec after the stimulus and duration of the potentials corresponded to a negative phase of the field potential. These responses remained unchanged when the hypoglossal or glossopharyngeal nerve was severed at its entrance into the brain stem. However, the reflex responses were almost completely abolished when the vagal nerve was cut intracranially, proximal to the nodosa ganglion. The record of Fig. 2C demonstrates that ipsilateral hypoglossal nerve stimulation produced no synaptic potentials when the vagal nerve was severed. In contrast, polysynaptic EPSPs could be evoked in this neuron by stimulation of the contralateral hypoglossal nerve (Fig. 2D) and increase of stimulus intensity added a spike potential to EPSPs (Fig. 2E). From these results it would be very likely that afferent fibers of a bypoglosso-facial reflex travel along the hypoglossal trunk and enter the brain stem mainly through the rootlets of the vagal nerve, presumably joining them at the level of the nodosa ganglion. This conclusion is consistent with the findings of Sauerland and Mizuno 9, and Hanson and Wid6nL There is increasing evidence that activation of the hypoglossal afferents produces facilitatory and/or inhibitory influences on the motor nuclei of the brain stem 4-v. The location of the cell bodies of hypoglossal afferents is still unknown, but it may be assumed that hypoglossal afferents are involved in a coordinate activity of the brain stem motor system.
144 The a u t h o r expresses his thanks to Prof. K. Sasaki for reading the m a n u s c r i p t a n d p r o v i d i n g valuable suggestions. T h a n k s are also due to Mr. T. A s a h a r a for his technical assistance.
1 BLOM, S., Afferent influences on tongue muscle activity, Acta physioL stand., 49, Suppl. 170 (1960) 1-97. 2 COOPER,S., Afferent impulses in the hypoglossal nerve on stretching the cat's tongue, J. Physiol. (Lond.), 126 (1954) 32P. 3 DOWNMAN,C. B. B., Afferent fibers of the hypoglossal nerve, J. Anat. (Lond.), 73 (1939) 387-395. 4 GREEN, J. D., AND NEGISm, K., Membrane potentials in hypoglossal motoneurons, J. NeurophysioL, 26 (1963) 835-856. 5 HANSON,J., AND WIDEN, L., Afferent fibers in the hypoglossal nerve of cat, Acta physiol, scand., 79 (1970) 24-36. 6 LINDQUIST,C., AND M~.RTENSSON,A., Reflex responses induced by stimulation of hypoglossal afferents, Acta physiol, scand., 77 (1969) 234-240. 7 NAKAMURA,Y., Possible afferent components in the hypoglossal nerve influencing the trigeminal monosynaptic reflex of the cat, Anat. Rec., 160 (1968) 399. 8 PORTER, R., Synaptic potentials in hypoglossal motoneurons, J. Physiol. (Lond.), 180 (1965) 209-224. 9 SAUERLAND,E. K., AND MZZtJNO,N., Hypoglossal nerve afferents: elicitation of a polysynaptic hypoglossolaryngeal reflex, Brain Research, 10 (1968) 256-258. 10 TANAKA,T., YU, H., AND KITAI,S. T., Trigeminal and spinal inputs to the facial nucleus, Brain Research, 33 0971) 504-508. 11 TARKHAN,A. A.~ Uber das Vorhandensein afferenter Fasern im Nervus hypoglossus, Arch. Psychiat. Nervenkr., 105 (1936) 475-483. 12 TARKHAN,A. A., AND ABou-EL-NAGA,T., Sensory fibers in the hypoglossal nerve, J, Anat. (Lond.), 81 (1947) 23-32.