Brain Research, 585 (1992) 377-380 © 1992 Elsevier Science Publishers B.V. All rights reserved 0006-8993/92/$05.00
377
BRES 25238
Afferent projections in the spinal accessory nerve to the facial motoneurons of the cat Tsutomu Tanaka, Toshihiro Asahara, Yoshihiro Nishimura, Kazuo Higuchi and Tetsuro Yamamoto Department of Physiology, School of Medicine, Mie Unh'ersity, Tsu, Mie-ken (Japan) (Accepted 17 March 1992)
Key words: Accessory afferent; Polysynaptic EPSP; Facial motoneuron; Solitary tract nucleus; Accessory-facial reflex; Cat
Stimulation of the accessory nerve evoked polysynaptic excitatory postsynaptic potentials (EPSPs) in the facial nucleus (FN) neurons of anesthetized cats. From the experiments with severance of CI-C3 dorsal roots, it is suggested that accessory afferents enter the brainstem through the accessory nerve. It was also found that stimulation of the solitary tract nucleus produced exclusively monosynaptic EPSPs in the FN neurons and the afferent volleys are most likely to be relayed at the solitary tract nucleus.
The spinal accessory nerve is usually regarded as a purely efferent nerve, supplying the trapezius (Trap) and sternocleidomastoid (SCM) muscles with motor fibers and the possible presence of afferents in 'the spinal accessory nerve has been a matter of interest s, but the available evidence is s o m e w h a t contradictory'~'4's'l~"17. Recently an electron microscopic study has shown that the feline accessory nerve contains 27% unmyelinated sensory axons I°, and another scanning laser confocal microscopic study identified clusters of sensory neurons consistently associated with the accessory nerve in adult rats mS.Electrophysiological experiments have proved an existence of sensory components in the spinal accessory nerve which have excitatory and inhibitory effects on the accessory motoneurons 9'~4. There is some evidence to suggest that afferents may exist also in other cranial motor nerves 5'~. Although the functional nature of the sensory fibers in the spinal accessory nerve is not completely agreed upon among investigators, it may be related to piai vasomotor ~°, nociception 1°'15 and proprioception ~6'~8. In a series of electrophysiological studies on the facial nucleus (FN) motoneurons of the cat, we have recently been studying the synaptic inputs of the FN motoneurons involved in neck reflex. The aim
of the present paper is to investigate synaptic actions of the spinal accessory afferents on the facial motoneurons in the cat. Adult cats (2.5-4.0 kg) were anesthetized with pentobarbital sodium (30 mg/kg) and were immobilized by administration of gallamine triethiodide under artificial respiration with air. Experimental design for stimulation and recording in this study, and a possible relationship of accessory afferents are shown diagrammatically in Fig. IC. For stimulation of the spinal accessory afferents, the nerve branches to the Trap and SCM muscles were dissected bilaterally at the point of their entrance into the musculature and mounted on bipolar chloridized silver wire electrodes. The stimulus used was of a strength which produced a maximum motor volley in the accessory nerve. The deep dorsal neck muscle nerves (nerves to splenius and biventer cervicis) were also dissected and mounted for stimulation. Stimuli consisted of rectangular pulses 0.05 ms in duration, delivered at the rate of 0.5/s. Activation of the fiber mediating the reflex responses required a stimulus strength about 2 times that required for activation of the accessory motor fibers and it is not clear whether the supramaximal stimulation could be sufficient to activate nociceptive axons. For intracellular recording
Correspondence: T. Tanaka, Department of Physiology, School of Medicine, Mie University, Tsu, Mie-ken, 514, Japan.
378
A
B
C
200 IV 400 ,~ 6 O0 it
800 I~I MO.
1000 ~ t200~
XI
t400 tl
(TRP.
ventrll ~
5reset
Fig. i. A. B: the depth profiles of field potentials recorded in the medial part of the facial nucleus. The field potentials were elicited by stimulation of the PA branch of the facial nerve (A), the Trap (B) branch of the accessory nerve on the ipsilateral side at the indicated depths. C: a schematic diagram of the experimental arrangement for stimulation and recording of a possible relationship of the accessory affcrents. ACC, accessory nucleus; FN, facial nucleus; ME, microelectrode; STN, solitary tract nucleus; Vii, facial nerve (PA, MB, and C branches): XI. accessory nerve (Trap and SCM branches). The voltage calibration and time scale shown in the lower right apply to A. B. In this ;rod the following figures arrows indicate the onset of stimulation. All traces were superimposed records.
from the FN motoneurons, the cerebellum was aspired and glass-pipettes containing 3 M KCI were inserted through the floor of the fourth ventricle, angled 600 above the horizontal. The FN motoneurons were identified by antidromic activation due to stimulation ap. plied to the branches of the peripheral facial nerve (VIi in Fig. IC). The facial nerve branches isolated were th¢ posterior auricular (PA), both the temporalzygomatic.orbital and the bucco-labial (MB), and the cervical (C). in some experiments, the C=-C~ dorsal roots on the ipsilateral side were sectioned, leaving the ventral root intact. The solitary tract nucleus at the level of the obex ipsilateral to the recording site of the FN was stimulated using bipok,r steel electrodes with an interpolar distance of 1.5 ram, After the experiment, the animal was perfuscd with 10% formalin and the stimulating site of the lower brainstem was checked by the blue reaction of iron ions with 0.2% potassium fcrrocynide. Figure I A shows a depth profile of antidromie field potentials following stimulation of the PA branch of the facial nerve, recorded at 200-#m intervals for a distance of 1,8 mm as the microelectrode was withdrawn from a ventral to dorsal position in the medial part of the FN. Response characteristics of the antidromic field potential in the FN have been described f' and previous studies showed that the PA branch was best represented in the medial part of the FN "~, In the
same microelectrodc track, field potentials were elicited by stimulating the ipsilateral Trap and SCM nerve branches, respectively. Stimulation of the Trap produced negative waves with a latency of around 3.0-3.5 ms and a duration of over 10 ms (Fig. 1B). The negative potentials were not always simple in configuration but sometimes showed a few negative peaks. These responses were maximally developed at around a depth of 1,400/~m, corresponding to the ventral part of the FN, and evoked action potentials were most frequently observed in this region. When the microelectrode was moved about 500 #m laterally from this microelectrode track, the Trap-induced field potentials were much smaller and less synchronized. It could be concluded, therefore, that the Trap inputs were localized more toward the medial and ventral aspects of the FN. Stimulation of the ipsilateral SCM produced slow negative waves similar in the shape and amplitude to those evoked by Trap stimulation. Slow negative field potentials in the medial part of the FN were also obtained by stimulation of the contralateral Trap and SCM nerve branches, respectively. Even though there were some differences in the latencies and the duration of the negative waves of the contralateral accessory nerve, patterns of amplitude changes of the negative wave, in general, paralleled those activated by ipsilaterai accessory nerve stimulation. Stimulation of the deep dorsal neck muscle nerves (splenius and biventer cervicis) produced slow negative field potentials in the facial nucleus (Fig. 3B), as well. The latency of the initial deflexion was between 2.5 and 3.0 ms. Supramaximal stimulation of the Trap on the ipsilatoral side evoked postsynaptic potentials in all 53 PA motoneurons. EPSPs were induced in the majority (83%) of these cells (Fig. 2A). The latency of the EPSPs in Fig. 2A was 3.8 ms. In 44 PA neurons, latencies of the EPSPs elicited by stimulation of the ipsilateral Trap nerve ranged from 2,5 to 5.0 ms (mean and S,D., 3.4 :i: 0,6 ms), as indicated in the histogram of Fig. 2F. The mean latency is longer by over 1 ms than the mean latency of the disynaptic EPSPs evoked in the FN neurons by stimulation of the upper cervical dorsal roots ~', suggesting that most of the Trap-induced EPSPs are of di- or polysynaptic origin. IPSPs (4 cells) and mixed EPSPs-IPSPs (5 cells) were occasionally noted (Fig. 2C), Stimulation of the ipsilateral SCM nerve branch also evoked postsynaptic potentials in a total of 60 PA neurons. EPSPs were induced in 53 cells (88%), An example of the EPSPs elicited by ipsilateral SCM stimulation in a PA neuron is illustrated in Fig. 2B. This cell responded to Trap nerve stimulation as well (Fig. 2A) and the convergence of the faeilitatory action of afferent components in each branch of the
379 accessory nerve to an FN neuron was usually observed. Latencies of the SCM-evoked EPSPs ranged from 2.5 to 5.0 ms (3.5 + 0.6 ms, n = 53). In a small fraction of PA neurons (12%), IPSPs or EPSPs-IPSPs were elicited by ipsilateral SCM stimulation. Stimulation of the contralateral accessory nerve (Trap and SCM) afferents produced polysynaptic PSPs in PA motoneurons. Eighty-three per cent of the 59 cells tested were depolarized by contralateral Trap stimulation and 10% were hyperpolarized or exhibited a depolarizing followed by a hyperpolarizing potential. Stimulation of the contralateral SCM produced EPSPs in 36 cells (84%) (Fig. 2D) and IPSPs in 7 cells (16%) (Fig. 2E). The mean latency of the EPSPs which responded to contralateral Trap stimulation was 4.4 ms (S.D.: 0.6 ms, n = 49) (Fig. 2G) and the EPSPs obtained by contralateral SCM stimulation had a mean latency of 4.5 ms (S.D.: 0.7 ms, n = 36), respectively. EPSPs were produced in PA neurons by stimulation of the deep dorsal neck muscle nerves (nerves to splenius and biventer cervicis). Their latencies ranged from 2.2 to 4.8 ms and the mean latency was 3.1 ms ( n - 61). This latency was shorter by 0.2-0.4 ms than the mean latency of the EPSPs evoked by the accessory nerve afferents. The synaptic potentials evoked by stimulation of the deep dorsal muscle nerves were exclusively depolarizing and no conspicuous hyperpolarization could be evoked in the PA neurons of this study, A peripheral afferent pathway for the accessory-facial
'°t'n
i
o:fl 2,5 3.5 4.5 5,5
C t
4mY IOmsec o~
3.0 4.0 5.0 6.0
msec
Fig. 2. Responses of intracellular potentials in 4 PA motoneurons (A,B,C,D,E) evoked by stimulation of Trap branch (A,C) and SCM branch (B,D,E) of the ipsilateral (A-C) and contralateral (D,E) accessory nerves. Lower traces of paired records are eatracellular control responses, EP$Ps (A,B,D), small EPSPs and subsequently IPSPs (C), and IPSPs (E). F, G: latency histograms of EPSPs evoked in PA neurons after stimulation of the Trap branch of the ipsilateral (F) and contralateral (G) accessory nerves, respectively. The ordinates show number of neurons and the abscissas latency in ms. Voltage calibration of 2 mV applies to A,B; 3 mV to D,E; and 4 mV to C. Time scale of 10 ms applies to A-E.
before section
after section
or--
2msec
4msec ~ Fig. 3. A-E: effects of sections of C]_ 3 dorsal roots on the accessory afferent responses. Field potentials recorded in the medial part of the FN following stimulation of the SCM (A and C) and of the splenius nerve branch (B and D) on the ipsilateral side, before (A, B) and after (C, D) severance of the C !_3 dorsal roots. SCM stimulation elicited EPSPs, after section of the C~_3 dorsal roots (E). F, G: synaptic responses in 2 PA neurons (F and G) evoked by stimulation of the solitary tract nucleus just caudal to the obex on the ipsilateral side. EPSPs (F) and action potentials superimposed on EPSPs with an increase in stimulus intensity and a faster sweep velocity (O). Voltage calibration of 5 mV applies to A-D, 6 mV to E and 4 mV to E-G. Time scale of 4 ms applies to A-F and 2 ms to G.
reflex could be different from that for the deep dorsal neck muscle afferents, which are mediated through C~-C 3 dorsal roots. The latter pathway is responsible for the activation of a spinofacial pathway m2. There is a question of whether the accessory afferents travel with the accessory nerve into the medulla oblongata or enter the spinal cord with the dorsal roots of upper cervical nerves. The accessory afferents might enter the central nervous system via the dorsal root C I, the vagus nerve or the bulbar part of the accessory nerve 2.4.~.~H in several experiments, attempts were made to identify the peripheral afferent pathway for the accessory-facial reflex. The accessory neree.evoked field potentials recorded in the FN could be elicited in the preparations in which the dorsal roots of Cj-C3 were interrupted. Fig. 3A and C show that the SCMevoked responses remained unchanged after the dorsal roots of C~-C3 were severed at their entrance into the spinal cord. In contrast, the field potentials evoked in the FN by stimulation of the dorsal neck muscle nerves were almost completely abolished after the rhizotomy (Fig. 3B, D). Intracellular recordings from the PA neurons in these animals revealed that the configuration of synaptic potentials is very similar to that of synaptic potentials obtained from animals with intact cervical dorsal nerves (Fig. 3E). In one preparation of this study, severance of the accessory nerve at its exit just distal to the jugular foramen abolished both the field potential and the synaptic discharge of the FN neurons due to volleys in the accessory afferents. While these results obviously do not rule out contribution by
380 a dorsal root loop, they demonstrate that some afferent component of an accessory-facial reflex enters the brainstem through the accessory nerve presumably via the jugular foremen. Under the present investigation, it was not possible to ascertain the central course of the accessory afferent nerve fibers using anterograde transport of wheat germ agglutinin conjugated to horseradish peroxidase, with particular attention to termination within the medulla. But since the effects were evoked polysynaptically and Windle t6 observed the sensory fibers passing into the solitary nucleus in silver preparations of cat embryos, the afferent volleys in the accessory nerve are most likely to be relayed in the solitary tract nucleus. Fig. 3F is a typical ;ecord oz" EPSPs evoked in a PA neuron by stimulation of the solitary tract nucleus. The duration of the EPSPs is over 10 ms. The action potentials elicited with a stronger stimulus in another PA neuron are also shown in Fig. 3G. Their onset latencies (Fig. 3F, G) were as short as 0.9 ms and 1.1 ms, respectively, indicating that they were monosynaptically evoked. This observation is consistent with the previous report t-~. We conclude that the feline spinal accessory nerve contains afferent fibers and activation of the accessory afferents produces predominantly a facilitatory effect upon the facial motor nucleus. The location of the cell bodies of accessory afferents could be in the accessory nerve, in the Ct ganglion or in the vagus complex t°,to. it is still unknown if these sensory axons enter the CNS through the spinal or the cranial root of the accessory nerve or may take use of other structures as a conduit (i,e. the vagus nerve or cervical ventral roots), The functional significance of the accessory.fricial reflex cannot be discussed at present, but integrative action in vasomotor ~°, nociceptiont", ~,~ and proprioception ~:~ appears to involve the reflex system.
I Brodal, A., Neurological Anatomy in Relation to Clinical Medicine, Oxford University Press, New York, 1981. 2 DuBois, F.S. and Foley, i.e., Experimental studies on the vagus and spinal accessory nerves in the cat, Anat. Rec., 64 (1936) 285 -307. 3 Fahmy, N., A note on the intracranial and extracranial parts of the IXth, Xth and Xlth nerves, J. Anat., 61 (1927) 298-301. 4 Fitzgerald, M.J.T., Comerford, P.T. and Tuffery, A.R., Sources of innervation of the neuromuscular spindles in sternomastoid and trapezius, J. Anat., 134 (1982) 471-490. 5 Goldberg, S.J., Hull, C.D. and Buchwald, N.A., Afferent projections in the abducens nerve: an intracellular study, Brain Res., 68 (1974) 205-214. 6 Kitai, S.T., Akaike, T., Bando, T., Tanaka, T., Tsukahara, N. and Yu, H., Antidromic and synaptic activation of the facial nucleus of cat, Brain Res., 33 (1971) 227-232. 7 Kitai, S.T., Tanaka, T., Tsukahara, N. and Yu, H., The facial nucleus of cat: antidromic and synaptic activation and peripheral nerve representation, Exp. Brain Res., 16 (1972) 161-183. 8 Pearson, A.A., The sensory accessory nerve in human embryos, J. Comp. Neurol., 68 (1938) 243-266. 9 Rapoport, S., Reflex connections of motoneurons of muscles involved in head movement in the cat, J. Physiol., 289 (1979) 311-327. I0 Risling, M., Hildebrand, C. and Uhler, G., Presence of unmyelinated axons in the spinal root of the feline accessory nerve, Brain Res., 342 (1985) 374-378. II Sasaki, K., Electrophysiological studies on oculomotor neurons of the cat, Jpn. J. Physiolo, 13 (1963) 287-302. 12 Tanaka, T., Further electrophysiological analysis of a spinofacial pathway in the cat, Brain Res., 138 (1977) 545-549, 13 Tanaka, T. and Asahara, T., Synaptic actions of vagal afferents on facial motoneurons in the cat, Brain Res., 212 (1981) 188-193. 14 Tanaka, T., Asahara. T., Nishimura, Y. and Higuchi, K., Influences of neck affe~ents on spinal accessory motoneurons, J. Physiol. Soc. Jpn,, 44 (1982) 365 (abstract). 15 Wetmore, C and Elde, R,, Detection and characterization of a sensory microganglion associated with the spinal accessory nerve: a scanning laser confocal microscopic study of the neurons and their processes, J, Comp, NcuroL, 305 (1991) 148-163. 16 Windle, W.F., The sensory components of the spinal accessory nerve, J. Comp, M,uml,, 53 (1931) 115-127, 17 Yee, J. ,rid C.orbin. K.B, The intramedullary course of the upper five cervical dors,l mot fibers in the rabbit, J, Cmnp, Heurol,, 70 (I 939) 297~304, 18 Yee, J,, Harrison, F, and Corbin, K,B,, The se.nsor? innervation of the spinal accessory and tongue musculature in the rabbit, J, Comp. Neural,, 70 (1939) 305-314.
377
BRES 25238
Afferent projections in the spinal accessory nerve to the facial motoneurons of the cat Tsutomu Tanaka, Toshihiro Asahara, Yoshihiro Nishimura, Kazuo Higuchi and Tetsuro Yamamoto Department of Physiology, School of Medicine, Mie Unh'ersity, Tsu, Mie-ken (Japan) (Accepted 17 March 1992)
Key words: Accessory afferent; Polysynaptic EPSP; Facial motoneuron; Solitary tract nucleus; Accessory-facial reflex; Cat
Stimulation of the accessory nerve evoked polysynaptic excitatory postsynaptic potentials (EPSPs) in the facial nucleus (FN) neurons of anesthetized cats. From the experiments with severance of CI-C3 dorsal roots, it is suggested that accessory afferents enter the brainstem through the accessory nerve. It was also found that stimulation of the solitary tract nucleus produced exclusively monosynaptic EPSPs in the FN neurons and the afferent volleys are most likely to be relayed at the solitary tract nucleus.
The spinal accessory nerve is usually regarded as a purely efferent nerve, supplying the trapezius (Trap) and sternocleidomastoid (SCM) muscles with motor fibers and the possible presence of afferents in 'the spinal accessory nerve has been a matter of interest s, but the available evidence is s o m e w h a t contradictory'~'4's'l~"17. Recently an electron microscopic study has shown that the feline accessory nerve contains 27% unmyelinated sensory axons I°, and another scanning laser confocal microscopic study identified clusters of sensory neurons consistently associated with the accessory nerve in adult rats mS.Electrophysiological experiments have proved an existence of sensory components in the spinal accessory nerve which have excitatory and inhibitory effects on the accessory motoneurons 9'~4. There is some evidence to suggest that afferents may exist also in other cranial motor nerves 5'~. Although the functional nature of the sensory fibers in the spinal accessory nerve is not completely agreed upon among investigators, it may be related to piai vasomotor ~°, nociception 1°'15 and proprioception ~6'~8. In a series of electrophysiological studies on the facial nucleus (FN) motoneurons of the cat, we have recently been studying the synaptic inputs of the FN motoneurons involved in neck reflex. The aim
of the present paper is to investigate synaptic actions of the spinal accessory afferents on the facial motoneurons in the cat. Adult cats (2.5-4.0 kg) were anesthetized with pentobarbital sodium (30 mg/kg) and were immobilized by administration of gallamine triethiodide under artificial respiration with air. Experimental design for stimulation and recording in this study, and a possible relationship of accessory afferents are shown diagrammatically in Fig. IC. For stimulation of the spinal accessory afferents, the nerve branches to the Trap and SCM muscles were dissected bilaterally at the point of their entrance into the musculature and mounted on bipolar chloridized silver wire electrodes. The stimulus used was of a strength which produced a maximum motor volley in the accessory nerve. The deep dorsal neck muscle nerves (nerves to splenius and biventer cervicis) were also dissected and mounted for stimulation. Stimuli consisted of rectangular pulses 0.05 ms in duration, delivered at the rate of 0.5/s. Activation of the fiber mediating the reflex responses required a stimulus strength about 2 times that required for activation of the accessory motor fibers and it is not clear whether the supramaximal stimulation could be sufficient to activate nociceptive axons. For intracellular recording
Correspondence: T. Tanaka, Department of Physiology, School of Medicine, Mie University, Tsu, Mie-ken, 514, Japan.
378
A
B
C
200 IV 400 ,~ 6 O0 it
800 I~I MO.
1000 ~ t200~
XI
t400 tl
(TRP.
ventrll ~
5reset
Fig. i. A. B: the depth profiles of field potentials recorded in the medial part of the facial nucleus. The field potentials were elicited by stimulation of the PA branch of the facial nerve (A), the Trap (B) branch of the accessory nerve on the ipsilateral side at the indicated depths. C: a schematic diagram of the experimental arrangement for stimulation and recording of a possible relationship of the accessory affcrents. ACC, accessory nucleus; FN, facial nucleus; ME, microelectrode; STN, solitary tract nucleus; Vii, facial nerve (PA, MB, and C branches): XI. accessory nerve (Trap and SCM branches). The voltage calibration and time scale shown in the lower right apply to A. B. In this ;rod the following figures arrows indicate the onset of stimulation. All traces were superimposed records.
from the FN motoneurons, the cerebellum was aspired and glass-pipettes containing 3 M KCI were inserted through the floor of the fourth ventricle, angled 600 above the horizontal. The FN motoneurons were identified by antidromic activation due to stimulation ap. plied to the branches of the peripheral facial nerve (VIi in Fig. IC). The facial nerve branches isolated were th¢ posterior auricular (PA), both the temporalzygomatic.orbital and the bucco-labial (MB), and the cervical (C). in some experiments, the C=-C~ dorsal roots on the ipsilateral side were sectioned, leaving the ventral root intact. The solitary tract nucleus at the level of the obex ipsilateral to the recording site of the FN was stimulated using bipok,r steel electrodes with an interpolar distance of 1.5 ram, After the experiment, the animal was perfuscd with 10% formalin and the stimulating site of the lower brainstem was checked by the blue reaction of iron ions with 0.2% potassium fcrrocynide. Figure I A shows a depth profile of antidromie field potentials following stimulation of the PA branch of the facial nerve, recorded at 200-#m intervals for a distance of 1,8 mm as the microelectrode was withdrawn from a ventral to dorsal position in the medial part of the FN. Response characteristics of the antidromic field potential in the FN have been described f' and previous studies showed that the PA branch was best represented in the medial part of the FN "~, In the
same microelectrodc track, field potentials were elicited by stimulating the ipsilateral Trap and SCM nerve branches, respectively. Stimulation of the Trap produced negative waves with a latency of around 3.0-3.5 ms and a duration of over 10 ms (Fig. 1B). The negative potentials were not always simple in configuration but sometimes showed a few negative peaks. These responses were maximally developed at around a depth of 1,400/~m, corresponding to the ventral part of the FN, and evoked action potentials were most frequently observed in this region. When the microelectrode was moved about 500 #m laterally from this microelectrode track, the Trap-induced field potentials were much smaller and less synchronized. It could be concluded, therefore, that the Trap inputs were localized more toward the medial and ventral aspects of the FN. Stimulation of the ipsilateral SCM produced slow negative waves similar in the shape and amplitude to those evoked by Trap stimulation. Slow negative field potentials in the medial part of the FN were also obtained by stimulation of the contralateral Trap and SCM nerve branches, respectively. Even though there were some differences in the latencies and the duration of the negative waves of the contralateral accessory nerve, patterns of amplitude changes of the negative wave, in general, paralleled those activated by ipsilaterai accessory nerve stimulation. Stimulation of the deep dorsal neck muscle nerves (splenius and biventer cervicis) produced slow negative field potentials in the facial nucleus (Fig. 3B), as well. The latency of the initial deflexion was between 2.5 and 3.0 ms. Supramaximal stimulation of the Trap on the ipsilatoral side evoked postsynaptic potentials in all 53 PA motoneurons. EPSPs were induced in the majority (83%) of these cells (Fig. 2A). The latency of the EPSPs in Fig. 2A was 3.8 ms. In 44 PA neurons, latencies of the EPSPs elicited by stimulation of the ipsilateral Trap nerve ranged from 2,5 to 5.0 ms (mean and S,D., 3.4 :i: 0,6 ms), as indicated in the histogram of Fig. 2F. The mean latency is longer by over 1 ms than the mean latency of the disynaptic EPSPs evoked in the FN neurons by stimulation of the upper cervical dorsal roots ~', suggesting that most of the Trap-induced EPSPs are of di- or polysynaptic origin. IPSPs (4 cells) and mixed EPSPs-IPSPs (5 cells) were occasionally noted (Fig. 2C), Stimulation of the ipsilateral SCM nerve branch also evoked postsynaptic potentials in a total of 60 PA neurons. EPSPs were induced in 53 cells (88%), An example of the EPSPs elicited by ipsilateral SCM stimulation in a PA neuron is illustrated in Fig. 2B. This cell responded to Trap nerve stimulation as well (Fig. 2A) and the convergence of the faeilitatory action of afferent components in each branch of the
379 accessory nerve to an FN neuron was usually observed. Latencies of the SCM-evoked EPSPs ranged from 2.5 to 5.0 ms (3.5 + 0.6 ms, n = 53). In a small fraction of PA neurons (12%), IPSPs or EPSPs-IPSPs were elicited by ipsilateral SCM stimulation. Stimulation of the contralateral accessory nerve (Trap and SCM) afferents produced polysynaptic PSPs in PA motoneurons. Eighty-three per cent of the 59 cells tested were depolarized by contralateral Trap stimulation and 10% were hyperpolarized or exhibited a depolarizing followed by a hyperpolarizing potential. Stimulation of the contralateral SCM produced EPSPs in 36 cells (84%) (Fig. 2D) and IPSPs in 7 cells (16%) (Fig. 2E). The mean latency of the EPSPs which responded to contralateral Trap stimulation was 4.4 ms (S.D.: 0.6 ms, n = 49) (Fig. 2G) and the EPSPs obtained by contralateral SCM stimulation had a mean latency of 4.5 ms (S.D.: 0.7 ms, n = 36), respectively. EPSPs were produced in PA neurons by stimulation of the deep dorsal neck muscle nerves (nerves to splenius and biventer cervicis). Their latencies ranged from 2.2 to 4.8 ms and the mean latency was 3.1 ms ( n - 61). This latency was shorter by 0.2-0.4 ms than the mean latency of the EPSPs evoked by the accessory nerve afferents. The synaptic potentials evoked by stimulation of the deep dorsal muscle nerves were exclusively depolarizing and no conspicuous hyperpolarization could be evoked in the PA neurons of this study, A peripheral afferent pathway for the accessory-facial
'°t'n
i
o:fl 2,5 3.5 4.5 5,5
C t
4mY IOmsec o~
3.0 4.0 5.0 6.0
msec
Fig. 2. Responses of intracellular potentials in 4 PA motoneurons (A,B,C,D,E) evoked by stimulation of Trap branch (A,C) and SCM branch (B,D,E) of the ipsilateral (A-C) and contralateral (D,E) accessory nerves. Lower traces of paired records are eatracellular control responses, EP$Ps (A,B,D), small EPSPs and subsequently IPSPs (C), and IPSPs (E). F, G: latency histograms of EPSPs evoked in PA neurons after stimulation of the Trap branch of the ipsilateral (F) and contralateral (G) accessory nerves, respectively. The ordinates show number of neurons and the abscissas latency in ms. Voltage calibration of 2 mV applies to A,B; 3 mV to D,E; and 4 mV to C. Time scale of 10 ms applies to A-E.
before section
after section
or--
2msec
4msec ~ Fig. 3. A-E: effects of sections of C]_ 3 dorsal roots on the accessory afferent responses. Field potentials recorded in the medial part of the FN following stimulation of the SCM (A and C) and of the splenius nerve branch (B and D) on the ipsilateral side, before (A, B) and after (C, D) severance of the C !_3 dorsal roots. SCM stimulation elicited EPSPs, after section of the C~_3 dorsal roots (E). F, G: synaptic responses in 2 PA neurons (F and G) evoked by stimulation of the solitary tract nucleus just caudal to the obex on the ipsilateral side. EPSPs (F) and action potentials superimposed on EPSPs with an increase in stimulus intensity and a faster sweep velocity (O). Voltage calibration of 5 mV applies to A-D, 6 mV to E and 4 mV to E-G. Time scale of 4 ms applies to A-F and 2 ms to G.
reflex could be different from that for the deep dorsal neck muscle afferents, which are mediated through C~-C 3 dorsal roots. The latter pathway is responsible for the activation of a spinofacial pathway m2. There is a question of whether the accessory afferents travel with the accessory nerve into the medulla oblongata or enter the spinal cord with the dorsal roots of upper cervical nerves. The accessory afferents might enter the central nervous system via the dorsal root C I, the vagus nerve or the bulbar part of the accessory nerve 2.4.~.~H in several experiments, attempts were made to identify the peripheral afferent pathway for the accessory-facial reflex. The accessory neree.evoked field potentials recorded in the FN could be elicited in the preparations in which the dorsal roots of Cj-C3 were interrupted. Fig. 3A and C show that the SCMevoked responses remained unchanged after the dorsal roots of C~-C3 were severed at their entrance into the spinal cord. In contrast, the field potentials evoked in the FN by stimulation of the dorsal neck muscle nerves were almost completely abolished after the rhizotomy (Fig. 3B, D). Intracellular recordings from the PA neurons in these animals revealed that the configuration of synaptic potentials is very similar to that of synaptic potentials obtained from animals with intact cervical dorsal nerves (Fig. 3E). In one preparation of this study, severance of the accessory nerve at its exit just distal to the jugular foramen abolished both the field potential and the synaptic discharge of the FN neurons due to volleys in the accessory afferents. While these results obviously do not rule out contribution by
380 a dorsal root loop, they demonstrate that some afferent component of an accessory-facial reflex enters the brainstem through the accessory nerve presumably via the jugular foremen. Under the present investigation, it was not possible to ascertain the central course of the accessory afferent nerve fibers using anterograde transport of wheat germ agglutinin conjugated to horseradish peroxidase, with particular attention to termination within the medulla. But since the effects were evoked polysynaptically and Windle t6 observed the sensory fibers passing into the solitary nucleus in silver preparations of cat embryos, the afferent volleys in the accessory nerve are most likely to be relayed in the solitary tract nucleus. Fig. 3F is a typical ;ecord oz" EPSPs evoked in a PA neuron by stimulation of the solitary tract nucleus. The duration of the EPSPs is over 10 ms. The action potentials elicited with a stronger stimulus in another PA neuron are also shown in Fig. 3G. Their onset latencies (Fig. 3F, G) were as short as 0.9 ms and 1.1 ms, respectively, indicating that they were monosynaptically evoked. This observation is consistent with the previous report t-~. We conclude that the feline spinal accessory nerve contains afferent fibers and activation of the accessory afferents produces predominantly a facilitatory effect upon the facial motor nucleus. The location of the cell bodies of accessory afferents could be in the accessory nerve, in the Ct ganglion or in the vagus complex t°,to. it is still unknown if these sensory axons enter the CNS through the spinal or the cranial root of the accessory nerve or may take use of other structures as a conduit (i,e. the vagus nerve or cervical ventral roots), The functional significance of the accessory.fricial reflex cannot be discussed at present, but integrative action in vasomotor ~°, nociceptiont", ~,~ and proprioception ~:~ appears to involve the reflex system.
I Brodal, A., Neurological Anatomy in Relation to Clinical Medicine, Oxford University Press, New York, 1981. 2 DuBois, F.S. and Foley, i.e., Experimental studies on the vagus and spinal accessory nerves in the cat, Anat. Rec., 64 (1936) 285 -307. 3 Fahmy, N., A note on the intracranial and extracranial parts of the IXth, Xth and Xlth nerves, J. Anat., 61 (1927) 298-301. 4 Fitzgerald, M.J.T., Comerford, P.T. and Tuffery, A.R., Sources of innervation of the neuromuscular spindles in sternomastoid and trapezius, J. Anat., 134 (1982) 471-490. 5 Goldberg, S.J., Hull, C.D. and Buchwald, N.A., Afferent projections in the abducens nerve: an intracellular study, Brain Res., 68 (1974) 205-214. 6 Kitai, S.T., Akaike, T., Bando, T., Tanaka, T., Tsukahara, N. and Yu, H., Antidromic and synaptic activation of the facial nucleus of cat, Brain Res., 33 (1971) 227-232. 7 Kitai, S.T., Tanaka, T., Tsukahara, N. and Yu, H., The facial nucleus of cat: antidromic and synaptic activation and peripheral nerve representation, Exp. Brain Res., 16 (1972) 161-183. 8 Pearson, A.A., The sensory accessory nerve in human embryos, J. Comp. Neurol., 68 (1938) 243-266. 9 Rapoport, S., Reflex connections of motoneurons of muscles involved in head movement in the cat, J. Physiol., 289 (1979) 311-327. I0 Risling, M., Hildebrand, C. and Uhler, G., Presence of unmyelinated axons in the spinal root of the feline accessory nerve, Brain Res., 342 (1985) 374-378. II Sasaki, K., Electrophysiological studies on oculomotor neurons of the cat, Jpn. J. Physiolo, 13 (1963) 287-302. 12 Tanaka, T., Further electrophysiological analysis of a spinofacial pathway in the cat, Brain Res., 138 (1977) 545-549, 13 Tanaka, T. and Asahara, T., Synaptic actions of vagal afferents on facial motoneurons in the cat, Brain Res., 212 (1981) 188-193. 14 Tanaka, T., Asahara. T., Nishimura, Y. and Higuchi, K., Influences of neck affe~ents on spinal accessory motoneurons, J. Physiol. Soc. Jpn,, 44 (1982) 365 (abstract). 15 Wetmore, C and Elde, R,, Detection and characterization of a sensory microganglion associated with the spinal accessory nerve: a scanning laser confocal microscopic study of the neurons and their processes, J, Comp, NcuroL, 305 (1991) 148-163. 16 Windle, W.F., The sensory components of the spinal accessory nerve, J. Comp, M,uml,, 53 (1931) 115-127, 17 Yee, J. ,rid C.orbin. K.B, The intramedullary course of the upper five cervical dors,l mot fibers in the rabbit, J, Cmnp, Heurol,, 70 (I 939) 297~304, 18 Yee, J,, Harrison, F, and Corbin, K,B,, The se.nsor? innervation of the spinal accessory and tongue musculature in the rabbit, J, Comp. Neural,, 70 (1939) 305-314.
