The influence of vagal afferent fiber activity on masticatory reflexes

EXPERIMENTAL

The

NEUROLOGY

27,

Influence

of on

M. Vctcrans

H.

A4drrtirGstratiors Plt>tsiology, School

545-553

(1970)

Vagal

Afferent

Masticatory

CHASE,

Activity

Reflexes

S. TORII,

AIYD Y.

Hospital, Scpulvcda; of Medicine, linivcrsity California

Keceked

Fiber

arrd

NAKAMURA

1

the* Dcpartrrtcrtts of .4natowy of (‘alifovrtia at Los Augelcs,

and

90024

Fchary

20. 1970

Single pulse, pulse train, and repetitive stimulation of the cut central end of the cervical vagus nerve initiated periods of depression and facilitation of monosynaptic (masseteric) and polysynaptic (digastric) brain stem reflexes. The excitation of cervical vagal fiber groups of different conduction velocities was examined during these variations in somatic reflex activity. The masseteric and digastric reflex responses were depressed or facilitated in conjunction with the excitation of cervical vagal fibers conducting between 10 and lSm/sec. It was concluded that these fibers were of thoracic origin and that on the basis of their conduction velocity were similar to the vagal fiber system whose excitation induces EEG patterns of activation. Introduction

Suprathreshold excitation of the cervical vagus nerve was shown to modify somatic reflex activity in the preceding paper (5). This level of vagal excitation yields central neural responses which reflect the massed discharge of all fiber groups within the cervical portion of the vagus nerve. However, one may determine the influence of specific abdominal and thoracic fiber groups on central neural activity by exciting the cervical vagus nerve with various levels of stimulation while monitoring the induced neurographic potentials. On the basis of their conduction velocities the neurographic. potentials of afferent fibers may be related to specific functions since thoracic and abdominal receptors give rise to vagal fiber groups of distinct diameters (11). In the past we have employed this technique to correlate the excitation of distinct visceral afferent fiber systems with 1 This research was supported by the United States l’eterans Administration and by grants from the U.S. Public Health Service (MH-10083) and the National Science Foundation (GF 262). Bibliographic assistance was received from the UCLA Brain Information Service, which is part of the Neurological Information Network of NINDS and is supported under Contract DHEW PH-43-66-59. Part of this research was carried out at Toho Univensity, Tokyo, Japan. 545

546

CFI;\SE,

TORII,

AND

NARAMURA

induced EEG synchronization and desynchronization (3,4,6). The present paper is an analysis of the vagal fiber systems whose excitation is correlated with the modulation of brain stem somatic reflex activity which we have previously described (5). Methods The animal preparations were also used in another investigation (5). Briefly, 26 adult cats were prepared in which the spinal cord was severed at C, and both vagal and sympathetic trunks were cut at a low cervical level. The right vagus nerve was isolated from the sympathetic trunk and surrounding perineurium and its cut central end was placed on a bipolar stimulating electrode. A bipolar recording electrode was placed on the nerve between the stimulating electrode and the brain. These surgical procedures were carried out while the animal was anesthetized with sodium methohoxital which was subsequently discontinued. The animal was then immobilized with Flaxedil. Wound margins, pressure points and the nerve trunks supplying these areas were infiltrated every 30 min with lidocaine throughout the entire experiment. The monosynaptic masseteric reflex (jaw closing) was induced by stimulation of the mesencephalic nucleus of the trigeminal nerve and was recorded along the ipsilateral masseter nerve. Stimulation of the inferior dental nerve induced the polysynaptic digastric reflex (jaw opening) which was recorded from the ipsilateral mylohyoid (digastric) nerve. Vagal and reflex excitation were carried out with Grass and Nihon Koden stimulators. The vagal neurogram and reflex responseswere displayed on an oscilloscope and photographed. A Nihon Koden computer (ATAC501) equipped with a paper write-out was used to obtain averaged reflex responses.The vagal neurogram was monitored both bipolarly and monopolarly (with the cervical musculature serving as the reference pole). At the termination of all experimental procedures, in order to calculate the conduction velocity of the various neurogram components, the conduction distance was measured from the cathode (cephalic pole) of the stimulating electrode to the cephalic pole of the recording electrode (which was the active pole when recording monopolarly j Results

Masseteric Reflex Response to Single Pulse Vagal Stiwdation: Neurogram Correlates. The masseteric reflex (Mass. R.) response during var-

ious vagal stimulation paradigms is shown in Fig. 1. At short vagal conditioning-test intervals (IO-35 msec) the Mass. R. was depressed (Fig. lB), while at long intervals (35-100 msecj it was facilitated (Fig. 1Cj.

VAGAL

NECROGRAM

547

CORRELATES

0

A

-l

-a MR

l-l

n

VAGUS FIG. 1. The experimental paradigms employed in testing the masseteric reflex response (A) to central vagal stimulation are depicted in B, C, and D. At short conditioning-test response intervals the masseteric reflex was depressed (B) ; at long intervals it was facilitated (C) as it was during repetitive (100 cps) vagal excitation (D). Mesencephalic trigeminal nucleus : 4 v, .5 msec, l/set (IO averaged responses) ; vagus : 10 v, 1 msec ; calibration : 200 pv, 5 msec.

Repetitive vagal stimulation (20-200 cps) resulted in tonic reflex facilitation ( Fig. ID ) . These reflex responses to vagal stimulation were correlated with the various components of the cervical vagal neurogram by gradually increasing the voltage applied to the vagus nerve while the duration of the stimulating pulse was held constant. The only consistent variations in the amplitude of the Mass. R. which were observed in conjunction with vagal excitation are those presented in Fig. 1. The neurographic correlates of vagal inhibition of the Mass. R. are shown in Fig. 2 (according to the paradigm in Fig. 1B). In Fig. 2A, the mean amplitude of the control response (I-open bar) was unchanged when preceded by a single vagal stimulus (II-hatched bar). Although there appears to be slight facilitation in B of Fig. 2, we were unable to reliably document an increase in reflex amplitude. The neurographic record (Fig. 2A, 13) indicated that the escitation of the rapidly conducting fiber systems, whose conduction velocities were greater than 20 m/set, was ineffective in reducing the amplitude of the Mass. R. In Fig. 2C. however, in conjunction with the excitation of a fiber group conducting between 10 and 15 m/set, depression of the Mass. R. was obtained. When the Mass. R. was induced according to the paradigms in Fig. IC and D, facilitation occurred which was correlated with the presence in the vagal neurogram of the potential indicated by the arrow in Fig. 2C. Reflex facilitation was absent when only those potentials were induced which are shown in Fig. 2A or B. In six of our animals the recurrent laryngeal nerve wa’s &mulated before severance of the ipsilateral vagus nerve. The induced activity was monitored along the intact cervical vagus nerve. No consistent variation in reflex amplitude occurred after recurrent laryngeal nerve excitation in conjunction with the stimulation paradigms depicted in Fig. I.

548

CHASE,

TORII,

Alr;D

KAKAMURA

A

B

C

FIG. 2. The mean amplitude of ten control masseteric reflex responses is depicted by the height of the open bar (I). The mean amplitude of a corresponding series of reflex responses preceded by a single vagal pulse is shown by the hatched bar (II). The neurograms in A, B, and C were obtained with a constant stimulus duration, but with increasing voltages (A,2 v, BJ.5 v, C,3 v). The reflex response was induced at a latency of 20 msec. Reflex depression was obtained in conjunction with the presence of the potential indicated by the arrow in C. Mesencephalic trigeminal nucleus: 5 v, .5 msec, l/set ; vagus : .5 msec; calibration : reflex 100 pv, neurogram (bipolar recording) 50 pv, 1 msec ; nerve length 5.2 cm. Digastric

Reflex

Response

to Single-Pulse

Vagaf

Stimdation:

Neuro-

gram Correlates. The modulation of the digastric reflex (Dig. R.) response was studied at short (3 msec) and long (30 msec) conditioningtest intervals (Fig. 3)) and during repetitive vagal stimulation (Fig. 4). As reported in the preceding paper (5), the Dig. R. was facilitated at short conditioning intervals (Fig. 311) and was depressed both at long conditioning intervals (Fig. 3111) and during repetitive vagal stimulation (Fig. 4). In Fig. 3I, the duration of the vagal stimulus was held constant while the voltage was increased in A through D. In Fig, 3II-a,e, there appears slight, if any, reflex facilitation; similarly, only a small amount of depression is present in Fig. SIII-a,e. In Fig. 3II-b,f, and 3111-b,f, facilitation and depression were observed together with the presence of the potential

VAGAL

NEL.ROGRAM

549

CORRELATES

a

d

h

d

h

FIG. 3. The digastric reflex response at short (column II) and long (column III) conditioning-test intervals was examined in conjunction with the induction of different vagal potentials (.4-D). Predominant facilitation (11-B) and depression (III-B) were obtained when the neurogram in B was induced. Although elevated levels of vagal excitation (I-C, I-D) resulted in the induction of higher threshold fiber groups there was no qualitative change in the response of the digastric reflex. Inferior dental nerve: 1.5 v, .Ol msec, l/set (ten averaged responses) ; vagus: A,2 v, B,2.4 v, C,3.2v, D,4 v, .l msec; calibration: reflex 100 pv, 2 msec, neurogram (bipolar recording) 50 gv, 1 msec ; nerve length 3.2 cm.

which is indicated by the arrow in Fig. 31-B. These in the neurogram responses were unchanged in direction or magnitude as stimulation was increased (Fig. 31-C and I-D). The critical fiber system, indicated by the in Fig. 31-B. had a conduction velocity of l%lO m/set.

arrow

Masscteric ad tion: Newo!gram

Digastric Rt$c.r Response to Repetitive Vagal StiwadaCovrelatcs. A correlative neurographic analysis was made

of the reflex responseswhich occurred during repetitive vagal stimulation. In Fig.

4T. the vagal

neurogram

represents

superimposed

traces

obtained

5.50

CHASE,

TORII,

AND

KAKAMURA

I

II OR : CONTROL

VACAL NEUROCRAM

Frc. 4. Repetitive vagal stimulation (100 cps) led to depression of a continuously evoked (l/set) digastric reflex and facilitation of the masseteric reflex (Fig. 1D). No change in digastric reflex amplitude was obtained until the neurogram in C was

induced. The potential 10-15

m/set.

This

same

indicated

by the arrow

neurogram

(C)

was

in C had a conduction also

correlated

with

velocity of

masseteric

reflx

facilitation (not shown). Inferior dental nerve: 1.6 v, .Ol msec, l/set (ten averaged responses) ; vagus: A,2 v, B,3.7 v, C,5 v, D&6 v, E, 10 v, 2 msec; calibration: reflex 100 pv, 5 msec, I.W, 5 msec ; nerve

neurogram (monopolar length 6.8 cm.

recording,

negative

deflection

upwards)

50

during the initial 10 seconds of 100 cps vagal stimulation. The recording in Fig. 4A was not correlated with any change in the amplitude of the Dig. R. In Fig. 4B, when the voltage was increased, another fiber group was activated but no reflex depression was observed. However, in conjunction with the induction of a potential whose conduction velocity was 10-15 m/set (arrow, Fig. 4C), depression of the Dig. R. occurred. In this series

\‘AGAI.

NECROGRAM

CORREIJATES

551

of experiments, we also induced the neurograms in Fig. 4A-E with single-pulse vagal stimulation and obtained early facilitation and late depression of the Dig. R. when the neurogram illustrated in Fig. 4C was induced. The presence of the neurographic potential shown by the arrow in Fig. 4C was also correlated with the modulation of the Mass. R. during all of the paradigms shown in Fig. 1. Discussion

The results presented in this paper indicate that somatic brain stem refles modulation is produced by cervical vagal afferents having a conduction velocity of lo-15 m/set. Since fibers issuing from the abdominal cavity conduct at less than 10 m/set (4)) one can exclude abdominal vagal afferents as being responsible for the visceral-somatic interactions presented in this paper. One can specify only the general origin of cervical vagal fibers by an analysis of their conduction velocities, since similar types of visceral receptors give rise to fibers of different diameters, e.g., systemic baroreceptors (12-53 m/set), left atria1 type-A fibers (13-27 m/set), etc. (11). We estimate from Paintal’s (11) data that pulmonary afferents conduct with a velocity of approximately 30 m/set, while those of cardiovascular origin 15 m/set. These data indicate that the somatic responses presented in this paper may be due to the excitation of cardiovascular afferents. It is known that carotid chemoreceptors and baroreceptors can influence somatic reflex activity (7-10,12-14,18,19). Thus, it is likely that systemic or aortic receptors of similar function also modify somatic activity, although not necessarily in a comparable fashion (12). Aortic baroreceptor afferents conduct considerably faster than the fibers which we found to be capable of influencing somatic activity (11). Since systemic chemoreceptor afferents and fibers of cardiac origin conduct within the general range of velocities which we found to be effective (ll), the responsible thoracic afferent fiber groups may originate in chemoreceptors or cardiac receptors, or both. We wonder why the induction of a single potential complex indicating excitation of fibers with a narrow range of conduction velocities yields widespread changes in EEG and somatic reflex activity (3, 4). No central responses were observed until the level of stimulation was raised so that activity occurred in fibers with a distinct conduction-velocity. As more fibers with this conduction velocity were excited no qualitative change in central response pattern occurred. This is especially striking since afferent fibers of similar diameter and conduction velocity arise from different receptors ( 11). Either the central responses are due to the excitation of a few fibers from specific receptors or the organization of visceral afferents is such that fibers with the same conduction velocity from different recep-

552

CHASE,

TORII,

AND

N.4KAMUR.i

tors exert similar central effects. The first explanation is possible, but it is improbable that the amplitude of the responsible neurographic potentials reflects the excitation of only a few fibers. The amplitude of these potentials is such that one would expect that fibers from many receptors contribute to them, although not all fibers which conduct within a specific range necessarily induce a similar central neural response. On the other hand, the latter possibility has a precedent, for group I fibers from different muscle groups have common conduction velocities, induce a similar response in homonymous and synergistic heteronymous muscles, different responses in antagonistic heteronymous muscles (17), and at the same time exert a CO~PUPLZO~ influence upon CNS activity regardless of their muscular site of origin (15,16). S imilar organization may exist within the visceral afferent system. We recently esamined the modulation of masseteric and digastric reflex activity during sleep and wakefulness in freely moving cats and found that reciprocal changes in amplitude take place during quiet sleep and wakefulness. Reflex modulation during the alert state, as compared with quiet sleep (1,2), was the same as that which was initiated by repetitive vagal stimulation of fiber groups which conduct at approximately 15 m/set (i.e., facilitation of the Mass. R. and depression of the Dig. R.). Fiber groups which conduct at approximately the same velocity of thoracic origin were found to produce desynchronization at cortical and thalamic levels, and theta activity within the hippocampus (3,6). Thus, reflex modulation produced by short periods of repetitive vagal stimulation is consistent both with vagal initiated patterns of EEG desynchronization and behavioral arousal. Therefore, we feel that the somatic reflex responsesdescribed in this and the preceding paper, and the EEG responsespreviously reported (3,6), are due to the excitation of a group of vagal afferents whose conduction velocities are similar. We believe these fibers are of thoracic origin and are capable, in addition to modulating somatic reflex of activity, of sinmhaneously initiating cortical and subcortical EEG patterns of activation. References 1. CHASE, M. H. The digastric reflex in the kitten and adult cat: paradoxical amplitude fluctuations during sleep and wakefulness. Arch. Ital. Biol., in press. 2. CHASE, M. H., D. J. MCGINTY, and M. B. STERMAN. 1968. Cyclic variation in the amplitude of a brain stem reflex during sleep and wakefulness. Experientia

3.

24 : 47-48.

CHASE, M. H., Y. NAKAMURA, C. D. CLEMENTE, and M. B. STERMAN. A&rent vagal stimulation : neurographic correlates of induced EEG chronization and desynchronization. Brain Res. 5 : 236249.

1967. syn-

VAGAI. NErROGRAh1

553

CORRELr\TES

M. H., and Y. NAKAMURA. 1968. Cortical and subcortical EEG patterns of response to afferent abdominal vagal stimulation: neurographic correlates. Physiol. Behav. 3 : 605-610. 5. CHASE, M. H., Y. NAKAMURA, and S. TORII. 1970. Afferent vagal modulation of brain stem somatic reflex activity. Exp. Newel. 27 : 534544. 6. CHASE, M. H., M. B. STERI\IAN, and C. D. CLEMENTE. 1966. Cortical and subcortical patterns of response to afferent vagal stimulation. Exp. Ncurol. 16:

4.

CHASE,

3G49. DELL,

P., and M. BONVALLET. 1966. Influences d’origine cardio-vasculaire et respiratoire sur I’activite somatique. Acta Neuroveg. 28 : 148-168. 8. FELIX, W. 19.51. Uber die Wirkung des Carotissinus auf Muskelleistung. Pfliigcrs Arch. 253 : 351-354. 9. KAUFI\~AN, W. 1938. Effects of chemical stimulation of the carotid body upon the reflex contraction of the tibialis anticus muscle. dmcr. J. Physiol. 123: 7.

677-686.

10. KOCH,

E. 1932. Die Irradiation

IVocltewchr.

der pressoreceptorischen

Kreislaufreflexe.

K/ill.

11 : 225-227.

11. PAINTAL, A. S. 1963. Vagal afferent fibers. Ergeb. Physiol. 52: 77-156. 12. PINOTTI, O., and L. GRANATA. 1953. Effetto della stimolazione dei chemocettori carotidei lsul riflesso lingua-mandibolare. Boll. Sot. Ital. Biol. Spcv 29: 3X377.

PINOTTI, O., and L. GKANATA. 1954. Azione inhibitrice dei pressocettori carotidei sul reflesso linguo-mandibolare. Boll. Sot. Ital. Biol. Sper. 30: 486-488. 14. PINOTTI, O., and L. GRANATA. 1955. Azione inhibitrice dei pressocettori carotidei sull’attivita motoria, spontanea e riflessa, nel cane. Arch. Sci. Biol. Bologna 13.

15.

39: 59-71. POMPEIANO,

O., and J. SWETT. 1962. EEG and behavioral manifestations of sleep induced by cutaneous nerve stimulation in normal cats. Arch. Ital. Biol.

100: 311-342. POMPEIANO, O.,

and J. SWETT. 1%2. Identification of cutaneous and muscular fibers producing EEG synchronization or arousal in normal cats. Arch. Ital. Biol. 100 : 343-380. 17. Rucn, T. C., and J. F. FULTON (Eds.). 1960. “Medical Physiology and Biophysics.” Saunders, Philadelphia, Pennsylvania. 18. SCHULTE, F. J., G. BUSH, and H. D. HENATSCH. 1959. Antriebssteigerungen lumbaler Extensor-Motoneurone bei Aktivierung der Chemoreceptoren im Glomus caroticum. Pfliigcrs *4rclz. 269 : 580-592. 19. TOURNADE, A., and J. MALMEJAC. 1929. Diversite des actions reflexes que declenche l’excitation du sinus carotidien et de son nerf. C. R. Sot. Biol. 100: 708-711. 16.

afferent