Brain'Research, 92 (1975) 149-152
149
© Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
Responses of motor cortex cells to tilt in the 'en,cephale isole' cat
N. BOISACQ-SCHEPENS AND M. ROUCOUX-HANUS* Laboratoire de Neurophysiologie, Universitd de Louvain, Facultd de Mddecine, Tour Claude Bernard, B - 1200 Bruxelles (Belgium)
(Accepted March 24th, 1975)
Numerous recent electrophysiological studies - - using either calorization, polarization of the labyrinth, or electrical stimulation of the whole vestibular nerve or of one of its ampullar components (see bibliography in refs. 8, 11, 12) - - have demonstrated the existence, in the cerebral cortex of the monkey and the cat, of a primary vestibular area. Moreover, some of these studies strongly suggested possible ascending projections from the ampullar system to the motor cortex in the cat. In contrast, projections from the otolithic organs - - whose natural stimulation by tilting of the animal was recently preferred to the difficult mechanical stimulation a - - have been less investigated. Until now, experiments in cats have reported on the unitary organization of otolithic inputs in the vestibular nuclei as well as in some subcortical relays (see bibliography in refs. 3, 6, 10). In the line of our previous work on motor cortex vestibular projections 11, we decided to investigate whether the static labyrinthine receptors influence this cortical area, mainly in the efferent zones devoted to the axial and proximal musculature. Results reported here have already been partly presented in abstract form g. In this experimental series, we prepared 'enc6phale isol6' cats (i.e., with spinal section at C1 level) during a short anaesthesia with ether or Brietal. A tracheotomy was performed and a venous cannula was placed in a hindlimb for easy curarization as well as an arterial cannula in the femoral artery for monitoring the blood pressure (AP). COg level and E E G were continuously monitored during the experiments. The cat was maintained in a semi-dark r o o m with its eyes covered. A large trepanation was performed on the right motor cortex where unitary recordings were confined to the precruciate area (see Fig. l) in area 6 and the medial part of area 4 following the cat cytoarchitectonic and m o t o r maps of Hassler and Muhs-Clement 7 and Nieoullon et al. 9, respectively. In brief, with such an 'enc6phale isol6' preparation in these experimental conditions, most of the inputs (visual, extero- and proprioceptive) signalling the orientation in space are eliminated while AP, E E G and COg are continuously checked. Our animals were placed in a stereotaxic apparatus on a tilting * Charg6 de Recherches, F.N.R.S., Belgium.
150 200
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Fig. 1. 'Enc~phale isol6' cat - - modulation of discharge frequency of a neurone from the right precruciate cortex during lateral tilt. On the right are shown cumulative frequency histograms with abscissa being time in seconds and ordinate the cumulated number of spikes (by addition of the number of spikes in each successive period of time). The line under the diagram shows the sequence of the stimulation. Thick line: histogram during controls and tilt to the left; dashed line: histogram during controls and tilt to the right. The slopes of the lines in the 4 control periods (A and D before tilt ; C and F after tilt) are identical and show how this unitary discharge is stable during the resting position. During a contralateral tilt (to the left) the discharge is decreased, while during an ipsilateral tilt (to the right) it is increased, as shown by the slopes of the lines (B and E respectively) during the two different stimulation periods. On the top left is shown a scheme of the cat's right motor cortex (after Woolsey14); points show the localization of our penetrations in the precruciate area. Bottom left shows examples of the spike discharge (after shaping in a Schmitt-trigger) of this neurone during the control periods (A, C, D and F) and during tilt in the two opposite directions (B and E).
table, m a n u a l l y driven (1 °/sec) a n d allowing inclinations to 25 ° a r o u n d the longitudinal (lateral tilt) or the transversal ( a n t e r o p o s t e r i o r tilt) axis. The s p o n t a n e o u s u n i t a r y a c t i v i t y o f cortical n e u r o n e s was r e c o r d e d b y means o f extracellular m i c r o p i p e t t e s ( i m p e d a n c e : 5-10 M~)) filled with a c o l o u r e d solution o f 5 % N a C I a n d 4 % P o n t a m i n e for m a r k i n g . I n some cases the m i c r o e l e c t r o d e was b r o u g h t up a n d w i t h d r a w n f r o m a cell to observe the effect o f these m e c h a n i c a l m o v e m e n t s on the s p o n t a n e o u s discharge. O n l y spikes whose a m p l i t u d e r e m a i n e d c o n s t a n t were analyzed further after shaping in a S c h m i t t - t r i g g e r before t a p e recording. Their activity is presented as frequency h i s t o g r a m s or c u m u l a t i v e frequency h i s t o g r a m s which m o r e clearly show modificat i o n s o f discharge p a t t e r n d u r i n g tilt. This p r e s e n t a t i o n o f our results is n o t s u p p o s e d to give a quantitative picture o f the o r g a n i z a t i o n o f the o t o l i t h i c i n p u t to the m o t o r cortex. O u t o f m o r e t h a n 70 cells r e c o r d e d in 8 cats, 37 were k e p t for a sufficiently long time to allow at least one c o m plete cycle o f s t i m u l a t i o n (firstly c o n t r o l lasting 3-5 min, then tilt m a i n t a i n e d for 2 - 3 min, a n d then b a c k to c o n t r o l for 3-5 min); o f these cells, 13 were dismissed for u n s t a b l e firing. A m o n g the a n a l y z e d n e u r o n e s (24), 16 showed, during tilt - - m a i n l y
151 lateral tilt (13/16) - - a modification of their discharge ranging from at least 20 ~ to 1 0 0 ~ (in two cases up to 300~). From nowon, our analysis will be devoted to a discussion of the qualitative significance of these observed fluctuations of unitary spontaneous discharge. Our results are presented in the light of previous work on tilt in primary vestibular relays a,10 which help to characterize an otolithic response. (1) The discharge rate in the tilted position shows no, or very slow, adaptation (observed in all cases). (2) The steady change in the discharge frequency occurring during tilt is repeatable (observed in 7 out of 9 tilting cycles tested for repeatability). (3) The magnitude of the response is a function of the degree of inclination (clearly related in 3 instances). (4) In some cases (4 neurones), the responses are reciprocally organized, i.e., increase of discharge for tilt in one direction and decrease for tilt in the opposite direction. Fig. 1 illustrates 2 of these characteristics, showing that there is no adaptation of the discharge frequency as long as the tilted position is maintained, and, also, that in this case the responses are reciprocally organized: increase of discharge for tilt to the right (ilzsilateral) and decrease of discharge for tilt to the left (contralateral). This type of response is reminiscent of the a-response in the classification of Peterson 1°. All these modifications of unitary frequency discharge were recorded without significant F E G changes (EEG was usually active) or blood pressure fluctuations. Although these results do not demonstrate that motor cortex neurones have functions similar to the otolithic-associated neurones in the vestibular nuclei, they strongly suggest that the activity of some cortical neurones (mainly from the axioproximal motor area) can be influenced by tilting. It is proposed that our qualitative analysis of the otolithic proFerties of response of these cortical neurones should be added to the now growing evidence for an ascending contribution of the vestibular system to the sensorimotor integration at the level of the cerebral cortex. Clear distinction between the two functions - - dynamic and static - - classically attributed to the ampullar receptors of the semi-circular canals and to the otolithic organs of the utriculus and the sacculus respectively is probably too simple, as recently suggested by Schor 13. With this idea in mind, and given recent evidence for vestibular, mostly ampullar, projections to the cerebral cortex, it seems worthwhile to investigate whether natural otolothic stimulation can modulate the excitability of some motor cortex n e u r o n e s - of interest is the zone of efferent projection to the axial and proximal musculature (area 6 and medial part of area 4) where the afferents concerned with the position of the head in gravitational field should be of functional importance. In the motor cortex, some responses we recorded show 'etolithic' properties identical to those observed for example, by Peterson 1° in the vestibular nuclei. These still preliminary data collected in our strictly controlled experimental conditions (enc6phale isol6' preparation deprived of visual input; CO2, FEG, blood pressure constant; stable firing rate of analyzed neurones) are, in our opinion, qualitative arguments which support the idea of functional influence of otolithic projections on the motor cortical area. As far as the pathways for otolithic input to the cerebral cortex are concerned, anatomical evidence for direct projections is lacking 4, and electrophysiological data are not numerous. Of particular importance in this discussion is the experimental
152 work o f P o m p e i a n o ' s school: they showed the effects of static tilts o n the activity o f n e u r o n e s i n some structures more directly related to the labyrinth, i.e., m a i n reticular f o r m a t i o n , precerebellar reticular nuclei as well as cerebellar fastigial nucleus 6. These reticular a n d cerebellar relays could be accounted for by long ascending otolithic projections to the m o t o r cortex via the thalamusS, 1~, possibly the VL nucleus as suggested by the a n a t o m o f u n c t i o n a l data (reviewed in ref. 1) concerning the precise o r g a n i z a t i o n of the cerebellothalamocortical pathways. The a u t h o r s wish to t h a n k Professor M. Meulders for advice a n d encouragem e n t d u r i n g the p r e p a r a t i o n of this work.
1 ANGAUT,P., Bases anatomo-fonctionnelles des interrelations c6r6bello-c6r6brales, J. Physiol. (Paris), 67 (1973) 53A-116A. 2 BOISACQ-SCHEPENS,N., ROUCOux-HANuS,M., ET DE WOLF,N., Effets d'une stimulation labyrinthique naturelle sur l'activit6 unitaire du cortex moteur chez le Chat 'enc6phale isol6', J. PhysioL (Paris), 69 (1974) 226A. 3 CURTOVS,I. S., AND MARKHAM,C. H., Convergence of labyrinthine influences on units in the vestibular nuclei of the cat. I. Natural stimulation, Brain Research, 35 (1971) 469-490. 4 GACEK,R., Anatomical demonstration of the vestibulo-ocular projections in the cat, Acta otolaryng. (Stockh.), Suppl. 293 (1971) 1-63. 5 GERNANDT,P. E., Discharges from utricular receptors in the cat, Exp. Neurol., 26 (1970) 203-219. 6 GHERLARDUCCI,B., POMPEIANO,O., AND SPYER,K. M., Distribution of the neuronal responses to static tilts within the cerebellar fastigial nucleus, Arch. ital. Biol., 112 (1974) 126-141. 7 HASSLER, R., UND MUHS-CLEMENT,K., Architektonischer aufbau des sensomotorischen und parietalen cortex der Katze, J. Hirnforsch., 6 (1964) 377-420. 8 JEANNEROD,M., MAGNIN, M., ET PUTKONEN,P. T. S., Int6gration thalamique et corticale des aff6rences vestibulaires, J. Physiol. (Paris), 66 (1973) 633-652. 9 NIEOULLON,A., RISPAL-PADEL,t., ET GRANGETTO,A., Repr6sentation motrice de la musculature corporelle 6tablie par stimulation intracorticale, J. Physiol. (Paris), 67 (1973) 297A. 10 PETERSON,B. W., Distribution of neural responses to tilting within vestibular nuclei of the cat, J. Neurophysiol., 33 (1970) 750-767. 11 ROUCOUX-HANuS,M. ET BOISACQ-SCHEPENS,N., Projections vestibulaires au niveau des aires corticales suprasylvienne et postcruci6e chez le Chat anesth6si6 au chloralose, Arch. itaL BioL, 112 (1974) 60-76. 12 SANS,A., Le Systdme Vestibulaire: Projections Primaires, Thalamiques, Corticales - - t~tude Neurophysiologique et Corrdlations Anatomo-fonctionnelles, Th6se en Sciences Naturelles, Montpellier, 1972. 13 SCHOR, R. H., Dynamic response of brain stem vestibular neurons, Abstr. 3rd Ann. Meet. Soc. Neurosci., San Diego, Calif., 1973, p. 143. 14 WOOLSEY,C. N., Organization of somatic sensory and motor area of the cerebral cortex. In H. F. HARLOWAND C. N. WOOLSEY(Eds.), Biological and Biochemical Basis of Behavior, University of Wisconsin Press, Madison, Wisc., 1958, pp. 63-82.
149
© Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
Responses of motor cortex cells to tilt in the 'en,cephale isole' cat
N. BOISACQ-SCHEPENS AND M. ROUCOUX-HANUS* Laboratoire de Neurophysiologie, Universitd de Louvain, Facultd de Mddecine, Tour Claude Bernard, B - 1200 Bruxelles (Belgium)
(Accepted March 24th, 1975)
Numerous recent electrophysiological studies - - using either calorization, polarization of the labyrinth, or electrical stimulation of the whole vestibular nerve or of one of its ampullar components (see bibliography in refs. 8, 11, 12) - - have demonstrated the existence, in the cerebral cortex of the monkey and the cat, of a primary vestibular area. Moreover, some of these studies strongly suggested possible ascending projections from the ampullar system to the motor cortex in the cat. In contrast, projections from the otolithic organs - - whose natural stimulation by tilting of the animal was recently preferred to the difficult mechanical stimulation a - - have been less investigated. Until now, experiments in cats have reported on the unitary organization of otolithic inputs in the vestibular nuclei as well as in some subcortical relays (see bibliography in refs. 3, 6, 10). In the line of our previous work on motor cortex vestibular projections 11, we decided to investigate whether the static labyrinthine receptors influence this cortical area, mainly in the efferent zones devoted to the axial and proximal musculature. Results reported here have already been partly presented in abstract form g. In this experimental series, we prepared 'enc6phale isol6' cats (i.e., with spinal section at C1 level) during a short anaesthesia with ether or Brietal. A tracheotomy was performed and a venous cannula was placed in a hindlimb for easy curarization as well as an arterial cannula in the femoral artery for monitoring the blood pressure (AP). COg level and E E G were continuously monitored during the experiments. The cat was maintained in a semi-dark r o o m with its eyes covered. A large trepanation was performed on the right motor cortex where unitary recordings were confined to the precruciate area (see Fig. l) in area 6 and the medial part of area 4 following the cat cytoarchitectonic and m o t o r maps of Hassler and Muhs-Clement 7 and Nieoullon et al. 9, respectively. In brief, with such an 'enc6phale isol6' preparation in these experimental conditions, most of the inputs (visual, extero- and proprioceptive) signalling the orientation in space are eliminated while AP, E E G and COg are continuously checked. Our animals were placed in a stereotaxic apparatus on a tilting * Charg6 de Recherches, F.N.R.S., Belgium.
150 200
i F I 1
~
150-
o
t'-'-" &,,.. , . J I'J
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,- .o" j
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.(
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20
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. . . . . 20
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Fig. 1. 'Enc~phale isol6' cat - - modulation of discharge frequency of a neurone from the right precruciate cortex during lateral tilt. On the right are shown cumulative frequency histograms with abscissa being time in seconds and ordinate the cumulated number of spikes (by addition of the number of spikes in each successive period of time). The line under the diagram shows the sequence of the stimulation. Thick line: histogram during controls and tilt to the left; dashed line: histogram during controls and tilt to the right. The slopes of the lines in the 4 control periods (A and D before tilt ; C and F after tilt) are identical and show how this unitary discharge is stable during the resting position. During a contralateral tilt (to the left) the discharge is decreased, while during an ipsilateral tilt (to the right) it is increased, as shown by the slopes of the lines (B and E respectively) during the two different stimulation periods. On the top left is shown a scheme of the cat's right motor cortex (after Woolsey14); points show the localization of our penetrations in the precruciate area. Bottom left shows examples of the spike discharge (after shaping in a Schmitt-trigger) of this neurone during the control periods (A, C, D and F) and during tilt in the two opposite directions (B and E).
table, m a n u a l l y driven (1 °/sec) a n d allowing inclinations to 25 ° a r o u n d the longitudinal (lateral tilt) or the transversal ( a n t e r o p o s t e r i o r tilt) axis. The s p o n t a n e o u s u n i t a r y a c t i v i t y o f cortical n e u r o n e s was r e c o r d e d b y means o f extracellular m i c r o p i p e t t e s ( i m p e d a n c e : 5-10 M~)) filled with a c o l o u r e d solution o f 5 % N a C I a n d 4 % P o n t a m i n e for m a r k i n g . I n some cases the m i c r o e l e c t r o d e was b r o u g h t up a n d w i t h d r a w n f r o m a cell to observe the effect o f these m e c h a n i c a l m o v e m e n t s on the s p o n t a n e o u s discharge. O n l y spikes whose a m p l i t u d e r e m a i n e d c o n s t a n t were analyzed further after shaping in a S c h m i t t - t r i g g e r before t a p e recording. Their activity is presented as frequency h i s t o g r a m s or c u m u l a t i v e frequency h i s t o g r a m s which m o r e clearly show modificat i o n s o f discharge p a t t e r n d u r i n g tilt. This p r e s e n t a t i o n o f our results is n o t s u p p o s e d to give a quantitative picture o f the o r g a n i z a t i o n o f the o t o l i t h i c i n p u t to the m o t o r cortex. O u t o f m o r e t h a n 70 cells r e c o r d e d in 8 cats, 37 were k e p t for a sufficiently long time to allow at least one c o m plete cycle o f s t i m u l a t i o n (firstly c o n t r o l lasting 3-5 min, then tilt m a i n t a i n e d for 2 - 3 min, a n d then b a c k to c o n t r o l for 3-5 min); o f these cells, 13 were dismissed for u n s t a b l e firing. A m o n g the a n a l y z e d n e u r o n e s (24), 16 showed, during tilt - - m a i n l y
151 lateral tilt (13/16) - - a modification of their discharge ranging from at least 20 ~ to 1 0 0 ~ (in two cases up to 300~). From nowon, our analysis will be devoted to a discussion of the qualitative significance of these observed fluctuations of unitary spontaneous discharge. Our results are presented in the light of previous work on tilt in primary vestibular relays a,10 which help to characterize an otolithic response. (1) The discharge rate in the tilted position shows no, or very slow, adaptation (observed in all cases). (2) The steady change in the discharge frequency occurring during tilt is repeatable (observed in 7 out of 9 tilting cycles tested for repeatability). (3) The magnitude of the response is a function of the degree of inclination (clearly related in 3 instances). (4) In some cases (4 neurones), the responses are reciprocally organized, i.e., increase of discharge for tilt in one direction and decrease for tilt in the opposite direction. Fig. 1 illustrates 2 of these characteristics, showing that there is no adaptation of the discharge frequency as long as the tilted position is maintained, and, also, that in this case the responses are reciprocally organized: increase of discharge for tilt to the right (ilzsilateral) and decrease of discharge for tilt to the left (contralateral). This type of response is reminiscent of the a-response in the classification of Peterson 1°. All these modifications of unitary frequency discharge were recorded without significant F E G changes (EEG was usually active) or blood pressure fluctuations. Although these results do not demonstrate that motor cortex neurones have functions similar to the otolithic-associated neurones in the vestibular nuclei, they strongly suggest that the activity of some cortical neurones (mainly from the axioproximal motor area) can be influenced by tilting. It is proposed that our qualitative analysis of the otolithic proFerties of response of these cortical neurones should be added to the now growing evidence for an ascending contribution of the vestibular system to the sensorimotor integration at the level of the cerebral cortex. Clear distinction between the two functions - - dynamic and static - - classically attributed to the ampullar receptors of the semi-circular canals and to the otolithic organs of the utriculus and the sacculus respectively is probably too simple, as recently suggested by Schor 13. With this idea in mind, and given recent evidence for vestibular, mostly ampullar, projections to the cerebral cortex, it seems worthwhile to investigate whether natural otolothic stimulation can modulate the excitability of some motor cortex n e u r o n e s - of interest is the zone of efferent projection to the axial and proximal musculature (area 6 and medial part of area 4) where the afferents concerned with the position of the head in gravitational field should be of functional importance. In the motor cortex, some responses we recorded show 'etolithic' properties identical to those observed for example, by Peterson 1° in the vestibular nuclei. These still preliminary data collected in our strictly controlled experimental conditions (enc6phale isol6' preparation deprived of visual input; CO2, FEG, blood pressure constant; stable firing rate of analyzed neurones) are, in our opinion, qualitative arguments which support the idea of functional influence of otolithic projections on the motor cortical area. As far as the pathways for otolithic input to the cerebral cortex are concerned, anatomical evidence for direct projections is lacking 4, and electrophysiological data are not numerous. Of particular importance in this discussion is the experimental
152 work o f P o m p e i a n o ' s school: they showed the effects of static tilts o n the activity o f n e u r o n e s i n some structures more directly related to the labyrinth, i.e., m a i n reticular f o r m a t i o n , precerebellar reticular nuclei as well as cerebellar fastigial nucleus 6. These reticular a n d cerebellar relays could be accounted for by long ascending otolithic projections to the m o t o r cortex via the thalamusS, 1~, possibly the VL nucleus as suggested by the a n a t o m o f u n c t i o n a l data (reviewed in ref. 1) concerning the precise o r g a n i z a t i o n of the cerebellothalamocortical pathways. The a u t h o r s wish to t h a n k Professor M. Meulders for advice a n d encouragem e n t d u r i n g the p r e p a r a t i o n of this work.
1 ANGAUT,P., Bases anatomo-fonctionnelles des interrelations c6r6bello-c6r6brales, J. Physiol. (Paris), 67 (1973) 53A-116A. 2 BOISACQ-SCHEPENS,N., ROUCOux-HANuS,M., ET DE WOLF,N., Effets d'une stimulation labyrinthique naturelle sur l'activit6 unitaire du cortex moteur chez le Chat 'enc6phale isol6', J. PhysioL (Paris), 69 (1974) 226A. 3 CURTOVS,I. S., AND MARKHAM,C. H., Convergence of labyrinthine influences on units in the vestibular nuclei of the cat. I. Natural stimulation, Brain Research, 35 (1971) 469-490. 4 GACEK,R., Anatomical demonstration of the vestibulo-ocular projections in the cat, Acta otolaryng. (Stockh.), Suppl. 293 (1971) 1-63. 5 GERNANDT,P. E., Discharges from utricular receptors in the cat, Exp. Neurol., 26 (1970) 203-219. 6 GHERLARDUCCI,B., POMPEIANO,O., AND SPYER,K. M., Distribution of the neuronal responses to static tilts within the cerebellar fastigial nucleus, Arch. ital. Biol., 112 (1974) 126-141. 7 HASSLER, R., UND MUHS-CLEMENT,K., Architektonischer aufbau des sensomotorischen und parietalen cortex der Katze, J. Hirnforsch., 6 (1964) 377-420. 8 JEANNEROD,M., MAGNIN, M., ET PUTKONEN,P. T. S., Int6gration thalamique et corticale des aff6rences vestibulaires, J. Physiol. (Paris), 66 (1973) 633-652. 9 NIEOULLON,A., RISPAL-PADEL,t., ET GRANGETTO,A., Repr6sentation motrice de la musculature corporelle 6tablie par stimulation intracorticale, J. Physiol. (Paris), 67 (1973) 297A. 10 PETERSON,B. W., Distribution of neural responses to tilting within vestibular nuclei of the cat, J. Neurophysiol., 33 (1970) 750-767. 11 ROUCOUX-HANuS,M. ET BOISACQ-SCHEPENS,N., Projections vestibulaires au niveau des aires corticales suprasylvienne et postcruci6e chez le Chat anesth6si6 au chloralose, Arch. itaL BioL, 112 (1974) 60-76. 12 SANS,A., Le Systdme Vestibulaire: Projections Primaires, Thalamiques, Corticales - - t~tude Neurophysiologique et Corrdlations Anatomo-fonctionnelles, Th6se en Sciences Naturelles, Montpellier, 1972. 13 SCHOR, R. H., Dynamic response of brain stem vestibular neurons, Abstr. 3rd Ann. Meet. Soc. Neurosci., San Diego, Calif., 1973, p. 143. 14 WOOLSEY,C. N., Organization of somatic sensory and motor area of the cerebral cortex. In H. F. HARLOWAND C. N. WOOLSEY(Eds.), Biological and Biochemical Basis of Behavior, University of Wisconsin Press, Madison, Wisc., 1958, pp. 63-82.
