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Responses evoked in neurones of the fastigial nucleus by cutaneous mechanoreceptors
523
BRAIN RESEARCH
Short Communications Responses evoked in neurone,s of the fastlgial nucleus by cutaneous mechanoreceptors Stimulation of cutaneous mechanoreceptors by taps to the toe pads, by pressure on the toe pads and by brief air jets to the hairy skin exerts excitatory and inhibitory actions on Purkyn~ cells of the lateral vermis of the cat cerebellum 6. These Purkynfi cells project directly to the neurones of the fastigial nucleus (see ref. 5, Ch. XII). We have therefore investigated the response of fastigial neurones using the same procedures as for the Purkyn~ cellsZ, 6. It proved much easier to record the spike responses of single units in isolation than with Purkyn~ cells. Altogether 460 fastigial neurones were investigated in our 25 experiments~ 17 being on decerebrate unanaesthetized cats, the
Responsesof fastigial neuronesto cutaneous mechanoreceptors
A FT50.2 mm
SR ST ~-.
PER ST
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.~"
~
FCP1.6 mm
,,,,""
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50 ms
F
FT2 ] . 6 ~ m..my /
, f ..... FT4
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.
'
50 ms Fig. 1. In A are specimen records of spike responses of a fastigial neurone, both spontaneous and evoked by a tap to toe 5 of the forefoot, that is shown in the lower traces. B and C give the poststimulus time histograms (PSTH) and cumulative frequency distributions (CFD) for responses evoked in this same neurone by stimulation of superficial radial (SR) and common peroneal nerves (PER). D and E are PSTHs and CFDs evoked by taps, as in A, to forefoot central pad, FCP, and hindfoot central pad, HCP. In F are the CFDs for taps to the 4 toe pads of the forefoot as shown, and similarly for the hindfoot in G. All PSTHs and CFDs are averages of 64 responses, there being the same time scale for all. Count scale for PSTHs give counts per bin of 0.5 msec, while count scale for CFDs gives counts per single response. Onsets of stimulation are signalled by short vertical lines, and with CFDs the background rate of firing is projected as the dotted lines.
Brain Research, 35 (1971) 523-527
524
SHORT COMMUNICATIONS
remaining being under light barbiturate or chloralose anaesthesia. The extreme lateral zone of the fastigial nucleus is the preferred site for projection from the foot pads, the zone ofhindfoot dominance tending to be anterior to that of the forefoot, as would be expected from anatomical and physiological studies 1,9, but many neurones were influenced from both hindfoot and forefoot on the ipsilateral side. In Fig. IA are specimen records of the spike responses of a fastigial neurone. A tap of 1.6 mm and 15 msec duration to the pad of toe 5 of the forefoot (FT5) appeared to cause a transient increase and later depression of the spontaneous firing. Averaging of 64 traces shows that there was in fact a large increase in firing, with a latency of 22 msec, followed by a virtual silence from 35 to 77 msec after the onset of the tap, as may be seen in the cumulative frequency distribution (CFD) of FT5 in Fig. 1F. This fastigial neurone exhibited an initial excitation and later inhibition for all pads of the forefoot, the central pad in D and the 4 toe pads in F, the strongest influences being from T3, T4 and T5. An even stronger E-I sequence was evoked from the superficial radial nerve (SR) in Fig. lB. Stimulation of the hindlimb nerve, common peroneal, PER, was less effective, but a clear E-I response is seen in C. Correspondingly, taps to the pads of the hindfoot were much less effective (E, G) than those to the forefoot (D, F). This fastigial neurone exhibited the same E-I sequence in response to air jet stimulation of hair receptors of both forelimb and hindlimb, the former being much more effective, but there was no tonic response to pressure of 2 sec duration on the central pads of either foot. Fig. I illustrates the usual type of response with an E-I sequence. Some neurones were selectively fired from hindfoot, others from forefoot, others again from both with dominance either from forefoot (Fig. 1) or hindfoot or approximate equivalence (Fig. 2C, D). Yet other neurones gave only inhibitory responses, or only excitatory responses (cJ~ Fig. 2C, D). These unusual observations are of importance because they illustrate the composite nature of the usual responses (cf. Fig. 1), which can be assumed to arise because of the convergence onto the same fastigial neurone of excitatory impulses from one pathway and inhibitory impulses from another. It has frequently been observed that inhibition begins about 2 msec before it is briefly overwhelmed by the strong excitation, only to reassert its action in the later prolonged inhibition characteristically seen in Fig. 1. This composite nature~of the E-I response can be revealed by increasing the repetition rate of the stimulation, which causes diminution of the inhibition relative to the excitation. Such a selective change may also be seen when the stimulus strength is reduced. According to the generally accepted mode of operation of the neurones of the intracerebellar nuclei, including the fastigial, incoming mossy and climbing fibres give collateral branches to these neurones (cf. ref. 5, Figs. 124, 141). The other important influence on these neurones is the inhibitory action by the synapses of Purkyn~ cell axons (cf. ref. 5, Fig. 132). This latter effect accounts quite satisfactorily for the observed inhibition on the fastigial neurones that is typically illustrated in Fig. 1. In parentheses, it should be noted that the inhibitory depression often observed for the Purkyn6 cell dischargeZ, 4 would be expected to result in a disinhibitory effect, which Brain Research, 35 (197I) 523-527
SHORT COMMUNICATIONS
525
Responses fastigial neurone to LRN stimulation A
LRN 9V I I_l tlrnVl+
Btr~ 'N ~
PSTH C
DpERST ~
........
120
.... Y, ~:~.
12
.¢~.,Z .............
50 ms
..~_z................. E
LRN 9V
G LRN 9 V . ~ •
........
LRN 5V
H I
lOms
LRN 9V(P)
)C
10 ms
Fig. 2. Responses of fastigial neuronelto~stimulation of lateral reticular nucleus (LRN). A, Specimen records of responses of a fastigial neurone to stimulation of LRN (9 V, 0.05 msec duration, 2 mm tip separation). B, PSTH and CFD for average of 32 responses as in A. C and D are PSTHs and CFDs similarly recorded but for responses evoked by stimulation of superficial radial (SR) and common peroneal (PER) nerves. E is similar to A but recorded at much faster sweep speed, F being the CFD. G is the CFD when the stimulus was reduced to 5 V and H when the stimulus of 9 V was applied slightly more posteriorly in the LRN. PSTHs and CFD of B to D have same scales, as also do the CFDs of F to H. I illustrates diagrammatically the essential features of the pathways to a fastigial neurone by cutaneous mechanoreceptors as modified in the light of the present investigation. PC, PA, Purkyn6 cell and axon; BC, basket cell; GrC, granule cells; PF, parallel fibres; MF, mossy fibres; CF, climbing fibre; FC, fastigial cell; LRC, lateral reticular cell; IOC, inferior olive cell.
w o u l d a c c o u n t for the p r o l o n g e d excitation o f the target neurones t h a t is sometimes o b s e r v e d (cf. Fig. 2D, a n d ref. 5, Ch. XIV). The latency o f the e x c i t a t o r y a c t i o n o f the nerve s t i m u l a t i o n is a b o u t 14 msec for b o t h nerves in Fig. 1B a n d C. Latencies o f the excitatory a c t i o n o f nerve volleys in Fig. 2C a n d D were 16 a n d 17 msec respectively. These latencies are m u c h longer t h a n w o u l d be expected if the excitation was p r o d u c e d by collaterals f r o m the fast m o s s y fiber inputs such as the d o r s a l spinocerebellar t r a c t ( D S C T ) a n d the cuneocerebellar t r a c t (CCT). F o r example, in the cerebellar cortex the latency is only 6-9 msec for mossy fibre impulses generated b y h i n d l i m b nerve s t i m u l a t i o n 2. I n averaged m a s s discharges r e c o r d e d f r o m the r e s t i f o r m b o d y the latency o f responses to nerve stimulat i o n was as b r i e f as 4.5-5 msec for the D S C T a n d 2.5-3 msec for the C C T 7. Y e t the excitation o f the fastigial n e u r o n e s regularly h a d a latency as long as 14-16 msec for b o t h h i n d l i m b a n d f o r e l i m b nerve s t i m u l a t i o n (cf. Figs. 1 a n d 2). T h e only exceptions were a r a r e l y o b s e r v e d slight excitation at 6-10 msec. Evidently collaterals f r o m the fast m o s s y fibre p a t h w a y s c a n n o t be directly responsible in the m a n n e r hitherto assumed. T w o p a t h w a y s for p r o d u c i n g the delayed excitation c o u l d be the climbing fibre p a t h w a y , a n d the reticulo-cerebellar p a t h w a y , which latter h i t h e r t o has n o t been specially p r o p o s e d for this role. The specimen records o f Fig. 2 A show t h a t stimulaBrain Research, 35 (1971) 523-527
526
SHORT COMMUNICATIONS
tion of the lateral reticular nucleus (LRN) by a stereotaxically inserted bipolar electrode excited the fastigial neurone with a very brief latency, which in the faster records of Fig. 2E is seen to vary from 2.3 to 3.7 msec. This fastigial neurone responded to nerve volleys with the characteristic long latencies of 16 msec and 17 msec in Fig. 2C and D respectively, which contrasts with early sharp excitation and later depression in Fig. 2B. This excitation by the L R N stimulus is well seen in the fast specimen records of Fig. 2E and the CFD derived therefrom in F, where there was a single response to almost every stimulus with a latency range of 2.3 to 3.8 msec. When the L R N stimulus was weaker (Fig. 2G) or applied to the nucleus more posteriorly (Fig. 2H) the initial response was less frequent, the deficiency being replaced by a response at a latency of 6.5-8 msec. The earlier and later responses of Fig. 2 F - H were regularly observed in the 60 fastigial neurones subjected to L R N stimulation. In the fast records of Fig. 2E a brief diphasic wave, positive-negative, immediately follows the stimulus artefact, the onset of the negative wave being at 0.4-0.5 msec, which is in good accord with the value of 0.6-0.8 msec observed by Sasaki and Strata 1° for the longer conduction distance for the L R N impulses to the cerebellar cortex. Thus this initial diphasic wave is attributable to a volley in the reticulo-cerebellar fibres. It is followed by a slower negative wave with a latency of 1.4 msec, from which the spikes arise. Evidently axon collaterals from the reticular cerebellar fibres are producing a depolarization of the fastigial neurones by synaptic excitation, and this in turn generates its discharge after a variable latency, which may be quite long after an immediately preceding discharge (cf. third record of Fig. 2E). The responses occurring with latencies in excess of 6.5 msec in Fig. 2G and H possibly arise when excitation is particularly effective on presynaptic fibres in the LRN, which then cause a delayed discharge via interneuronal relays. Volleys in both forelimb and hindlimb nerves evoke discharges from L R N to the cerebellum with latencies of7-11 msec, as tested by averaging of potentials produced by mass discharges 7, and with latencies usually of 10-20 msec for individual units 8. The time for generation of an impulse discharge by a fastigial neurone was at least 1.5 msec after an L R N stimulus. Addition onto the latency for L R N discharge gives latencies of the excitatory responses of fastigial neurones which are in satisfactory agreement with those observed (cf. Figs. 1B, C and 2C, D). In addition excitatory action by climbing fibre collaterals would contribute, because excitation of the contralateral inferior olive also evoked discharge of fastigial neurones, and this pathway also has the necessary long latency. Only part of our investigations is reported here. Cutaneous mechanoreceptors have been shown to influence fastigial neurones via the pathways that in essentials are shown in the form of the amended diagram in Fig. 2I. This structural design has the functional feature that the excitation of fastigial neurones is delayed so that, in response to some peripheral stimulus, it is produced almost synchronously with the inhibitory action evoked by the fast mossy fibre pathways via the Purkyn~ cell discharge.
Brain Research, 35 (1971) 523-527
527
SHORT COMMUNICATIONS
This work was supported by a g r a n t from the N a t i o n a l Institute of Neurological Diseases a n d Stroke, G r a n t No. R01NB0822101,2,3 a n d by generous research s u p p o r t by Dr. H e n r y C. a n d Bertha H. Buswell F u n d to J. C. Eccles a n d H. T~tbo~ikov~. N. H. Sabah is a postdoctoral fellow, U H F G r a n t No. F T F - 3 - U B - 7 0 . Departments of Physiology attd Biophysics, School of Medicine, State University of New York, Buffalo, N. Y. 14214 (U.S.A.)
JOHN C. ECCLES NASSIR H. SABAH HELENA T~BOR.~KOVA_
I BRODAL,A., Anatomical studies of cerebellar fibre connections with special reference to problems of functional localization. In C. A. Fox AND R. S. SNIDER (Eds.), Progress in Brain Research, VoL 25, Elsevier Amsterdam, 1967, pp. 135-173. 2 ECCLES,J. C., FABER,D. S., MURPHY,J. T., SABAH,N. H., ANDT/~BOP,IKOV.~,H., Afferent volleys in limb nerves influencingimpulse discharges in cerebellar cortex. I. In mossy fibers and granule cells, Exp. Brain Res., 13 (1971) 15-35. 3 ECCLES,J. C., FABER,D. S., MURPHY,J. T., SABAH,N. H., ANDT.~BOP,IKOVA,H., Afferent volleys in limb nerves influencingimpulse discharges in cerebeUar cortex. II. In Purkyn6 cells, Exp. Brain Res., 13 (1971) 36--53. 4 ECCLES,J. C., FABER,D. S., MURVHY,J. T., SABAH,N. H., AND T/,BOfiiKOVA,H., Investigations on integration of mossy fiber inputs to Purkyn~ cells in the anterior lobe, Exp. Brain Res., 13 (1971) 54-77. 5 ECCLES,J. C., ITo, M., AND SZENT~GOTI-tAI,J., The Cerebellum as a Neuronal Machine, Springer, Berlin, 1967, 335 pp. 6 ECCLES, J. C., SABAH, N.H., SCRMIDT, R. F., AND T~BO~iICOV/~,H., Cerebellar Purkyn~ cell responses to cutaneous mechanoreceptors, Brain Research, 30 (1971) 419-424. 7 GRANT, G., OSCARSSON,O., AND ROS~N, I., Functional organization of the spinoreticulo-cerebellar path with identification of its spinal component, Exp. Brain Res., 1 (1966) 306-319. 80SCARSSON, O., AND ROSEN, I., Response characteristics of reticulo-cerebellar neurones activated from spinal afferents, Exp. Brain Res., 1 (1966) 320-328. 9 POMPEIANO, O., Functional organization of the cerebellar projections to the spinal cord. In C. A. Fox AND R. S. SNIDER (Eds.), Progress in Brain Research, Vol. 25, Elsevier, Amsterdam, 1967, pp. 282-321. 10 SASAKI,K., ANt)SXRATA,P., Responses evoked in the cerebellar cortex by stimulating mossy fiber pathways to the cerebellum, Exp. Brain Res., 3 (1967) 95-110. (Accepted October llth, 1971)
Brain Research, 35 (1971) 523-527
BRAIN RESEARCH
Short Communications Responses evoked in neurone,s of the fastlgial nucleus by cutaneous mechanoreceptors Stimulation of cutaneous mechanoreceptors by taps to the toe pads, by pressure on the toe pads and by brief air jets to the hairy skin exerts excitatory and inhibitory actions on Purkyn~ cells of the lateral vermis of the cat cerebellum 6. These Purkynfi cells project directly to the neurones of the fastigial nucleus (see ref. 5, Ch. XII). We have therefore investigated the response of fastigial neurones using the same procedures as for the Purkyn~ cellsZ, 6. It proved much easier to record the spike responses of single units in isolation than with Purkyn~ cells. Altogether 460 fastigial neurones were investigated in our 25 experiments~ 17 being on decerebrate unanaesthetized cats, the
Responsesof fastigial neuronesto cutaneous mechanoreceptors
A FT50.2 mm
SR ST ~-.
PER ST
• , ... .
.~"
~
FCP1.6 mm
,,,,""
Z~lEr_¢E>- . . ~-',~:'t~'~'~
_,
j
dl
HCP1.6 mm
50 ms
F
FT2 ] . 6 ~ m..my /
, f ..... FT4
Gr21. H
..........
HT3
........
,f .... , ....-"~"
......
FT3 r...- ..........Y
50 ms
FT5
...' r-...-
HT4
..............
.
'
50 ms Fig. 1. In A are specimen records of spike responses of a fastigial neurone, both spontaneous and evoked by a tap to toe 5 of the forefoot, that is shown in the lower traces. B and C give the poststimulus time histograms (PSTH) and cumulative frequency distributions (CFD) for responses evoked in this same neurone by stimulation of superficial radial (SR) and common peroneal nerves (PER). D and E are PSTHs and CFDs evoked by taps, as in A, to forefoot central pad, FCP, and hindfoot central pad, HCP. In F are the CFDs for taps to the 4 toe pads of the forefoot as shown, and similarly for the hindfoot in G. All PSTHs and CFDs are averages of 64 responses, there being the same time scale for all. Count scale for PSTHs give counts per bin of 0.5 msec, while count scale for CFDs gives counts per single response. Onsets of stimulation are signalled by short vertical lines, and with CFDs the background rate of firing is projected as the dotted lines.
Brain Research, 35 (1971) 523-527
524
SHORT COMMUNICATIONS
remaining being under light barbiturate or chloralose anaesthesia. The extreme lateral zone of the fastigial nucleus is the preferred site for projection from the foot pads, the zone ofhindfoot dominance tending to be anterior to that of the forefoot, as would be expected from anatomical and physiological studies 1,9, but many neurones were influenced from both hindfoot and forefoot on the ipsilateral side. In Fig. IA are specimen records of the spike responses of a fastigial neurone. A tap of 1.6 mm and 15 msec duration to the pad of toe 5 of the forefoot (FT5) appeared to cause a transient increase and later depression of the spontaneous firing. Averaging of 64 traces shows that there was in fact a large increase in firing, with a latency of 22 msec, followed by a virtual silence from 35 to 77 msec after the onset of the tap, as may be seen in the cumulative frequency distribution (CFD) of FT5 in Fig. 1F. This fastigial neurone exhibited an initial excitation and later inhibition for all pads of the forefoot, the central pad in D and the 4 toe pads in F, the strongest influences being from T3, T4 and T5. An even stronger E-I sequence was evoked from the superficial radial nerve (SR) in Fig. lB. Stimulation of the hindlimb nerve, common peroneal, PER, was less effective, but a clear E-I response is seen in C. Correspondingly, taps to the pads of the hindfoot were much less effective (E, G) than those to the forefoot (D, F). This fastigial neurone exhibited the same E-I sequence in response to air jet stimulation of hair receptors of both forelimb and hindlimb, the former being much more effective, but there was no tonic response to pressure of 2 sec duration on the central pads of either foot. Fig. I illustrates the usual type of response with an E-I sequence. Some neurones were selectively fired from hindfoot, others from forefoot, others again from both with dominance either from forefoot (Fig. 1) or hindfoot or approximate equivalence (Fig. 2C, D). Yet other neurones gave only inhibitory responses, or only excitatory responses (cJ~ Fig. 2C, D). These unusual observations are of importance because they illustrate the composite nature of the usual responses (cf. Fig. 1), which can be assumed to arise because of the convergence onto the same fastigial neurone of excitatory impulses from one pathway and inhibitory impulses from another. It has frequently been observed that inhibition begins about 2 msec before it is briefly overwhelmed by the strong excitation, only to reassert its action in the later prolonged inhibition characteristically seen in Fig. 1. This composite nature~of the E-I response can be revealed by increasing the repetition rate of the stimulation, which causes diminution of the inhibition relative to the excitation. Such a selective change may also be seen when the stimulus strength is reduced. According to the generally accepted mode of operation of the neurones of the intracerebellar nuclei, including the fastigial, incoming mossy and climbing fibres give collateral branches to these neurones (cf. ref. 5, Figs. 124, 141). The other important influence on these neurones is the inhibitory action by the synapses of Purkyn~ cell axons (cf. ref. 5, Fig. 132). This latter effect accounts quite satisfactorily for the observed inhibition on the fastigial neurones that is typically illustrated in Fig. 1. In parentheses, it should be noted that the inhibitory depression often observed for the Purkyn6 cell dischargeZ, 4 would be expected to result in a disinhibitory effect, which Brain Research, 35 (197I) 523-527
SHORT COMMUNICATIONS
525
Responses fastigial neurone to LRN stimulation A
LRN 9V I I_l tlrnVl+
Btr~ 'N ~
PSTH C
DpERST ~
........
120
.... Y, ~:~.
12
.¢~.,Z .............
50 ms
..~_z................. E
LRN 9V
G LRN 9 V . ~ •
........
LRN 5V
H I
lOms
LRN 9V(P)
)C
10 ms
Fig. 2. Responses of fastigial neuronelto~stimulation of lateral reticular nucleus (LRN). A, Specimen records of responses of a fastigial neurone to stimulation of LRN (9 V, 0.05 msec duration, 2 mm tip separation). B, PSTH and CFD for average of 32 responses as in A. C and D are PSTHs and CFDs similarly recorded but for responses evoked by stimulation of superficial radial (SR) and common peroneal (PER) nerves. E is similar to A but recorded at much faster sweep speed, F being the CFD. G is the CFD when the stimulus was reduced to 5 V and H when the stimulus of 9 V was applied slightly more posteriorly in the LRN. PSTHs and CFD of B to D have same scales, as also do the CFDs of F to H. I illustrates diagrammatically the essential features of the pathways to a fastigial neurone by cutaneous mechanoreceptors as modified in the light of the present investigation. PC, PA, Purkyn6 cell and axon; BC, basket cell; GrC, granule cells; PF, parallel fibres; MF, mossy fibres; CF, climbing fibre; FC, fastigial cell; LRC, lateral reticular cell; IOC, inferior olive cell.
w o u l d a c c o u n t for the p r o l o n g e d excitation o f the target neurones t h a t is sometimes o b s e r v e d (cf. Fig. 2D, a n d ref. 5, Ch. XIV). The latency o f the e x c i t a t o r y a c t i o n o f the nerve s t i m u l a t i o n is a b o u t 14 msec for b o t h nerves in Fig. 1B a n d C. Latencies o f the excitatory a c t i o n o f nerve volleys in Fig. 2C a n d D were 16 a n d 17 msec respectively. These latencies are m u c h longer t h a n w o u l d be expected if the excitation was p r o d u c e d by collaterals f r o m the fast m o s s y fiber inputs such as the d o r s a l spinocerebellar t r a c t ( D S C T ) a n d the cuneocerebellar t r a c t (CCT). F o r example, in the cerebellar cortex the latency is only 6-9 msec for mossy fibre impulses generated b y h i n d l i m b nerve s t i m u l a t i o n 2. I n averaged m a s s discharges r e c o r d e d f r o m the r e s t i f o r m b o d y the latency o f responses to nerve stimulat i o n was as b r i e f as 4.5-5 msec for the D S C T a n d 2.5-3 msec for the C C T 7. Y e t the excitation o f the fastigial n e u r o n e s regularly h a d a latency as long as 14-16 msec for b o t h h i n d l i m b a n d f o r e l i m b nerve s t i m u l a t i o n (cf. Figs. 1 a n d 2). T h e only exceptions were a r a r e l y o b s e r v e d slight excitation at 6-10 msec. Evidently collaterals f r o m the fast m o s s y fibre p a t h w a y s c a n n o t be directly responsible in the m a n n e r hitherto assumed. T w o p a t h w a y s for p r o d u c i n g the delayed excitation c o u l d be the climbing fibre p a t h w a y , a n d the reticulo-cerebellar p a t h w a y , which latter h i t h e r t o has n o t been specially p r o p o s e d for this role. The specimen records o f Fig. 2 A show t h a t stimulaBrain Research, 35 (1971) 523-527
526
SHORT COMMUNICATIONS
tion of the lateral reticular nucleus (LRN) by a stereotaxically inserted bipolar electrode excited the fastigial neurone with a very brief latency, which in the faster records of Fig. 2E is seen to vary from 2.3 to 3.7 msec. This fastigial neurone responded to nerve volleys with the characteristic long latencies of 16 msec and 17 msec in Fig. 2C and D respectively, which contrasts with early sharp excitation and later depression in Fig. 2B. This excitation by the L R N stimulus is well seen in the fast specimen records of Fig. 2E and the CFD derived therefrom in F, where there was a single response to almost every stimulus with a latency range of 2.3 to 3.8 msec. When the L R N stimulus was weaker (Fig. 2G) or applied to the nucleus more posteriorly (Fig. 2H) the initial response was less frequent, the deficiency being replaced by a response at a latency of 6.5-8 msec. The earlier and later responses of Fig. 2 F - H were regularly observed in the 60 fastigial neurones subjected to L R N stimulation. In the fast records of Fig. 2E a brief diphasic wave, positive-negative, immediately follows the stimulus artefact, the onset of the negative wave being at 0.4-0.5 msec, which is in good accord with the value of 0.6-0.8 msec observed by Sasaki and Strata 1° for the longer conduction distance for the L R N impulses to the cerebellar cortex. Thus this initial diphasic wave is attributable to a volley in the reticulo-cerebellar fibres. It is followed by a slower negative wave with a latency of 1.4 msec, from which the spikes arise. Evidently axon collaterals from the reticular cerebellar fibres are producing a depolarization of the fastigial neurones by synaptic excitation, and this in turn generates its discharge after a variable latency, which may be quite long after an immediately preceding discharge (cf. third record of Fig. 2E). The responses occurring with latencies in excess of 6.5 msec in Fig. 2G and H possibly arise when excitation is particularly effective on presynaptic fibres in the LRN, which then cause a delayed discharge via interneuronal relays. Volleys in both forelimb and hindlimb nerves evoke discharges from L R N to the cerebellum with latencies of7-11 msec, as tested by averaging of potentials produced by mass discharges 7, and with latencies usually of 10-20 msec for individual units 8. The time for generation of an impulse discharge by a fastigial neurone was at least 1.5 msec after an L R N stimulus. Addition onto the latency for L R N discharge gives latencies of the excitatory responses of fastigial neurones which are in satisfactory agreement with those observed (cf. Figs. 1B, C and 2C, D). In addition excitatory action by climbing fibre collaterals would contribute, because excitation of the contralateral inferior olive also evoked discharge of fastigial neurones, and this pathway also has the necessary long latency. Only part of our investigations is reported here. Cutaneous mechanoreceptors have been shown to influence fastigial neurones via the pathways that in essentials are shown in the form of the amended diagram in Fig. 2I. This structural design has the functional feature that the excitation of fastigial neurones is delayed so that, in response to some peripheral stimulus, it is produced almost synchronously with the inhibitory action evoked by the fast mossy fibre pathways via the Purkyn~ cell discharge.
Brain Research, 35 (1971) 523-527
527
SHORT COMMUNICATIONS
This work was supported by a g r a n t from the N a t i o n a l Institute of Neurological Diseases a n d Stroke, G r a n t No. R01NB0822101,2,3 a n d by generous research s u p p o r t by Dr. H e n r y C. a n d Bertha H. Buswell F u n d to J. C. Eccles a n d H. T~tbo~ikov~. N. H. Sabah is a postdoctoral fellow, U H F G r a n t No. F T F - 3 - U B - 7 0 . Departments of Physiology attd Biophysics, School of Medicine, State University of New York, Buffalo, N. Y. 14214 (U.S.A.)
JOHN C. ECCLES NASSIR H. SABAH HELENA T~BOR.~KOVA_
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Brain Research, 35 (1971) 523-527