Cerebellar Purkyneˇcell responses to cutaneous mechanoreceptors

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419

Cerebellar Purkyn6 cell responses to cutaneous mechanoreceptors Volleys in cutaneous afferent nerves powerfully influence the spontaneous discharges of some Purkyn~ cells in the anterior lobe2, 3. There may be excitation by the climbing fiber pathways or excitation and/or inhibition by mossy fiber pathways, or various combinations of these effects. This survey has been fully reported 4-6 and forms the basis for extensive investigations on the responses evoked in Purkyn6 cells by adequate stimulation of various mechanoreceptors. Investigations on cutaneous input to the cerebellum were conducted first by Adrian 1 and Snider and Stowel112, but were imprecise both in the mechanical stimulation and in the cerebellar recording. This preliminary report will be restricted to the effects produced by mossy fiber pathways from mechanoreceptors of the pads and the adjacent hairy skin in the hindlimb and forelimb. The 51 experiments have been performed either on unanesthetized cats that were decerebrated during an initial ether or halothane anesthesia (28), or on cats anesthetized by light pentothal (17) or by chloralose (6). All cats were immobilized by Flaxedil and there was careful control of blood pressure, end-tidal COg and rectal temperature. The general experimental procedures for averaging the responses of individual Purkyn~ cells and the criteria for identification of these cells (just over 400 in all) have already been described 2-4. The left forepaw or hindpaw was fixed rigidly with the pads upward for optimal application of the mechanical stimuli. It was important that these stimuli were applied in a precisely reproducible manner with control by a master-timer (Digitimer) in order that the responses of the individual Purkyn~ cell could be averaged by the special purpose computer (Fabri-Tek 1062). Two types of stimulus were applied to the mechanoreceptors of the pads: servo-controlled displacement pulses; and weights that were released under gravity and lifted pneumatically. The displacement pulses could be sinusoidal, square or with a ramp leading edge and could be applied singly or repetitively. Displacements ranging between 4/~m and 2 mm were applied with a cylindrical probe of 5 mm diameter and repeated at the rate of l/sec. Weights of 50 g2 kg were applied with a cylindrical probe of 1 sq. cm area for a duration of 2 sec and at the rate of 1 every 6 sec. When applied to the central toe pad of the cat's hindfoot, displacement stimuli evoke activity in populations of cutaneous mechanoreceptors ranging from about 10-20 units with the smallest stimuli to up to several hundred units with the largestTM. This activity is mainly evoked in Pacinian corpuscles, though also in the rapidly adapting mechanoreceptors (RA-receptors) of the hairless skin of the central pad. Stimuli of constant weight applied to the area of the hindlimb central pad evoke an initial discharge in the different types of mechanoreceptor units, but about 0.5 sec after stimulus onset the stimulus-evoked activity consists almost exclusively of impulses from slowly adapting mechanoreceptors, designated SA-receptors by J~iniget al. s. Typically for constant weight stimuli of 100-1000 g the stimulus evoked a total afferent outflow ranging from some hundred to some thousand impulses per sec in the first few seconds after stimulus onsetS,1L In Fig. 1A are 3 records of the sinusoidal pulse applied to toe 3 and the resulting Brain Research, 30 (1971) 419-424

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Fig. 1. Purkyn6 cell excitation by cutaneous mechanoreceptors. A, Three records, upper traces, of extracellular spike responses of a cerebellar Purkyn~ cell in response to a brief mechanical stimulus of 1.6 mm amplitude, lower traces, to toe 3 of the forelimb, B shows the poststimulus time histogram (PSTH) made by averaging 64 of these responses in 0.5 rnsec bins, and the cumulative frequency distribution (CFD) derived from the PSTH. Arrows mark onsets of mechanical stimuli, C, D and E are the PSTHs and CFDs for similar mechanical stimuli to toes 2, 4 and 5. F, G is similar to A, B, but for stimulation of t he central pad. H, I and J show PSTHs and CFDs for graded stimulation of the central pad as indicated. All PSTHs haxe the same scale. I0 counts per bin. and all CFD~ have same scale of 5 counts per trace. Time for all PSTHs and CFDs are the same as indicated.

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Fig. 2. Purkyn6 cell responses to mechanoreceptors in another experiment. A gives P S T H and C F D for averaging o f 64 traces in response to a stimulus o f the sciatic nerve o f 5T. B shows two traces o f responses o f a Purkyn6 cell to a mechanical pulse of 1.6 m m to the central pad o f the hindfoot, C giving the P S T H and C F D for the average o f 64 such responses• In D - F at the much slower time base are the responses to a 2 see application o f the indicated weights (horizontal bars), there being averaging o f 16 traces in 20 msec bins• The P S T H is scaled as frequency per sec per trace as shown by the H z scale• The C F D scale is given for impulses per single trace• G - L is similar to A - F but is for another Purkyn6 cell 580 p m deeper in the same track• Arrows indicate times o f onset of the brief mechanical stimuli• Times for D - F and J - L are the same• Same count scales for PSTHs o f A, C, G, I and C F D s o f C, G, I.

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responses of a single Purkyn6 cell that had such a low background frequency that no spike appeared before the brief burst of spike responses evoked by the stimulus. In 13 are the poststimulus time histogram (PSTH) and the cumulative frequency distribution (CFD) derived by the averaging of 64 responses in 256 bins of 0.5 msec duration. The stimulus to toe 3 evoked on the average 3-4 discharges from this Purkyn~ cell. In F and G there is a similar series for the same stimulus applied to the central foot pad, there being on the average 4-5 discharges per stimulus. In C, D and E are also the PSTHs and CFDs respectively for the other toes, T2, T4 and T5. The responses were smaller - - 3-2 discharges on the average, but in general it can be seen that the responses were similar in their time courses. Fig. 1H-J also shows the effect of increasing the amplitude of the displacement applied to the central pad. In Fig. 1H there was still a small response of about 1.5 impulses on the average for a tap of 0.1 m m excursion. With a 4-fold increase in stimulus, there was approximately a doubling of the response in I. A further increase to 1.0 m m excursion gave no further increase in response, J, which remained well below the response to 1.6 m m in Fig. I G, but this discrepancy can be attributed to the low background of discharge in J (cJi Fig. 6H, I, ref. 5). Fig. 2 illustrates the responses that cutaneous mechanoreceptors evoked in two Purkyn~ cells ( A - F and G - L ) that were only 0.58 m m apart on the same microelectrode track. The responses to a single sciatic nerve volley (about 5T stimulus) were very different. In A it was a prolonged burst of over 10 impulses, while in G, there was a brief initial response of less than one on the average and then an inhibition for about 25 msec. B and H each give two specimen records of the responses evoked by a 1.6 mm pulse to the central pad, and in C and 1 are the PSTHs and CFDs for the average of 64 of these responses. There was a small increase in the spike discharges of about two on the average in C, and a late slow increase of about the same order in I. D - F and J - L display for each cell the responses, PSTH and CFD, evoked by a prolonged steady pressure of 2 sec duration at the time indicated. There is a remarkable difference in the two cells. In D a weight of 500 g approximately doubled the rate of the background discharge, from about 25/sec to 50/sec in the PSTH, and this rate was still over 40/sec when the pressure was removed 2 sec later. This effect is also displayed by the increased slope of the CFD. In E and F weights of 200 g and 100 g had similar but smaller effects. By contrast in the other Purkyn~ cell the weight of 500 g (J) reduced the frequency of discharge from about 20/sec to 10/sec for the whole period of its application. On careful inspection there seems to be an initial conflict of excitation and inhibition, the inhibition taking up to 1 sec to reach its full effectiveness. As the weight was removed there was a significant brief rebound up to an average frequency of 30/sec. With smaller weights, 200 g and 100 g, there was in K and L a corresponding diminution in the inhibitory slowing. The inhibitory effects in J - L thus are virtually mirror images of the respective excitatory effects in D - F. The responses evoked by the sciatic nerve volley in A and G seem to be correlatable with the respective responses of the two cells to pressure in D and J, there being a powerful and prolonged excitation in A and a mixed excitation-inhibition in G. However, there is no such correlation with responses evoked by displacement pulses to the central pads, where there was excitation in both C and I in contrast to the excitation in Brain Research, 30 (1971) 419 424

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423

D and inhibition in J. It can be assumed that the observed responses in I and J are the net results of an excitation-inhibition conflict, as already suggested for J. All of the responses in Fig. 2 were evoked by the mossy fiber input to the cerebellar cortex. The same stimulation of the slowly adapting cutaneous mechanoreceptors gave, via the mossy fiber input, excitation (D-F) on one Purkyn~ cell and inhibition (J-L) on the other only 580 pm distant transversely across the folium. This distance is well within the range of the transverse distribution of inhibition by basket cells 7. Figs. 1 and 2 give specimens of simple responses of Purkyn~ cells to brief mechanical stimuli and to prolonged pressure, but they are inadequate in conveying the full range of the responses exhibited in our experiments. Most Purkyn6 cells have been purely phasic in their responses to a mossy fiber input from mechanical stimuli. For example a cell responding as in Fig. 1 will exhibit no steady change in frequency during pressure applied as in Fig. 2, but only brief responses at the onset and end of pressures. Another common phasic response is an inhibition of about 50 msec duration in response to brief mechanical stimuli, and only brief on- and off-responses to the pressure which may be excitatory or inhibitory. Other cells give mixed phasic and tonic responses, there being all degrees of admixture, with excitatory and inhibitory components. When hairy skin of the paws is excited by brief jets of air, or by the brief mechanical stimuli, there are often mossy fiber evoked responses that may be excitatory or inhibitory, usually resembling those from the adjacent pads. Often the responses evoked from the toes and central pad have been much more discriminating than in Fig. 1 and than reported by Kitai et al. 9. This preliminary report illustrates the sensitivity and the subtlety of the responses of individual Purkyn~ cells to adequate stimulation of cutaneous mechanoreceptors. Probably the pathways for mossy fiber input from the skin of the fore- and hindlimbs would be preponderantly by the cuneocerebellar and the dorsal spinocerebellar tracts respectively10, and these would be the pathways subserving the fine discriminations that we have often observed. It is evident that cutaneous inputs must play an important role in the functional performance of the cerebellum. This work was supported by a grant from the National Institute of Neurological Diseases and Stroke, Grant No. R01NB0822101,2,3 and by generous research support by Dr. Henry C. and Bertha H. Buswell Fund to J. C. Eccles, R. F. Schmidt and H. Tfibo[ikovfi. N. H. Sabah is a postdoctoral fellow, UHF Grant No. FTF-3-UB-70. Departments of Physiology and Biophysics, School of Medicine, State University of New York, Buffalo, N.Y. 14214 (U.S.A.)

J. C. ECCLES N. H. SABAH R. F. SCHMIDT H. TABOI'~IKOVA

1 ADRIAN, E. D., Afferent areas in the cerebellum connected with the limbs, Brain, 66 (1943) 289-315. 2 ECCLES,J. C., FABER,D. S., MURPHY,J. T., SABAH,N. H., AND TABoii,fKOV./~,H., Firing patterns of Purkinje cells in response to volleys from limb nerves, Brain Research, 14 (1969) 222-226. 3 ECCLES,J. C., FABER,D. S., MURPHY, J. T., SABAH,N. H., AND T~BOftfKOV~,H., The integrative

Brain Research, 30 (1971) 419-424

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performance of the cerebellar Purkyn6 cell. In P. ANDERSEN AND J. K. S. JANSEN (Eds.), Excitato~v

Synaptic Mechanisms, Universitetsforlaget, Oslo, 1970, pp. 223-236. 4 ECCLES, J. C., FABER, D. S., MURPHY, J. T., SABAH, N. H., AND T~,aO~iKOV~,, H., Afferent volleys in limb nerves influencing impulse discharges in cerebellar cortex. 1. In mossy fibers and granule cells, Exp. Brain Res., 13 (1971) in press. 5 ECCLES, J. C., FAaER, D. S., MugPtaV, J. T., SABAH, N. H., AND T~,BO0,iKOV/,, H., Afferent volleys in limb nerves influencing impulse discharges in cerebellar cortex. II. In Purkyn6 cells, Exp. Brahz Res., 13 (1971) in press. 6 EC('LES, J. C., FABER, D. S., MURPHY, J. T., SABAH, N. H., AND T~,Bo~,iKOV~,, H., Investigations on integration of mossy fiber inputs to Purkyn6 cells in the anterior lobe, Exp. Brain Res., 13 (1971) in press. 7 ECCLES, J. C., SASAKI, K., AND STRATA, P., The profiles of physiological events produced by a parallel fibre volley in the cerebellar cortex, Exp. Brain Res., 2 (1966) 18-34. 8 J.~NIG, W., SCHMIDT, R. F., AND ZIMMERMANN, M., Single unit responses and the total afferent outflow from the cat's foot pad upon mechanical stimulation, E~cp. Brain R:,s'., 6 (1968) 10O-115. 9 K1TM, S. T., T~,Bo~iKov~., H., TSUKAHARA, N., ANt) ECCLES, J. C., The distribution to the cerebellar anterior lobe of the climbing and mossy fiber inputs from the plantar and palmar cutaneous afferents, Exp. Brain Res., 7 (1969) 1-10. 10 OSCARSSON, O., Functional organization of the spino- and cuneo-cerebellar tracts, Physiol. Rev., 45 (1965) 495-522. 11 SCHMIDT, R. F., Spinal cord afferents: Functional organization and inhibitory control. In M. A. B. BRAZIER (Ed.), The Interneuron, UCLA Forum in Medical Sciences, Los Angeles, 1969, pp. 209-229. 12 SN~DER, R. S., AND STOWELL, A., Receiving areas of the tactile, auditory and visual systems in the cerebellum, J. Neurophysiol., 7 (1944) 337-357. (Accepted April 26th, 1971)

Brain Research, 30 (1971) 419-424