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The fine topography of climbing fiber projections to the paramedian lobule of the cerebellum in the cat
The
Fine Topography Paramedian T. S.
Crntre
de Rccherche Uhersitt! Rcceivcd
of Climbing
Lobule MILES,
October
Projections
of the Cerebellum
J. P.
eu Scierrces de Montrtal,
Fiber
LUND,
Nmvologiqucs Montrtal,
7, 1977;
rcuisiorr
AND
J.
in the Cat COURVILLE
1
et Faculth de Midccim P.Q. H3C 3T8 Cartada receizled
to the
Dcccmbcr
Derltaire,
21, 1977
Motor cortex and somatosensory afferents of climbing fibers (CFs) which terminate in the two rostra1 folia of the paramedian lobule of the cerebellum were studied in pentobarbital-anesthetized cats. CF responses were elicited in all but one Purkinje cell recorded within the paramedian lobule by stimulation of one or more pericruciate sites; 67% of these also had a peripheral receptive field or were excited by stimulation of the superior radial, sciatic, or infraorbital nerve. CFs responded to stimulation of peripheral nerves at either short latency (14 to 34 ms) or long latency (<120 ms), or gave a mixture of both responses. Those responding at long latency generally did not have peripheral receptive fields. The forelimb was predominantly represented in folia a and b, although it was noted that the relationship between bodily representation and surface landmarks was variable between cats. The proximal portion of the limb was represented medially and the distal part laterally. A clear relation was established between the location of the lowest-threshold cortical site and the peripheral afferents which evoked a CF response in any given cell. The cortical site was generally in a region controlling movements of the body parts on which the peripheral receptive field was found. Two populations of CFs exist in this region, one receiving convergent and complementary cortical and peripheral information and another which, under the present experimental conditions, is excited only from the cortex. It is suggested that these two populations may project as interdigitating bands which run across the long axis of the folia. Abbreviations : CF-climbing fiber; MF-mossy fiber. 1 This research was supported by the Canadian Medical Research Council. Dr. Miles’s permanent address is Department of Human Physiology and Pharmacology, University of Adelaide, South Australia. Drs. Lund and Courville are members of the M.R.C. Group in Neurological Sciences. We wish to thank Dean J.-P. Lussier for arranging Dr. Miles’s visit. The illustrations w-ere prepared by Mr. G. Filosi. The technical assistance of J. Jodoin is gratefully acknowledged. Dr. Y. Lamarre provided valuable discussions and helpful criticisms. 151 0014~4%6/78/0601-0151$02.00/0 Copyright @ 1978 by Academic Press, Inc. AI1 rights of reproduction in any form reserved.
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INTRODUCTION A comparison of the somatotopography of cortical and peripheral projections to the cerebellar cortex was first made by Adrian (1). On the basis of evoked potential recordings, he concluded that an area of the cerebellar anterior lobe which receives a peripheral input also receives an input from the motor cortical area in which the peripheral part is represented. Somatotopical convergence was also observed in the paramedian lobule (2). The principle of somatotopical distribution of cortical or peripheral inputs to various cerebellar regions has been amply confirmed in other evoked potential studies (12, 17, 18, 28). However, the aforementioned reports were published prior to the characterization of mossy fiber (MF) and climbing fiber (CF) evoked potentials (15). Recently the convergence of corresponding cerebral and peripheral inputs onto CFs terminating in the anterior lobe was shown in evoked potential studies (22, 24, 26). Single CFs may also respond to both projections (21), and the convergence is somatotopically arranged. Allen et al. (3) showed a convergence from forelimb nerves and forelimb motor cortical area into lobule V. Hind limb nerves and the corresponding cortical motor area converge into lobules III and IV. Miles and Wiesendanger (23) showed a similar convergence of inputs from the face and from the cortical Sl face area to CFs supplying the corresponding cerebellar cortical area in lobule VI. In this latter study, however, it was reported that approximately 30% of Purkinje cells responded only to peripheral stimulation. Fifty-four percent of the CFs recorded by Miller et ~2. (24) received convergent inputs. In view of the somatotopical distribution of cerebrocerebellar projections, a lack of cortical input might be explained by an improperly placed cortical stimulus (24). An alternative possibility is that two or more populations of CFs to a given region exist, some responding to the periphery alone and others to convergent inputs. In the present experiments, stimulation of a larger number of cortical sites allowed these possibilities to be tested. In addition, an attempt was made to analyze the topography of cortical and peripheral projections to a population of Purkinje cells in more detail than previously attempted. The paramedian lobule, which receives both cortical and peripheral inputs, was chosen because the folia are small and easily identified ; thus each recording site could be precisely located. METHODS Preparation of aninzals. These experiments were carried out on six adult cats, anesthetized initially with an intraperitoneal injection of pentobarbital sodium (35 mg/kg). A moderate depth of anesthesia was maintained by the administration of supplementary throughout each experiment
FINE
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intravenous doses of pentobarbital. Gallamine triethiodide was administered intravenously to paralyze the cats throughout the recording period. Artificial ventilation was adjusted to maintain the end-tidal COz concentration at 4.0 +- 0.2%. Mean arterial blood pressure was monitored and exceeded 100 mm Hg throughout each experiment. The deep colonic temperature was maintained at 37 + 1°C. The right paramedian lobule of the cerebellum was exposed by a small craniotomy, and the exposed pial surface was protected from dehydration with small pieces of plastic film.
Stirntllation Procedures. An array of eight electrodes (two in the hind limb, four in the forelimb, and two in face or jaw areas) was placed for stimulation of the contralateral somatomotor cortex. The frontal sinus was widely opened and fine stainless-steel pin electrodes were advanced through small burr holes into the skull to penetrate the dura and pass into the cortex (25). Individual electrodes were selected for monopolar cathodic stimulation (four shocks, 500 Hz, 0.1 ms, to as much as 500 PA) of the cortex by means of a rotary switch. The somatotopic position of each cortical electrode was determined by recording the field potential evoked in that electrode by stimulation of the various peripheral nerves; the precise position of each electrode tip was checked in a postmortem inspection of the cortical surface and in histologic sections. Electrodes from which no evoked potentials could be recorded either were found to be in an area damaged during the insertion of the electrode or were inserted deep into the white matter. The latter were often effective stimulation points. Sleeve electrodes were placed around the superficial radial and sciatic nerves on both sides. Parallel bipolar pin electrodes were inserted permucosally to stimulate the infraorbital branches of both trigeminal nerves. The presence of reflex responses to stimulation of all nerves (<3 V, I-ms pulse) was confirmed before gallamine was given. The extent of the peripheral receptive field for each cell was mapped using small probes and cotton-wool wisps and by manipulation of joints and muscles. In selected cases, an electrically timed mechanical stimulator, which delivered taps to the skin, was used to determine the latency to natural stimulation of the CF peripheral field. Record&g Procedures. Extracellular recordings of Purkinje cell activity were made with glass micropipets filled with a 2 M NaCl solution (DC resistance, 2 to 10 Ma). Electrical signals were amplified and immediately photographed on Polaroid film or stored on magnetic tape. Whenever possible, each cell was tested for responses to each of the cortical and peripheral electrodes in turn. The threshold current which evoked a CF response from each cortical site was measured.
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1.5 mm
2.4
mm
4.6
mm
‘,L. pm. I
FIG. 1. This photograph and the corresponding line drawing of a sagittal section through the medial one-third of the paramedian lobule show the position and orientation of portions of three separate electrode tracks (indicated by broken lines in the drawing). In folium a, a single electrode track was made at an angle of about 30” relative to the vertical axis. It is barely visible in the section, except for a dark spot corresponding to a small hemorrhage. No Purkinje cell recordings were obtained along this track. In folium b, the tracks are at an angle of about 45” to the vertical. A portion of one track is seen superficially and another one is visible deeply, where it passes through a deep folium of crus 2 (Cr 2). The neurons with peripheral sensory fields which were recorded in this region of folium b responded to probing
FINE
TOPOGRAPHY
OF
PARAXEDIAK
LOBULE
cLI~IBING
FIBERS
15 5
SCIATIC N-25
FIG. 2. Histograms of the minimum latency tion. The few Purkinje cells which responded in more than one histogram.
of CF responses to peripheral stimulato more than one input are represented
In an attempt to define the detailed pattern of CF inputs to folia a and b of the paramedian lobule (19), microelectrode penetrations were made at intervals of 200 pm in the transverse plane, perpendicular to the surface. The accurate identification of the folial layer from which responseswere evoked was aided by breaking many electrodes in situ and Ieaving the of the ipsilateral forelimb, but not to light tactile stimulation. at a depth of 4.6 mm was clearly in crus 2. L.pm-paramedian lateralis ; Fl-flocculus ; Pfl-paraflocculus.
The neuron recorded lobu!e: Nl-nucleus
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tips in the tissue during fixation. Histologic sections of the cerebellum were prepared for analysis of recording sites in all cases. Analysis of Data. Poststimulus time histograms were made on an Ortec histogram unit from the magnetic tape recordings. The Mann-Whitney U-test was used to establish the probability that short- or long-latency CF responses to electrical stimulation were related to the presence or absence of a peripheral receptive field. A sign test was used to determine the probability that Purkinje cells which received a CF input from a peripheral nerve also obtained their cortical input from a region activated by the same peripheral input. RESULTS Eighty-seven tracks were made into folia a and b of the paramedian lobule of the cerebellar cortex. One hundred and twenty-eight Purkinje cells were readily identified by the characteristic “complex spike” discharge in response to a CF input (14). During the examination of the histologic sections of all cerebella, a number of electrode tracks were recognized. Because all tracks in a series were parallel and depth measurements had been made during recording, it was possible to estimate whether a particular neuron was recorded within one of the Purkinje cell layers of the paramedian lobule or within deeper adjacent folia which were penetrated upon occasion (Fig. l), It was found that 123 CF responses were recorded in the first two folia of the paramedian lobule, four were located in crus 1, and one in crus 2. Ninety-eight percent of all neurons isolated were activated by one or more of the inputs tested. Among the 71 Purkinje cells which were completely tested, 65% rkceived CFs from both cortex and periphery, 31% from cortex alone, and 4% (three neurons) solely from the periphery. Two of these three neurons were in crus 1 and had receptive fields on the back of the animal (Fig. 1) ; no stimulating electrode was placed in the corresponding projection area of the pericruciate cortex [medial anterior sigmoid gyrus (25)]. Thus, all but one cell recorded in the paramedian lobule received a CF from the somatomotor cortex. Two major populations of CFs were therefore defined, one excited from the cortex and the other by both cortical and peripheral inputs. Climbing Fiber Responses to Peripheral Stiwaulation. The majority of Purkinje cells activated from the periphery received a CF input only from the ipsilateral forelimb (56%) ; 19’7’0 were activated from the hind limb alone, and 13% from both ipsilateral limbs. The remaining 12% responded to various combinations of trigeminal and spinal inputs, including 5% which gave CF responses to contralateral stimulation. In general, superficial radial nerve-evoked CF responses were found throughout the breadth of both folia a and b, and cells receiving sciatic and trigeminal inputs were observed infrequently. However, the hind limb
FINE
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representation was predominant in folia a and b of one cat. Of all Purkinje cells found in this investigation which responded to sciatic stimulation, 54% were recorded in this animal. It is of interest that the single CF response which could be evoked from a restricted forepaw field in this
Al
L1 c’v
i
-
c2-
I
IOD T 1 -
FIG. 3. Poststimulus time histograms for three Purkinje cells, with inserts showing examples of the CF responses of these cells. In A and B, the cortex was stimulated, and in C, the sciatic nerve. A total of 128 trials was accumulated for each histogram; the first bin gives the vertical calibration in events per bin. The time indicated below each histogram is also the horizontal calibration for the corresponding filmed records. The vertical calibration applies to all filmed records; positive up in all figures.
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experiment was found to be in the adjacent folium of crus 2. This point will be reexamined in the Discussion. In addition, in all cases, receptive fields on the proximal part of the forelimb or hind limb were predominantly represented in the medial half of the lobule, whereas the lateral half received CFs which had distally situated receptive fields (Fig. 6, compare the receptive fields in the medial and lateral regions of the folium). The number of Purkinje cells and their minimal response latencies to stimulation of each ipsilateral peripheral nerve are given in Fig. 2. Purkinje cells can be divided into three groups based on CF response latency measurements. The large majority (74%) of all Purkinje cells responding to the superficial radial nerve did so at short latency (14 to 34 ms, Fig. 2), although some of these also fired a second time at long latency (120 to 250 ms) after stimulation, These responses were of the type illustrated in Figs. 3A and B. Sixteen percent of the superficial radial nerve-activated cells
0
50
60
70
SO 100 110 120 130 140 150
60
THRESHOLD
CURRENT
160 170 190 190 2003210
(PA.)
CORTEX N=70
25
20 I: Ii B
5
"
15 II,<%... 0 16
16
20
22
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26
26
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LATENCY
32
34
150 160 170 160 190200250
(msec)
4. Histograms of the cortical threshold current and of the minimum latency responses to cortical stimulation (measured from the first shock). Note that the binwidths for the latter are 2 ms for latencies up to 34 ms, and 10 ms for latencies greater than 120 ms. FIG.
of
CF
wve CF responses only at long latencies. ~1 similar distributioll of Intenb ties of sciatic-evoked CF responses occurred (I;ig. 2). .ln example of a long-latency sciatic response is shown in Fig. 3C. In the group of l’urkinje cells respondin g after long latency, there were significantly more receiving CFs without peripheral receptive fields than there tvere receiving CPs with peripheral fields, for both the superficial radial nerve (,P < 0.05) and the sciatic nerves (I’ < 0.01). Eleven of the fifteen cells responding to the superficial radial nerve at a latency of > 120 ms had ii0 receptive fields, and seven of the eight cells responding to sciatic stimulation at long latency were not excited by natural stimulation. X11 three short-latency trigeminal responses were also driven by tactile stimulation of the face, whereas three of the five long-latency responses \vere not. However, the Purkinje cells which responded at long latency to stimulation of peripheral nerves also responded at long latency to natural stimulation of their receptive fields when these were present. Gentle mechanical stimulation of hair or skin was an adequate stimulus for eliciting CF responses in 7376 of the cells which had peripheral fields. In 24% of the cases, particularly those with peripheral fields proximally situated on the limb, firm pressure KLS needed to evoke the response, suggesting that deep pressure receptors or possibly muscle receptors may have I)eeu the effective stimulus. Three percent responded only to pinching or pricking of the skin. The extent of the skin area from which CF responses were evoked was generally small. For the limbs, the peripheral skin fields were most coiim~only restricted to a small area of the dorsal and/or ventral surface of the foot, or to the skin inside the ell)ow or knee. Occasionally “stocking”or “glove”-type peripheral fields were observed. Only three cells were found which could be activated from a large skin area extending over more than one limb. Three of the four neurons which hat1 Cl: receptive fields on the face \vere also activated from skin fields on the forelimb. Cli/ilbiPlg Fiber Xcs~ollsrs fo Corticd Sti~lrulatioa. Stimulation of one or nlore cortical sites (mean = 2.3) at intensities of less than 500 ,uA was effective in evoking CF responsesin 96% of the cells tested. The number of cortical sites from which a neuron could be activated was unrelated to the presence, absence,or variety of peripheral inputs or to the latency of CF responses. For any given cell, honever, the threshold current for eliciting a response from one specific cortical stimulus site (the “best point”) was significantly less than the threshold current for adjacent stimulus sites in the cortex. Figure 4 ~1~0~~s the amplitudes of the threshold currents at the cortical best point for 67 cells. The minimum CF respome latency, measured from the first pulse of the stimulus train delivered to the best point for each Purkinje cell, is also shown. X11of the eight neurons
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(11%) activated by CFs at long latency (150 to 250 ms) following cortical stimulation also received long-latency peripheral CF inputs. It can be seen that the modal latency of cortically evoked CF responses is 20 to 22 ms, using the convention of measurement from the first cortical shock. Several cells were tested for their response to cortical stimulation at constant intensity, but with different numbers of shocks. Some CFs were activated by a minimum of two cortical shocks, others by three. Increasing the number of stimulus shocks did not alter the latency in these cells. This suggests that the corticofugal cells were activated not by the first stimulus shock, but rather when their excitability was raised sufficiently by subsequent shocks to bring them above their firing threshold. Comparison of Cortical and Perifheral Isputs. A clear correlation was observed between the location of the cortical “best point” and the peripheral input which excited an individual CF (Fig. 5). Generally, the best point was in a region where stimulation could be expected to cause movement of the body part containing the peripheral field. Although movements were
P --i
B
y-s-=-
SRN
T
FIG. 5. Convergence of peripheral and cortical inputs to three Purkinje cells. Two CFs were recorded on a track at the anterolateral corner of folium a. The most superficially situated had a receptive field on the ventrolateral surface of the forepaw (forelimb prone) and had a cortical “best point,” E, where a negative potential was evoked by stimulation of the superficial radial nerve (SRN). The other CF responded to poking the foot pads of the hind paw (hind limb plantar) and to stimulation of cortical point B, which received an input from the sciatic nerve (SCI). Another CF, recorded in the middle of the folium and at its caudal border, had a discontinuous tactile receptive field on both the ipsilateral forepaw and chin. It also received a convergent input from control point H, where negative fields were recorded after stimulation of both the SRN and the infraorbital nerve (V).
FINE
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not evoked because of curarization, 7870 of the Purkinje cells receiving a CF peripheral input from superficial radial trigeminal, or sciatic nerves had their best point at a cortical site where a potential could be evoked by stimulation of the same nerve (Figs. SB, IX). It is statistically unlikely that such .a relationship could arise by chance (P < 0.01). The remaining EC/o of the cells, for which the hest-point electrode was often deep in the \\-hite matter (see methods), responded to an adjacent “second-best” cortical point, receiving the corresponding peripheral input. If a Purkinje cell received a CF input which could be activated from two peripheral nerves, the cortical best point was generally in a region from which evoked potentials from both nerves could be recorded (Fig. SH ). The precision of this point-to-point, cortico-olivocerebellar projection is shown by the fact that the threshold for cortical activation of CF responses in 84% of the cells was in the range of 60 to 200 PA for the hest cortical site; from cortical sites within 1 to 2 mm of the best point, the threshold stimulus current was significantly greater. As previously stated, the predominant input to the two folia came from the ipsilateral forepaw. The CF peripheral fields of all Purkinje cells recorded in one cat are shown in Fig. 6, together with the anatomic location of the recording points. In this folium, the forelimb was exclusively represented and the somatotopography was the most ordered of all the cases examined. The map illustrates the general finding that the CF somatosensory receptive fields on both forelimb and hind limb tended to move distally (cf. tracks 1 and 7) and posteroventrally (cf. 7 and 17) as the electrode was moved from the medial to the lateral margin of the folia. Accompanying this shift in peripheral receptive fields is a movement of the cortical best point from electrodes B and C, which are in regions which probably control the more proximal muscles of the limb, to electrode IL Stimulation at this point would be expected to elicit movements of the elbow or wrist (25). DISCUSSION The present investigation shows that, with one exception, all CFs terminating in paramedian folia a and b of the cat are activated from the motor cortex. These results differ from those obtained in lobule VI and adjacent anterior lobe areas of the cat by Miles and Wiesendanger (23), who found that about 30% of the CFs excited by stimulation of trigeminal nerves did not receive a convergent input from stimulation of a single point in the Sl sensorimotor cortex. Whether these remaining Purkinje cells in the regions they investigated receive an input from other unstimulated regions of the Sl face area or from the more anterior jaw and face motor areas is unknown. In the intermediate zone of the anterior lobe, Allen et al. (4) did not report such a universal cortical input. Although
162
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12345678
.,
LUND,
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AND 9
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II
12
13 I,
14
FIG. 6. Diagrammatic representation of 17 electrode tracks made into folium a of the right paramedian lobule. The positions of the points of entry of the tracks on the surface of the folium are shown in the inset. The location of each Purkinje cell in the vertical and mediolateral plane is indicated by a circle. The filled circles represent neurons in which a CF response was elicited by both cortical and superficial radial nerve stimulation, and the open circles indicate neurons that were excited only from the cortex. Peripheral receptive fields and their sensory modalities are indicated on the diagrams of the ipsilateral forelimb. The cortical best points for each track are shown below. Their positions are approximately those shown in Fig. 4, with one important difference: Electrode B was more laterally placed and superficial radial nerve-evoked potentials were recorded from it.
the percentage of Purkinje cells responding to stimulation of the tortes is not given, their Fig. 4 shows that not all received CFs which were activated from the cortex. In the present study, the motor cortical CF input to paramedian folia a and b is principally from the area known to control the forelimb (25, 29). Furthermore, it was observed that the more medial parts of the cortical forelimb area, where stimulation evoked movements of the shoulder, project to the medial half of the paramedian lobule. The lateral half of the paramedian folia a and b receives CF inputs from cortical regions controlling elbow or wrist movements. The presence of barbiturate anesthesia is known to raise the stimulus threshol’d of the motor cortex for eliciting
FIA-iE
TOi’I~GK.\I’HY
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P~\R‘\.lIEL)IAN
LC)BULE
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other responses such as movement (25 j. Despite this, 457; of the CFs were activated by cathodal currents of
from
one experiment
to another.
Variations
in the arrange-
ment of the cells in relation to surface landmarks are not surprising because the pattern of folding of the folia of this region is known to be inconstant (19). The present data suggest furthermore that the functional representation may also vary in relation to surface landmarks. This is exemplified bq one case in which the posterior limb was predominantly represented in the first two folia of the paramedian lobule, and the forelimb in the adjacent folium of crus 2. In this case, the proximal regions of the hind limb were again represented medially and the distal regions laterally. Despite such variability the convergence of cortical and peripheral body parts through single CFs was generally maintained (e.g., Fig. 5j . The majority of CF responsesin the paramedian lobule were found to have input characteristics which were qualitatively similar to those of Purkinje cells in the pars intermedia of the anterior lobe and hemisphere, that is, a low threshold to natural stimulation of skin and hair, and restricted peripheral fields (16, 20, 23). Those properties of paramedian Purkinje cells were expected, since it is known that CFs which project to the anterior cerebellar lobe send collaterals to synapseonto Purkinje cells in the paramedian lobule and elsewhere (7). Hence the present observations lend further support to the supposition that climbing fibers can transmit precise somatosensorv information to the cerebellar cortex, which ma\
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serve to keep the Purkinje cell informed of peripheral events during movements (9, 16, 21, 23). The range of latencies at which CF responses were evoked in Purkinje cells by stimulation of limb nerves was similar to that reported in other studies on the cerebellar anterior lobe and the paramedian lobule (6, 7. 13, 21, 24). However, there was no obvious grouping of cells according to the criteria of latency and/or presence or absence of contralateral inputs which would confirm the results that Armstrong et al. (6) obtained in their evoked potential studies. Figure 4 shows that the modal latency of cortically evoked responses is 20 ms, measured from the first pulse of the train. However, in agreement with Allen et al, (3), it w.as shown in a few Purkinje cells that at least two or three cortical shocks were required to elicit CF responses; that is, temporal summation in corticofugal neurons was necessary. Although only a few cells were tested in this way, it is reasonable to assume that the convention used in describing the latency of cortically evoked CF responses overestimates their true latency (measured from the time of activation of corticofugal neurons) by 2 to 4 ms. Thus the true modal latency is probably about 16 to 17 ms, as was found in the anterior lobe (3). In addition to the cells whose latencies to peripheral or cortical stimulation were within the range of 15 to 32 ms, a number of cells were found to respond only after 120 to 250 ms. This applied mainly to peripheral stimulation. The fact that the majority of these neurons had no demonstable peripheral field is probably significant. It is highly unlikely that such long latencies are due to a circuitous route to the inferior olive because even quite short polysynaptic paths are depressed by barbiturate anesthesia. The more common phenomenon of an early excitation of CFs, followed by a long period of inhibition, then a rebound excitation has been observed before (9, 16, 21, 23). The inhibition has been ascribed to inhibitory mechanisms within the olive (5, 14). It seems probable that this feedforward inhibition explains the very long latency of some CFs to stimulation. That is, olivary neurons which h,ave not themselves been initially excited may receive recurrent inhibition resulting from discharge of adjacent climbing fibers. At the end of this long inhibitory period, the olivary cells then undergo rebound excitation, giving an apparent response latency of 120 to 250 ms. It is worth noting that long-latency cells were observed in five of six cats, and that adjacent cells responded at the more common short latencies. This observation indicates that the long-latency phenomenon was not due to some idiosyncrasy of a particular experimental preparation. Furthermore, long-latency responses were produced in several neurons by timed mech.anical stimulation of skin. This rules out the possibility that the phenomenon was due to the nonphysiologic nature of electrical stimulation.
In the anterior lobe, Miles and II’iesendanger (23) found a population of CFs which could be excited only from the periphery. In contrast, we found a number of CFs which were excited only from the cortex. The latter had no receptive fields arid could not he excited hy the stiniulation of a nerve which activated others on adjacent penetrations, although it is possible that they lnay he activated hy stimulation of other peripheral nerves. If one CF on a track was of this type, then there was a high luwl~al~ility that the other cells recortled along the saiiie penetration would also he without a peripheral input (Fig. 6, tracks 10 ant1 16), and in two atljacent tracks it was also often found that cells responded in the salne
manner. There was therefore a certain indication that populations of cells activatetl by corte*x alone alternate across the folia with cell groups responding to both cortex and periphery (Fig. 6). This observation can he correlated with the data of Armstrong ct al. (8), who showed electrophysiologically that groups of olivary cells project CFs in long strips oriented l~erpendicularly to the long axes of the folia. They described six distinct bands in the anterior lobe and three in the paraineclian lohule. L+cordiiig to a study of the olivocerehellar projection 1)~ the method of injection of tritiated amino acids and radioautography ( IO), these strips of projection inight he even siiialler (0.4 to 0.8 inni), and consequently more numerous, than those observed 1)~ electrol~hysiologica1 mappings. In the anterior lobe, it was estimated that as niany as 12 different hantls could exist, and in the paraniedian lohule, there coultl he six to eight separate strips. It was also concludetl in that study that the intercligitating strips originate from distinct sites in the olive. The present tlata are not sufficiently detailed to pemlit an exact tlelirieatioii of the projection strips in the paranieclian lobule. However, it is suspected that the sets of interdigitating bands of CFs might correspontl on the one hand to a group of olivary cells receiving
convergent inputs from the cortex and periphery, and on the other hand to another
group
influenced
1)~ the cortex
hut without
a peripheral
input.
REFERENCES 1.
E. D. 1943. Afferent areas in the cerebellum connected with the limbs. 289315. 2. ALRI?-FESSAKD, D., AND T. S~ABO. 1954. Observations sur I’interaction des aff&ences d’origines pCriphCrique et corticale destinPes Z?II’&orce c&Cbelleuse du chat. J. Pl~ysiol. (Paris) 46: 225-229. 3. ALLEN, G. I., G. B. AZZENA, AND T. OHICO. 1974. Cerebellar Purkyne cell responses to inputs from sensorimotor cortex. Exp. Bvni~ RES. 20: 239-254. 4. ALLEN, G. I., G. B. AZZENA, AND T. OHNO. 1974. Somatotopically organized inputs from fore- and hindlimb areas of sensorimotor cortex to cerebellar Purkyne cells. E.zp. Brabt Rrs. 20 : 255-272. 5. ARMSTRONG, D. M., AND R. J. HARVEY. 1966. Responses in the inferior olive to stimulation of the cerebellar and cerebral cortices in the cat. J. Physiol. (Lo&m) 187 : 553-574. ADRIAN,
Brain
66:
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6. ARMSTRONG, D. M., R. J. HARVEY, AND R. F. SCHILD. 1973. Spino-olivocerebellar pathways to the posterior lobe of the cat cerebellum. Exp. Braira Res. 18: l-18. 7. ARMSTRONG, D. M., R. J. HARVEY, AND R. F. SCHILD. 1973. Branching of inferior olivary axons to terminate in different folia, lobules or lobes of the cerebellum. Brain Res. 54: 365-371. 8. ARMSTRONG, D. M., R. J. HARVEY, AND R. F. SCHILD. 1974. Topographical localization in the olivo-cerebellar projections: An electrophysiological study in the cat. J. Comp. Neztrol. 154: 287-302. 9. COOK, J. R., AND M. WIESENDANGER. 1975. Input from trigeminal cutaneous afferents to neurones of the inferior olive in rats. Exp. Brain Res. 26: 193-202. 10. COURVILLE, J. 1975. Distribution of olivocerebellar fibers demonstrated by a radioautographic tracing method. Brain Res. 95 : 253-263. 11. COURVILLE, J., AND F. FARACO-CANTIN. 1976. Cerebellar corticonuclear projections demonstrated by the horseradish peroxidase method. Neurosci. Abstr. 2 : 108. 12. Dow, R. S. 1939. Cerebellar action potentials in response to stimulation of various afferent connections. J. Neurophysiol. 2 : 543-555. 13. ECCLES, J. C., D. S. FABER, J. T. MURPHY, N. H. SABAH, AND H. TABOR~KOVA. 1971. Afferent volleys in limb nerves influencing impulse discharges in cerebellar cortex. II. In Purkyne cells. Exp. Bruin Res. 13 : 36-53. 14. ECCLES, J. C., R. LL~NAS, AND K. SASAKI. 1966. The excitatory synaptic action of climbing fibres on the Purkinje cells of the cerebellum. J. Physiol. (London) 182 : 268-296. 1.5. ECCLES, J. C., L. PROVINI, P. STRATA, AND H. TABOR~KOVA. 1968. Analysis of electrical potentials evoked in the cerebelldr anterior lobe by stimulation of hindlimb and forelimb nerves. Exp. Brain Res. 6 : 171-194. 16. ECCLES, J. C., N. H. SABAH, R. F. SCHMIDT, AND H. TABOK~KOV~. 1972. Cutaneous mechanoreceptors influencing impulse discharges in cerebellar cortex. III. In Purkyne cells by climbing fiber input. Exp. Brain Res. 15: 484-497. 17. HAMPSON, J. L. 1949. Relationships between cat cerebral and cerebellar cortices. J. Neurophysiol. 12: 37-50. 18. JANSEN, J. 1957. Afferent impulses to the cerebellar hemispheres from the cerebral cortex and certain subcortical nuclei. Acta Phyysiol. Stand. (Suppl.) 143 : l-99. 19. LARSELL, 0. 1953. The cerebellum of the cat and the monkey. J. Comp. Nrurol. 99 : 135-200. 20. LEICHT, R., M. J. ROWE, AND R. F. SCHMIDT. 1973. A cutaneous convergence onto the climbing fibre input to cerebellar Purkyne cells. J. Physiol. (London) 228 : 601-618. 21. LEICHT, R., M. J. ROWE, AND R. F. SCHMIDT. 1973. Cortical and peripheral modification of cerebellar climbing fibre input from cutaneous mechanoreceptors. J. Physiol. (London) 228 : 619-635. 22. MILES, T. S., AND H. WIESENDANGER. 1975. Organization of climbing fibre projections to the cerebellar cortex from trigeminal cutaneous afferents and from the Sl face area of the cat. J. Physiol. (London) 245: 409-424. 23. MILES, T. S., AND M. WIESENDANGER. 1975. Climbing fibre inputs to cerebellar Purkinje cells from trigeminal cutaneous afferents and the Sl face area of the cerebral cortex in the cat. J. Physiol. (London) 245 : 425-445. 24. MILLER, S., N. NEZLINA, AND 0. OSCARSSON. 1969. Projection and convergence patterns in climbing fibre paths to the cerebellar anterior lobe activated from the cerebral cortex and the spinal cord. Brain Res. 14: 230-233. 2.5. NIEOULLON, A., AND L. RISPAL-PADEL. 1976. Somatotopic localization in cat motor cortex. Brain Res. 105 : 405422.
24. PROVINI, L., S. REDMAN, AKD P. STRATA. 1968. hlossy and climbing fibre organization on the anterior lobe of the cerebellum activated by forelimb and hindlimb areas of the sensorimotor cortex. Exp. Brairr I&x 6: 216-233. 27. KANCK, J. B. 1975. Which elements are excited in electrical stimulation of mammalian central nervous system : A review. Brclilr IIcs. 98 : 417-440. 28. SNIDER, R. S., AKD A. STO\VELL. 1944. Receiving areas of the tactile, auditory and visual systems in the cerebellum. J. Nczrvoplzyvsiol. 7 : 331-357. 29. WOOLSEY, C. N. 1958. Organization of somatic sensory and motor areas of the cerebral cortex. Pages 63-81 in H. F. HARLOW AK;D C. N. WOOLSEY, Eds., Biological crud Biochemical Basrs of flekn~iour. L~nirersitp of Wisconsin Press. Madison.
Fine Topography Paramedian T. S.
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Motor cortex and somatosensory afferents of climbing fibers (CFs) which terminate in the two rostra1 folia of the paramedian lobule of the cerebellum were studied in pentobarbital-anesthetized cats. CF responses were elicited in all but one Purkinje cell recorded within the paramedian lobule by stimulation of one or more pericruciate sites; 67% of these also had a peripheral receptive field or were excited by stimulation of the superior radial, sciatic, or infraorbital nerve. CFs responded to stimulation of peripheral nerves at either short latency (14 to 34 ms) or long latency (<120 ms), or gave a mixture of both responses. Those responding at long latency generally did not have peripheral receptive fields. The forelimb was predominantly represented in folia a and b, although it was noted that the relationship between bodily representation and surface landmarks was variable between cats. The proximal portion of the limb was represented medially and the distal part laterally. A clear relation was established between the location of the lowest-threshold cortical site and the peripheral afferents which evoked a CF response in any given cell. The cortical site was generally in a region controlling movements of the body parts on which the peripheral receptive field was found. Two populations of CFs exist in this region, one receiving convergent and complementary cortical and peripheral information and another which, under the present experimental conditions, is excited only from the cortex. It is suggested that these two populations may project as interdigitating bands which run across the long axis of the folia. Abbreviations : CF-climbing fiber; MF-mossy fiber. 1 This research was supported by the Canadian Medical Research Council. Dr. Miles’s permanent address is Department of Human Physiology and Pharmacology, University of Adelaide, South Australia. Drs. Lund and Courville are members of the M.R.C. Group in Neurological Sciences. We wish to thank Dean J.-P. Lussier for arranging Dr. Miles’s visit. The illustrations w-ere prepared by Mr. G. Filosi. The technical assistance of J. Jodoin is gratefully acknowledged. Dr. Y. Lamarre provided valuable discussions and helpful criticisms. 151 0014~4%6/78/0601-0151$02.00/0 Copyright @ 1978 by Academic Press, Inc. AI1 rights of reproduction in any form reserved.
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INTRODUCTION A comparison of the somatotopography of cortical and peripheral projections to the cerebellar cortex was first made by Adrian (1). On the basis of evoked potential recordings, he concluded that an area of the cerebellar anterior lobe which receives a peripheral input also receives an input from the motor cortical area in which the peripheral part is represented. Somatotopical convergence was also observed in the paramedian lobule (2). The principle of somatotopical distribution of cortical or peripheral inputs to various cerebellar regions has been amply confirmed in other evoked potential studies (12, 17, 18, 28). However, the aforementioned reports were published prior to the characterization of mossy fiber (MF) and climbing fiber (CF) evoked potentials (15). Recently the convergence of corresponding cerebral and peripheral inputs onto CFs terminating in the anterior lobe was shown in evoked potential studies (22, 24, 26). Single CFs may also respond to both projections (21), and the convergence is somatotopically arranged. Allen et al. (3) showed a convergence from forelimb nerves and forelimb motor cortical area into lobule V. Hind limb nerves and the corresponding cortical motor area converge into lobules III and IV. Miles and Wiesendanger (23) showed a similar convergence of inputs from the face and from the cortical Sl face area to CFs supplying the corresponding cerebellar cortical area in lobule VI. In this latter study, however, it was reported that approximately 30% of Purkinje cells responded only to peripheral stimulation. Fifty-four percent of the CFs recorded by Miller et ~2. (24) received convergent inputs. In view of the somatotopical distribution of cerebrocerebellar projections, a lack of cortical input might be explained by an improperly placed cortical stimulus (24). An alternative possibility is that two or more populations of CFs to a given region exist, some responding to the periphery alone and others to convergent inputs. In the present experiments, stimulation of a larger number of cortical sites allowed these possibilities to be tested. In addition, an attempt was made to analyze the topography of cortical and peripheral projections to a population of Purkinje cells in more detail than previously attempted. The paramedian lobule, which receives both cortical and peripheral inputs, was chosen because the folia are small and easily identified ; thus each recording site could be precisely located. METHODS Preparation of aninzals. These experiments were carried out on six adult cats, anesthetized initially with an intraperitoneal injection of pentobarbital sodium (35 mg/kg). A moderate depth of anesthesia was maintained by the administration of supplementary throughout each experiment
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intravenous doses of pentobarbital. Gallamine triethiodide was administered intravenously to paralyze the cats throughout the recording period. Artificial ventilation was adjusted to maintain the end-tidal COz concentration at 4.0 +- 0.2%. Mean arterial blood pressure was monitored and exceeded 100 mm Hg throughout each experiment. The deep colonic temperature was maintained at 37 + 1°C. The right paramedian lobule of the cerebellum was exposed by a small craniotomy, and the exposed pial surface was protected from dehydration with small pieces of plastic film.
Stirntllation Procedures. An array of eight electrodes (two in the hind limb, four in the forelimb, and two in face or jaw areas) was placed for stimulation of the contralateral somatomotor cortex. The frontal sinus was widely opened and fine stainless-steel pin electrodes were advanced through small burr holes into the skull to penetrate the dura and pass into the cortex (25). Individual electrodes were selected for monopolar cathodic stimulation (four shocks, 500 Hz, 0.1 ms, to as much as 500 PA) of the cortex by means of a rotary switch. The somatotopic position of each cortical electrode was determined by recording the field potential evoked in that electrode by stimulation of the various peripheral nerves; the precise position of each electrode tip was checked in a postmortem inspection of the cortical surface and in histologic sections. Electrodes from which no evoked potentials could be recorded either were found to be in an area damaged during the insertion of the electrode or were inserted deep into the white matter. The latter were often effective stimulation points. Sleeve electrodes were placed around the superficial radial and sciatic nerves on both sides. Parallel bipolar pin electrodes were inserted permucosally to stimulate the infraorbital branches of both trigeminal nerves. The presence of reflex responses to stimulation of all nerves (<3 V, I-ms pulse) was confirmed before gallamine was given. The extent of the peripheral receptive field for each cell was mapped using small probes and cotton-wool wisps and by manipulation of joints and muscles. In selected cases, an electrically timed mechanical stimulator, which delivered taps to the skin, was used to determine the latency to natural stimulation of the CF peripheral field. Record&g Procedures. Extracellular recordings of Purkinje cell activity were made with glass micropipets filled with a 2 M NaCl solution (DC resistance, 2 to 10 Ma). Electrical signals were amplified and immediately photographed on Polaroid film or stored on magnetic tape. Whenever possible, each cell was tested for responses to each of the cortical and peripheral electrodes in turn. The threshold current which evoked a CF response from each cortical site was measured.
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1.5 mm
2.4
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FIG. 1. This photograph and the corresponding line drawing of a sagittal section through the medial one-third of the paramedian lobule show the position and orientation of portions of three separate electrode tracks (indicated by broken lines in the drawing). In folium a, a single electrode track was made at an angle of about 30” relative to the vertical axis. It is barely visible in the section, except for a dark spot corresponding to a small hemorrhage. No Purkinje cell recordings were obtained along this track. In folium b, the tracks are at an angle of about 45” to the vertical. A portion of one track is seen superficially and another one is visible deeply, where it passes through a deep folium of crus 2 (Cr 2). The neurons with peripheral sensory fields which were recorded in this region of folium b responded to probing
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FIG. 2. Histograms of the minimum latency tion. The few Purkinje cells which responded in more than one histogram.
of CF responses to peripheral stimulato more than one input are represented
In an attempt to define the detailed pattern of CF inputs to folia a and b of the paramedian lobule (19), microelectrode penetrations were made at intervals of 200 pm in the transverse plane, perpendicular to the surface. The accurate identification of the folial layer from which responseswere evoked was aided by breaking many electrodes in situ and Ieaving the of the ipsilateral forelimb, but not to light tactile stimulation. at a depth of 4.6 mm was clearly in crus 2. L.pm-paramedian lateralis ; Fl-flocculus ; Pfl-paraflocculus.
The neuron recorded lobu!e: Nl-nucleus
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tips in the tissue during fixation. Histologic sections of the cerebellum were prepared for analysis of recording sites in all cases. Analysis of Data. Poststimulus time histograms were made on an Ortec histogram unit from the magnetic tape recordings. The Mann-Whitney U-test was used to establish the probability that short- or long-latency CF responses to electrical stimulation were related to the presence or absence of a peripheral receptive field. A sign test was used to determine the probability that Purkinje cells which received a CF input from a peripheral nerve also obtained their cortical input from a region activated by the same peripheral input. RESULTS Eighty-seven tracks were made into folia a and b of the paramedian lobule of the cerebellar cortex. One hundred and twenty-eight Purkinje cells were readily identified by the characteristic “complex spike” discharge in response to a CF input (14). During the examination of the histologic sections of all cerebella, a number of electrode tracks were recognized. Because all tracks in a series were parallel and depth measurements had been made during recording, it was possible to estimate whether a particular neuron was recorded within one of the Purkinje cell layers of the paramedian lobule or within deeper adjacent folia which were penetrated upon occasion (Fig. l), It was found that 123 CF responses were recorded in the first two folia of the paramedian lobule, four were located in crus 1, and one in crus 2. Ninety-eight percent of all neurons isolated were activated by one or more of the inputs tested. Among the 71 Purkinje cells which were completely tested, 65% rkceived CFs from both cortex and periphery, 31% from cortex alone, and 4% (three neurons) solely from the periphery. Two of these three neurons were in crus 1 and had receptive fields on the back of the animal (Fig. 1) ; no stimulating electrode was placed in the corresponding projection area of the pericruciate cortex [medial anterior sigmoid gyrus (25)]. Thus, all but one cell recorded in the paramedian lobule received a CF from the somatomotor cortex. Two major populations of CFs were therefore defined, one excited from the cortex and the other by both cortical and peripheral inputs. Climbing Fiber Responses to Peripheral Stiwaulation. The majority of Purkinje cells activated from the periphery received a CF input only from the ipsilateral forelimb (56%) ; 19’7’0 were activated from the hind limb alone, and 13% from both ipsilateral limbs. The remaining 12% responded to various combinations of trigeminal and spinal inputs, including 5% which gave CF responses to contralateral stimulation. In general, superficial radial nerve-evoked CF responses were found throughout the breadth of both folia a and b, and cells receiving sciatic and trigeminal inputs were observed infrequently. However, the hind limb
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representation was predominant in folia a and b of one cat. Of all Purkinje cells found in this investigation which responded to sciatic stimulation, 54% were recorded in this animal. It is of interest that the single CF response which could be evoked from a restricted forepaw field in this
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FIG. 3. Poststimulus time histograms for three Purkinje cells, with inserts showing examples of the CF responses of these cells. In A and B, the cortex was stimulated, and in C, the sciatic nerve. A total of 128 trials was accumulated for each histogram; the first bin gives the vertical calibration in events per bin. The time indicated below each histogram is also the horizontal calibration for the corresponding filmed records. The vertical calibration applies to all filmed records; positive up in all figures.
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experiment was found to be in the adjacent folium of crus 2. This point will be reexamined in the Discussion. In addition, in all cases, receptive fields on the proximal part of the forelimb or hind limb were predominantly represented in the medial half of the lobule, whereas the lateral half received CFs which had distally situated receptive fields (Fig. 6, compare the receptive fields in the medial and lateral regions of the folium). The number of Purkinje cells and their minimal response latencies to stimulation of each ipsilateral peripheral nerve are given in Fig. 2. Purkinje cells can be divided into three groups based on CF response latency measurements. The large majority (74%) of all Purkinje cells responding to the superficial radial nerve did so at short latency (14 to 34 ms, Fig. 2), although some of these also fired a second time at long latency (120 to 250 ms) after stimulation, These responses were of the type illustrated in Figs. 3A and B. Sixteen percent of the superficial radial nerve-activated cells
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4. Histograms of the cortical threshold current and of the minimum latency responses to cortical stimulation (measured from the first shock). Note that the binwidths for the latter are 2 ms for latencies up to 34 ms, and 10 ms for latencies greater than 120 ms. FIG.
of
CF
wve CF responses only at long latencies. ~1 similar distributioll of Intenb ties of sciatic-evoked CF responses occurred (I;ig. 2). .ln example of a long-latency sciatic response is shown in Fig. 3C. In the group of l’urkinje cells respondin g after long latency, there were significantly more receiving CFs without peripheral receptive fields than there tvere receiving CPs with peripheral fields, for both the superficial radial nerve (,P < 0.05) and the sciatic nerves (I’ < 0.01). Eleven of the fifteen cells responding to the superficial radial nerve at a latency of > 120 ms had ii0 receptive fields, and seven of the eight cells responding to sciatic stimulation at long latency were not excited by natural stimulation. X11 three short-latency trigeminal responses were also driven by tactile stimulation of the face, whereas three of the five long-latency responses \vere not. However, the Purkinje cells which responded at long latency to stimulation of peripheral nerves also responded at long latency to natural stimulation of their receptive fields when these were present. Gentle mechanical stimulation of hair or skin was an adequate stimulus for eliciting CF responses in 7376 of the cells which had peripheral fields. In 24% of the cases, particularly those with peripheral fields proximally situated on the limb, firm pressure KLS needed to evoke the response, suggesting that deep pressure receptors or possibly muscle receptors may have I)eeu the effective stimulus. Three percent responded only to pinching or pricking of the skin. The extent of the skin area from which CF responses were evoked was generally small. For the limbs, the peripheral skin fields were most coiim~only restricted to a small area of the dorsal and/or ventral surface of the foot, or to the skin inside the ell)ow or knee. Occasionally “stocking”or “glove”-type peripheral fields were observed. Only three cells were found which could be activated from a large skin area extending over more than one limb. Three of the four neurons which hat1 Cl: receptive fields on the face \vere also activated from skin fields on the forelimb. Cli/ilbiPlg Fiber Xcs~ollsrs fo Corticd Sti~lrulatioa. Stimulation of one or nlore cortical sites (mean = 2.3) at intensities of less than 500 ,uA was effective in evoking CF responsesin 96% of the cells tested. The number of cortical sites from which a neuron could be activated was unrelated to the presence, absence,or variety of peripheral inputs or to the latency of CF responses. For any given cell, honever, the threshold current for eliciting a response from one specific cortical stimulus site (the “best point”) was significantly less than the threshold current for adjacent stimulus sites in the cortex. Figure 4 ~1~0~~s the amplitudes of the threshold currents at the cortical best point for 67 cells. The minimum CF respome latency, measured from the first pulse of the stimulus train delivered to the best point for each Purkinje cell, is also shown. X11of the eight neurons
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(11%) activated by CFs at long latency (150 to 250 ms) following cortical stimulation also received long-latency peripheral CF inputs. It can be seen that the modal latency of cortically evoked CF responses is 20 to 22 ms, using the convention of measurement from the first cortical shock. Several cells were tested for their response to cortical stimulation at constant intensity, but with different numbers of shocks. Some CFs were activated by a minimum of two cortical shocks, others by three. Increasing the number of stimulus shocks did not alter the latency in these cells. This suggests that the corticofugal cells were activated not by the first stimulus shock, but rather when their excitability was raised sufficiently by subsequent shocks to bring them above their firing threshold. Comparison of Cortical and Perifheral Isputs. A clear correlation was observed between the location of the cortical “best point” and the peripheral input which excited an individual CF (Fig. 5). Generally, the best point was in a region where stimulation could be expected to cause movement of the body part containing the peripheral field. Although movements were
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FIG. 5. Convergence of peripheral and cortical inputs to three Purkinje cells. Two CFs were recorded on a track at the anterolateral corner of folium a. The most superficially situated had a receptive field on the ventrolateral surface of the forepaw (forelimb prone) and had a cortical “best point,” E, where a negative potential was evoked by stimulation of the superficial radial nerve (SRN). The other CF responded to poking the foot pads of the hind paw (hind limb plantar) and to stimulation of cortical point B, which received an input from the sciatic nerve (SCI). Another CF, recorded in the middle of the folium and at its caudal border, had a discontinuous tactile receptive field on both the ipsilateral forepaw and chin. It also received a convergent input from control point H, where negative fields were recorded after stimulation of both the SRN and the infraorbital nerve (V).
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not evoked because of curarization, 7870 of the Purkinje cells receiving a CF peripheral input from superficial radial trigeminal, or sciatic nerves had their best point at a cortical site where a potential could be evoked by stimulation of the same nerve (Figs. SB, IX). It is statistically unlikely that such .a relationship could arise by chance (P < 0.01). The remaining EC/o of the cells, for which the hest-point electrode was often deep in the \\-hite matter (see methods), responded to an adjacent “second-best” cortical point, receiving the corresponding peripheral input. If a Purkinje cell received a CF input which could be activated from two peripheral nerves, the cortical best point was generally in a region from which evoked potentials from both nerves could be recorded (Fig. SH ). The precision of this point-to-point, cortico-olivocerebellar projection is shown by the fact that the threshold for cortical activation of CF responses in 84% of the cells was in the range of 60 to 200 PA for the hest cortical site; from cortical sites within 1 to 2 mm of the best point, the threshold stimulus current was significantly greater. As previously stated, the predominant input to the two folia came from the ipsilateral forepaw. The CF peripheral fields of all Purkinje cells recorded in one cat are shown in Fig. 6, together with the anatomic location of the recording points. In this folium, the forelimb was exclusively represented and the somatotopography was the most ordered of all the cases examined. The map illustrates the general finding that the CF somatosensory receptive fields on both forelimb and hind limb tended to move distally (cf. tracks 1 and 7) and posteroventrally (cf. 7 and 17) as the electrode was moved from the medial to the lateral margin of the folia. Accompanying this shift in peripheral receptive fields is a movement of the cortical best point from electrodes B and C, which are in regions which probably control the more proximal muscles of the limb, to electrode IL Stimulation at this point would be expected to elicit movements of the elbow or wrist (25). DISCUSSION The present investigation shows that, with one exception, all CFs terminating in paramedian folia a and b of the cat are activated from the motor cortex. These results differ from those obtained in lobule VI and adjacent anterior lobe areas of the cat by Miles and Wiesendanger (23), who found that about 30% of the CFs excited by stimulation of trigeminal nerves did not receive a convergent input from stimulation of a single point in the Sl sensorimotor cortex. Whether these remaining Purkinje cells in the regions they investigated receive an input from other unstimulated regions of the Sl face area or from the more anterior jaw and face motor areas is unknown. In the intermediate zone of the anterior lobe, Allen et al. (4) did not report such a universal cortical input. Although
162
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FIG. 6. Diagrammatic representation of 17 electrode tracks made into folium a of the right paramedian lobule. The positions of the points of entry of the tracks on the surface of the folium are shown in the inset. The location of each Purkinje cell in the vertical and mediolateral plane is indicated by a circle. The filled circles represent neurons in which a CF response was elicited by both cortical and superficial radial nerve stimulation, and the open circles indicate neurons that were excited only from the cortex. Peripheral receptive fields and their sensory modalities are indicated on the diagrams of the ipsilateral forelimb. The cortical best points for each track are shown below. Their positions are approximately those shown in Fig. 4, with one important difference: Electrode B was more laterally placed and superficial radial nerve-evoked potentials were recorded from it.
the percentage of Purkinje cells responding to stimulation of the tortes is not given, their Fig. 4 shows that not all received CFs which were activated from the cortex. In the present study, the motor cortical CF input to paramedian folia a and b is principally from the area known to control the forelimb (25, 29). Furthermore, it was observed that the more medial parts of the cortical forelimb area, where stimulation evoked movements of the shoulder, project to the medial half of the paramedian lobule. The lateral half of the paramedian folia a and b receives CF inputs from cortical regions controlling elbow or wrist movements. The presence of barbiturate anesthesia is known to raise the stimulus threshol’d of the motor cortex for eliciting
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other responses such as movement (25 j. Despite this, 457; of the CFs were activated by cathodal currents of
from
one experiment
to another.
Variations
in the arrange-
ment of the cells in relation to surface landmarks are not surprising because the pattern of folding of the folia of this region is known to be inconstant (19). The present data suggest furthermore that the functional representation may also vary in relation to surface landmarks. This is exemplified bq one case in which the posterior limb was predominantly represented in the first two folia of the paramedian lobule, and the forelimb in the adjacent folium of crus 2. In this case, the proximal regions of the hind limb were again represented medially and the distal regions laterally. Despite such variability the convergence of cortical and peripheral body parts through single CFs was generally maintained (e.g., Fig. 5j . The majority of CF responsesin the paramedian lobule were found to have input characteristics which were qualitatively similar to those of Purkinje cells in the pars intermedia of the anterior lobe and hemisphere, that is, a low threshold to natural stimulation of skin and hair, and restricted peripheral fields (16, 20, 23). Those properties of paramedian Purkinje cells were expected, since it is known that CFs which project to the anterior cerebellar lobe send collaterals to synapseonto Purkinje cells in the paramedian lobule and elsewhere (7). Hence the present observations lend further support to the supposition that climbing fibers can transmit precise somatosensorv information to the cerebellar cortex, which ma\
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AND
COURVILLE
serve to keep the Purkinje cell informed of peripheral events during movements (9, 16, 21, 23). The range of latencies at which CF responses were evoked in Purkinje cells by stimulation of limb nerves was similar to that reported in other studies on the cerebellar anterior lobe and the paramedian lobule (6, 7. 13, 21, 24). However, there was no obvious grouping of cells according to the criteria of latency and/or presence or absence of contralateral inputs which would confirm the results that Armstrong et al. (6) obtained in their evoked potential studies. Figure 4 shows that the modal latency of cortically evoked responses is 20 ms, measured from the first pulse of the train. However, in agreement with Allen et al, (3), it w.as shown in a few Purkinje cells that at least two or three cortical shocks were required to elicit CF responses; that is, temporal summation in corticofugal neurons was necessary. Although only a few cells were tested in this way, it is reasonable to assume that the convention used in describing the latency of cortically evoked CF responses overestimates their true latency (measured from the time of activation of corticofugal neurons) by 2 to 4 ms. Thus the true modal latency is probably about 16 to 17 ms, as was found in the anterior lobe (3). In addition to the cells whose latencies to peripheral or cortical stimulation were within the range of 15 to 32 ms, a number of cells were found to respond only after 120 to 250 ms. This applied mainly to peripheral stimulation. The fact that the majority of these neurons had no demonstable peripheral field is probably significant. It is highly unlikely that such long latencies are due to a circuitous route to the inferior olive because even quite short polysynaptic paths are depressed by barbiturate anesthesia. The more common phenomenon of an early excitation of CFs, followed by a long period of inhibition, then a rebound excitation has been observed before (9, 16, 21, 23). The inhibition has been ascribed to inhibitory mechanisms within the olive (5, 14). It seems probable that this feedforward inhibition explains the very long latency of some CFs to stimulation. That is, olivary neurons which h,ave not themselves been initially excited may receive recurrent inhibition resulting from discharge of adjacent climbing fibers. At the end of this long inhibitory period, the olivary cells then undergo rebound excitation, giving an apparent response latency of 120 to 250 ms. It is worth noting that long-latency cells were observed in five of six cats, and that adjacent cells responded at the more common short latencies. This observation indicates that the long-latency phenomenon was not due to some idiosyncrasy of a particular experimental preparation. Furthermore, long-latency responses were produced in several neurons by timed mech.anical stimulation of skin. This rules out the possibility that the phenomenon was due to the nonphysiologic nature of electrical stimulation.
In the anterior lobe, Miles and II’iesendanger (23) found a population of CFs which could be excited only from the periphery. In contrast, we found a number of CFs which were excited only from the cortex. The latter had no receptive fields arid could not he excited hy the stiniulation of a nerve which activated others on adjacent penetrations, although it is possible that they lnay he activated hy stimulation of other peripheral nerves. If one CF on a track was of this type, then there was a high luwl~al~ility that the other cells recortled along the saiiie penetration would also he without a peripheral input (Fig. 6, tracks 10 ant1 16), and in two atljacent tracks it was also often found that cells responded in the salne
manner. There was therefore a certain indication that populations of cells activatetl by corte*x alone alternate across the folia with cell groups responding to both cortex and periphery (Fig. 6). This observation can he correlated with the data of Armstrong ct al. (8), who showed electrophysiologically that groups of olivary cells project CFs in long strips oriented l~erpendicularly to the long axes of the folia. They described six distinct bands in the anterior lobe and three in the paraineclian lohule. L+cordiiig to a study of the olivocerehellar projection 1)~ the method of injection of tritiated amino acids and radioautography ( IO), these strips of projection inight he even siiialler (0.4 to 0.8 inni), and consequently more numerous, than those observed 1)~ electrol~hysiologica1 mappings. In the anterior lobe, it was estimated that as niany as 12 different hantls could exist, and in the paraniedian lohule, there coultl he six to eight separate strips. It was also concludetl in that study that the intercligitating strips originate from distinct sites in the olive. The present tlata are not sufficiently detailed to pemlit an exact tlelirieatioii of the projection strips in the paranieclian lobule. However, it is suspected that the sets of interdigitating bands of CFs might correspontl on the one hand to a group of olivary cells receiving
convergent inputs from the cortex and periphery, and on the other hand to another
group
influenced
1)~ the cortex
hut without
a peripheral
input.
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24. PROVINI, L., S. REDMAN, AKD P. STRATA. 1968. hlossy and climbing fibre organization on the anterior lobe of the cerebellum activated by forelimb and hindlimb areas of the sensorimotor cortex. Exp. Brairr I&x 6: 216-233. 27. KANCK, J. B. 1975. Which elements are excited in electrical stimulation of mammalian central nervous system : A review. Brclilr IIcs. 98 : 417-440. 28. SNIDER, R. S., AKD A. STO\VELL. 1944. Receiving areas of the tactile, auditory and visual systems in the cerebellum. J. Nczrvoplzyvsiol. 7 : 331-357. 29. WOOLSEY, C. N. 1958. Organization of somatic sensory and motor areas of the cerebral cortex. Pages 63-81 in H. F. HARLOW AK;D C. N. WOOLSEY, Eds., Biological crud Biochemical Basrs of flekn~iour. L~nirersitp of Wisconsin Press. Madison.