395

J. Physiol. (1977), 265, pp. 395-428 With 3 plates and 9 text-ft gurea Printed in Great Britain

CONNEXIONS FROM LARGE, IPSILATERAL HIND LIMB MUSCLE AND SKIN AFFERENTS TO THE ROSTRAL MAIN CUNEATE NUCLEUS AND TO THE NUCLEUS X REGION IN THE CAT

By H. JOHANSSON AND H. SILFVENIUS From the Department of Physiology, University of Umed, S-901 87 Umea, Sweden

(Received 21 April 1976) SUMMARY

1. Evidence is presented for an input from ipsilateral hind limb group I muscle afferents and low threshold cutaneous afferents, to cells in the rostral division of the main cuneate nucleus (rMCN) and in the region of the descending vestibular nucleus and the nucleus X of Brodal & Pompeiano (1957a), the (DV-X). 2. Thirteen group I-rMCN cells were recorded from. The functional properties of these cells were similar to those of nucleus Z (Landgren & Silfvenius, 1971; Johansson & Silfvenius, 1977a, b). The cells were monosynaptically linked to spinal dorsolateral fascicle (DLF) fibres. Nine cells projected to the contralateral thalamus, i.e. a second group I hind limb bulbothalamic tract is described. Ten cells were synaptically activated from the ipsilateral cerebellum from the anterior projection zone of the dorsal spinocerebellar tract (DSCT). Axon-collateral activation by DSCT fibres was established for two of these cells. They were both bulbothalamic relay cells. For the remaining eight cells, activated from the cerebellum, this was not proven. These cells could, however, either be linked to DSCT fibres or to short axon-collaterals of a cell body of unknown location. A projection from the rMCN to the cerebellum is described and agrees with recent anatomical findings. Two cells were not excited from the cerebellum. 3. Four rMCN cells were activated by cutaneous afferents with their secondary axons in the DLF. Suggestive evidence for a bulbothalamic cutaneous hind limb path via the rMCN is presented. Two cells were activated from the cerebellum, presumably via axon-collaterals of nonsegmental cells. 4. Eight group I-DV-X cells were recorded from. They were monosynaptically linked to spinal DLF fibres and resembled functionally the

H. JOHANSSON AND H. SILFVENIUS 396 nucleus Z and rMCN cells when stimulated from the periphery. Two cells projected to the contralateral thalamus, and two others were synaptically excited. Seven cells were activated from the ipsilateral cerebellum. Two of them projected to the cerebellum, and three were synaptically activated by axon-collaterals of an undefined non-segmental cell. 5. Two DV-X cells which were activated by cutaneous afferents possibly had their spinal fibres deep in the dorsal column. Both were activated from the cerebellum, one by collaterals of a spinal axon. The functional organization of the three juxtaposed medullary nuclei, Z, rMCN and DV-X is discussed. INTRODUCTION

A proprio- and an exteroceptive input to nucleus Z in the cat medulla oblongata from ipsilateral hind limb afferents has been described (Landgren & Silfvenius, 1971; Johansson & Silfvenius, 1973, 1977a, b). This input is partly mediated via axon-collaterals of the dorsal spinocerebellar tract (DSCT). In analysing the recording positions made in the region of nucleus Z. we have observed that cells, which are located lateral and rostrolateral to nucleus Z receive an input similar to that forwarded to nucleus Z from the lumbar spinal segments. Thus cells in the rostral division of the main cuneate nucleus, rMCN, and in the region of the descending vestibular nucleus and the nucleus X of Brodal & Pompeiano (1957 a), the DV-X, also receive excitation from group I hind limb muscle afferents and from low threshold cutaneous afferents. In this report we describe the functional properties of the cells recorded from in these two nuclear regions and their relation to the contralateral thalamus and the ipsilateral cerebellum. A preliminary report has been published (Johansson & Silfvenius, 1975). METHODS The experimental procedures were similar to those employed in the two preceding reports (Johansson & Silfvenius, 1976a, b). Six cats were used. In three of them, a number of nucleus Z cells were recorded from. Those cells are included in our

previous reports. Delimitation of medullary nuclei. From the histological serial transverse sections, stained with Luxol fast blue (Kluver & Barrera, 1953), every second one was analysed with regard to the presence of and to the spatial relation between the relevant medullary nuclei listed below. By determining the medial and lateral borders, as well as the positions of the rostral and caudal poles of every nucleus present in each section studied, a horizontal diagram could be compiled of the medullary nuclei identified. Their interrelation and their distances to the obex and the mid line were thus specified. Before the horizontal diagrams were drawn, corrections were made for shrinkage as described earlier (Johansson & Silfvenius, 1977a). The following medullary nuclei were identified and delimited.

HIND LIMB INPUT TO rMCN AND X

397

Nucleus Z: Z of Brodal & Pompeiano (1957a). Nucleu X: X of Brodal & Pompeiano (1957 a). As Brodal & Pompeiano point out, X fuses at its caudal pole with the rostral tip of Z. The cells of X are, however, distinguishable from those of Z, and X is located rostrolaterally to Z as it extends ventrally under the restiform body. Only the caudomedial portion of X is considered of relevance in this report. Ventrally X borders the external cuneate nucleus (ECN) and the rostral division of the main cuneate nucleus (rMCN). Medial to X is the descending vestibular nucleus (DV). The ro8tral division of the MCN. This nucleus contains clustered cells arranged irregularly and the rMCN was thus differentiated from the Z and X nuclei. In most of the cats there was a cell-free zone between the rMCN and the Z. The external cuneate nucleus, ECN. This nucleus is partly covered dorsally by the rMCN and lies ventrolateral to it. The large and regularly grouped cells of the ECN made it possible to separate it from the rMCN. The gracile nucleus, G. This structure was delimited from the Z by the cell free zone as reported by Brodal & Pompeiano (1957a). The descending vestibular nucleus, DV. The cells of the caudal part of the DV are similar to those of the medial vestibular nucleus (MV) and the border between these two nuclei was therefore difficult to establish in the present sections. In this material the caudal pole of the DV was located ventrolaterally to the rostral tip of the Z, a finding which agrees with the diagrams of Pompeiano & Walberg (1957). The nucleus f of Meessen & Olszewski. This nucleus, described in 1949 by the authors mentioned, is located in the caudal portion of the DV and is easily distinguishable from the DV by containing clusters of large cells. Sometimes two f nuclei are present in the DV (Walberg, Pompeiano, Brodal & Jansen, 1962). The medial vestibular nucleus, MV. See text under the DV. The superior vestibular nucleus, SV. The caudal pole of the SV is located rostromedially to the Z (Brodal, Pompeiano & Walberg, 1962). Some of the present cells localized rostrolateral to Z could possibly be SV neurones. Calculation of unit locations. The localization of units recorded from were made as described by Johansson & Silfvenius (1977a). The marking tracks and a number of penetration tracks were identified in three of the six medullae which were cut at a 30 degrees caudal angle in relation to the vertical plane. The other three blocks were cut transversely and vertically in order to identify histologically the position of the cerebellar depth stimulating electrodes used in these three cats. The marking tracks in the vertically cut blocks were identified by the holes they left in the medullae from their penetrations in it, 30 degrees from behind. The other penetration tracks were not identified histologically in the medullae. However, both they and the marking tracks were defined in two of the blocks in the overlying ventral remnant of the cerebellum. By calculating the directional angle, from a number of successive sections, which the penetration and the marking tracks made through the posterior cerebellum and referring these to the position of the marking track as it touched the dorsal surface of the medulla oblongata, the medullary elongations of the penetration tracks were estimated (cats D, E in Fig. 1). In animal A the cerebellum was not included in the block and the only histological reference here was the marking track. RESULTS

Recording positions in the medulla oblongata. Electrical stimulation of ipsilateral hind limb group I muscle afferents and of low threshold cutaneous afferents activates cells at short latency in the region adjacent to nucleus Z, of the cat medulla oblongata. The regions from which the

398 H. JOHANSSON AND H. SILFVENIUS activity was recorded are shown in Text-fig. 1 which also illustrates the spatial relation of Z to surrounding superficial medullary nuclei. The horizontal diagrams of Text-fig. 1 are compiled from histological serial transverse sections, each diagram delimiting Z and illustrating its distance (mm) from the histologically defined obex (ordinate) and from the mid line (abscissa) in the six cats studied. The diagrams also give the location D A 4-

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399 HIND LIMB INPUT TO rMCN AND X in the medulla oblongata of the marking micro-electrode track (0), the histologically verified penetrations (U) and the histologically undefined stereotactic tracks (e). With the marking tracks as reference the stereotactic locations of hind limb group I and of skin activated medullary neurones are plotted on the diagrams. The locations in diagrams A-F of cells indicate that medullary neurones lateral to Z are excited by ipsilateral hind limb group I (-) and by skin afferents (A). Diagrams A-D and F of Fig. 1 show the distribution of seventeen hind limb activated cells. Thirteen were activated by group I muscle afferents, and four by cutaneous afferents. All seventeen cells were localized to the rostral division of the main cuneate nucleus, rMCN, its mediorostral border marked in Text-fig. 1 with the interrupted line. In two animals the marking tracks were in the rMCN at a level caudal to Z (see Text-fig 1 A-B, and arrows in PI. 1, 1-2) and in the two other cats more rostrally, at the rostrolateral border of Z (see Text-fig. 1 C-D, arrows in P1. 2, 3-4). The AP co-ordinates, relative to the histologically determined obices of the seventeen cells encountered in the four cats ranged between 1780 and 3750,m (mean 2730,um, S.D. + 615) and their distances from the mid line between 2800 and 3950 gm (mean 3460 jsm, S.D. + 315). Four of the group I cells were marginal cells, three rostral ones in cat B and the caudal one in Text-fig. 1. Horizontal diagrams compiled from serial transverse histological sections of six cats, showing the right nucleus Z and surrounding superficial medullary nuclei. Ordinate and abscissa show Ap. and lat. distances in mm from the histologically determined obices. The following nuclei are delineated: nucleus Z = continuous line, rMCN = the rostral division of the main cuneate nucleus: interrupted line. In rMCN the continuous line marks the medial border of the external cuneate nucleus (ECN) G = the rostral pole of the gracile nucleus: thin line, X = the caudomedial portion of nucleus X of Brodal & Pompeiano (1957 a): dotted line, DV = the caudal part of the descending vestibular nucleus: dot-dash-dot interrupted line, f = the group f of Meessen & Olszewski (1949) situated in the DV: thin line. A-D and F = distribution of thirteen neurones activated by ipsilateral hind limb group I afferents (-), the caudal cell in F included in this group, and of four cells activated by skin fibres (A), all are considered to be located in the rMCN. E-F: location of eight group I neurones (-) and of two cutaneous (A) cells in the DV-X region. The histologically verified location of the marking micro-electrode track in each experiment is illustrated with (0). Histologically verified micro-electrode tracks are marked (D), and stereotactic, histologically undefined tracks are marked with (e). These verified and unverified tracks were used in histological identification of the various nuclear regions.

H. JOHANSSON AND H. SILFVENIUS cat F. They could equally well be DV-f-X cells, but are included among the rMCN cells as they fall within its borders and because the rMCN here was histologically more prominent than the DV-f-X. From the agreement in distribution between histological and stereotactic tracks and the location of the stereotactic tracks within the rMCN we conclude that the cells located lateral to Z and recorded from in these four cats represent rMCN neurones. They are probably not cells of the external cuneate nucleus, ECN (see Text-fig. 1A-D, Pls. 1, 2) as they were located medial to the medial border, marked in the diagrams by the rostrocaudally oriented line traversing the rMCN. In two of the six cats, ten other cells were found that were activated by the same modality afferents, but these cells were located rostrolateral to Z as shown in Text-fig. 1 E-F. Eight were excited by ipsilateral group I muscle afferents and two by low threshold cutaneous fibres. Diagram E and arrows in PI. 3, 5 demonstrate that in this particular cat the histological tracks were distributed rostrally to Z. In cat F the histological tracks were located somewhat more caudally, but still in the region to Z (cf. Fig. IF and arrows in P1. 3, 6). The AP co-ordinates, relative to the histologically defined obex, of the ten cells found in these two cats ranged between 3480 and 5000 jtm (mean 4020 ,pm, S.D. + 600), and their distances from the mid line varied between 3700 and 4680 jsm (mean 4100 jum, S.D. + 410. The cytoarchitecture of the region where the ten cells were located represents three different nuclear structures, i.e. the descending vestibular nucleus (DV) the nucleusf of Meessen & Olszewski (1949), which is situated within the caudal portion of the DV, and the nucleus X of Brodal & Pompeiano (1957a). Although the cells were localized to these three structures, this region will in the following be referred to only as the DV-X (see text from p. 19). Fig. 2, which is a horizontal anatomical diagram of the medulla oblongata in the cat, modified from Brodal & Pompeiano (1957a) and from Walberg et al. (1962), and into which the f has been incorporated, approximates the detailed findings illustrated in Text-fig. 1 and in the Plates. It shows the distribution within the rMCN and the DV-X of hind limb activated group I (filled circles) and skin activated cells (triangles). Functional properties of rMCN cells. The recordings made from the rMCN cells were mainly extracellular. No axonal discharges were registered. The rMCN cells activated by ipsilateral hind limb group I muscle afferents will in the following be called group I-rMCN cells and the neurones excited by low threshold cutaneous fibres cut.rMCN cells. The thresholds at which the thirteen group I-rMCN cells were activated are shown in Text-fig. 3A. A single group I volley in an ipsilateral hind limb muscle nerve evoked usually one to two action potentials in the cells. 400

HIND LIMB INPUT TO rMCN AND X 401 The first group I spike potential appeared at stimulation intensities between 0-8 and 1P86 times threshold (T) of the afferent volley recorded on the lumbar spinal cord (Text-fig. 3A black bars). The low thresholds indicate that group Ia fibres excite these cells, which may in addition receive convergence from group Ib fibres as suggested by an additional spike observed in about every second cell (see Text-fig. 4A). The group I-rMCN cells easily followed high frequency stimulation (2-500/sec) and show thereby high synaptic efficacy (see Text-fig. 4C).

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The thresholds and the discharge pattern of group I-rMC7N cells are thus similar to those of Z and of the segmental group DSCT cells (Lundberg & Oscarsson, 1960; Landgren & Silfvenius, 1971; Johansson & Silfvenius, 1977 a, b). lThe latencies of the first group I spike in these cells ranged between

402 H. JOHANSSON AND H. SILFVENIUS 6-4 and 9-2 msec, and of the second spike between 7-2 and 10 msec (see Text-fig. 3B, filled and open columns). The interval between the two group I spikes varied between 0-8 and 2-5 msec. The short orthodromic (OD) 4

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Text-fig. 3. Threshold and latency histograms of rMCN and DV-X cells. A, filled columns indicate T of first group I spike in thirteen rMCN cells, numbers below Ts show those of a second group I spike in nine cells. Hatched columns show Ts of four skin activated rMCN cells. B, filled columns show latencies of first group I spike in thirteen rMCN cells, open bars those of the second I spike in nine cells. Hatched bars show latencies of four skin activated rMCN cells. C, filled columns indicate T of first group I spike in eight DV-X cells, numbers below Ts those of the second group I spike evoked in five cells. Hatched bars indicate T of two cutaneous cells in the DV-X. D, latencies of first group I spike in eight DV-X cells are marked with filled columns, latencies of second group 1 spike in five cells marked with open bars. Hatched columns are for two cutaneous cells. E, latency correlation in rMCN and DV-X cells between the first spike potential evoked by peripheral (P) and ipsilateral DLF-C1 stimulation. (0*) = group I rMCN cells, (*) = group I DV-X cells, (A) = DLF-linked skin cells in rMCN, (O) = DC-linked skin cells in DV-X.

HIND LIMB INPUT TO rMCN AND X 403 latencies strongly suggest that the group I-rMCN cells are directly linked to ascending spinal fibres, an assumption substantiated by the short latencies of the spikes evoked in the cells by electrical stimulation of the ipsilateral dorsolateral fascicle (DLF) at the first cervical segment as described below. A PBSt

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-Text-fig. 4. Records from three group I-rMCN cell8. A, electrical stimulation strength series of a cell linked to PBSt afferents. B, excitatory convergence pattern of the same cell shown in A. C-E, unitary discharges of another cell linked to Q afferents, C, to repetitive afferent stimulation, D, to repetitive DLF-C1 stimulation and E, to single DC-C1 stimulation. F-G, synaptic potentials in a third cell activated by G-S afferents and by DLF-C1 stimulation, DC-C1 stim. evokes an i.p.s.p. H-I, records from an orthodromic-thalamic collision test performed on the same cell with records in F-G, illustrating that the thalamic spike evoked is AD. Voltage, 200 #aV for A-B, 400 #aV for C-I; timer, 8 msec.

Spinal location of fibres to group I-rMCN cells. Electrical stimulation of the ipsilateral DLF and the dorsal column (DC) at the C, segment performed on ten group I-rMCN cells, demonstrated that at C1, the group I fibres exciting the rMCN cells are located superficially in the DLF, close to the entry zone of the C1 dorsal rootlets. The negative current pulses necessary to activate the cells were under optimal conditions low, 25-30 ,uA. These axons thus have an identical location at this spinal level to those of the DSCT (Busch, 1961; Grant, 1962; Holmqvist & Oscarsson,

H. JOHANSSON AND H. SILFVENIUS 1963). The latencies of the DLF-C1 evoked spikes in the group I-rMCN cells ranged between 0 5 and 1-6 msec, mean 1-25 msec. The cells with DLF-C0 latencies < 1 msec are undoubtedly monosynaptically excited by spinal fibres. Records of the DLF-C1 potentials are shown in Text-fig. 4D, F. The comparison between the OD and the DLF-C0 latencies made in Text-fig. 3E (filled circles) suggests both mono- and disynaptic coupling to spinal DLF fibres among group I-rMCN cells. Occasionally a group I-rMCN cell could also be activated by DC-C1 stimulation as is shown in Text-fig. 4D-E; the DC-C1 latency of this cell was 2-0 msec, i.e. 0 4 msec longer than the DLF-C1 latency. Excitatory convergence from unknown DC fibres thus occurs in group I-rMCN cells. Short glimpses of intracellular recording could at times show inhibition by DC fibres and excitation by DLF fibres in group I-rMCN cells as is seen in records F and G of Text-fig. 4. The latency of the i.p.s.p., 2-4 msec, is suggestive of disynaptic inhibition from the DC fibres, by a cell presumably located in the rMCN. Spatial excitatory convergence. The group I-rMCN cells had restricted receptive fields, usually being excited only by one muscle nerve as is shown in records under B in Text-fig. 4 and in Table 1. They are in this respect similar to the DSCT and Z group I cells. The modality 8pecificity of the group I-rMCN cell8 was prominent as Table 1 and Text-fig. 4B demonstrate. At times the cells did, however, receive excitation from other types of afferents. Excitatory convergence, from presumed group II muscle afferents, was seen in six of the thirteen cells while only two received co-excitation from cutaneous afferents. The group II linked cells discharged an additional spike potential at stimulation intensities between 1-8 and 3-7T. Such effects were evoked only from other muscle nerves than the one supplying the group I activation. The latencies of these spikes varied between 11 and 22 msec. The excitation from skin afferents of the group IrMCN cells arrived 3 msec later than the group I activation suggesting additional delay in the cutaneous path to these rMCN cells. Thalamic activation of group I-rMCN cells. Electrical stimulation in the contralateral tegmentum-thalamus activated seven of the thirteen group I-rMCN cells at short latency. The threshold currents ranged between 115 and 1300,uA (mean 670 1tA) and the latencies between 1-2 and 2-3 msec (mean 1-7 msec). With the collision test (Darian-Smith, Phillips & Ryan, 1963) it was shown that all the cells were antidromically (AD) activated from the contralateral rostral stimulation. Records from the collision performed on one cell are shown in Text-fig. 4H-I. They show that the thalamic spike, thal, is blocked at an interval succeeding the orthodromic (OD) spike, which is slightly longer than twice the latency of 404

405 HIND LIMB INPUT TO rMCN AND X the thalamic spike, a time which suffices for AD activation of the group I-rMCN cell from the thalamus. The locations of the tips of the thalamic electrodes AD-activating the group I-rMCN cell are shown in the histological diagrams of Text-fig. 5. A is a transverse section diagram from one animal, diagrams B-C are from another cat. The black dots mark the positions of the stimulating electrode tips. Diagram A represents TABLE 1. Characteristics of neurones located rostro-lateral to nucleus Z, presumably in the rMCN

DLF-C,

rMCN-cells Group I activated

Cutaneous cells

Cell A1 2 3 4 5 6 7 8 9 10 11 12 13 BI 2 3 4

Aff. Threshold Lat. Excit. converg. lat. nerve (T) (msec) (T/msec) (msec) 1-2 90 Su 1.4/12.6 PBSt 0*5 09 G-S 6.8 8-2 G-S 0.8 1.1 PBSt 1.0 8*0 1-6 Q 1X4 8X8 1-6 1.0 G-S 8-0 1.0 Q 1P86 6-4 1-5 6-4 1*4 Q PBSt 1.0 9.0 Su 2/11.5 +G-S 2/12 1.3 1.0 7.4 PBSt 1-4 192 G-S 9.2 1*2 Q 1i3 6-7 1*3 G-S 7*0 1*4 Su 1.1 11.2 1.6 Su 1-4 7.8 + Su 1.0 10*0 SP 1-2 13-0 1-4

the most caudal plane, roughly corresponding to a level between frontal planes A 4-5 according to Berman (1968). Diagram B approximates a plane between A 5-2 and 6-4 and C corresponds to a plane rostral to A 6-4 of Berman. The diagrams indicate that group I-rMCN cell are AD activated from the central tegmental field (CTF) and from stimulation in the lateral thalamus, from the region of the medial geniculate nucleus (MGM). These locations agree well with those from which group I relay cells of Z were AD invaded as well as with the anatomically defined Z efferents (Landgren & Silfvenius, 1971; Grant, Boivie & Silfvenius, 1973; Johansson & Silfvenius, 1977 a). The DLF-C1 latencies, measured in five of the cells activated from the contralateral thalamus, ranged between 1 . 1 and 1*6 msec suggesting that at least some cells were monosynaptically linked to spinal fibres. The rMCN could thereby function as another medullary relay nucleus for information from ipsilateral hind limb group I muscle afferents to contralateral rostral levels, in addition to the nucleus Z.

406 H. JOHANSSON AND H. SILFVENIUS A combined AD-trans-synaptic activation from the contralateral thalamus was observed in one group I-rMGN cell. The trans-synaptic activation of the cell was achieved by stimulation in the lateral part (0), and AD activation again from the medial (i) part of the central tegmental field. The two points are interconnected in Text.-fig. 5C A

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Text-fig 5. Diagrams of transverse histological sections from four cats in which electrical stimulation in the contralateral thalamus activated rMCN and DV-X cells AD and trans-synaptically. Diagram A represents one animal, diagrams B-C another. A-C, diagrams showing points from which group I-rMCN cells were AD activated (@). Open triangle shows the location of the electrode tip which activated one cutaneous rMCN cell AD (A). Trans-synaptic activation was obtained from the point marked (0). D-E, diagrams showing location of stimulating electrode tips from which AD activation of DV-X group I cells was obtained (0). Trans-synaptic activation was obtained from the point marked (0). Scale in mm. Abbreviations: CTF, central tegmental field; EW, Edinger-Westphal nucleus; ICA, interstitial nucleus of Cajal; NR, nucleus Ruber; MGM, magnocellular part of medial geniculate body; D, nucleus of Darkschewitsch; VB, ventrobasal complex.

diagram. As the latencies of the thalamic spikes were equal, the observation indicates a descending tract to the group I-rMCN activated cell. Group I-rMCN cells and the cerebellum. The functional relation of group I-rMCN cells to the ipsilateral cerebellum was investigated with electrical surface or depth stimulation. This method showed that ten of the twelve cells tested were activated at short latency from the cerebellum

407 HIND LIMB INPUT TO rMCN AND X while two cells were uninfluenced even by high intensity stimulation (5 mA). Five of the ten cells activated from the cerebellum were bulbothalamic relay cells. The points in the anterior cerebellar lobe from which seven group I-rMCN cells were activated are shown in Text-fig. 6A, which is a diagram of the anterior lobe based on a photograph taken from a lateral-anterior direction. The circle symbols refer to points from which group I-rMCN cells were activated at short latency. They are all located in the projection field of the DSCT. The thresholds for activating the cells ranged between 260 and 1000 ,A (mean 660 4A). Surface negative and sometimes surface positive pulses were delivered, and the thresholds of the positive pulses were about half the values of the negative ones, suggesting a deep location of the terminals connecting the group I-rMCN cells with the cerebellum. Cerebellar depth stimulation aiming at the restiform body (RB) was used on two cells and the locations of the negative electrode tips activating cells are marked in Text-fig. 6D, showing that they were close to the restiform body. The black dots mark the negative tips, the dotted circles, Text-fig. 6B-C, the positive ones. The points are interconnected for the two cells stimulated. The thresholds for activating the two group I-rMCN cells by restiform body stimulation were 36 and 200 puA, indicating a relatively close position of the electrodes to the pertinent fibres. The latencies of the cerebellarly evoked spikes in the ten group I-rMCN cells were short, 0-8-2-4 msec (mean 1P6 msec) indicating a 'direct' connexion between group I-rMCN cells and the cerebellum. The connexion could either be a cuneocerebellar tract or a mossy fibre axon-collateral connexion. The collision test was applied to specify this connexion. If a group I-rMCN cell were AD activated from the cerebellum, the OD-cerebellum collision sequence would establish it by extinguishing the cerebellar spike during an interval of the order of twice the latency of the cerebellar spike. The reversed collision sequence, cerebellum-OD, would not give information on a possible AD activation from the cerebellum, but would establish whether synaptic activation of a cell takes place by spinal or other fibres with cerebellar axon-collaterals (Johansson & Silfvenius, 1976b).

The OD-cerebellum collision was performed on eight group I-rMCN cells, six of them being bulbothalamic relay cells. None of the relay cells projected to the cerebellum. Two non-relay cells did, however, project as demonstrated with surface and depth stimulation. The cerebellar spike, evoked with anterior lobe stimulation from the point marked with arrow a in Text-fig. 6A, was extinguished during an interval slightly longer than twice the latency of the cerebellar spike. The result of this collision is shown in Text-fig. 7K-M. The latency of the cerebellar spike in this cell activated by gastrocnemius-soleus afferents was 0-8 msec and did not show

408 H. JOHANSSON AND H. SILFVENIUS any jitter. This result establishes that there is a group I hind limb cerebellar projection from the rMCN to the anterior lobe. The other cerebellum-AD activated cell is referred to later in the text. The other six cells were not AD invaded from the cerebellum. Records from a bulbo-

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HIND LIMB INPUT TO rMCN AND X 409 thalamic group I-rMCN cell in which the collision test gave evidence only for soma refractoriness of the cell are shown in Text-fig. 7A-F, the cell being synaptically activated both from the cerebellum and the periphery. The cell was linked to gastrocnemius-soleus afferents and stimulated at 2T. A just suprathreshold stimulation in the anterior cerebellar lobe in the point marked with arrow b in Text-fig. 6A with a strength of + 300,sA evoked a 1-8 msec cerebellar spike, record B of Text-fig. 7 showing that the cerebellar spike fails to appear only during the soma refractory period of the rMCN cell. Surface cathodal stimulation in the same cerebellar point at - 600 ,sA (T = 400 ,tA) gives another cerebellar spike with a latency of 2-4 msec. The collision does not block this cerebellar spike either, as is seen in the record series D-F of Text-fig. 7. Increasing the cathodal current fourfold did not alter the collision result. This type of collision result indicates synaptic activation of the group IrMCN cell from the cerebellum. The same result was obtained in the other five cells tested. The reversed collision sequence, i.e. cerebellum-OD, would give information whether (a) a group I-rMCN cell is activated synaptically by axon-collaterals of long spinal fibres, i.e. by the DSCT, or (b) whether it receives excitation via short axon-collaterals of a cell body of unknown location. This collision sequence was tested on seven cells. Evidence for axon-collateral activation by DSCT fibres was observed in two bulbothalamic group I-rMCN relay cells. Records from one of them are shown in Text-fig. 7 G-I. This cell was activated by quadriceps afferents and stimulated at 2T in the collision test. RB stimulation Text-fig. 6. Diagrams of cerebellar surface and depth stimulation points from which rMCN and DV-X cells were activated at short latency. A, diagram of the ipsilateral anterior cerebellar lobe, based on a photograph taken from an ant.-lat. oblique angle. 0 = point from which axoncollateral activation was obtained in a group I-rMCN relay cell; 0 = points of short latency activation of group I-relay cells; arrow a marks the location from which one cell was AD invaded; arrow b, see Text. L = short latency activation of a cutaneous non-relay cell in rMCN; * = DSCT axon-collateral activation of a group I relay cell in the DV-X; O = short latency cerebellar activation of a group I non-relay cell in the DV-X. O = AD activated group I non-relay cell in DV-X. B-G, diagrams of transverse histological sections of the cerebellum in which depth stimulation activated rMCN (B-D) and DV-X cells (E-G). Black dots indicate cathode, dot-filled circles anode. A in G marks cathode for a DSCT activated cutaneous cell in DV-X. For details see Text. Abbreviations: CBM, medial cerebellar nucleus; CBL, lateral cerebellar nucleus; INA, ant. part of interpositus nucleus; INP, post. part of IN; RB, restiform body; IC, inferior colliculus. Scale in mm.

410 H. JOHANSSON AND H. SILFVENIUS (T = - 200 ,tA) at a 300 /LA strength evoked a spike in the cell after 1 msec. By placing the cerebellar spike before the OD spike, the latter is blocked at an spike interval of 8 msec, which demonstrates that the cell is synA

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411 HIND LIMB INPUT TO rMCN AND X optically activated by axon-collaterals of the DSCT. The other bulbothalamic relay cell activated by DSCT axon-collaterals was excited from the anterior cerebellar lobe (point, *), at a threshold of 1000 jA, shown in Text-fig. 6A. Five other group I-rMCN-cells were proven with the same collision sequence not to be DSCT axon-collateral activated. In three of them the OD spike was blocked at intervals shorter than the latency of the cerebellar spike indicating synaptic linkage between these rMCN cells and the cerebellum. This synaptic coupling could be mediated either by activated cerebellar terminals of axon-collaterals of uncollided DSCT fibres, or by axon-collaterals of a cell-body of unknown location and an undefined primary afferent input. In the other two cells, the cerebellar spike blocked the OD potential at intervals longer than twice the latency of the cerebellar spike, but less than the expected conduction times in the long secondary ascending spinal fibres ( = OD latency - (conduction time in primary afferents + segmental synaptic delay + medullary synaptic delay)). This type of response also suggests synaptic activation of the rMCN cells by axon-collaterals of a cell body of unknown location. This cell would itself receive monosynaptic excitation from ascending spinal group I hind limb fibres. Evidence in support of these two suggested connexions are provided in Text-fig. 7N-I. The non-relay cell is orthodromically linked to gastrocnemius-soleus group I afferents which evoked two spikes with a latency of 7-2 and 8-2 msec. A low current stimulation, 60 gA, in the restiform body, RB-anterior interpositus nucleus, INA, region (T = 36 ,uA) (cf. Text-fig. 6C-D, black dot, negative electrode) evokes a cerebellar spike in the cell after 0-8 msec and with a 'fixed' latency. The OD-cerebellum collision sequence establishes that the cerebellar spike is AD as it fails to appear during an interval slightly longer than twice its latency as seen in records U-V. Reversing the collision sequence results in a blockage of the OD-spike only during a short interval compatible with soma refractory period (records S-T) which is also in agreement with AD activation of the cell from the cerebellum. The OD2 potential is, however, blocked (records Q-T) at an interval of 1*8 msec, compatible with cerebellar axon-collateral activation of another cell exciting the cell recorded from. Record S shows extinction of the cerebellar spike and T of both OD spikes. This interpretation of course implies that two cerebellar impulses travel towards the rMCN cell, the second one not appearing because it arrives during the refractory period of the rMCN cell recorded from, caused by its AD invasion. The results thus entitle us to conclude that group I-rMCN cells may receive excitation from the cerebellum; (a) synaptically via axon-collaterals of the DSCT, (b) synaptically via short axon-collaterals of a cell itself activated by group I

412 H. JOHANSSON AND H. SILFVENIUS hind limb afferents, and (c) antidromically from the anterior cerebellar lobe or the restiform body region. Alternatives a and b apply to relay cells, c to non-relay cells. Three group I-rMCN cell-8 were not excited from the cerebellum, such

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I Text-fig. 8. Records from cells in the rMCN and DV-X activated by cutaneous afferents. A, excitatory convergence pattern of a Su activated cell in the rMCN. B, the same cell as in A, unresponsive to thalamic stimulation, C, short latency Cb ant. lobe activation of the same Su activated cell. D-G, records of a rMCN cell excited by SP afferents (D); unresponsive to thalamic stimulation (E); but activated by Cb depth stimulation (F-G), Ha-e, extracellular records from a rMCN cell activated by Su afferents and projecting to the contralateral thalamus (a-b). The cell was activated from DLF-C1 (c) and DC-C1 (d) stimulation, but unresponsive to cerebellar stimulation (e). f-h are intracellular records, (f) showing a multicomponent e.p.s.p. and (g) action potentials while the cell is deteriorating, h shows an i.p.s.p. evoked by forelimb SR afferents. Ka-f records from a DV-X cell linked to Su fibres, presumably located in DC (f). It was a non-relay cell (b) activated trans-synaptically from the cerebellum (c-d) by DSCT axon-collaterals. Voltages: 400 #sv for A-C, Ha-e, K e-f, 1 mV for D-G, Hf-h, 2 mV for Hg and Ka-d. Timer, 4 msec for D-G, Ha-b, Ka-f, 8 msec for A-C, and 10 msec for Hc-g.

413 HIND LIMB INPUT TO rMCN AND X neurones may have ramifications exclusively within the rMCN, or may receive cerebellar inhibition. Cutaneous rMCN cells. Four cells in the rMCN were excited by ipsilateral hind limb cutaneous afferents of low electrical threshold. These cut. rMCN cells were located among the group I-rMCN cells as is evident from Text-fig. 1 (A-D, triangles). The cut. rMCN cells discharged two to four spike potentials to a single volley in the primary afferents. Records from a typical unit are shown in Text-figf. 8H. The records a-b show the cell responding to a 2T volley in sural afferents, three spikes appearing, the latency of the first one being 11 msec. The shortest unitary latency observed was 7 8 msec (cf. Text-fig. 3 C, hatched bars) which suggests that monosynaptic linkage of cut. rMCN cells to spinal fibres does exist. The location of the spinal fibres activating the cut. rMCN cells was superficially in the DLF at Cl level. The cells could be activated from this location at latencies between 1P6 and 2 msec (cf. Text-fig. 3E, triangles). At times a cut. rMCN cell was also activated from a DC-Cl stimulation but at a latency longer than from the DLF-C1 stimulation as shown in Fig. 8Hc-d. Excitatory spatial convergence from other cutaneous nerves was not observed in these cells (see Text-fig. 8A and Table 1B). Other modality afferents at times exhibited weak synaptic contact with the cells recorded from (Su+ G-S in Text-fig. 8A). Thalamic activation of cut. rMCN cells. Three out of four cut. rMCN cells were tested with stimulation in the contralateral thalamus. One was AD invaded from the medial geniculate nucleus (see Text-fig. 5 C, triangle) as is shown in Text-fig. 8H a-b. Synaptic activation from the thalamus was not observed. Cerebellar activation with electrical stimulation in the RB region was seen in two of three cells tested (see Text-fig. 8 C, D-G). The non-relay cell, the records of which are in D-G, was linked to superficial peroneal afferents and had an OD latency of 13-14-8 msec, being activated with lower threshold from the DC-C1 stimulation, 1-8 msec, than from the DLF. Restiform body stimulation at 180 ,uA from the point shown in Text-fig. 6B-D (@) activated the cell after 1 msec. The collision test revealed synaptic excitation from the cerebellum but not proven to occur via axon-collaterals of the DSCT. Whether the cell was AD invaded from the cerebellum was not tested. Cerebellar lobe collision was not performed on the cell with records in A-C. Functional properties of DV-X cells. The location of the ten cells considered to be D V-X cells is shown in Text-fig. 1 E-F. Eight of them were activated by ipsilateral hind limb group I muscle afferents, and are bere called group I-DV-X cells (see Text-fig. 1 E-F, large dots) and two cells

414 H. JOHANSSON AND H. SILFVENIUS here called cut. D V-X cells were excited by similar, low threshold cutaneous afferents. (See Text-fig. 1 E-F, triangles). Although the groupf is located in the caudal part of the DV which receives a spinal afferent input, it has been stated that f appears not to receive spinal afferents (Walberg, Bowsher & Brodal, 1958; Brodal et al. 1962). According to the findings of

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395 J. Physiol. (1977), 265, pp. 395-428 With 3 plates and 9 text-ft gurea Printed in Great Britain CONNEXIONS FROM LARGE, IPSILATERAL HIND LIMB MUS...
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