J. Phyaiol. (1983), 337, pp. 257-285 257 With 8text-figure8 PrintedinGreatBritain ORIGIN AND SAGITTAL TERMINATION AREAS OF CEREBRO-CEREBELLAR CLIMBING FIBRE PATHS IN THE CAT BY GERT ANDERSSON AND JUDITH NYQUIST* From the Institute ofPhysiology, University ofLund, S6lvegatan 19, S-223 62 Lund, Sweden (Received 2 July 1982) SUMMARY 1. Climbing fibre responses were recorded in the cerebellar anterior lobe on stimulation ofthe cerebral cortex. A zonal pattern was demonstrated in the cortical projection, which was related to the cerebellar sagittal zones, as identified from peripheralclimbingfibreinput. Inallzones,exceptc2, aco-variationoftheresponses evokedonperipheral nervestimulationandonstimulationofthecorrespondingpart ofthe sensorimotor cortex was found. 2. There was a bilateral projection to the a, b, c2 and dl zones which also, to a varyingextent, receiveabilateralperipheral input. The x, cl and c3 zones, receiving an ipsilateral peripheral input, were activated exclusively from the contralateral cortex. 3. Stimulation ofthe posterior sigmoid gyrus (p.s.g.) evoked responses in all the zones. These responses had, in all zones except dl, lower thresholds and shorter latencies than the responses from other cortical areas. 4. Two separate p.s.g areas were shown to project to the pars intermedia zones (cl, c2, c3 and dl), the lateral area to the caudal parts and the medial area to the rostralpartsofthezones. In contrast, the bzonereceivedaprojectionfrom onlyone p.s.g. area, centred between, but overlapping, the two areas projecting to the pars intermedia zones. 5. Stimulationoftheanteriorsigmoidgyrusevokedshort-latencyresponsesinthe dl zone and long-latency responses in all other zones. 6. Stimulation of the first and second somatosensory areas (SI and SII) was generally less effective in evoking climbing fibre responses than was stimulation of the p.s.g. The only exception was the c2 zone, in which responses were evoked from the SII with nearly as low thresholds and short latencies as on p.s.g. stimulation. 7. From the parietal cortex, long-latency responses were regularly evoked in the dl zone and less frequently in the a, b and c2 zones. * Present address: Dept. of Clinical Research, Scripps Clinic and Research Foundation, 10666 North Torrey Pines Road, LaJolla, CA 92037, U.S.A. 9 PHiY 337 258 G. ANDERSSON AND J. NYQUIST INTRODUCTION It is now generally accepted that the inferior olive is the major, ifnot exclusive, source ofclimbing fibres to the cerebellum (Szentagothai & Rajkovits, 1959; Eccles, Ito & Szentagothai, 1967; Armstrong, 1974; Desclin, 1974; Batini, Corvisier, Destombes, Gioanni & Everett, 1976). The functional role of the olivo-cerebellar systemis, however, stillunknowndespitemanyanatomicalandphysiologicalstudies (for references see Eccles et al. 1967; Armstrong, 1974; Brodal & Kawamura, 1980; Ito, 1980; Oscarsson, 1980). It has been proposed that the climbing fibres convey signals about errors in motor performances (Barmack & Hess, 1980; Barmack & Simpson, 1980; Ito, 1980). These signals would induce plastic changes in the cerebellar cortex and result in improved performance (Marr, 1969; Albus, 1971; Ito, 1980). It has been suggested that the errors are detected by the inferior olive acting as a comparator (Miller & Oscarsson, 1970; Oscarsson, 1980); the inferior olive receivesinformationfromdescendingpathwayswhichmediatemotorcommandsand from ascending pathways which monitor the on-going activity in the lower motor centres andthe resulting movement. From a comparison ofthese converging inputs, the inferior olive might detect deviations from the intended movement and inform the cerebellum about these deviations. The ascending paths, the spino-olivo-cerebellar paths (s.o.c.p.s), have been thoroughlyinvestigatedwithrespecttotheirfunctionalproperties, aswellastotheir origin and course in the spinal cord and their termination areas in narrow sagittal strips in the cortex ofthe cerebellar anterior lobe (for references see Ekerot, Larson & Oscarsson, 1979; Armstrong & Schild, 1980; Oscarsson, 1980). The climbing fibre input to the cerebellum from the cerebral cortex has also been described in several reports (Provini, Redman & Strata, 1968; Miller, Nezlina & Oscarsson, 1969a; Allen, Azzena & Ohno, 1974a, b; Miles & Wiesendanger, 1975a, b; Sasaki, Oka, Matsuda, Shimono & Mizuno, 1975; Oka, Yasuda, Jinnai & Yoneda, 1976; Oka, Jinnai, & Yamamoto, 1979; Rowe, 1977a), as have the responses in the inferiorolive to stimulation ofthe cerebral cortex (Armstrong & Harvey, 1966; Crill & Kennedy, 1967; Crill, 1970). These studies showed a topographically organized convergenceofperipheraland cortical climbingfibre inputstothe cerebellum, which isconsistentwiththecomparatorhypothesis. However, inthesestudies, theclimbing fibre responses were not localized in relation to the sagittal zones which presumably constitute the functional units ofthe cerebellar cortex, each controlling aparticular motor mechanism (Oscarsson, 1980). Nor were the cortical areas giving rise to the climbing fire responses adequately delineated. Therefore, the present study was conducted in order to investigate the cortico- olivo-cerebellar projection to each ofthe climbing fibre zones in the anterior lobe. Particular emphasis was laid on the projection from the pericruciate cortex to the forelimb parts of the zones. A preliminary report has been given (Andersson & Nyquist, 1980). CEREBRO-CEREBELLAR CLIMBING FIBRE PROJECTION 259 METHODS Theexperimentswereperformedonthirtyadultcats(20-3-5kg),twenty-oneunderpentobarbi- tone (iNitial dose40mg/kgintraperitoneally) andnine under chloralose anaesthesia (initial dose 80mg/kgintravenously). Allanimalsweregivensupplementarydoses(2-5mg/kg)ofpentobarbi- tone as required. The animals were paralysed with gallamine triethiodide and artificially ventilated. Thearterialbloodpressure, end-expiratory CO concentrationandrectal temperature werecontinuouslymonitoredandkeptwithinphysiologicallimits.Theanaesthesiawasmaintained at such a level that the size ofthe pupils was always minimal. The sciatic and ulnar nerves were dissected bilaterally and mounted for stimulation. In some experiments, thesuperficialanddeepradialnerveswerealsostimulated. Theinfraorbitalbranches ofthetrigeminalnerves(henceforwardreferredtoastrigeminalnerves)werestimulatedbilaterally withneedleelectrodes insertedthroughthegingiva (Andersson & Eriksson, 1981). Allnerveswere stimulated at 20 times nerve threshold. Craniotomies were performed to expose the cerebellar anterior lobe on the left side and the cerebral pericruciate cortex bilaterally. In many cases, the cerebral exposure was extended to includeothercorticalareassuchastheparietalcortexandthefirstandsecondsomatosensoryareas. Photographs were taken ofthe cerebellar and cerebral surfaces and the exposed parts were then coveredwithwarmmineraloiltopreventdrying.Climbingfibreresponseswererecorded,withsilver ballelectrodes, aspositivefieldpotentialsatthesurface. Insomeexperiments,theactivityofsingle Purkinje cells was recorded with glass micropipettes filled with a 3M-KC1 solution and having a resistance of3-6MCI. The cerebral cortex was stimulated either at the surface with mono-polar silver ball electrodes oratadepth of2mmwithaneedleelectrode, insulatedexceptforthetip andhavingaresistance of10-20kQ. Thecortexwasstimulated cathodallywith 1-5squarepulsesof200,ssdurationusing aconstant current stimulator. The train frequency was 300-400Hz and the stimuli were applied 1-2timespers. Themaximalstimulusintensitiesemployedwere3mAforsurfacestimulationand, withtheexceptionoftheexperimentillustratedinFig. 1, 1-5mAforintracorticalstimulation. Prior to cortical stimulation, the responses on peripheral nerve stimulation were recorded at the stimulation site on the surface ofthe cerebral cortex. RESULTS (1) Functional properties ofcortico-cerebellar pathway8 The cortico-olivo-cerebellar projection was studied by recording climbing fibre responses evoked in the cerebellum on stimulation of the cerebral cortex. Fig. 1A shows records of responses recorded, under pentobarbitone anaesthesia, from the cerebellar surface on stimulation ofthe contralateral cerebral cortex with different stimulusintensitiesandnumberofshocks. Thecortexwasstimulatedcathodallywith a mono-polar needle electrode at a depth of 2 mm. The evoked responses consisted oftwo components: a short-latency (3 ms), slowly rising positivity due to synaptic activation of granule cells by a mossy fibre input and a second, steeply rising positivity at 13-16 ms due to a climbing fibre activation ofPurkinje cells (Eccles et al. 1967; Eccles, Provini, Strata & Taborikova 1968; Armstrong & Harvey, 1968; Provini et al. 1968; Allen, Azzena & Ohno, 1972; Allen et al. 1974a). Inmostpreparationsunderbarbiturate anaesthesia, themossyfibreresponseswere considerably smaller than illustrated here. Under chloralose anaesthesia, they were large and almost concealed the climbing fibre responses. Since the aim ofthe present study was to investigate the latter, small amounts ofpentobarbitone (2-5 mg/kg at intervals of 1-2 h) were given in these cases in order to depress the mossy fibre responses. Under such circumstances, it has been proposed that the amplitude ofthe climbing fibre responses is increased (Gordon, Rubia & Strata, 1973; Allen, Azzena 9-2 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~Al 260 0. ANDERSSON AND J. NYQUIST A 1-2mA 0*45mA 11200y 10 ms B 300- ;IV A0 100- C.f. M.f. /~~~~0 ,*-A3Sh. A_ 3sh. Ao A-A1 sh. o--o 1 sh. C 0*2 0-5 1.0 2*0 3S0 17- 15- C.f. A-A 3sh. ms \ 1 sh. 13- A. 0*2 0-5 1.0 2.0 3*0 mA Fig. 1. Characteristics of mossy and climbing fibre responses to cerebral intracortical stimulation. A, responses recorded at the cerebellar surface to stimulation of the contralateralposteriorsigmoidgyruswithanintracorticalneedleelectrode.Asingleshock (sh.) and a train ofthree shocks were used with varying intensities. Positivity upwards, as in all Figures. B, relation between stimulus strength and the amplitudes ofclimbing fibre (c.f.) and mossy fibre (m.f.) responses. C, relation between stimulus strength and climbing fibre response latencies. Pentobarbitone anaesthesia. & Ohno, 1979). Additionally, pentobarbitone stabilized the climbing fibre respons which fluctuate markedly under chloralose anaesthesia (Oscarsson & Sj6lund, 197', With a high stimulus strength (e.g. 1-2 mA), the climbing fibre response stimulationofthecortexwithoneorthreeshockswasessentially thesame (Fig. 1 / In some cases, as in the example illustrated in Fig. 1, the climbing fibre response evokedwithtwoshocksweresimilartothoseevokedwiththreeshocks.Inothercas stimulation with three shocks evoked climbing fibre responses with slightly lo, thresholds and larger amplitudes than those evoked with two shocks. An increase the number ofshocks above three did not reduce the threshold further nor incres the amplitudes. Therefore, stimulation with three shocks was regularly employee CEREBRO-CEREBELLAR CLIMBING FIBRE PROJECTION 261 When using lower stimulus intensities (e.g. 045 mA; Fig. 1A), there was a clear difference between responses evoked with one and with three shocks. This is further illustrated in Fig. 1B and C. In B, the mossy fibre and climbing fibre response amplitudesareplottedagainstthestimulationstrengths.Theclimbingfibreresponses evoked with a single shock were much smaller than those evoked with three shocks at low stimulation strengths. At higher stimulus intensities, the amplitudes did not grow further when the stimulus strength or the number of shocks was increased, implying a maximal activation ofthe cortico-olivo-cerebellar pathway. In contrast, the amplitudes of the mossy fibre responses (dashed lines) were increased both by increasing the stimulus strength and the number ofstimuli at all strengths tested. Thelatenciesofthe corticallyevokedclimbingfibreresponsesdepended, toalarge extent, on the stimulus intensities used (Fig. 1C). When the stimulus intensity was increased from threshold, there was a gradual decrease ofthe latencies, by several milliseconds, to aplateau level, inthis case 13 ins. Thereafter, further increase ofthe stimulation strength resulted in no, or only a very slight, decrease ofthe response latency. Atlowstrengths, theresponselatencytoasingleshockwaslongerthanthat to double or triple shock stimulation. In this study, the response latencies used for analysis (cf. Fig. 3) were measured from the first shock at stimulation strengths where a moderate increase in strength did not reduce the latency further, i.e. when the synaptic delays in the cortico- olivo-cerebellar pathways had become minimal. Usually, thisplateau on the latency curvewasreachedbetween05and1 0mAwithintracorticalstimulationandbetween 1.0 and 2-5 mA with surface stimulation, if the stimulus electrode was optimally located. (2) Identification ofcortico-olivo-cerebellar projections The mapping of the climbing fibre input to the cerebellar anterior lobe from the cerebral cortex was achieved using two different strategies: (a), ten to twenty stationarystimuluselectrodeswereplacedatfixedsitesonthecerebralsurface,which were identified by the surface configuration (sulci andgyri) and the peripheral input (e.g. maximal responses in the first somatosensory area). A recording electrode was moved in small steps (0-3-1 mm) along the surface ofa cerebellar folium. Peripheral nerves were stimulated in order to identify the cerebellar sagittal zones (Oscarsson, 1973; Oscarsson & Sj6lund, 1977; Ekerot & Larson, 1979a), and then the responses elicited from different cortical stimulation sites were studied. The amplitudes ofthe responsesfromperipheralnervesandcorticalstimulationsiteswereplottedalongthe folium (seeFig. 2A), andthecortical inputtoeachzonewasspecified. (b), Stationary recording electrodes were placed on the cerebellar surface. The recording sites were chosen sothateachelectrode recorded maximalresponsesevokedfromtheperiphery in each ofthe studied zones. Either a silver ball electrode for surface stimulation or a needle electrode for intracortical stimulation was moved in small steps along the cerebral cortex. The responses evoked from peripheral nerves were recorded at each cortical stimulation site prior to stimulation ofthat site. Based on the results from thisprocedure, a mapofthe cerebral cortical area(s) projecting toacertainzonewas constructed (see Fig. 5). Thefirst strategy permitted an analysisofthezonal organization intheprojection from any stimulation site. However, it was notpossible to determine the borders of 262 0. ANDERSSON AND J. NYQUIST the cerebral cortical areas projecting to the cerebellum, nor to locate the minimum threshold sites. When a response was evoked from a certain site only with a rather high stimulus intensity, it was impossible to know whether the response was due to stimulusspreadtoanadjacentcorticalareaortoactivationofacortico-olivo-cerebellar pathway originating at the site, but requiring much spatial summation. With the second strategy, itwas possible to delineate the cerebral projecting areas as well as to identify the sites from where responses with minimal thresholds and latencies and maximal amplitudes were evoked. On the other hand, responses could be recorded from only a few sites in each cerebellar zone. In most experiments, the recording session was begun with the first strategy and, when it had been determined in which zones the most favourable recordings could be made in that particular experiment, the second strategy was employed. Fig. 2 shows an experiment in which the caudal folium oflobule Vc was mapped with a surface recording electrode. Fig. 2B is a map of the left cerebellar anterior lobe.Thepointsandstarsalongthefoliumindicatetherecordingsitesandthe borders between the sagittal zones are indicated with dotted lines. Fig. 2C shows the cortical stimulation sites and the responses evoked at these sites on stimulation ofthe ulnar (upper traces) and sciatic (lower traces) nerves contralateral to the cortical sites. In Fig. 2A, the amplitudes of the climbing fibre responses recorded from lobule Vc are plotted and the latencies indicated. The cerebral cortex was stimulated at a strength of 1-5 mA with surface electrodes. To the right are records from the sites markedwith stars. Responses in successively more lateral zones, identified from their peripheral inputs, will be described. The a zone in the medial vermis receives a long-latency hind-limb input which is often bilateral (Oscarsson & Sj6lund, 1977). In the experiment illustrated in Fig. 2, no, or only small, responses were evoked in the lateral part of the zone on cerebral corticalstimulation. Moremedially,responseswereevokedfromnearlyallstimulation sites. In fact, when the stimulation strength was 2 mA, responses were evoked from all ten stimulation sites. The x zone, lateral to a, receives information from the ipsilateral forelimb and the Fig. 2. Distribution of climbing fibre responses elicited in one folium on stimulation of peripheral nerves and cerebral cortex. A, distribution of climbing fibre responses along one folium in the cerebellum. Response latencies indicated. To the right, records of responses in four of the zones, recorded at the sites indicated with stars to the left and in B. B, left cerebellar anteriorlobe, lobulesIV-Vc (nomenclature ofLarsell, 1953). Dots and stars indicate recording sites. C, cerebral cortex and stimulation sites (viewed from dorsal direction). At each site, records ofresponses evoked on contralateral ulnar nerve #uV. (uppertraces) andsciaticnerve(lowertraces)stimulationareshown. Verticalbars200 Open circle indicates stimulation site in the contralateral lateral gyrus. Names of gyri indicated totheleftandnamesofsulci totheright, accordingtoHassler& Muhs-Clement (1964). D, records ofresponses evoked in the dl zone in lobule IV (indicated with a ring in B). Abbreviations: i, ipsilateral; c, contralateral; Trig., trigeminal; Uln., ulnar; Sci., sciaticnerves;SIflandSIIfl,forelimbareasoffirstandsecondsomatosensoryareas;p.c.d. post-cruciate dimple; P.s.g., posterior sigmoid gyrus; 1 lateral; m medial; A.s.g., anterior sigmoidgyrus; Cor, coronalgyrus; Esyl,ectosylviangyrus;Ssyl, suprasylviangyrus; L.g. lateral gyrus; lat, lateral sulcus; ssyl, syprasylvian sulcus; p.g., paravermal groove; ml, midline;i.c.,inferiorcolliculus. CerebellarzonesaccordingtoOscarsson(1980).Chloralose anaesthesia. CEREBRO-CEREBELLAR CLIMBING FIBRE PROJECTION 263 A ill c3 c2i clPI9 b+x .a ml c3 c2 b+x a iTrig. a L i s 43p .9 cTrig. iUln. 1. .i 37. .--- _ - 20 41 cUln. 32 33 - 20 isci t.30 2C 33 cSci 128~~- cSlfl 3027 25«,22 v , r , , , _ iSlfl cSllfl ts-.27e1 iSlIfl c.p.c.d. I i.p.c.d. cP.s.g.l I iP.s.g.l 18 . ! cP.s.g.m /~~~~; ~ I 200pV iP.s.g.m r, I * * ,I200pV 1mm 10ms s IV .10M *Sg~ ~~~~~cL.g. short-latencyresponse (16-17 ms)onipsilateralulnarnervestimulationinthemedial part ofthe b+x division in Fig. 2A can be ascribed to the activation of Purkinje cellsinthexzone (Ekerot & Larson, 1979a; Andersson & Eriksson, 1981). When the x zone was as narrow as in Fig. 2A, it was impossible to separate it spatially from the medial part of the b zone when the responses were recorded only as surface potentials (Andersson & Eriksson, 1981). 2. ANDERSSON AND J. NYQUIST 264 Thebzoneinthelateralvermisreceivesabilateralinputfromtheperiphery. Caudal body segments project to the lateral part, and more rostral segments project to successivelymoremedialparts (micro-zones) ofthezone (Oscarsson & Sj6lund, 1977; Andersson & Oscarsson, 1978; Andersson & Eriksson, 1981). In the experiment illustrated in Fig. 2, responses were evoked in the lateral part ofthe b zone from the medial posterior sigmoid gyrus (p.s.g.) electrodes, bilaterally. Overlapping these responses but shifted slightly medially, responseswererecordedonstimulationofthe forelimb parts of both posterior sygmoid gyri (contralateral and ipsilateral p.s.g.l. and post-cruciate dimple (p.c.d.)). Due to the difficulty in separating the responses in the medial b zone and the x zone, it was not possible to ascribe the responses from the SI and SII specifically to one of these zones. However, when the stimulation strength was increased to 2-5 mA, more laterally located responses of considerably larger amplitudes, with latencies of 22-25 ms, were evoked bilaterally from the SI and SII electrodes. This suggests that the responses were evoked in the b zone. The cl zoneinthe medialparsintermediareceives, inlobuleVc, ipsilateralforelimb input (Ekerot & Larson, 1979a). In the illustrated experiment (Fig. 2A), responses were evoked on cortical stimulation only from the forelimb part ofthe contralateral p.s.g. The c2 zone receives a long-latency input from all four limbs without a distinct somatotopy (Larson, Miller & Oscarsson, 1969b; Ekerot & Larson, 1979a). In addition, in the present investigation, responses were recorded on ipsilateral and, usuallyalso,contralateraltrigeminalnervestimulation(latencies15-22and18-28 ms, respectively). In the illustrated experiment (Fig. 2A), the forelimb site in the contralateralsecondsomatosensoryarea(cSIIfl)wastheonlycorticalsitefromwhich a climbing fibre response could be evoked in the c2 zone. The c3 zone receives, in lobule V, a short-latency climbing fibre input mainly from the ipsilateral forelimb (Ekerot & Larson, 1979a, b). In the present investigation, a short-latency response (9-12 ms) in a lateral strip in the c3 zone (in lobule Vc) was also frequently observed on stimulation of the ipsilateral trigeminal nerve. In the experiment of Fig. 2, short-latency responses (14 ms) were evoked mainly from the forelimb parts of the contralateral first somatosensory area (cSIfl) and posterior sigmoid gyrus (cp.s.g.l and cp.c.d.). Thedl zone receivesipsilateral forelimb input in lobule V andipsilateral hind-limb inputinlobuleIVand,tosomeextent,inlobuleV(Larson,Miller& Oscarsson, 1969a; Ekerot & Larson, 1979a). In the present study, responsestocontralateral limb nerve stimulation were also occasionally evoked (latencies 21-24 ms). In the experiment illustrated in Fig. 2, the dl zone was very narrow in lobule Vc and no, or only very small, responses were recorded there. Recordings were also made from a folium in lobule IV in this experiment. Here the dl zone was wider, and at the site indicated with an open circle in Fig. 2B, a large response from the ipsilateral sciatic nerve was evoked. In 2D, records are shown of responses evoked at this site. Of the cortical stimulation sites in C, only the medial one on the contralateral p.s.g. (cp.s.g.m) was effectiveinevokingaresponse (latency 15 ms). Inaddition, stimulationoftherostral part ofthe lateral gyrus (cl.g.), indicated with a circle in 2C, evoked a response at a latency of 19 ms. Stimulation ofthis site evoked no responses in lobule Vc. The results presented in Fig. 2 demonstrate that climbing fibre responses can be CER'."EBRO-CEREBELLAR CLIMBING FIBRE PROJECTION 22665 evoked from several cerebral-cortical areas. The histograms in Fig. 3 show the response latencies to stimulation of the different areas in the contralateral and ipsilateral cortex, displayed above and below the abscissa, respectively. The values wereobtained by selecting, ineach experiment, the shortest response latency ineach zone. The characteristics of the projections from each cortical area will be briefly described. P.S.g. A.s.g. Sifi C.g. SIl Par 10 15 20 25>2510 15LL20L2I5 110.145-240-425 1L0 L1L5 I20~O25 10 L15 20425I10 K1K5'.2L0I{25I>25ms 51-lJ L JI cI a y2~~~~~~~1 r bC4= "6 LL~~AILLLI LLL2~LIL1 IJ LL l~l 5 5. LaceIfclmigfberepne Fig 3. osimlto fceerlcre.-otaaea scond soatsesoy re; ar pritacrtx shorte 3Latenciesthalmtbhngberesponseesvoke frmuatom othe corticalaorteas.Cota Therespnwatbntipasorahia orandizsateioninlothperojctios.fnromtexpesg.Simulationh oforthestlateral part sevokted frespose ionte. xumbersidiacarte ofthewbmand caudariparts othpasitreiezacohez.onIens exceptc2, these resposnsheoswertrehldlrgramltdtessame distributionpattern asthatfound onstimulation oflargeforelimb nerves (cf. Ekerot 266 G. ANDERSSON AND J. NYQUIST & Larson, 1979a). Correspondingly, the medial part ofthe p.s.g. projected to the a, the lateral part ofthe b and rostral parts ofthe pars intermedia zones, which are the main hind-limb receiving areas in the cerebellar anterior lobe (see below, section 3). Stimulation of the ipsilateral p.s.g. was almost as effective as stimulation of the contralateral p.s.g. in evoking responses in the a and b zones. Responses were less frequently seen in the c2 and dl zones. In the zones receiving only ipsilateral peripheral input, small responses were observed only occasionally on stimulation of the ipsilateral p.s.g. From a large area in the anterior sigmoid gyrus (a.s.g.), short-latency responses (12-13 ms) were evoked exclusively in the dl zone (Fig. 3). These responses had as low, or lower, thresholds and as large amplitudes as those evoked from the p.s.g. Responseswere alsoevoked in the dl zone on ipsilateral a.s.g. stimulation. These had approximately 5 ms longer latencies and 2-3 times higher thresholds than the contralaterally evoked responses. In the other zones, responses were evoked only from the lateral part ofthe a.s.g. These responses had considerably longer latencies than those evoked from the p.s.g. (Fig. 3). At least in the b zone (the only zone tested), these long-latency responses were evoked also from the ipsilateral a.s.g. The first somatosensory area (SI) was stimulated with a fixed stimulus electrode where maximal surface positive responseswereevoked on forelimb nerve stimulation (Fig. 2). The climbing fibre responses evoked from SI occurred exclusively in the forelimb parts ofthe zones. The responses had higher thresholds and longer latencies than those evoked from the p.s.g. (Fig. 3). In three experiments, intracortical stimulation was employed in the parts ofSI with maximal responses on stimulation ofthe ulnar and superficial radial nerves. Climbing fibre responses were only evoked with stimulus strengths which were considerably higfier than those required in the p.s.g. Whenfixedstimuluselectrodeswereused, itwasnotalwayspossibletodetermineiftheresponses from the SI electrode were due to activation ofthe SI only, or to stimulus spread into the p.s.g. area, particularly in those cases where the distance between theSI and p.s.g. area was short and thethresholdforevoking aresponse from theSI was considerably higherthan thatfrom the p.s.g. area. Therefore, only the latencies ofthose responses that could be clearly ascribed to activation oftheSI, i.e. if they had only slightly higher thresholds than the responses from the p.s.g., are included in Fig. 3 and only positive findings are considered. On stimulation of the coronal gyrus at the site of maximal responses evoked on trigeminal nerve stimulation, climbing fibre responses were evoked in the b, c2, c3 anddl zones. Thelatencieswereusually 15-20 ms (Fig. 3). Theresponseswereevoked in those parts of the zones receiving a trigeminal input. The occasional absence of responses in the b and c3 zones, indicated in the latency histograms in Fig. 3, reflects theinclusionofexperimentsinwhichrecordingsweremadeonlyfrom'non-trigeminal' parts of the zones. Thedl zonehasnotpreviouslybeenreportedtoreceive atrigeminal input. Inthepresent study, climbingfibreresponseswereevokedinthedl zoneonstimulationoftheipsilateraltrigeminalnerve in five (oftwenty-five tested) cats (latencies 17-24ms). The second somatosensory area (SII), in the anterior ectosylvian gyrus (see Jones & Powell, 1973) was stimulated in thirteen cats. Climbing fibre responses were regularly evoked in the c2 zone and less frequently in the other zones (Figs. 2 and