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1. Corticocortical synaptic influences on pyramidal neurones in the precentral motor cortex of monkeys were examined using intracellular recordings. Corticocortical afferents from the postarcuate premotor area and the somatic sensory cortical areas were activated by bifocal stimulation of the cortical surface. Neurones that were found to respond orthodromically to such stimuli were labelled by intracellular ionophoresis of horseradish peroxidase. 2. Almost all neurones that were penetrated satisfactorily and labelled successfully were found to be pyramidal neurones located in lamina III or lamina V. Some labelled neurones in lamina V were also characterized as pyramidal tract neurones (PTNs) by antidromic activation from the cerebral peduncles or medullary pyramids. 3. Pyramidal neurones located in lamina III and lamina V (including PTNs) were excited at short latency by stimulation of the premotor cortex (1.1-4.0 ms) and somatosensory cortex (1.1-6.5 ms). There were no statistical differences in the distributions of latencies of corticocortical EPSPs between those evoked in lamina III neurones and those recorded in lamina V neurones, or between corticocortical EPSPs evoked from the premotor cortex in comparison with those from the somatosensory cortex. Excitatory responses to stimulation of the premotor area were usually more difficult to evoke and smaller in amplitude than those produced by stimulation of the somatosensory areas. 4. Corticocortical EPSPs were often followed by IPSPs. The amplitudes of the EPSPs and IPSPs could be increased by increasing the stimulus intensity. In a few neurones IPSPs that were not preceded by EPSPs were recorded.

1. Recordings were made of the natural discharges of 388 pallidal neurones in awake, free-to-move monkeys in order to describe the discharge properties of such neurones in relation to normal movement performance. 2. Of the 388 neurones, 156 discharged only in association with one direction of movement of the forelimb about a specific joint. If that movement was not taking place the neurone would not discharge. 3. All joints and directions of movement for the upper limb were represented by clusters of cells within the pallidal population. 4. Twenty-nine per cent of neurones co-varied with movement of both contralateral and ipsilateral limb for the same direction of movement about a given joint; distal movements were represented with similar frequency to proximal movements in this group. 5. Afferent information provided by natural stimulation of peripheral receptors did not directly influence either the discharging or non-discharging pallidal neurones. 6. Movement related neurones were regionally organized and were found in the posterior part of the pallidum.

1. Four male monkeys (M. fascicularis) were trained in a movement performance task which involved pulling a horizontal lever into a target zone and then collecting, from one of a variety of positions, a small food reward. The same animals were also trained to sit quietly and accept passive manipulation and natural stimulation of the arm and hand while remaining relaxed. 2. After complete bilateral section of the cuneate fasciculi or division of a major part of these dorsal column afferents at C1-C2 or at C5 level, the animals were still able to perform movement tasks normally. Disturbance of discrimination ability was revealed after vision was occluded it the animal was required to detect differences in texture with only a small cutaneous area in contact with the object (e.g. using only the tip of the index finger). Contactual-placing reactions could be performed in the absence of vision and the movements the animal made in these reactions were well controlled and appropriately directed. Minimal disturbance of contact placing was noticed if the surface touched was on the hand or fingers or if the reaction involved crossed placing. 3. An examination of the natural discharges of 342 percentral neurones revealed that the patterns of activity exhibited in relation to complex movements were indistinguishable from patterns recorded in normal monkeys carrying out similar tasks. 4. Discharges of ninety-one of 321 precentral neurones could be produced by appropriate natural stimuli delivered within the cell's afferent input zone at the periphery. The zone from which a given cell could be influenced was usually limited and its location could be on any part of the contralateral forelimb. However, all but nine of these responses were found in animals in which a small proportion of the cuneate fibres remained intact. In an animal with histologically proven complete section of the cuneate faciculi very few (nine of 171) precentral neurones were influenced by natural activation of peripheral receptors in the forelimb. The zones from which these few afferent inputs were found could all have been proximal to the level of the cuneate lesion. 5. The very small number of responding pre-central neurones found in an animal with complete section of the cuneate fasciculi made it likely that the dorsal columns provide the major pathway for effects from circumscribed peripheral receptors in the forelimb to influence precentral neurones. However, even in an animal with complete interruption of cuneate fibres, a proportion of post-central neurones could still be influenced by natural activation of peripheral receptors within restricted regions of the forelimb. Hence the 'sensory' cortex was still in receipt of afferent projections which could be revealted readily by the tests used. 6...

1. Conscious monkeys were trained with food rewards to perform movement tasks with the left forelimb and to accept manipulation of the joints and muscles and natural non-noxious stimulation of the skin of all four limbs. 2. Recordings were made from 217 cells situated in the left interpositus and dentate nuclei of the cerebellum. The identity of seventy-seven cells as cerebellar projection neurones was definitively established by activating them antidromically from the brachium conjunctivum near the contralateral red nucleus. 3. Modulation in the natural activity of 129 of these crebellar nuclear cells (sixty in interpositus; sixty-nine in dentate) occurred in a reproducible manner in temporal association with a phase of the self-paced movement tasks performed by the animal using the ipsilateral arm and hand. The discharges during motor performance of forty-two dentate and forty-five interpositus cells were shown to be associated with movement about a particular joint or region of the forelimb whenever that movement occurred. 4. Cells whose discharges were related to proximal joint movements (shoulder, elbow) and cells related to distal joint movements (wrist, fingers) were encountered in both the dentate and interposed nuclei. 5. The cells were tonically active at rest. Most commonly, accelerations in the discharge were related to movement of a joint or the limb in one direction and a reduction or cessation of activity accompanied movement in the opposite direction. 6. For some cells, variation of the amount of discharge demonstrated during movement performance could be related to the range of the movement or its duration, more activity being characteristic of more prolonged movement performance through larger angles of joint displacement. 7. The dentate and interpositus cells whose discharges were most strongly and consistently related to movements of the forelimb were concentrated in the mid region and caudal half of either nucleus. 8. None of seventy-three dentate neurones examined showed appreciable responses to stimulation of the skin or manipulation of joints and muscles of the fore- or hind limbs and only two cells responded to unexpected perturbation of movement performance. 9. No influence resulting from peripheral afferent input from the ipsilateral forelimb was detected in any interpositus cell whose firing was unchanged during ipsilateral arm movements. 10. Of the sixty interpositus cells whose discharge rates changed during motor performance, twenty-eight were demonstrated to be in receipt of input from receptors in the ipsilateral hand or arm, which could be activated by brisk tapping of the skin and sometimes by gentle squeezing of the forearm. 11. In the passive relaxed animal, manipulation of joints was ineffective in modifying the discharges of most interpositus neurones and, in all cases, prolonged pressure upon the skin elicited only transient responses...

1. Pyramidal neurones in the precentral motor area of the monkey were studied using intracellular techniques. Pyramidal tract neurones (PTNs) were identified by antidromic activation from the cerebral peduncles or medullary pyramids. Orthodromic responses were recorded in PTNs and in other pyramidal neurones when antidromic volleys were set up by stimulation of the peduncles or pyramids. The neurones were then labelled by intracellular ionophoresis of horseradish peroxidase and their morphology examined. All neurones studied were identified as pyramidal cells according to their morphology. 2. Six pyramidal neurones located in lamina V were well stained; they included two fast PTNs and two slow PTNs. The morphology of all pyramidal neurones in this lamina (fast PTNs, slow PTNs and those pyramidal cells that were not antidromically characterized) was essentially similar. A single apical dendrite branched as it ascended and its terminals arborized subpially. Numerous lateral and oblique dendrites branched from the apical dendrites in lamina V and near its border with lamina III: short basal dendrites arborized in the vicinity of the soma in lamina V. Long basal dendrites had a wider field of arborization in lamina V and sometimes extended into lamina VI. 3. Three to five collaterals arose from the axon of lamina V cells in the cortex and arborized in laminae V and VI. Short collateral branches arborized in the vicinity of the soma in the region of the basal and lateral dendrites. Long collateral branches could be traced over long distances (often more than 1 mm). One pyramidal neurone in this lamina (a fast PTN) lacked short collateral branches from the axon. 4. Four pyramidal neurones in lamina III were stained well. The dendritic morphology of all these neurones was similar. Apical dendrites branched as they ascended and terminated subpially. Lateral and basal dendrites formed a column of dendritic branches around the soma. No long basal dendrites were seen. 5. The number and arborization of intracortical collaterals from the axon of lamina III cells varied widely; from three to twelve collaterals arose from the axon. The biggest arbor of collateral branches involved all the cortical laminae and was about 3 mm wide mediolaterally, while the smallest arbor was restricted mainly to lamina III in the vicinity of the soma. One neurone in this lamina also lacked short collateral branches from the axon. 6. Antidromic volleys from the pyramidal tract evoked excitatory responses in fast PTNs, predominantly inhibitory responses in slow PTNs and either excitatory or inhibitory responses in other pyramidal neurones in lamina V.(ABSTRACT TRUNCATED AT 400 WORDS)

1. Monkeys were trained to perform a stereotyped movement task in return for food rewards. On completion of training a headpiece which allowed microelectrode access to the thalamus for single cell recordings during performances of the task was attached at a surgical operation. The location of each cell studied was determined by histological examination of the fixed brain and precise identification of electrode tracks. 2. Ninety-three of ninety-seven cells discharging in association with arm movements but not responding to natural activation of peripheral receptors in the forelimb were located predominantly in the rostral part of nucleus VPLo and the caudo-ventral part of VLo. These cells appeared to be associated with active movement in one direction of a specific joint. 3. 52% of these 'motor' cells discharged in association with movements of either forelimb. The other 48% discharged in association with movement of the contralateral arm and hand only. 4. Sixty cells responded to stimulation of deep receptors or to passive limb manipulation in the relaxed and cooperative animal. These cells were predominantly located in the ventro-caudal part of VPLo and all the responses were obtained from contralateral receptors. Their discharges during performances of the motor task were indistinguishable from those of 'motor' cells. 5. Ninety-two cells were driven by limb manipulation and by natural activation of superficial cutaneous receptors and these were found predominantly in VPLo and VPLc. All responses were from contralateral receptors. These cells discharged during performance of the motor task; for some of them, their afferent input zones were not being stimulated by contact with the manipulandum when their 'motor' discharges commenced. 6. Although responses in each of the above groups of cells were sought by imposing a sudden perturbation of the limb during the performance of the active movement task, no responses were seen.

1. Conscious monkeys were trained with food rewards to perform movement tasks with the left hand and to accept manipulation of the joints and muscles and natural non-noxious stimulation of the skin of both forelimbs.2. Recordings were made from 230 Purkinje cells situated in the paravermal region of lobules V and VI or immediately adjacent folia of the left cerebellum in a region from 2 to 7 mm from the mid line. These neurones were all in a zone which was demonstrated to receive inputs from the ipsilateral hand and which is known to receive projections, via the pontine nuclei from the ;arm area' of motor cortex in the right hemisphere.3. Modulation of the natural activity of 182 of these 230 Purkinje cells (79%) occurred in a reproducible manner in temporal association, each with a particular phase of the self-paced movement tasks performed by the animal using the ipsilateral arm and hand. The patterns of modulation of Purkinje cell firing in this limited zone of cerebellar cortex could be classified into one of four groups, and each cell's discharge was associated with a particular aspect of movement such as general arm flexion, shoulder retraction, elbow extension or elbow flexion whenever it occurred.4. The cells were spontaneously active at rest. Most commonly, marked accelerations of the discharge were related to one direction of the particular aspect of movement and a reduction of activity or even total silence accompanied movement in the opposite direction.5. Variation of the amount of discharge demonstrated during a movement performance with which this discharge was characteristically associated could be related to the range of the movement or its duration, more activity being characteristic of more prolonged movement performance through larger angles of joint displacement.6. Both simple spikes and complex spikes of some cells showed characteristic modulation of their activity during the monkey's self-initiated movements. Cells whose simple spikes did not change in frequency during the movement task, also showed no modification of complex spike discharge.7. Of the 182 neurones whose discharges changed during active movement performance, 105 (roughly 60%) were demonstrated to be in receipt of an input from peripheral receptors in the hand which could be activated by brisk tapping of the skin or brushing of hairs. In contrast, none of the Purkinje cells whose discharges were unchanged during arm movements could be demonstrated to receive such an input.8. Movement of joints through their full range and prodding of muscles were completely ineffective stimuli for causing changes in Purkinje cell firing in this zone of the cerebellar cortex while the animal was passive and relaxed. Imposed perturbations of movement performance injected unexpectedly during the execution of a movement task were also ineffective in modifying the discharge of these Purkinje cells in relation to the task.

A cooling plate was implanted over the forelimb representation in area 2 of the post-central region of cerebral cortex in two monkeys. Recordings were made of the discharges of thirty-seven movement-related neurones (thirty-four precentral and three post-central) in the forelimb motor representation of the cerebral cortex during active and passively imposed limb movements before, during and after cooling area 2 and local surrounding regions. Perfusion of the cooling plate with ice-cooled water for 3-5 min caused marked clumsiness of the conscious animal's forelimb movement and anaesthesia of the contralateral hand. Cooling of area 2 did not reduce the responses of area 4 cells to passive joint movements, nor did it alter the over-all pattern of activity of these cells during self-initiated lever pulling while that could still be performed. Cooling of area 2 did cause a significant increase in background cellular discharge in area 4 while the animal was at rest. Afferent impulses which are generated by passive joint movement and which have been shown to influence cells in area 4 of the conscious monkey at short latencies are probably not transmitted through cortico-cortical connexions from area 2.

1. Reflex discharge of some hypoglossal neurones innervating the intrinsic musculature of the tongue occurred following single stimuli delivered to low threshold afferents in the ipsilateral or contralateral lingual nerves of decerebrate cats.2. The discharge produced by contralateral lingual nerve stimuli was smaller and had a longer latency than that produced by ipsilateral lingual nerve stimuli. The timing of the ipsilateral response suggested that two synapses were included in the reflex arc.3. Intracellular recordings in hypoglossal motoneurones revealed complex depolarizing and later hyperpolarizing synaptic respones following single nerve stimuli. The direction of the early synaptic action in any cell was the same whether low threshold afferents in the ipsilateral or the contralateral lingual nerve were stimulated. The timing of these synaptic responses and the occurrence of ripples on the rising phase of many of the depolarizing potentials was consistent with the presence of an internuncial neurone in the pathway from the afferent fibres to the motoneurone.4. Recordings were made of the discharges of neurones in or near the sensory nucleus of the spinal trigeminal tract caused by lingual nerve stimuli. The latencies of these discharges and the characters of the bursts of responses which followed single lingual nerve stimuli indicated that these neurones could be the internuncials on the reflex pathway.5. The relationship of these findings to the observations of Miller & Sherrington (1915) on the ;licking' reflex is discussed.

1. Corticomotoneuronal EPSPs have been generated in lumbar motoneurones of the monkey by single and paired corticospinal volleys. The facilitation of the second of a pair of EPSPs with respect to the size of the first has been measured.2. The relationship between the degree of facilitation of the second response and the interval between the two volleys has been studied. Average facilitation of minimal EPSPs was found to be maximal about 2 msec after the arrival of the corticospinal volley and to decay roughly exponentially thereafter with a time constant of about 10 msec.3. The degree of facilitation varied from one minimal corticomotoneuronal EPSP to another but this facilitation was not statistically correlated with the time course of the individual EPSPs.4. Significant facilitation (0.4) was still present 10 msec after a corticospinal volley so that this phenomenon could play a part in the initiation of motoneuronal discharge by corticospinal activity at natural frequencies of the order of 100 impulses/sec.

1. Intracellular recordings were made of minimal corticomotoneuronal e.p.s.p.s in lumbar motoneurones of anaesthetized monkeys. For intervals of 2 msec and greater between paired cortical shocks, the average time course of facilitation of the second e.p.s.p. with respect to the first could be fitted closely by a negative exponential with a time constant of 10 msec.2. In the same motoneurones, ;triplets' of corticomotoneuronal e.p.s.p.s were generated by delivering three identical stimuli to the motor cortex. Considering the triplet as a conditioning e.p.s.p. followed by a test pair, the facilitation of the third e.p.s.p. with respect to the second was measured for various combinations of test and conditioning intervals. In each case the amplitude of the third e.p.s.p. was also compared with that of the first (conditioning) e.p.s.p.3. The effect of a brief conditioning interval was to reduce considerably the facilitation of the third e.p.s.p. with respect to the second at all test intervals from 2 to 50 msec. Combinations of brief conditioning intervals (e.g. 2 or 5 msec) and long test intervals (e.g. 20 or 50 msec) caused the third e.p.s.p. to be smaller than the second. As the conditioning interval lengthened, facilitation in the test pair increased towards the unconditioned values at all test intervals.4. Facilitation of the third e.p.s.p. with respect to the first could be described approximately as the linear addition of two facilitation components, one due to the conditioning input and one due to the first stimulus of the test pair. Each component followed the same negative exponential time course as found for an isolated pair of e.p.s.p.s and each of the first two inputs contributed to the facilitation of the third e.p.s.p. as if the other of these two inputs had not occurred.

1. Two groups of mechanoreceptors in the tongue have been identified by recording afferent discharges in single nerve fibres dissected from the lingual nerves of anaesthetized cats. The group of superficially situated, rapidly adapting mechanoreceptors had faster conduction velocities than the presumed deeply situated, slowly adapting endings.2. Stretch of the tongue musculature did not prove to be as efficient in activating the endings as local deformation, although it was possible to excite some of the presumed deeply situated endings in this way.3. No mechanoreceptor fibres could be identified in filaments dissected from the hypoglossal nerves in the same experiments.4. Twitches of the tongue musculature produced by stimulation of the hypoglossal nerve were able to cause discharge of superficial and presumed deeply situated mechanoreceptors during the tension change.5. The timing of the discharges in response to twitch was such that it could account for the delay in synaptic potentials produced in hypoglossal motoneurones when supramaximal stimuli were applied to the hypoglossal nerve.6. The possible significance of mechanoreceptor discharges in reflex activation of tongue motor units is discussed.

1. Group Ia EPSPs were recorded from lumbosacral motoneurones in anaesthetized cats after almost complete section of the relevant dorsal roots. The EPSPs were usually of small amplitude (median value of 230 muV) and an averaging device was used to improve the definition of their time course.2. From a total of over 500 averaged EPSPs a smaller number (342) were subjected to analysis. The other EPSPs were rejected either because they showed signs of multiple origin in the rising phase of their time course (see Methods) or because the resting membrane potential of the cell was less than 50 mV. All the selected EPSPs had their rise time (from the 10 to the 90% level) and half-width measured, and a semilogarithmic plot of their decay time course was made.3. 252 of the EPSPs showed an exponential decline in their later time course and the slope of this line was used to give an estimate of the membrane time constant. The range of the time constant for different motoneurones was 2.3-12.9 msec, with a mean value of 5.8 msec.4. In ten cells an EPSP was recorded which was judged to be generated exclusively by synaptic knobs located on the soma. On this assumption measurements of the normalized rise time, half-width and break point time were used to estimate alpha, rho(infinity) and L by the method suggested in Jack & Redman (1971b). The estimated value of alpha ranged from 18 to 65. A positive correlation was found between alpha and tau(m), indicating that for these EPSPs the duration of current injection was independent of the membrane time constant. The peak time of the wave form of current injection was between 0.1 and 0.25 msec. The estimates of rho(infinity) were not thought to be very accurate. A lower limit of 4 was assumed and the highest measured value was 12, but in three cells the time course of the EPSP could not be fitted even with a very high value of rho(infinity). Some possible explanations for this discrepancy are mentioned in the Discussion. The electrotonic length of the dendrites (L) was usually greater than 1.0 lambda and ranged between 0.75 and 1.5 lambda. Evidence for an open-circuit termination of the dendrites was found in some cells.5. The normalized values of the rise time and half-width were used to make an electrotonic distance allocation to the 246 EPSPs which were judged to be non-somatic. The method of allocation was not precise because individual values of rho(infinity) and L were not available for these motoneurones. Instead, a maximum possible range was assumed: for rho(infinity), 4-25; for L, 0.75-1.5. The range of alpha was also assumed, from 12 to 100. With these values the motoneurone model (Jack & Redman, 1971b) was used to set limits within which the normalized rise time and half-width of all EPSPs, generated by current at a single point, should lie. Twenty of the 246 EPSPs lay outside these boundary lines and hence they did not receive a distance allocation. The remaining 226 were assigned values between 0.2 and 1.6 lambda (in 0.2 lambda steps); the majority of the allocations (183) were to the proximal electrotonic part of the dendrites (0.2, 0.4 or 0.6 lambda). The relationship of these distance allocations to the histological results of Conradi (1969) is discussed.6. It is concluded that there is no good evidence against the view that the main time course of minimal Ia EPSPs can be explained by their generation by a brief pulse of synaptic current and subsequent passive spread.

1. Contractions of the small muscles of the foot producing flexion and adduction of the hallux were elicited by brief trains of electrical stimulation of a motor point on the precentral gyrus of anaesthetized monkeys and these contractions were recorded myographically.2. The cortical stimulus intensities necessary to produce minimal muscle contractions were measured for different frequencies of stimulation at the cortical point, and the latency of the minimal muscle contraction was measured in each case.3. Section of all the relevant lumbar and sacral dorsal roots had no effect on the threshold stimulus currents necessary to produce minimal contractions or on the latencies of these responses. Hence, in the anaesthetized monkey, the power of the cortico-fusimotor activity stirred up by electrical stimulation of the cortex is inadequate to influence significantly the motor responses of the most accessible muscles.

1. Intracellular records were made of the synaptic potentials produced in hypoglossal motoneurones when electrical stimuli were applied to the cat's motor cortex.2. The depolarizing synaptic responses reached their maxima in steps similar to those produced in the same motoneurones by suprathreshold stimulation of the lingual nerve.3. Interneurones in or near the spinal trigeminal nucleus were caused to discharge by cortical shocks adequate to produce synaptic potentials in hypoglossal motoneurones. The latencies and patterns of repetitive responses in these interneurones were consistent with their possible role as internuncial cells in the motor pathway from cerebral cortex to hypoglossal motoneurones.4. Individual interneurones in or near the spinal trigeminal nucleus could be caused to discharge both by stimulation of the lingual nerve and by stimulation of the motor cortex. This convergence suggests a common function for the internuncial cell in reflexly and cortically induced excitation of hypoglossal motoneurones and would allow the internuncial cell to be a site for integration of spatially separated excitatory influences.