Relationship of the discharges of cortical neurones to movement in free-to-move monkeys

Motor cortical and other cortical interneuronal networks that generate very high frequency waves

A remarkable feature of motor cortical organization in higher mammals is that a brief electrical stimulus elicits in the pyramidal tract and corticospinal tract an unrelayed direct (D) wave followed by multiple indirect (I) waves at frequencies as high as 500-700 Hz. This review presents some conclusions regarding very high frequency synchronous activity in mammalian cortex: (1) Synchrony in repetitive I discharges is extraordinary in humans and monkeys, less in cats and still less in rats, being there represented by a delayed broad wave; such phylogenetic trends have important implications for the suitability of lower mammalian species for studies of high frequency cortical networks in the human brain; (2) The evidence from microstimulation at different cortical depths and pial cooling favors a vertically oriented chain of interneurons that centripetally excite corticospinal neurons as the basis for inter-I wave periodicity and synchrony; (3) Significantly, the I wave periodicity is conserved despite wide changes in stimulus parameters; (4) Synchronous high frequency activity similar to that of I waves can be recorded from other neocortical areas such as visual and somatosensory cortex; however, evidence is still lacking that the output neurons of these cortical regions have synchronized discharges comparable to I waves; (5) In limbic cortices, the frequency of synchronous neural activity is lower than that in motor cortex or related cortices and periodicity is not conserved with changes in stimulus parameters, indicating a lack of the neocortical interneuronal substrate in limbic cortex; (6) We propose that the very high frequency synchronous activity of motor cortical output reflects a computational function such as a "clock," quantizing times at which inputs would interact preferentially yielding synchronous output discharges. Such circuitry, if a general feature of neocortex, would facilitate rapid communication of significant computations between cortical regions.

Timing of finger opening and ball release in fast and accurate overarm throws

How precisely does the CNS control the timing of finger muscle contractions in skilled movements? For overarm throwing, it has been calculated that a ball release window of less than 1 ms is needed for accuracy in long throws. The objective was to investigate the timing precision of ball release and finge opening for 100 overarm throws made using only the arm. Subjects sat with a fixed trunk and threw balls fast and accurately at a 6-cm-square target when it was 1.5, 3.0 and 4.5 m away. Three-dimensional angular positions in space of the clavicle, upper arm, forearm, hand and distal phalanx of the middle finger were simultaneously recorded at 1000 Hz using the magnetic-field search-coil technique. Ball release was determined by pressure-sensitive microswitches on the proximal and distal phalanges of the middle finger (proximal and distal triggers). Variability of ball release, defined in terms of the standard deviation (SD) of the means of release times, was different when synchronized to different hand kinematic parameters. It was highest to the start of movement (when the hand started rotating vertically forward and up around a space-fixed horizontal axis) and was lowest when synchronized to the moment near ball release when the hand was vertical. These values did not depend on target distance. When throws were synchronized to vertical hand position, and SDs were averaged across the 10 subjects, the average interval for 95% of the throws (4xSD) was 9.6 ms for ball release and 10.0 ms for onset of finger opening. Thus, two independent measures of timing precision gave similar results. It is concluded that for 100 fast and accurate throws made by male recreational ball players, timing of finger opening and ball release was controlled precisely but not to fractions of a millisecond.

The influence of changes in discharge frequency of corticospinal neurones on hand muscles in the monkey

1. The possibility that the discharge pattern of monkey corticomotoneuronal cells influences the degree to which they facilitate their target hand muscles was tested by compiling spike-triggered averages of EMG recorded from these muscles. 2. Records were made from area 4 corticomotoneuronal cells in three conscious macaque monkeys while they performed a precision grip between index finger and thumb. Simultaneous EMG recordings were made from up to six different intrinsic hand muscles. Twenty cells which produced clear post-spike facilitation of one or more muscles were selected for further analysis. 3. Spikes recorded from these cells were grouped according to the occurrence of a previous spike in the periods 0-10 ms, 10-20 ms, and so on up to 60-70 ms before the trigger spike. The post-spike period in which no additional spikes were allowed to fall was kept at either 12.5 or 25 ms. 4. Spikes selected in this way produced a transient facilitation of their target muscle EMG activity. The peak amplitude of this facilitation was normalized as a percentage of modulation of the background EMG level. The background level was determined from a period in the average to which the cell could not have contributed, because of the post-trigger spike interval. We verified that the percentage of modulation was not influenced by the overall level of EMG activity, since, for a given interval, the modulation was the same whether the relevant spikes were selected during periods of high- or low-level EMG activity. 5. The relative amplitude of the post-spike facilitation (i.e. the percentage of modulation) showed marked variation with interspike interval. A full analysis was completed for seventeen neurones. Spikes with the shortest intervals (less than 10 ms) usually produced the strongest effects, and evidence is presented that this was due to temporal summation and facilitation at the corticomotoneuronal synapse. Mid-range intervals (10-40 ms) were generally far less effective, although they constituted the highest proportion of cell activity. 6. A striking finding was the strong facilitation generated by the longer interspike intervals (40-70 ms). Although the absolute size of this post-spike effect was much smaller than that of the shortest intervals, its percentage of modulation was similar. It is suggested that this enhanced facilitation results from a combination of lower frequency discharge among the active motoneurones, and increased synchrony in the corticomotoneuronal input to them. 7. All of the above results were confirmed by examining cross-correlations between single corticomotoneuronal cells and single motor units in their target muscle.(ABSTRACT TRUNCATED AT 400 WORDS)

The Florey lecture, 1987. Corticomotoneuronal projections: synaptic events related to skilled movement

During infancy, children develop an expanding repertoire of movement skills in parallel with the maturation in their brains of direct nerve-fibre connections between the cerebral cortex and motoneurons in the spinal cord. These corticomotoneuronal connections are characteristic of primates and can be studied in monkeys; in these animals, refinement in the control of movements of the hand is also associated with increasing development of corticomotoneuronal connections. In monkeys, motoneurons innervating distally acting muscles are preferentially excited by convergent activities in corticomotoneuronal fibres. This excitation has been demonstrated to be effective in natural functional states when a conscious monkey is performing learned movement tasks. Extensive intraspinal arborizations of individual corticomotoneuronal fibres could permit the engagement of large numbers of local motoneurons and related interneurons by each of these fibres. Abolition of corticomotoneuronal influences, after section of the pyramidal tracts, causes a permanent deficit in fractionation of use of muscles of the forelimb and an inability to carry out independent movements of the fingers.

Discharges of pyramidal tract and other motor cortical neurones during locomotion in the cat

A method is described for chronically implanting fine flexible microwires into cat motor cortex, which permitted extracellular recordings to be made from 165 single neurones. Most units were recordable for 12 h and some for up to 2 days. Of the neurones tested, 57% were shown to project to the medullary pyramid (pyramidal tract neurones, p.t.n.s). Antidromic latencies corresponded to a range of conduction velocities from 63 to 9 m/s. In the animal at rest neurones discharged at rates from 0.5 to 44 impulses/s. During locomotion at 0.5 m/s (a slow walk) 56% of cells discharged faster than at rest and 80% showed frequency modulations time-locked to the step cycle. Most fired one discrete burst of impulses per step or one peak period superimposed on a maintained discharge. In different cells peak activity occurred at widely different times during the step cycle. A few cells peaked twice per step. Peak rates (averaged over twenty steps) ranged from 10 to over 120 impulses/s, the values for most slow-axon p.t.n.s (conduction velocity less than 21 m/s) being lower than for any of the fast-axon p.t.n.s. For locomotion at speeds between 0.37 and 1.43 m/s a roughly linear relationship existed between discharge rate and speed in 14% of cells. However, the changes were modest and in most cells both mean rate and peak rate were unrelated to speed. In some cells discharge phasing was fixed (relative to the step cycle in the contralateral forelimb); in others there were progressive phase shifts (or more complex changes) as speed increased. During locomotion up a 10 degrees incline discharge phasings were the same as on the flat in all of the twenty-seven neurones studied and most showed no substantial change in mean rate or peak rate (although there were substantial increases in limb muscle electromyogram amplitudes).

The discharges during movement of cells in the ventrolateral thalamus of the conscious monkey

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.

Relationship between the activity of precentral neurones during active and passive movements in conscious monkeys

Recordings have been made from neurones in the precentral cortex of conscious monkeys carrying out a stereotyped movement task for food rewards. The activity of each movement related neurone was investigated while the monkey performed a wide variety of movements in order to collect the food reward placed in a number of different positions. Of 362 task-related neurones, 176 neurones showed definite modulation of their discharge frequency which could be related to the performance of specific voluntary movements about one joint, or at a number of associated digit joints. These neurones could therefore be classified to have dis­charges associated with particular voluntary movements of shoulder, elbow, wrist, hand or fingers. The responses of the same neurones were then examined with natural stimulation of the skin, joints and muscles of the arm while the animal sat relaxed as it had been trained to do. 149 of the 176 neurones responded to afferent input from particular peripheral territories. 130 of the 176 neurones had afferent inputs from zones which were anatomically closely related to the joint involved in the specific active movement with which the neurone’s natural discharges were clearly associated. An analysis of the anatomical distribution in the motor cortex of the cells exhibiting given input and output associations revealed that not all the members of a restricted local population of neurones shared the same peripheral territory. When they did, the direction of movement of that territory which was associated with discharge of the cell could be the same or opposite under active and passive conditions. Moreover, not all the neurones with a particular association with active movement of a joint and an input from that same region were aggregated in the one restricted local area of the motor cortex; some members of the group sharing these input/output characteristics could be situated up to 5 mm from the main aggregation. An attempt is made to distinguish between the properties of receiving neurones and output neurones of the precentral gyrus.

Relationship of neuronal discharges in the precentral gyrus of monkeys to the performance of arm movements

Recordings have been made from 162 pyramidal tract neurones which discharged bursts of nerve impulses in characteristic temporal association with performances of a stereotyped motor task by monkeys. Clinical evaluation of the relationship between discharges of the neurones and free movement led to the view that each cell's firing was associated with a characteristic aspect of movement performance and the contraction of a particular group of muscles. Quantitative evaluation of these relationships led to the conclusion that coding of the recruitment of motor units to the movement task could have been conferred by the number of pyramidal tract neurones discharging to those motoneurone targets. A ramp of "recruitment" of pyramidal tract neurones preceded the development of a ramp of force by about 100 msec. This general conclusion was supported by the observations made in a single animal in which orderly discharge of precentral neurones in relation to a sterotyped movement performance was clearly evident.

The effect of a preceding stimulus on temporal facilitation at corticomotoneuronal synapses

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.