Motor control: forcing neurons to change.
New research shows that the properties of cells in motor cortex change during learning of new tasks, shedding new light on the neural basis of motor adaptation.
Adaptation, Physiological↗
Biomedical subjects
R N Lemon
Publications and source records attributed to R N Lemon.
Synchronization in monkey motor cortex during a precision grip task. I. Task-dependent modulation in single-unit synchrony.
Neural synchronization in the cortex, and its potential role in information coding, has attracted much recent attention. In this study, we have recorded long spike trains (mean, 33,000 spikes) simultaneously from multiple single neurons in the primary motor cortex (M1) of two conscious macaque monkeys performing a precision grip task. The task required the monkey to use its index finger and thumb to move two spring-loaded levers into a target, hold them there for 1 s, and release for a food reward. Synchrony was analyzed using a time-resolved cross-correlation method, normalized using an estimate of the instantaneous firing rate of the cell. This was shown to be more reliable than methods using trial-averaged firing rate. A total of 375 neurons was recorded from the M1 hand area; 235 were identified as pyramidal tract neurons. Synchrony was weak [mean k' = 1.05 +/- 0.04 (SD)] but widespread among pairs of M1 neurons (218/1359 pairs with above-chance synchrony), including output neurons. Synchrony usually took the form of a broad central peak [average width, 18.7 +/- 8.7 (SD) ms]. There were marked changes during different phases of the task. As a population, synchrony was greatest during the steady hold period in striking contrast to the averaged cell firing rate, which was maximal when the animal was moving the levers into target. However, the modulation of synchrony during task performance showed considerable variation across individual cell pairs. Two types of synchrony were identified: oscillatory (with periodic side lobes in the cross-correlation) and nonoscillatory. Their relative contributions were quantified by filtering the cross-correlations to exclude either frequencies from 18 to 37 Hz or all higher and lower frequencies. At the peak of population synchrony during the hold period, about half (51.7% in one monkey, 56.2% in the other) of the synchronization was within this oscillatory bandwidth. This study provides strong support for assemblies of neurons being synchronized during specific phases of a complex task with potentially important consequences for both information processing within M1 and for the impact of M1 commands on target motoneurons.
Action Potentials↗
Investigation into non-monosynaptic corticospinal excitation of macaque upper limb single motor units.
There has been considerable recent debate as to relative importance, in the primate, of propriospinal transmission of corticospinal excitation to upper limb motoneurons. Previous studies in the anesthetized macaque monkey suggested that, compared with the cat, the transmission of such excitation via a system of C3-C4 propriospinal neurons may be relatively weak. However, it is possible that in the anesthetized preparation, propriospinal transmission of cortical inputs to motoneurons may be depressed. To address this issue, the current study investigated the responses of single motor units (SMUs) to corticospinal inputs in either awake (n = 1) or lightly sedated (n = 3) macaque monkeys. Recordings in the awake state were made during performance of a precision grip task. The responses of spontaneously discharging SMUs to electrical stimulation of the pyramidal tract (PT) via chronically implanted electrodes were examined for evidence of non-monosynaptic, presumed propriospinal, effects. Single PT stimuli (up to 250 microA; duration, 0.2 ms, 2 Hz) were delivered during steady discharge of the SMU (10-30 imp/s). SMUs were recorded from muscles acting on the thumb (adductor pollicis and abductor pollicis brevis, n = 18), wrist (extensor carpi radialis, n = 29) and elbow (biceps, n = 9). In all SMUs, the poststimulus time histograms to PT stimulation consisted of a single peak at a fixed latency and with a brief duration [0.74 +/- 0.25 (SD) ms, n = 56], consistent with the responses being mediated by monosynaptic action of cortico-motoneuronal (CM) impulses. Later peaks, indicating non-monosynaptic action, were not present even when the probability of the initial peak response was low and when there was no evidence for suppression of ongoing SMU activity following this peak (n = 20 SMUs). Even when repetitive (double-pulse) PT stimuli were used to facilitate transmission through oligosynaptic linkages, no later peaks were observed (16 SMUs). In some thumb muscle SMUs (n = 8), responses to PT stimulation were compared with those evoked by transcranial magnetic stimulation, using a figure-eight coil held over the motor cortex. Responses varied according the orientation of the coil: in the latero-medial position, single peak responses similar to those from the PT were obtained; their latencies confirmed direct excitation of CM cells, and there were no later peaks. In the posterio-anterior orientation, responses had longer latencies and consisted of two to three subpeaks. At least under the conditions that we have tested, the results provide no positive evidence for transmission of cortical excitation to upper limb motoneurons by non-monosynaptic pathways in the macaque monkey.
Anesthesia↗
Human cortical muscle coherence is directly related to specific motor parameters.
Cortical oscillations have been the target of many recent investigations, because it has been proposed that they could function to solve the "binding" problem. In the motor cortex, oscillatory activity has been reported at a variety of frequencies between approximately 4 and approximately 60 Hz. Previous research has shown that 15-30 Hz oscillatory activity in the primary motor cortex is coherent or phase locked to activity in contralateral hand and forearm muscles during isometric contractions. However, the function of this oscillatory activity remains unclear. Is it simply an epiphenomenon or is it related to specific motor parameters? In this study, we investigated task-dependent modulation in coherence between motor cortex and hand muscles during precision grip tasks. Twelve right-handed subjects used index finger and thumb to grip two levers that were under robotic control. Each lever was fitted with a sensitive force gauge. Subjects received visual feedback of lever force levels and were instructed to keep them within target boxes throughout each trial. Surface EMGs were recorded from four hand and forearm muscles, and magnetoencephalography (MEG) was recorded using a 306 channel neuromagnetometer. All subjects showed significant levels of coherence (0.086-0.599) between MEG and muscle in the 15-30 Hz range. Coherence was significantly smaller when the task was performed under an isometric condition (levers fixed) compared with a compliant condition in which subjects moved the levers against a spring-like load. Furthermore, there was a positive, significant relationship between the level of coherence and the degree of lever compliance. These results argue in favor of coherence between cortex and muscle being related to specific parameters of hand motor function.
Adult↗
Striking differences in transmission of corticospinal excitation to upper limb motoneurons in two primate species.
There is considerable debate as to the relative importance, for cortical control of upper limb movements, of direct cortico-motoneuronal (CM) versus indirect, propriospinal transmission of corticospinal excitation to cervical motoneurons. In the cat, which has no CM connections, a significant proportion of corticospinal excitation reaches forelimb motoneurons via a system of C(3)-C(4) propriospinal neurons (PN). In contrast, in the macaque monkey most motoneurons receive direct CM connections, and, under the same experimental conditions as in the cat, there is little evidence for PN transmission. We have investigated corticospinal transmission in the New World squirrel monkey (Saimiri sciureus) because its CM projections are weaker than in the macaque. Intracellular recordings were made from motoneurons identified from the ulnar, median, and deep radial (DR) nerves in four adult squirrel monkeys under chloralose anesthesia and neuromuscular paralysis. Responses to stimulation of the contralateral medullary pyramid were recorded before and after a lesion to the dorsolateral funiculus (DLF) at C(5), designed to interrupt direct corticospinal inputs to the lower cervical segments and unmask PN-mediated effects. This lesion greatly reduced the proportion of motoneurons showing either CM EPSPs or disynaptic IPSPs, but the proportion showing late EPSPs with segmental latencies beyond the monosynaptic range, evoked by repetitive but not single PT stimuli, was unaffected: 23 of 29 motoneurons (79%) before and 32 of 37 (86%) after the lesion; 41% of these late EPSPs had strictly disynaptic latencies after the lesion, only 14% before. These results are in striking contrast to the macaque (late EPSPs in only 18% of motoneurons before a C(5) lesion, 19% after it). Transmission of the late EPSPs via C(3)-C(4) PNs in the squirrel monkey was indicated by their absence after an additional C(2) DLF lesion. Nearly all tested motoneurons also responded with short latency EPSPs to stimulation in the ipsilateral lateral reticular nucleus. By analogy with the cat, these EPSPs probably reflect antidromic activation of ascending collaterals of C(3)-C(4) PNs with monosynaptic connections to motoneurons; the EPSPs were significantly smaller than in the cat but larger than in the macaque. These results suggest that the positive correlation across species between more advanced hand function and the strength of the CM system is accompanied by a negative correlation between hand function and the strength of the PN system. We hypothesize that in primates with more advanced hand function, the CM system effectively replaces PN-mediated control. This would include a contribution to the control of reaching movements, which are said to be specifically under the control of the PN system in the cat, and we speculate that these differences may be related to the degree of dexterity exhibited by the different species. This interpretation of the results predicts that in man, where the CM system is highly developed, the PN system is unlikely to be responsible for significant transmission of cortical commands to upper limb motoneurons.
Animals↗
Precise spatiotemporal repeating patterns in monkey primary and supplementary motor areas occur at chance levels.
Precise spatiotemporal patterns in neural discharge are a possible mechanism for information encoding in the brain. Previous studies have found that such patterns repeat and appear to relate to key behavioral events. Whether these patterns occur above chance levels remains controversial. To address this question, we have made simultaneous recordings from between two and nine neurons in the primary motor cortex and supplementary motor area of three monkeys while they performed a precision grip task. Out of a total of 67 neurons, 46 were antidromically identified as pyramidal tract neurons. Sections of recordings 60 s long were searched for patterns involving three or more spikes that repeated at least twice. The allowed jitter for pattern repetition was 3 ms, and the pattern length was limited to 192 ms. In all 11 recordings analyzed, large numbers of repeating patterns were found. To assess the expected chance level of patterns, "surrogate" datasets were generated. These had the same moment-by-moment modulation in firing rate as the experimental spike trains, and matched their interspike interval distribution, but did not preserve the precise timing of individual spikes. The number of repeating patterns in 10 randomly generated surrogates was used to form 99% confidence limits on the repeating pattern count expected by chance. There was close agreement between these confidence limits and the number of patterns seen in the experimental data. Analysis of high complexity patterns was carried out in four long recordings (mean duration 23.2 min, mean number of neurons simultaneously recorded 7.5). This analysis logged only patterns composed of a larger number (7-11) of spikes. The number of patterns seen in the surrogate datasets showed a small but significant excess over those seen in the original experimental data; this is discussed in the context of surrogate generation. The occurrence of repeating patterns in the experimental data were strongly associated with particular phases of the precision grip task; however, a similar task dependence was seen for the surrogate data. When a repeating pattern was used as a template to find inexact matches, in which up to half of the component spikes could be missing, similar numbers of matches were found in experimental and surrogate data, and the time of occurrence of such matches showed the same task dependence. We conclude that the existence of precise repeating patterns in our data are not due to cortical mechanisms that favor this form of coding, since as many, if not more, patterns are produced by spike trains constructed only to modulate their firing rate in the same way as the experimental data, and to match the interspike interval histograms. The task dependence of pattern occurrence is explicable as an artifact of the modulation of neural firing rate. The consequences for theories of temporal coding in the cortex are discussed.
Action Potentials↗
Multiple single unit recording in the cortex of monkeys using independently moveable microelectrodes.
Simultaneous recording from multiple single neurones presents many technical difficulties. However, obtaining such data has many advantages, which make it highly worthwhile to overcome the technical problems. This report describes methods which we have developed to permit recordings in awake behaving monkeys using the 'Eckhorn' 16 electrode microdrive. Structural magnetic resonance images are collected to guide electrode placement. Head fixation is achieved using a specially designed headpiece, modified for the multiple electrode approach, and access to the cortex is provided via a novel recording chamber. Growth of scar tissue over the exposed dura mater is reduced using an anti-mitotic compound. Control of the microdrive is achieved by a computerised system which permits several experimenters to move different electrodes simultaneously, considerably reducing the load on an individual operator. Neurones are identified as pyramidal tract neurones by antidromic stimulation through chronically implanted electrodes; stimulus control is integrated into the computerised system. Finally, analysis of multiple single unit recordings requires accurate methods to correct for non-stationarity in unit firing. A novel technique for such correction is discussed.
Animals↗
Experiments using transcranial magnetic brain stimulation in man could reveal important new mechanisms in motor control.
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Electromagnetic Fields↗
Task-dependent modulation of 15-30 Hz coherence between rectified EMGs from human hand and forearm muscles.
1. Recent reports have shown task-related changes in oscillatory activity in the 15-30 Hz range in the sensorimotor cortex of human subjects and monkeys during skilled hand movements. In the monkey these oscillations have been shown to be coherent with oscillatory activity in the electromyographic activity of hand and forearm muscles. 2. In this study we investigated the modulation of oscillations in the electromyogram (EMG) of human volunteers during tasks requiring precision grip of two spring-loaded levers. 3. Two tasks were investigated: in the 'hold' task, subjects were required to maintain a steady grip force (ca 2.1 N or 2.6 N) for 8 s. In the 'ramp' task, there was an initial hold period for 3 s (force ca 2.1 N) followed by a linear increase in grip force over a 2 s period. The task ended with a further steady hold for 3 s at the higher force level (ca 2.6 N). 4. Surface EMGs were recorded from five hand and forearm muscles in 12 subjects. The coherence of oscillatory activity was calculated between each muscle pair. Frequencies between 1 and 100 Hz were analysed. 5. Each subject showed a peak in the coherence spectra in the 15-30 Hz bandwidth during the hold task. This coherence was absent during the initial movement of the levers. During the ramp task the coherence in the 15-30 Hz range was also significantly reduced during the movement phase, and significantly increased during the second hold period, relative to the initial hold. 6. There was coherence between the simultaneously recorded magnetoencephalogram (MEG) and EMG during steady grip in the hold task; this coherence disappeared during the initial lever movement. Using a single equivalent current dipole source model, the coherent cortical activity was localized to the hand region of the contralateral motor cortex. This suggests that the EMG-EMG coherence was, therefore, at least in part, of cortical origin. 7. The results are discussed in terms of a possible role for synchrony in the efficient recruitment of motor units during maintained grip.
Adult↗
Neural control of dexterity: what has been achieved?
This chapter reviews progress made in our understanding of the neural control of dexterity. It stresses the increasing benefit derived by uniting the different disciplines concerned with the study of the hand. It highlights the study of natural movements and of the importance of tackling the function of the interface between the neural control system and the biomechanical apparatus of the hand and arm. It also highlights the distributed nature of the control system, its utilisation of complex spatio-temporal representations and its dependence on sensory input. It concludes by pointing out the lessons that have been learned from two fields of work: the development of motor skill and the comparative study of dexterity in different primate species
Animals↗
The role of synchrony and oscillations in the motor output.
There is currently much interest in the synchronisation of neural discharge and the potential role it may play in information coding within the nervous system. We describe some recent results from investigations of synchronisation within the motor system. Local field potentials (LFPs) and identified pyramidal tract neurones (PTNs) were recorded from the primary motor cortex of monkeys trained to perform a precision grip task. The LFPs showed bursts of oscillatory activity at 20-30 Hz, which were coherent with the rectified electromyographs (EMG) of contralateral hand and forearm muscles. This oscillatory synchronisation showed a highly specific task dependence, being present only during the part of the task when the animal maintained a steady grip and not during the movement phases before or after it. PTNs were phase-locked to LFP oscillations, implying that at least part of the coherence between cortical activity and EMG was mediated by corticospinal fibres. The phase locking of the PTNs to LFP oscillations produced task-dependent oscillatory synchronisation between PTN pairs, as assessed by the single-unit cross-correlation histogram. Recordings were also made from normal human subjects performing a precision grip similar to that used in the monkey recordings. Pairs of EMGs recorded from intrinsic hand and forearm muscles showed 20-30 Hz coherence, which modulated during task performance, being present only during periods of steady contraction. We suggest that these changes in EMG-EMG synchronisation reflect changing levels of synchronous drive from the corticospinal system. The generation of oscillations in the cortex is discussed in the light of results from a model of local cortical circuits. Other modelling work has shown that synchrony in the corticospinal inputs could act to recruit motoneurones more efficiently, producing more output force from a muscle than asynchronous inputs firing at the same mean rate. A speculative hypothesis is presented on the role of synchronous oscillations in the motor system, which is consistent with experimental observations to date.
Animals↗
The contribution of fast corticospinal input to the voluntary activation of proximal muscles in normal subjects and in stroke patients.
Although it is well known that the corticospinal system exerts more influence over distal (hand and fingers) than proximal (elbow and shoulder) upper limb muscles, differences in the importance of this system for voluntary activation of these muscle groups have not been demonstrated directly. Two investigations were carried out to provide a quantitative comparison of the contribution of fast corticospinal inputs to voluntary activity in proximal and distal muscles of normal subjects. The first study confirmed that the rate of increase in the amplitude of EMG responses to transcranial magnetic stimulation (TMS) with voluntary activation of the muscles was significantly greater in a hand muscle (first dorsal interosseous, 1DI) than in biceps, which was in turn greater than that for deltoid. The second study demonstrated that this result reflected a genuine difference in corticospinal influence over these muscles and was not due to differences in the pattern and type of motor unit recruitment in proximal vs distal muscles. The voluntary activation of a pair of low-threshold single motor units (SMUs) in 1DI and deltoid was compared with their response to TMS. In both muscles only a small amount of additional effort was required to recruit the second SMU; increments were typically within 1% of maximum voluntary contraction, as assessed from EMG measurements. Subjects were asked to voluntarily discharge the lower threshold SMU at a steady rate, and then the threshold of responses of this SMU and that of the second unit to TMS were determined. In 1DI, only small increments in TMS intensity above the threshold for the first SMU were required to activate the second unit [mean 1.4% maximum stimulator output (MSO), SD +/- 1.0%, n = 7 subjects]. In contrast, in deltoid a significantly greater intensity increase was needed (mean 6%, SD +/- 1.2%, MSO n = 7, P < 0.001). Similar results were obtained when TMS thresholds of motor unit pairs were assessed in relaxed subjects. These experiments support the hypothesis that the fast corticospinal input that can be activated by TMS is of greater importance for the voluntary activation of hand than of shoulder muscles. This hypothesis served as a basis for testing deltoid responses in three stroke patients. In two patients smaller responses to TMS were obtained on the affected side than on the unaffected side during the production of equivalent voluntary contractions, suggesting that the patients achieved these contractions using inputs other than the fast corticospinal elements excited by TMS.
Adult↗
Does a C3-C4 propriospinal system transmit corticospinal excitation in the primate? An investigation in the macaque monkey.
1. Synaptic responses to electrical stimulation of the contralateral pyramidal tract were measured in intracellular recordings from 206 upper limb motoneurones in ten chloralose-anaesthetized macaque monkeys. The objective was to search for evidence of a disynaptic excitatory pathway via C3-C4 propriospinal interneurones similar to that in the cat. 2. In monkeys with intact spinal cords, only a small proportion of motoneurones (19%) responded with late EPSPs to repetitive stimulation of the pyramid; only 3% had segmental latencies that were appropriate for a disynaptic pathway. 3. From previous studies in the cat, it was expected that a lesion to the dorsolateral funiculus (DLF) at C5 would interrupt the corticospinal input to the spinal segments supplying upper limb muscles, whilst leaving intact excitation transmitted via a C3-C4 propriospinal system, the descending axons of which travel in the ventral part of the funiculus. In five of the monkeys a lesion was made to the DLF at C5 which spared the ventrolateral columns. It severely reduced the monosynaptic EPSPs and disynaptic IPSPs evoked from the pyramidal tract that were present in the intact monkey spinal cord, and which might have masked the presence of disynaptic EPSPs. However, even after the lesion the proportion of motoneurones with such late EPSPs was still low (18%); 14% of motoneurones had EPSPs within the disynaptic range. 4. In addition, some EPSPs with relatively long segmental latencies (> 1.1 ms) were recorded before and after the C5 lesions, but since these effects could be evoked by single stimuli, had stable latencies and did not facilitate with repetitive shocks, it is likely that they represent monosynaptic EPSPs evoked by slowly conducting corticospinal fibres which survived the lesions. 5. In seven of the monkeys motoneurone responses to stimulation of the ipsilateral lateral reticular nucleus (LRN) were also tested. Most motoneurones showed EPSPs with short latencies (1.2-2.5 ms) and other properties characteristic of monosynaptic activation. This is consistent with the presence of collaterals of C3-C4 propriospinal neurones to the LRN, as demonstrated in the cat. 6. These short-latency EPSPs evoked from the LRN were just as common before (77%) as after (75%) the C5 lesion. They had small amplitudes both before (mean +/- s.d. 1.1 +/- 0.59 mV) and after (1.2 +/- 0.72 mV) the lesion. Unlike the situation in the cat, only a small proportion (16%) of motoneurones activated from the LRN showed late EPSPs after repetitive stimulation of the pyramid. 7. The results provide little evidence for significant corticospinal excitation of motoneurones via a system of C3-C4 propriospinal neurones in the monkey. The general absence of responses mediated by such a system in the macaque, under experimental conditions similar to those in which they are seen in the cat, show that extrapolation of results from the cat to the primate should be made with considerable caution.
Animals↗
The importance of the cortico-motoneuronal system for control of grasp.
Our recent work has revealed new evidence of the importance of direct cortico-motoneuronal (CM) connections for voluntary control of the hand. Most of these connections are derived from corticospinal neurons located in the M1 hand area, although there are some much smaller contributions from other secondary motor areas, such as the supplementary motor area (SMA). Intracellular recordings show that 75% of upper limb motoneurons in the chloralose-anaesthetized macaque monkey receive a monosynaptic projection from the corticospinal tract; evidence for non-monosynaptic, propriospinal excitatory influences from the corticospinal tract was conspicuously lacking in these anaesthetized preparations. Moreover, in the conscious monkey, hand and arm muscle motor unit responses to corticospinal tract input are dominated by single, brief peaks compatible with monosynaptic excitation. CM excitatory post-synaptic potentials, recorded from a comparable sample of hand and arm motoneurons in anaesthetized macaque and squirrel monkeys, were found to be larger and faster rising in the macaque, which is by far the more dexterous of the two species. CM cells facilitating a given muscle in the conscious macaque are distributed over a wide region of M1 cortex, and each contributes a particular pattern of discharge during a skilled task. In addition to their direct effects on target muscles there may be weaker but potentially important effects that derive from the synchronous binding of assemblies of output neurons. Synchronous oscillations between these neurons are particularly prevalent during steady grip, but disappear during digit movement.
Animals↗
An investigation of the intrinsic circuitry of the motor cortex of the monkey using intra-cortical microstimulation.
The motor cortex contains a distributed map of muscles, with a single muscle represented over a wide cortical area. We have searched for inter-connections between distant sites projecting to common muscles by delivering pairs of 20-microA single-pulse intracortical microstimuli (ICMS) to sites separated by 1.5-2 mm in the hand-area primary motor cortex of two macaque monkeys performing a precision grip task. The facilitation of hand- and forearm-muscle rectified EMG was measured. When stimuli were delivered simultaneously, responses were quantified using a technique to correct for non-linearities inherent in the use of averaged, rectified EMG. A spatial facilitation was seen for such simultaneous stimuli; however, it was of the same magnitude as that occurring when ICMS was paired with stimulation of corticospinal axons in the pyramidal tract (PT), so that it was likely to be spinal in origin. When two such distant sites were stimulated separated by a 10- or 20-ms delay, the second response scaled with the level of background EMG in the same way as a response to the PT stimulus. By contrast, when the same site was stimulated twice with these delays, the second response showed a facilitation compared with a similarly timed PT response. There would therefore appear to be a local facilitation of the cortical output at these intervals, which is not seen between distant sites. Antidromically identified pyramidal-tract neurones (PTNs) were recorded whilst stimuli were delivered to a cortical site, with a distance between stimulating and recording electrodes of also 1.5-2 mm. The most common response was a facilitation followed by a suppression. Six of eleven PTNs showed a facilitation in their discharge following this stimulation (maximum connection strength s=0.19), 8/11 showed a suppression (maximum s=0.16). It is concluded that powerful inter-connections do exist between distributed parts of the motor output and that there is widespread cortical activation after even a single ICMS pulse. However, these inter-connections do not lead to interactions between cortical outputs following stimulation, as assessed from the EMG. It is proposed that this is likely to reflect differences in the summation of output cells to local versus remote stimulation.
Animals↗
Computer simulation of post-spike facilitation in spike-triggered averages of rectified EMG.
When the spikes of a motor cortical cell are used to compile a spike-triggered average (STA) of rectified electromyographic (EMG) activity, a post-spike facilitation (PSF) is sometimes seen. This is generally thought to be indicative of direct corticomotoneuronal (CM) connections. However, it has been claimed that a PSF could be caused by synchronization between CM and non-CM cells. This study investigates the generation of PSF using a computer model. A population of cortical cells was simulated, some of which made CM connections to a pool of 103 motoneurons. Motoneurons were simulated using a biophysically realistic model. A subpopulation of the cortical cells was synchronized together. After a motoneuron discharge, a motor unit action potential was generated; these were summed to produce an EMG output. Realistic values were used for the corticospinal and peripheral nerve conduction velocity distribution, for slowing of impulse conduction in CM terminal axons, and for the amount of cortical synchrony. STA of the rectified EMG from all cortical neurons showed PSF; however, these were qualitatively different for CM versus non-CM cells. Using an epoch analysis to determine reliability in a quantitative manner, it was shown that the onset latency of PSF did not distinguish the two classes of cells after 10,000 spikes because of high noise in the averages. The time of the PSF peak and the peak width at half-maximum (PWHM) could separate CM from synchrony effects. However, only PWHM was robust against changes in motor unit action-potential shape and duration and against changes in the width of cortical synchrony. The amplitude of PSF from a CM cell could be doubled by the presence of synchrony. It is proposed that, if a PSF has PWHM < 7 ms, this reliably indicates that the trigger is a CM cell projecting to the muscle whose EMG is averaged. In an analysis of experimental data where macaque motor cortical cells facilitated hand and forearm muscle EMG, 74% of PSFs fulfilled this criterion. The PWHM criterion could be applied to other STA studies in which it is important to exclude the effects of synchrony.
Action Potentials↗
Coherent oscillations in monkey motor cortex and hand muscle EMG show task-dependent modulation.
1. Recordings were made of local field potential (slow waves) and pyramidal tract neurone (PTN) discharge from pairs of sites separated by a horizontal distance of up to 1.5 mm in the primary motor cortex of two conscious macaque monkeys performing a precision grip task. 2. In both monkeys, the slow wave recordings showed bursts of oscillations in the 20-30 Hz range. Spectral analysis revealed that the oscillations were coherent between the two simultaneously recorded cortical sites. In the monkey from which most data were recorded, the mean frequency of peak coherence was 23.4 Hz. 3. Coherence in this frequency range was also seen between cortical slow wave recordings and rectified EMG of hand and forearm muscles active during the task, and between pairs of rectified EMGs. 4. The dynamics of the coherence were investigated by analysing short, quasi-stationary data segments aligned relative to task performance. This revealed that the 20-30 Hz coherent oscillations were present mainly during the hold phase of the precision grip task. 5. The spikes of identified PTNs were used to compile spike-triggered averages of the slow wave recordings. Oscillations were seen in 11/17 averages of the slow wave recorded on the same electrode as the triggering spike, and 11/17 averages of the slow wave recorded on the distant electrode. The mean period of these oscillations was 45.8 ms. 6. It is concluded that oscillations in the range 20-30 Hz are present in monkey motor cortex, are coherent between spatially separated cortical sites, and encompass the pyramidal tract output neurones. They are discernable in the EMG of active muscles, and show a consistent task-dependent modulation.
Animals↗
Postnatal development of corticospinal projections from motor cortex to the cervical enlargement in the macaque monkey.
The postnatal development of corticospinal projections was investigated in 11 macaques by means of the anterograde transport of wheat germ agglutin-horseradish peroxidase injected into the primary motor cortex hand area. Although the fibers of the corticospinal tract reached all levels of the spinal cord white matter at birth, their penetration into the gray matter was far from complete. At birth, as in the adult, corticospinal projections were distributed to the same regions of the intermediate zone, although they showed marked increases in density during the first 5 months. The unique feature of the primate corticospinal tract, namely direct cortico-motoneuronal projections to the spinal motor nuclei innervating hand muscles, was not present to a significant extent at birth. The density of these cortico-motoneuronal projections increased rapidly during the first 5 months, followed by a protracted period extending into the second year of life. The densest corticospinal terminations occupied only 40% of the hand motor nuclei in the first thoracic segment at 1 month, 73% at 5 months, and 75.5% at 3 years. A caudo-rostral gradient of termination density within the hand motor nuclei was present throughout development and persisted into the adult. As a consequence, the more caudal the segment within the cervical enlargement, the earlier the adult pattern of projection density was reached. No transitory corticospinal projections were found. The continuous postnatal expansion of cortico-motoneuronal projections to hand motor nuclei in primates is in marked contrast to the retraction of exuberant projections that characterizes the development of other sensory and motor pathways in subprimates.
Animals↗