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R N Lemon

Publications and source records attributed to R N Lemon.

At least 73 records · Page 4Linked to original sources
PubMed1993

Task dependence of responses in first dorsal interosseous muscle to magnetic brain stimulation in man.

1. The response of the first dorsal interosseous (1DI) muscle to non-invasive magnetic and scalp electrical stimulation of the brain have been investigated during performance of different manual tasks. 2. The six tasks tested required activation of the 1DI muscle, either in isolation (during abduction of the index finger) or as part of a more complex pattern of muscle synergies (e.g. during power grip). The level of 1DI EMG activity across tasks was kept constant by providing subjects with visual feedback of their muscle activity. 3. In every subject (n = 14) magnetic stimulation produced larger responses during performance of complex tasks than during the simple index abduction task. The pooled results from all subjects showed that four of the five complex tasks were associated with significantly larger 1DI responses (paired t test, P < 0.05). 4. These results were confirmed at the single motor unit level for nine motor units recorded from six subjects. Subjects were requested to produce a steady discharge of the same motor unit during performance of different tasks. The probability of motor unit discharge in response to magnetic stimulation was significantly greater during complex tasks (rotation or pincer grips) than during abduction. 5. Scalp electrical stimulation was performed in three subjects with the cathode at the vertex and the anode over the contralateral motor cortex. The pattern of response amplitudes in the different tasks tended to parallel that obtained for magnetic stimulation, but the task-related differences were smaller. 6. These results suggest that during performance of the different tasks, the corticospinal volleys evoked by magnetic stimulation may vary in amplitude. The task-related cortical mechanisms that may contribute to this variability are discussed.

Adult↗
PubMed1993

Recruitment of motor units in response to transcranial magnetic stimulation in man.

1. Short-latency responses of single motor units (SMUs) and surface electromyographic activity (EMG) to transcranial magnetic stimulation (TMS) were examined in five different hand and forearm muscles of human subjects. 2. The response probability, P (number of extra spikes in the response peak above background per stimulus), was, in general, higher at the lower voluntary discharge rate of the motor unit than at the higher rate. 3. Increasing the strength of TMS increased the response probability of a tonically firing motor unit and at the same time recruited new units which discharged phasically during the response peak. This demonstrates rate coding and recruitment of motor units by excitatory inputs resulting from TMS when the motoneurone pool is tonically facilitated by a constant voluntary drive. 4. Next, TMS was delivered without any voluntary facilitation of motoneurones. The order of recruitment for up to four different motor units discharged by TMS was compared to that observed with voluntary input. The threshold of recruitment for each of the two inputs was estimated from the surface EMG value at which the unit was recruited. For these motoneurone pools (eleven sets of observations), the order of recruitment was the same with TMS and voluntary inputs. 5. From these data it is concluded that, despite the complex and phasic nature of the descending corticospinal volleys generated by TMS, it produces orderly recruitment and rate coding of motoneurones similar to that found for voluntary activation.

Action Potentials↗
PubMed1993

Contribution of the monkey corticomotoneuronal system to the control of force in precision grip.

1. The contribution of 33 corticomotoneuronal (CM) cells, recorded in the primary motor cortex, to the production of precision grip force has been investigated in four monkeys (Macaca nemestrina). These CM cells were shown, by spike-triggered averaging, to facilitate electromyographic (EMG) activity of hand and forearm muscles. 2. Single-cell recordings were obtained as the monkey performed a low force precision grip task under either isometric or auxotonic conditions. The monkey had to produce independent control of the forces exerted by the thumb and index finger and maintain them for 1-1.5 s. Steady force segments of data were selected trial-by-trial from these hold periods. For each segment the following mean values were determined: 1) CM cell firing rate, 2) EMG activity of facilitated muscles, and 3) index finger, thumb, and total force. 3. Of the 33 CM cells, 18 had a phasic-tonic pattern of discharge during the task, 7 were tonic, 5 had a ramplike increase, and 3 were deactivated during the hold period. 4. Of the 33 cells analyzed, 11 showed a significant positive (P < 0.05) correlation of their mean firing rate with static force; 4 of them had high correlation coefficients (P < 0.001). There was a considerable trial-by-trial variability in the cells' activity-force relationship. Six CM cells had significant negative correlations between their activity and isometric force (5 at the P < 0.001 level), showing lower firing rates with higher forces. 5. The force sensitivity of the CM cells, calculated from the rate-force slopes, was higher for either the thumb or the index finger force. Under isometric conditions the mean rate-force slopes, calculated from the best correlated digit force, was 32.4 Hz/N for eight positively correlated cells and -21.3 Hz/N for the cells with a negative correlation. 6. Correlation between CM cell spike activity and force was more common among neurons with slowly conducting axons (4/6 correlated) than for those with fast axons (13/27). 7. Significant correlations between target muscle EMG and force were always positive. The correlations between CM cell firing rate and target muscle EMG were comparable with those found between firing rate and force. Three of the CM cells with a negative correlation to force also had a negative correlation with EMG in one of their target muscles. 8. Each CM cell facilitated the EMG activity of one to five target muscles; postspike facilitation (PSF) was most common among intrinsic hand muscles (68/82 CM cell/muscle combinations).(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1992

The development of corticospinal projections to tail and hindlimb motoneurons studied in infant macaques using magnetic brain stimulation.

The EMG responses evoked in tail, leg and foot muscles by magnetic stimulation (MS) of the brain were investigated in two male macaque monkeys under ketamine sedation. The animals were studied longitudinally over a period of 7 months (from 2.75 to 9.75 months old). MS was generally ineffective in eliciting responses when the animals were 2.75-4.5 months old, even at maximum stimulator output (1.5 Tesla). After this time the threshold for evoking EMG responses decreased considerably, and there was an increase in the probability of occurrence of the responses. These age-related changes plateaued at about 7.5 months, after which they remained fairly constant at adult levels. In both animals the maturation of these responses in tail and hindlimb muscles occurred later than in forelimb muscles.

Afferent Pathways↗
PubMed1992

The development of cortico-motoneuronal projections investigated using magnetic brain stimulation in the infant macaque.

1. The effects of magnetic brain stimulation on electromyographic (EMG) activity recorded from arm and hand muscles have been investigated in five infant and six adult macaque monkeys under ketamine sedation. 2. In the adults, brief, short-latency EMG responses could be readily evoked with magnetic stimuli of 40-50% of the maximum stimulator output (1.5 T). 3. In a cross-sectional study of five infant macaques, it was difficult to evoke EMG responses in young infants (less than 5 months old). Clear short-latency responses were first evoked in an animal 5.75 months old. This change was accompanied by an increase in the probability of occurrence of the responses. 4. In a longitudinal study of two infant monkeys over a period ranging from 2.5 to 14.5 months of age we found that clear short-latency responses were first evoked at 4 and at 5.5 months, respectively. In both animals there was a steady fall in response threshold which reached the adult range at 6.5 and 8 months, respectively. EMG responses in animals older than 8 months were indistinguishable from those in adults. 5. In the longitudinal study we also noted that the latency of EMG responses to magnetic brain stimulation declined with age. Since there were no comparable changes in the peripheral conduction time in these animals, we attribute this result to a decrease in central conduction time. 6. Parallel behavioural observations of the natural behaviour of the same animals within a colony indicated that mature precision movements of the fingers were not used until 5-6 months of age. 7. In two adult monkeys, the latency of EMG responses evoked in the extensor digitorum and first dorsal interosseous muscles by direct stimulation of the corticospinal tract, via electrodes implanted in the medullary pyramids, was found to be 0.7-1.7 ms shorter than that of responses evoked by magnetic stimuli. It is argued that at least the earliest component of these latter responses is conducted over the cortico-motoneuronal pathway. 8. The mechanisms likely to contribute to the late appearance of EMG responses to brain stimulation are discussed. One of these is probably the establishment of mature cortico-motoneuronal connections, which are not present at birth.

Aging↗
PubMed1990

Non-invasive brain stimulation reveals reorganized cortical outputs in amputees.

EMG responses to non-invasive electromagnetic brain stimulation (EMS) were recorded from arm muscles of congenital amputees. Responses were obtained with lower thresholds on the amputated than on the intact side and were evoked from a larger cortical area. Contracting muscles showed increased responses to EMS; the increase was more pronounced on the amputated side. Similar findings were obtained in one traumatic amputee who suffered an early amputation, but not in another patient with a late amputation. We conclude that in congenital amputees there is substantial reorganisation of the corticospinal system and that this may also occur in early traumatic cases.

Adult↗
PubMed1990

Recording and identification of single motor units in the free-to-move primate hand.

A new technique is described for recording the activity of single motor units in human or monkey hand muscles. A pair of microwire electrodes is introduced into the muscle using a fine needle. After insertion, the needle can be completely removed, leaving the recording microwires in situ. The method allows stable recording of a motor unit during natural movement of the hand and fingers. The identity of a given single motor unit was reflected in the form and amplitude of the motor unit-triggered average (MU-TA), derived by averaging the unrectified surface EMG recorded from the muscle with discharges of the motor unit. The MU-TA of a given unit remained constant despite variations in the form and size of its action potential. Inspection of successive MU-TAs increased confidence that records were taken from one and the same unit over long recording periods. Control experiments in human first dorsal interosseous showed that the peak-to-peak amplitude of the MU-TA was highly correlated with both the twitch force (r = 0.65-0.92, mean 0.82, six subjects) and force threshold (r = 0.62-0.93, mean 0.83) of a given unit. Similar findings were obtained for human abductor pollicis brevis (AbPB) motor units. In the monkey, AbPB motor units which were recruited early in a precision grip task and which discharged steadily during the grip had smaller MU-TAs than late-recruited, phasic units. The combination of methods described in this paper enable a single motor unit to be identified and recognised. The relative size of the unit, which is an important parameter in most motor unit studies, can be reliably estimated from the amplitude of the MU-TA. This allows indirect assessment of motor unit size in a free-to-move animal.

Action Potentials↗
PubMed1990

Excitation of the corticospinal tract by electromagnetic and electrical stimulation of the scalp in the macaque monkey.

1. The responses evoked by non-invasive electromagnetic and surface anodal electrical stimulation of the scalp (scalp stimulation) have been studied in the monkey. Conventional recording and stimulating electrodes, placed in the corticospinal pathway in the hand area of the left motor cortex, left medullary pyramid and the right spinal dorsolateral funiculus (DLF), allowed comparison of the actions of non-invasive stimuli and conventional electrical stimulation. 2. Responses to electromagnetic stimulation (with the coil tangential to the skull) were studied in four anaesthetized monkeys. In each case short-latency descending volleys were recorded in the contralateral DLF at threshold. In two animals later responses were also seen at higher stimulus intensities. Both early and late responses were of corticospinal origin since they could be completely collided by appropriately timed stimulation of the pyramidal tract. The latency of the early response in the DLF indicated that it resulted from direct activation of corticospinal neurones: its latency was the same as the latency of the antidromic action potentials evoked in the motor cortex from the recording site in the DLF. 3. Scalp stimulation, which was also investigated in three of the monkeys, evoked short-latency volleys at threshold and at higher stimulus intensities these were followed by later waves. The short-latency volleys could be collided from the pyramid and, at threshold, had latencies compatible with direct activation of corticospinal neurones. The longer latency volleys were also identified as corticospinal in origin. 4. The latency of the early volley evoked by electromagnetic stimulation remained constant with increasing stimulus intensities. In contrast, with scalp stimulation above threshold the latency of the early volleys decreased considerably, indicating remote activation of the corticospinal pathway below the level of the motor cortex. In two monkeys both collision and latency data suggest activation of the corticospinal pathway as far caudal as the medulla. 5. The majority of fast corticospinal fibres could be excited by scalp stimulation with intensities of 20% of maximum stimulator output. Electromagnetic stimulation at maximum stimulator output elicited a volley of between 70 and 90% of the size of the maximal volley evoked from the pyramidal electrodes. 6. Electromagnetic stimulation was also investigated in one awake monkey during the performance of a precision grip task. Short-latency EMG responses were evoked in hand and forearm muscles. The onsets of these responses were approximately 0.8 ms longer than the responses evoked by electrical stimulation of the pyramid.(ABSTRACT TRUNCATED AT 400 WORDS)

Action Potentials↗
PubMed1990

Contributions to the history of psychology: LXVII. Henricus (Hans) Kuypers F.R.S. 1925-1989.

Hans Kuypers, who died in September 1989, was one of the founding fathers of neuroscience. Kuypers studied medicine in Leiden, The Netherlands and trained as a neuroanatomist with Nauta. His early work at Maryland and at Cleveland led to a wealth of new discoveries about descending motor pathways. Subsequently at Rotterdam, where he was foundation professor, and at Cambridge, England he searched for new brain probes, including retrograde fluorescent tracers and viruses for labelling chains of interconnected neurons. He saw the need to relate new neuroanatomical findings to brain function: in particular, how the brain steers hand and arm movements. He will be greatly missed by a long and distinguished list of collaborators on every continent.

England↗
PubMed1989

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)

Animals↗
PubMed1989

Preparation of visually cued arm movements in monkey. Involvement of inferior parietal cortex.

Single-unit activity was recorded in monkey inferior parietal lobule (IPL) during performance of a visually cued limb motor task. Many neurons in the IPL modulated their activity just after the visual cue was presented, similar to the neuronal activity observed in the premotor cortex in a previous experiment. It is suggested that IPL neurons are involved in preparation of visually cued limb movement. The present results are discussed in view of a possible role for IPL and premotor cortex in processing visual information for use by the primary motor area.

Animals↗
PubMed1987

Cross-correlation reveals facilitation of single motor units in thenar muscles by single corticospinal neurones in the conscious monkey.

The functional connections between corticospinal neurones and motor units of the monkey's hand muscles were investigated by constructing cross-correlograms of activity recorded from pyramidal tract neurones and from single motor units in the contralateral thenar muscles during performance of a precision grip between thumb and index finger. Only those neurones which produced postspike facilitation of the surface-recorded electromyogram (EMG) of these muscles were selected for analysis. Positive correlations were observed for 11/15 selected neurones, and the form of the correlation was suggestive of monosynaptic action. Corticospinal cells which produced a correlation peak often did so with all concurrently-sampled motor units.

Animals↗
PubMed1987

The effects upon the activity of hand and forearm muscles of intracortical stimulation in the vicinity of corticomotor neurones in the conscious monkey.

Corticomotor (CM) neurones were identified in three conscious macaque monkeys by the presence of post-spike facilitation (PSF) in spike-triggered averages of e.m.g. recorded from intrinsic hand and forearm muscles during performance of a precision grip task. Post-spike effects were compared with those produced by single-pulse intracortical microstimulation (ICMS), with strengths of 4-20 microA, delivered at the site of 47 CM cells. Most muscles facilitated by a CM cell were also facilitated by ICMS at the site of the cell. ICMS effects were stronger: at 10 microA, the amplitude of ICMS-evoked facilitation was on average 2.8 times greater than PSF, and 6.9 times greater at 20 microA. Onset latency of ICMS-evoked facilitation was consistently longer (by 1.7 and 1.3 ms at 10 and 20 microA respectively) than PSF, and it is suggested that this results from the indirect, trans-synaptic excitation of CM cells by ICMS. Post-spike suppression was rarely seen (7/421 compared to 105/421 cases of PSF). In contrast, suppression and facilitation were equally common in response to ICMS. The synaptic mechanisms underlying these effects were explored in 5 anaesthetised macaque monkeys. ICMS facilitated a greater proportion of the tested muscles than did the CM cell recorded at the stimulus site. The results suggest the juxtaposition in the motor cortex of CM neurones with different 'muscle fields'. The merits of STA and ICMS for exploring cortical organisation are discussed.

Animals↗
PubMed1986

Corticospinal facilitation of hand muscles during voluntary movement in the conscious monkey.

1. The method of spike-triggered averaging has been used to detect a direct influence of pyramidal tract neurones on the activity of hand and forearm muscles in conscious monkeys trained to perform repetitive movements of the hand and fingers. Gross electromyograms (e.m.g.s) from individual muscles were rectified and synchronously averaged with respect to the discharge of single, antidromically identified pyramidal tract cells in the 'hand' area of the pre-central gyrus. 2. The presence in an average of a post-spike facilitation which could be revealed reproducibly from successive epochs of recording and was clearly larger than the biggest fluctuations seen in pseudo-randomly triggered averages of the same e.m.g. data, was taken to indicate a direct cortico-motoneuronal excitatory influence. 3. 55% of cortical neurones analysed showed post-spike facilitation in one or more recorded muscle and 7% showed post-spike suppression. In terms of the total number of muscle-neurone combinations analysed, the proportions showing post-spike effects were 18 and 1% respectively. These figures have been influenced by the pre-selection of neurones for analysis according to restrictive criteria. The neurones selected (a) were recorded at cortical loci where weak intracortical microstimulation could evoke finger movements, (b) could be activated antidromically at short latency by medullary pyramidal tract stimulation, (c) showed natural discharge activity which was clearly modulated in relation to voluntary finger movements, and (d) were located in the anterior bank of the central sulcus. The results provide some evidence to vindicate these criteria. 4. The strongest post-spike facilitation observed had a peak which was 42% higher than the average pre-spike level of e.m.g. activity, but most were within the range 5-20%. Facilitation peaks below about 3% could not have been resolved from the 'noise' in the averages. The mean latency from cell discharge in the cortex to the start of the post-spike facilitation was 11.2 ms (range 7.4-17.2) for intrinsic hand muscles and 9.8 ms (range 4.1-15.0) for forearm muscles. These latencies were compared with the latencies of responses to intracortical microstimulation and to stimulation of the medullary pyramidal tract. 5. Evidence was obtained suggesting that the latency for cortico-motoneuronal activation of an individual motor unit was commonly subject to considerable variability and that different motor units of a muscle could be facilitated by the one cortical neurone at different latencies. These factors are thought to contribute to an elongation of the time course of post-spike facilitation.(ABSTRACT TRUNCATED AT 400 WORDS)

Action Potentials↗
PubMed1986

Selective facilitation of different hand muscles by single corticospinal neurones in the conscious monkey.

1. Post-spike facilitation of e.m.g. activity by monkey motor cortex neurones has been investigated in different hand and forearm muscles. 2. Seventy-eight neurones were recorded concurrently with between five and ten different muscles. Forty-seven neurones were identified as cortico-motor by the presence of post-spike facilitation in the spike-triggered average of at least one of the tested muscles. 3. All forty-seven cortico-motor neurones showed clear increases in activity during performance of a precision grip task by the monkey, and all of them were co-activated with the sampled muscles. 4. To assess the divergence of facilitation from a single cortico-motor neurone to different muscles, spike-triggered averages were constructed with all of the concurrently recorded muscles. The number of muscles in the sample, and the number of muscles showing post-spike facilitation, were corrected by excluding any post-spike facilitation which could have arisen by cross-talk between the different pairs of e.m.g. electrodes. 5. Most cortico-motor neurones produced post-spike facilitation in a restricted number of tested muscles. The mean number of post-spike facilitation-bearing muscles per cortico-motor cell rose from 1.4 +/- 0.5 (S.D.) when five muscles were sampled to 2.0 +/- 1.5 when ten were sampled. On average, each cortico-motor neurone produced post-spike facilitation in 27% of the tested muscles. Only three of forty-seven cortico-motor neurones gave post-spike facilitation in half or more of the tested muscles. 6. The distribution pattern of post-spike facilitation among the muscles sampled with a given cortico-motor neurone was not altered when the spike-triggered averages were constructed from cortico-motor cell and e.m.g. activity recorded during two different phases of the precision grip task, or during performance of a quite different, power grip, task. 7. Cortico-motor cells which produced post-spike facilitation in two or more different muscles often did so in muscles with synergistic functions. 8. It is suggested that cortico-motor neurones may contribute to relatively independent finger movements by virtue of their selective facilitation of hand muscles leading to a fractionated pattern of muscle activity.

Action Potentials↗
PubMed1984

Cortical afferents and efferents of monkey postarcuate area: an anatomical and electrophysiological study.

A study has been made of the corticocortical efferent and afferent connections of the posterior bank of the arcuate sulcus in the macaque monkey. The distribution of efferent projections to the primary motor cortex (MI) was studied by injecting three different fluorescent retrograde tracers into separate regions of MI. The resultant labeling showed a discrete and topographically organized projection: neurons lying below the inferior limb of the arcuate sulcus project into the MI face area, while neurons located in the posterior bank of the inferior limb of the arcuate sulcus and in the arcuate spur region project into the MI hand area. These findings were confirmed electrophysiologically by demonstrating that postarcuate neurons could only be activated antidromically by stimulation within restricted regions of MI. HRP injections within postarcuate cortex indicated that afferents to this region arise from a number of cortical areas. However, the largest numbers of labeled neurons were found in the posterior parietal cortex (area 7b; PF) and in the secondary somatosensory region (SII). Neurons in both 7b (PF) and SII could be antidromically activated by postarcuate stimulation. It was further shown that stimulation of area 7b (PF) gives rise to short-latency synaptic responses in postarcuate neurons, including some neurons with identified projections to MI. The results are discussed in relation to the possible function of the postarcuate region of the premotor cortex in the sensory guidance of movement.

Animals↗
PubMed1983

Antidromic excitation of motoneurons by intramuscular electrical stimulation.

The terminal branches of motoneurons can be excited by electrical current pulses delivered via needles inserted intramuscularly. Observations are reported which indicate that tungsten semi-microelectrodes suitably positioned in a muscle belly can excite antidromically a large proportion of the alpha-motoneurons of that muscle, at stimulus strengths weak enough to avoid spread of excitation to motor units of adjacent muscles. Some gamma-motoneurons and afferent axons can also be excited by intramuscular stimulation. This technique of antidromic excitation will find application in electrophysiological studies on motoneurons of small muscles, such as intrinsic hand and foot musculature, where the individual muscle nerves are too fragile to be dissected and mounted on stimulating electrodes in the conventional manner.

Animals↗