Complete unilateral section of the pyramidal tract at the medullar level in Macaca mulatta

Differential impact of unilateral stroke on the bihemispheric motor cortex representation of the jaw and tongue muscles in young and aged rats

Introduction

Dysphagia commonly occurs after stroke, yet the mechanisms of post-stroke corticobulbar plasticity are not well understood. While cortical activity associated with swallowing actions is bihemispheric, prior research has suggested that plasticity of the intact cortex may drive recovery of swallowing after unilateral stroke. Age may be an important factor as it is an independent predictor of dysphagia after stroke and neuroplasticity may be reduced with age. Based on previous clinical studies, we hypothesized that cranial muscle activating volumes may be expanded in the intact hemisphere and would contribute to swallowing function. We also hypothesized that older age would be associated with limited map expansion and reduced function. As such, our goal was to determine the impact of stroke and age on corticobulbar plasticity by examining the jaw and tongue muscle activating volumes within the bilateral sensorimotor cortices.

Methods

Using the middle cerebral artery occlusion rat stroke model, intracortical microstimulation (ICMS) was used to map regions of sensorimotor cortex that activate tongue and jaw muscles in both hemispheres. Young adult (7 months) and aged (30 months) male F344 × BN rats underwent a stroke or sham-control surgery, followed by ICMS mapping 8 weeks later. Videofluoroscopy was used to assess oral-motor functions.

Results

Increased activating volume of the sensorimotor cortex within the intact hemisphere was found only for jaw muscles, whereas significant stroke-related differences in tongue activating cortical volume were limited to the infarcted hemisphere. These stroke-related differences were correlated with infarct size, such that larger infarcts were associated with increased jaw representation in the intact hemisphere and decreased tongue representation in the infarcted hemisphere. We found that both age and stroke were independently associated with swallowing differences, weight loss, and increased corticomotor thresholds. Laterality of tongue and jaw representations in the sham-control group revealed variability between individuals and between muscles within individuals.

Conclusion

Our findings suggest the role of the intact and infarcted hemispheres in the recovery of oral motor function may differ between the tongue and jaw muscles, which may have important implications for rehabilitation, especially hemisphere-specific neuromodulatory approaches. This study addressed the natural course of recovery after stroke; future work should expand to focus on rehabilitation.

A robust role for motor cortex

The role of motor cortex in non-primate mammals remains unclear. More than a century of stimulation, anatomical and electrophysiological studies has implicated neural activity in this region with all kinds of movement. However, following the removal of motor cortex, rats retain most of their adaptive behaviors, including previously learned skilled movements. Here we revisit these two conflicting views of motor cortex and present a new behavior assay, challenging animals to respond to unexpected situations while navigating a dynamic obstacle course. Surprisingly, rats with motor cortical lesions show clear impairments facing an unexpected collapse of the obstacles, while showing no impairment with repeated trials in many motor and cognitive metrics of performance. We propose a new role for motor cortex: extending the robustness of sub-cortical movement systems, specifically to unexpected situations demanding rapid motor responses adapted to environmental context. The implications of this idea for current and future research are discussed.

Behaviorally Selective Engagement of Short-Latency Effector Pathways by Motor Cortex

Blocking motor cortical output with lesions or pharmacological inactivation has identified movements that require motor cortex. Yet, when and how motor cortex influences muscle activity during movement execution remains unresolved. We addressed this ambiguity using measurement and perturbation of motor cortical activity together with electromyography in mice during two forelimb movements that differ in their requirement for cortical involvement. Rapid optogenetic silencing and electrical stimulation indicated that short-latency pathways linking motor cortex with spinal motor neurons are selectively activated during one behavior. Analysis of motor cortical activity revealed a dramatic change between behaviors in the coordination of firing patterns across neurons that could account for this differential influence. Thus, our results suggest that changes in motor cortical output patterns enable a behaviorally selective engagement of short-latency effector pathways. The model of motor cortical influence implied by our findings helps reconcile previous observations on the function of motor cortex.

Transient middle cerebral artery occlusion disrupts the forelimb movement representations of rat motor cortex

Infarcts from proximal middle cerebral artery (MCA) stroke can produce impairments in motor function, particularly finger movements in humans and digit flexion in rats. In rats, the extent of neural damage may be limited to basal ganglia structures or may also include portions of the frontal and parietal cortex in severe cases. Although the primary motor cortex (M1) is anatomically spared in proximal MCA occlusion, its functional integrity is suspect because even a small subcortical infarct can damage neural circuits linking M1 with basal ganglia, brainstem, and spinal cord. This motivated the present study to investigate the neurophysiological integrity of M1 after transient proximal MCA occlusion. Rats, preoperatively trained and non-preoperatively trained to reach for food, received extensive reach training/testing with the contralateral-to-lesion paw for several weeks after MCA occlusion. The forelimb movement representations were assayed from the ipsilateral-to-lesion M1 with intracortical microstimulation approximately 10 weeks after MCA occlusion. Digit flexion was impaired during food grasping in rats with relatively small subcortical infarcts and was completely abolished in rats that sustained at least moderate subcortical damage. Corresponding forelimb movement representations ranged from abnormally small to absent. The results suggest that ischemia in subcortical territories of the MCA does not spare the neurophysiological properties of M1 despite its apparent anatomical intactness, probably because of damage sustained to its descending fibers. Thus, M1 dysfunction contributes to the impairments that ensue from proximal MCA occlusion, even when the infarct is limited to subcortical regions.

Chapter 2 Comparative anatomy and physiology of the corticospinal system

The corticospinal tract provides the most direct pathway over which the cerebral cortex controls movement. In rodents and marsupials this influence is exerted largely upon interneurons in the dorsal horn of the spinal gray matter. However, ascending the phylogenetic scale through carnivores and primates, the number of corticospinal axons grows and corticospinal terminations shift progressively toward the interneurons of the intermediate zone and ventral horn, ultimately forming increasing numbers of synaptic terminations directly on the motoneurons themselves. Based on this phylogenetic trend, humans are believed to have more direct corticomotoneuronal synapses than any other species, consistent with observations that humans suffer more extensive loss of motility from lesions of the corticospinal tract than do other mammals. Beyond this phylogenetic trend, studies of the corticospinal system in animals have provided insight into the motor abnormalities that result from corticospinal lesions in humans. Corticospinal lesions impair many functionally related muscles and movements in parallel, both because of the divergent output from single corticomotoneuronal cells to multiple motoneuron pools, and because of the convergent input to different motoneuron pools from large, overlapping cortical territories. Furthermore, the weakness, slowness and inflexible, stereotyped movements that remain after corticospinal lesions reflect the loss of input to spinal interneurons and motoneurons from corticospinal neurons, the discharge frequency of which varies with the force, direction and speed of both gross and fine movements. That these deficits resulting from corticospinal lesions are more prominent in humans than in animals indicates, moreover, that animals make greater use of additional descending pathways to control movement. Animal studies have shown that although the bulk of the corticospinal tract arises from the primary motor cortex, this projection is not the only route via which the brain controls movement. Adjacent areas in the frontal and parietal lobes also contribute axons to the corticospinal tract, as well as having corticocortical connections with the motor cortex. Furthermore, the motor cortex and premotor cortex both project to the red nucleus and to the pontomedullary reticular formation, from which the rubrospinal and reticulospinal tracts arise. However, given the limitations on experimental studies in humans, comparative animal studies of the distributed descending system through which the brain controls movement continue to provide deeper understanding and insight into the deficits resulting from human corticospinal lesions, whether caused by stroke, tumor, multiple sclerosis, trauma or ALS.

A unilateral section of the corticospinal tract at cervical level in primate does not lead to measurable cell loss in motor cortex

The effects of a unilateral interruption of the dorsolateral funiculus at cervical level on the survival of neurons in the motor cortex were investigated in macaque monkeys. The lesion was made on the left side at the transition region between the 7(th) and 8(th) cervical segments, above the motoneurons controlling hand muscles. As a result, the homolateral hand became paretic, although an incomplete recovery of manual dexterity took place during 2 months post-lesion. A quantitative anatomical assessment of pyramidal neurons in layer V was performed in the hindlimb area of the primary motor cortex and in the supplementary motor area (SMA proper). The pyramidal neurons were visualized using the marker SMI-32 and thus included the subpopulation of corticospinal neurons. These quantitative data demonstrated that the vast majority of the axotomized corticospinal (CS) neurons did not degenerate. Rather, their somata shrank, compared to the opposite hemisphere or to intact monkeys. This conclusion is in contrast to some previous studies in monkeys that argued for a substantial degeneration of motor cortex neurons as a result of transection of the corticospinal tract; yet in agreement with others that concluded the survival of most CS neurons. The survival of the majority of CS axotomized neurons is also consistent with the observation of numerous CS axons 1 mm above the cervical hemisection.

Comparison of electrical thresholds for evoking movements from sensori-motor areas of the cat cerebral cortex and its relation to motor training

Motor maps and electrical thresholds for evoking movements from motor areas of the cerebral cortex were evaluated in normal cats by using intracortical microstimulation techniques. Stainless steel chambers were implanted over craniotomies in adult cats trained to perform reaching and retrieval movements with their forelimbs. Prehensile motor training was continued and movement performance monitored for about 6-10 weeks during which the cortex was progressively explored with sharp tungsten electrodes inserted into cortical gyri (anterior and posterior sigmoid, and coronal) and the banks of sulci (cruciate, presylvian and coronal). Twice weekly, under light general anaesthesia, 3-4 tracks were made in either hemisphere till about 50 tracks were made in each hemisphere. Mean thresholds for evoking forelimb movements from different cytoarchitectonic areas (4gamma, 4delta, 6agamma and 3a) were compared and no consistent or significant differences were observed between the different areas. In the animals (4/6) which used either forelimb to perform the tasks, there were no consistent differences in the mean thresholds for evoking forelimb movements from the two hemispheres. However, in 2 animals, which used their right forelimbs predominantly or exclusively to perform all the tasks, mean thresholds for evoking forelimb movements was significantly higher in areas 4gamma and 6agamma of the left hemisphere (compared to the right); no consistent differences in the mean thresholds for evoking hindlimb or facial movements were observed between the two hemispheres. These findings suggest that ICMS thresholds for evoking forelimb movements may be similar in different sensorimotor areas of the cat cerebral cortex, and these thresholds could be influenced by motor training.

Reduced muscle selectivity during individuated finger movements in humans after damage to the motor cortex or corticospinal tract

We investigated how damage to the motor cortex or corticospinal tract affects the selective activation of finger muscles in humans. We hypothesized that damage relatively restricted to the motor cortex or corticospinal tract would result in unselective muscle activations during an individuated finger movement task. People with pure motor hemiparesis attributed to ischemic cerebrovascular accident were tested. Pure motor hemiparetic and control subjects were studied making flexion/extension and then abduction/adduction finger movements. During the abduction/adduction movements, we recorded muscle activity from 3 intrinsic finger muscles: the abductor pollicis brevis, the first dorsal interosseus, and the abductor digit quinti. Each of these muscles acts as an agonist for only one of the abduction/adduction movements and might therefore be expected to be active in a highly selective manner. Motor cortex or corticospinal tract damage in people with pure motor hemiparesis reduced the selectivity of finger muscle activation during individuated abduction/adduction finger movements, resulting in reduced independence of these movements. Abduction/adduction movements showed a nonsignificant trend toward being less independent than flexion/extension movements in the affected hands of hemiparetic subjects. These changes in the selectivity of muscle activation and the consequent decrease in individuation of movement were correlated with decreased hand function. Our findings imply that, in humans, spared cerebral motor areas and descending pathways that remain might activate finger muscles, but cannot fully compensate for the highly selective control provided by the primary motor cortex and the crossed corticospinal system.

Differential impairment of individuated finger movements in humans after damage to the motor cortex or the corticospinal tract

The purpose of this study was to quantify the long-term loss of independent finger movements in humans with lesions relatively restricted to motor cortex or corticospinal tract. We questioned whether damage to the motor cortex or corticospinal tract would permanently affect the ability to move each finger to the same degree or would affect some fingers more than others. People with pure motor hemiparesis due to ischemic cerebrovascular accident were used as our experimental sample. Pure motor hemiparetic and control subjects were tested for their ability to make cyclic flexion/extension movements of each finger independently. We recorded their finger joint motion using an instrumented glove. The fingers of control subjects and of the unaffected hands (ipsilateral to the lesion) of hemiparetic subjects moved relatively independently. The fingers of the affected hands (contralateral to the lesion) of hemiparetic subjects were differentially impaired in their ability to make independent finger movements. The independence of the thumb was normal; the independence of the index finger was slightly impaired, while the independence of the middle, ring, and little fingers was substantially impaired. The differential long-term effects of motor cortical or corticospinal damage on finger independence may result from rehabilitative training emphasizing tasks requiring independent thumb and index movements, and from a greater ability of the spared components of the neuromuscular system to control the thumb independently compared with the other four fingers.

Partial inactivation of the primary motor cortex hand area: effects on individuated finger movements

After large lesions of the primary motor cortex (M1), voluntary movements of affected body parts are weak and slow. In addition, the relative independence of moving one body part without others is lost; attempts at individuated movements of a given body part are accompanied by excessive, unintended motion of contiguous body parts. The effects of partial inactivation of the M1 hand area are comparatively unknown, however. If the M1 hand area contains the somatotopically ordered finger representations implied by the classic homunculus or simiusculus, then partial inactivation might produce weakness, slowness, and loss of independence of one or two adjacent digits without affecting other digits. But if control of each finger movement is distributed in the M1 hand area as many studies suggest, then partial inactivation might produce dissociation of weakness, slowness, and relative independence of movement, and which fingers movements are impaired might be unrelated to the location of the inactivation along the central sulcus. To investigate the motoric deficits resulting from partial inactivation of the M1 hand area, we therefore made single intracortical injections of muscimol as trained monkeys performed visually cued, individuated flexion-extension movements of the fingers and wrist. We found little if any evidence that which finger movements were impaired after each injection was related to the injection location along the central sulcus. Unimpaired fingers could be flanked on both sides by impaired fingers, and the flexion movements of a given finger could be unaffected even though the extension movements were impaired, or vice versa. Partial inactivation also could produce dissociated weakness and slowness versus loss of independence in a given finger movement. These findings suggest that control of each individuated finger movement is distributed widely in the M1 hand area.

Object size as a determinant of grasping in infancy

The early development and patterns of development of prehensile ability were examined. Infants 5, 7, and 9 months old were presented five objects, 0.5, 1.0, 3.5, 7.0, and 14.0 cm in diameter. The findings revealed that infants as young as 5 months old were able to differentiate grip configurations as a function of object size. The number of grasps involving the two or three most radial digits (thumb, index finger, and long finger) increased greatly over this age span. At 9 months of age these kinds of grasps were 10 times more frequent than at 5 months of age. However, at each age level, when only the two or three most radial digits were used, the reaches were typically directed at the two smallest objects. These findings suggest that it was not a perceptual problem that the younger infants were facing, nor was the problem knowing when to use different kinds of grasps; rather, the problem was one of cortico-motoneural connections, which are better established in older infants. The findings also suggest that traditionally described sequential development of infants' prehension is rigid and conservative. The discrepancy with earlier results may also be attributed to the difference in the objects' sizes and the way they were presented.

Evoked potentials from direct cerebellar stimulation for monitoring of the rodent spinal cord

Although the assessment of spinal cord function by electrophysiological techniques has become important in both clinical and research environments, current monitoring methods do not completely evaluate all tracts in the spinal cord. Somatosensory and motor evoked potentials primarily reflect dorsal column and pyramidal tract integrity, respectively, but do not directly assess the status of the ventral funiculus. The present study was undertaken to evaluate the use of evoked potentials, elicited by direct cerebellar stimulation, in monitoring the ventral component of the rodent spinal cord. Twenty-nine rats underwent epidural anodal stimulation directly over the cerebellar cortex, with recording of evoked responses from the lower thoracic spinal cord, both sciatic nerves, and/or both gastrocnemius muscles. Stimulation parameters were varied to establish normative characteristics. The pathways conducting these "posterior fossa evoked potentials" were determined after creation of various lesions of the cervical spinal cord. The evoked potential recorded from the thoracic spinal cord consisted of five positive (P1 to P5) and five negative (N1 to N5) peaks. The average conduction velocity (+/- standard deviation) of the earliest wave (P1) was 53 +/- 4 m/sec, with a latency of 1.24 +/- 0.10 msec. The other components followed within 4 msec from stimulus onset. Unilateral cerebellar stimulation resulted in bilateral sciatic nerve and gastrocnemius muscle responses; there were no significant differences (p greater than 0.05) in the thresholds, amplitudes, or latencies of these responses elicited by right- versus left-sided stimulation. Recordings performed following creation of selective lesions of the cervical cord indicated that the thoracic response was carried primarily in the ventral funiculus while the sciatic and gastrocnemius responses were mediated through the dorsal half of the spinal cord. It is concluded that the posterior fossa evoked potential has research value as a method of monitoring pathways within the ventral spinal cord of the rat, and should be useful in the study of spinal cord injury.

Motor responses after transcranial electrical stimulation of cerebral hemispheres with a degenerated pyramidal tract

Motor responses were evoked in the thenar muscles by transcranial electrical cortex stimulation in 5 stroke patients with an isolated lacuna in the internal capsule, in whom wallerian degeneration of the pyramidal tract was demonstrated in vivo. Suprathreshold stimulation of the affected hemisphere elicited bilateral motor responses; whereas, stimulation at identical intensities of the undamaged hemisphere yielded strictly unilateral responses in the contralateral hand, like the responses of all normal control subjects. Focused magnetic brain stimulation was performed in 1 patient and gave identical results. Because muscular excitability to cortical stimulation is preserved in spite of pyramidal tract disruption, other pathways must bypass the lesion. Because of the bilaterality of responses, we suggest polysynaptic corticoreticulospinal connections.

Computed tomographic studies of the basis pedunculi in chronic hemiplegic patients: topographic correlation between cerebral lesion and midbrain shrinkage

A computed tomographic method for analyzing the shrinkage of the basis pedunculi (BP) due to the secondary degeneration of the descending fibers was applied in correlation to the site of cerebral lesions in 89 chronic hemiplegic patients. Cerebral lesions in the anterior corona radiata or the anterior limb of the capsula interna caused shrinkage of the medial BP. Lesions in the central corona radiata or the genu and posterior limb of the capsula interna caused shrinkage of the central BP, while lesions of the posterior corona radiata or the posterior limb of the capsula interna caused shrinkage of the lateral BP. These results suggested that CT images are able to reveal the principle sites of atrophy of the descending fiber tracts in chronic hemiplegia.

Pyramidotomy abolishes the abnormal movements evoked by intracortical microstimulation in adult rats that sustained neonatal cortical lesions

Unilateral sensorimotor cortical lesions in newborn rats result in the development of an anomalous ipsilateral corticospinal tract originating from the opposite unablated hemisphere. Intracortical microstimulation of the intact hemisphere in adult rats that sustained such lesions demonstrated a reduction in current threshold levels needed to evoke ipsilateral forelimb movements. Disruption of the low-threshold ipsilateral movements by medullary pyramidotomy as observed in the present study suggests that these movements were mediated by the anomalous ipsilateral corticospinal tract fibers which traverse the medullary pyramid.

Forelimb motor cortical projections in normal rats and after neonatal hemicerebellectomy: an anatomical study based upon the axonal transport of WGA/HRP

Cerebral cortical projections from the forelimb motor cortex, as defined by intracortical microstimulation where movements were evoked at low current intensities (less than 15 microA), were examined in normal rats and in adult rats that sustained neonatal hemicerebellectomy. The distribution pattern of cortical efferent projections in normal rats generally appeared more restricted than previously described. This restricted distribution is attributed to the use of WGA/HRP as the axonal tracing method and to the electrophysiological definition of the injection site as the motor cortex. The observed remodeling of the corticobulbar projections, seen after cerebellar lesions in the young, largely confirmed previous reports. Moreover, no alterations in the laterality of distribution in corticospinal projection were found. Aberrant corticospinal projections were sought in an effort to provide an anatomical basis to a previous description of abnormally low-threshold ipsilateral forelimb responses evoked from the motor cortex in adult rats after neonatal cerebellar lesions. This apparent absence of corticospinal tract remodeling after neonatal hemicerebellectomy suggests that the abnormal responses are mediated by the normal corticospinal pathways. This possibility is discussed in terms of an alteration in the spinal circuits that may change the responsiveness of spinal motoneurons to a given pyramidal discharge.

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)

A study of forelimb movements evoked by intracortical microstimulation after hemicerebellectomy in newborn, young and adult rats

Microstimulation of the cerebral motor cortex in normal adult rats evokes low-threshold contralateral and high-threshold ipsilateral forelimb movements. The present study examined the effects of hemicerebellar ablation at different postnatal ages on the current threshold values needed to evoke forelimb movements by intracortical stimulation. Animals that received hemicerebellar lesions at various ages were electrophysiologically tested 4-6 months postoperatively. In all groups, including non-lesion control animals, forelimb movements contralateral to the stimulating electrode were evoked at threshold values of 7-11 microA. Ipsilateral forelimb movements for control animals as well as those receiving cerebellar lesions at 45 or 120 days of age showed significantly higher mean threshold current values, ranging from 38 to 45 microA. In contrast, the mean threshold current values for ipsilateral forelimb movements in adult animals sustaining hemicerebellar lesions at 2, 10 or 21 days of age were significantly lowered, ranging from 16 to 22 microA. Secondary lesions of spared cerebellar tissue or callosal fibers in adult animals that had sustained hemicerebellectomy at two days of age had no effect on the current intensities needed to evoke forelimb responses. In comparison, lesions of the cerebral cortex contralateral to the stimulated cortex increased the threshold for ipsilateral movements and medullary pyramidal lesions ipsilateral to the stimulated cortex both reduced the number of responses and increased the threshold current intensities needed to evoke them. These data indicate that hemicerebellectomy within 3 weeks of age can induce electrophysiological alterations in the responses mediated by the corticospinal tract. These results support previous suggestions of the cerebral cortical involvement in compensation for neonatal cerebellar lesions.

Intracortical stimulation in pyramidotomized monkeys

In order to examine, separately, the organizations of pyramidal and extrapyramidal projections from the primary motor cortex, efforts were made to map the forelimb area of two rhesus monkeys with microstimulation before and after unilateral pyramidotomy. However, microstimulation was not effective in evoking motor responses following complete pyramidal tract section. Movements were evoked using a modified intracortical electrode with a large exposed tip and using stimulation parameters similar to those used for surface stimulation. The results from this modified intracortical stimulation generally agree with those from surface stimulation studies in that: (1) the extrapyramidal topography is similar to the normal motor cortex topography and (2) while peripheral responses can be evoked from the cortex following pyramidotomy, greater spatial and temporal summation are necessary to evoke these responses. In addition, the modified intracortical technique revealed a more widespread post-pyramidotomy digit representation than observed previously with surface stimulation. Results from an incomplete pyramidal tract lesion suggest that recovery of motor function may include plastic changes in surviving corticospinal axons.

The formation of finger grip during prehension. A cortically mediated visuomotor pattern

The pattern of finger grip formation during natural prehension movements was described in normal subjects with the help of a quantified film technique. Movements were studied in one condition with visual feedback from the moving hand available, and one condition without visual feedback. The studied parameters, including the maximum size of the anticipatory grip and the final size of the grip before contact with the object, were not affected by shifting from one condition of visual feedback to the other. The same technique was applied to a group of patients with cerebral lesions. In two patients with unilateral lesions involving the motor cortex, grip formation with the hand contralateral to the lesion, was found to be severely affected, in that fingers and particularly the index finger, remained stretched until contact with the object was made. In two patients with unilateral lesions in the posterior parietal cortex, grip formation of the contralateral hand was absent specifically in the no-visual feedback condition. The same result was obtained in two other patients with a lesion (subcortical in one case, cortical in the other) of somatosensory pathways corresponding to one hand. These results are interpreted as evidence for the role of cerebral cortex in the control of finger grip formation during prehension of visual objects. Integration at cortical level of visual and somatosensory cues from the involved hand is a necessary condition for grip formation to be adapted to the grasp.

The corticomotoneuronal component of the pyramidal tract: corticomotoneuronal connections and functions in primates

Corticomotoneuronal fibers make up a functional component of the pyramidal tract-corticospinal system which is characteristic of primates. The corticomotoneuronal fibers include large, rapidly conducting axons. They arise from somatotopically arranged areas of precentral cortex and the largest concentration of pyramidal cells of origin in the deep part of lamina V is in area 4. Their influence is exerted contralaterally on the spinal cord, where monosynaptic excitation of spinal motoneurons occurs. Motoneurons innervating distally acting muscles are preferentially excited and marked convergence of corticomotoneuronal influences occurs on these. The excitatory post-synaptic potentials in these motoneurons are characterized by the property of temporal facilitation. Intraspinal divergence of the terminal arborizations of individual corticomotoneuronal fibers could permit the engagement of large populations of motoneurons and also the activation of excitatory and inhibitory interneurons and propriospinal neurons for that region of the spinal cord. Corticomotoneuronal synapses may be located more distally on the dendrites of motoneurons than are the monosynaptic connections from group Ia afferents. The corticomotoneuronal excitation has been demonstrated to be effective in natural functional states when the conscious animal is performing learned movement tasks. Abolition of corticomotoneuronal influences causes a permanent deficit in the fractionation of use of distal muscles and an inability to carry out independent movements of the fingers.

Sizes, laminar and topographic origins of cortical projections to the major divisions of the red nucleus in the monkey

The retrograde transport of horseradish peroxidase was used to study the topographic and laminar origins of the cortical projections to the parvocellular and the magnocellular divisions of the red nucleus in Macaca mulatta and Macaca fascicularis. Approximately 90% of the corticorubral projection is directed to the parvocellular division of the nucleus. Corticoparvocellular (CRp) neurons are pyramidally shaped, are smaller in size than corticospinal neurons, and are more numerous. They are found principally in sublamina Va of cytoarchitectonic areas 4 and 6, and in moderate quantities in sublamina Vb of posterior area 8 and area 5. In areas 4 and 6, the cells are grouped in clusters of three to 15 neurons each and are arranged in cellular bands of varying rostrocaudal thickness which course mediolaterally. With respect to functionally defined zones, CRp neurons are found throughout the supplementary motor area and the precentral motor cortex. In addition, they are found in parts of areas 5, 6, and 24 that project to these cortical motor areas, and that are thought to have "premotor" or movement-programming functions. The corticomagnocellular (CRm) projection arises principally from cells in sublamina Vb of the precentral arm and leg areas (area 4), and from adjacent parts of posterior area 6, CRm cells are pyramidally shaped, and their size distribution is bimodal, with peaks that correspond, respectively, to the modal diameters of CRp and of corticospinal neurons. These results and those of previous studies suggest that CRm neurons are involved principally in the control of hand and foot movements, with little effect on more proximal musculature. The massive CRp projection, however, is clearly part of a large cerebrocerebellar communication system, with motor and/or movement programming functions that have yet to be clearly defined.

A behavioral analysis of complete unilateral section of the pyramidal tract at the medullary level in Macaca mulatta

Ten Macaca mulatta monkeys were operantly conditioned to perform three motor paradigms designed to evaluate single and combination finger movements. Eight of these monkeys were retested after left medullary pyramidotomy; 2 monkeys underwent left medullary pyramidotomy prior to conditioning. All animals were tested for three years after operation. Monkeys with a completely sectioned medullary pyramid could, with time, perform difficult motor paradigms that required: (1) both individual and combination finger movements; (2) proximal upper extremity motor control; (3) thumb and index finger pincer grasp; and (4) the ability to preprogram and then execute a precision hand movement. The greater the extent of pyramidal tract destruction, the longer the time necessary for recovery of both discrete finger movement and pincer grasp, the greater the effort needed to attain recovery of hand function, and the weaker the affected musculature. The 2 animals in which pyramidotomy of at least 70% of the tract preceded efforts at operant conditioning learned and performed difficult motor paradigms. In all animals, neurological examination revealed that the most enduring and functionally most important deficit that interferes with hand function following pyramidotomy is loss of contactual hand orienting responses and failure of reflex sensorimotor adjustments.

Influence of pyramidotomy on limb flexion movements induced by cortical stimulation and on associated postural adjustment in the cat

Flexion movements induced by cortical stimulation and the associated postural adjustments in bilaterally pyramidotomised cats have been studied by means of an apparatus which measures separately the changes of force under each limb in the upright position of the animal. The results show: (1) The general motor behaviour of the animal is not affected by the lesion. The principal deficit is loss of tactile placing reaction in the forelimbs; there also appears a state akin to a vestibular syndrome when a light restraining harness is placed around the back and chest. (2) Stimulation of the motor cortex continues to evoke flexion movements of the contralateral limbs together with associated postural adjustment. Coordination between movement and postural adjustment is generally similar to that observed before pyramidal section, and response thresholds are unchanged. (3) Measurements reveal great quantitative modification both of the movement and the postural adjustment after the lesion. Weight shift latencies are increased and more dispersed in time, while the weight shifts themselves are reduced in amplitude and speed. (4) All these changes are more marked in the case of forelimb, rather than hindlimb, flexion, emphasising the greater degree of pyramidal influence on forelimb activity in the normal animal. (5) The results as a whole underline the ability of non-pyramidal systems, under the control of the motor cortex, to bring about both limb flexion and the associated postural adjustments in the absence of the corticospinal pathway.

Exaggeration of knee-jerk following spinal hemisection in monkeys

In 18 monkeys ipsilateral flaccid hemiplegia of the hind-limb was produced by spinal cord hemisection at T8. As a result of systematic observations of various phenomena subsequent to the hemisection for several months, it was found that an ipsilateral marked exaggeration of the knee-jerk gradually developed in 2--3 weeks after the infliction, along with slight atrophy of the hind-limb muscles on the same side. The exaggeration reached a maximum in about 6 weeks and persisted thereafter for several months until the final experiments. The exaggeration of knee-jerk was confirmed by measuring quantitatively photographed trajectories of a small lamp attached to the malleolus, a specially devised hammer being used. The trajectories showed larger amplitudes, shorter rise times and lower thresholds on the hemisected side. Achilles tendon reflex was also observed to be hyperactive. However, there was no evidence of other pathological phenomena such as clonus or Babinski's sign in the present experiment. It was noted that total spinal cord transection one segment below the hemisected site did not abolish the exaggeration of knee-jerk. Another noticeable finding was that quadriceps afferent volleys picked up from the L6 dorsal root following tendon taps were smaller on the hemisected side, in spite of the fact that distinct augmented reflex potentials were observed in the ipsilateral L6 ventral root. These results strongly suggest that the exaggeration of knee-jerk was not induced by a release from tonic supraspinal inhibition nor by augmented quadriceps afferent volleys, but by some neural mechanisms which developed gradually within the lumbo-sacral segments below the hemisected site.