Forelimb Area Research Articles

Induction of c- fos expression in cervical spinal interneurons after kainate stimulation of the motor cortex in the rat

The expression of the immediate-early gene c- fos was used as a marker of neuronal activity to investigate the cervical spinal interneuron populations involved in the corticomotoneuronal pathway. Adult rats received unilateral kainate injections in the forelimb area of the primary motor cortex. After a survival period of 90 min, during which the animals showed vehement twitching of the contralateral forelimb, the rats were perfused and their brains and cervical spinal cords processed for Fos-like immunoreactivity. In the cervical spinal cord Fos-like immunoreactive neurons were found bilaterally in the dorsal horn and in the intermediate zone, though contralaterally significantly more labelled nuclei were encountered in two different areas. One area closely resembles the corticospinal terminal field as demonstrated with anterograde horseradish-peroxidase tract-tracing and the other reflecting primary afferent and noxious sensory neurons in the dorsal horn. Thus by monitoring the evoked expression of the immediate-early gene c- fos, structural components of the rat motor system can be identified.

Induction of c-fos expression in cervical spinal interneurons after kainate stimulation of the motor cortex in the rat

The expression of the immediate-early gene c-fos was used as a marker of neuronal activity to investigate the cervical spinal interneuron populations involved in the corticomotoneuronal pathway. Adult rats received unilateral kainate injections in the forelimb area of the primary motor cortex. After a survival period of 90 min, during which the animals showed vehement twitching of the contralateral forelimb, the rats were perfused and their brains and cervical spinal cords processed for Fos-like immunoreactivity. In the cervical spinal cord Fos-like immunoreactive neurons were found bilaterally in the dorsal horn and in the intermediate zone, though contralaterally significantly more labelled nuclei were encountered in two different areas. One area closely resembles the corticospinal terminal field as demonstrated with anterograde horseradish-peroxidase tract-tracing and the other reflecting primary afferent and noxious sensory neurons in the dorsal horn. Thus by monitoring the evoked expression of the immediate-early gene c-fos, structural components of the rat motor system can be identified.

Metabolic activity pattern in the motor and somatosensory cortex of monkeys performing a visually guided reaching task with one forelimb.

The [14C]deoxyglucose method was used to map the metabolic activity in the primary somatosensory and motor cortex in monkeys (Macaca nemestrina) performing a unimanual task. The task required visually guided reaching and target holding at a rate of about 10 movements per min. The entire dorsoventral extent of the cortical region lying between the posterior crown of the arcuate and the anterior crown of the intraparietal sulci was reconstructed on the sagittal plane, from horizontal sections aligned on the fundus of the central sulcus. The metabolic mapping of the control monkey demonstrated homogeneous activity all around the central sulcus, bilaterally. The mapped activity in the performing monkeys displayed two different patterns. The first pattern, contralateral to the moving forelimb, was characterized by several discrete regions of increased metabolic activity, which were symmetrically distributed in a mirror image fashion around the fundus of the central sulcus. These activated regions correspond to the lower body, forelimb, and mouth areas of representation of body parts in previously reported maps in primary motor and somatosensory cortical areas. The second activity pattern ipsilateral to the moving forelimb, displayed activated somatosensory and motor regions corresponding only to the lower body, and mouth representations. Our study provides a continuous, high resolution map of activity pattern in the entire primary motor and somatosensory cortices, which demonstrates that the reaching forelimb is controlled by a discrete subregion in the contralateral somatosensorimotor cortex, whereas other subregions of body representation are actively involved, bilaterally, during the performance of a relatively simple motor behaviour.

Varieties of paw and digit movement during spontaneous food handling in rats: Postures, bimanual coordination, preferences, and the effect of forelimb cortex lesions

This study describes how rats use their paws and digits when handling a wide range of foodstuffs, including food pellets, grapes, sunflower seeds, shelled and unshelled peanuts, and different sized pastas, etc. Analysis of videorecordings show that the rats display digit postures that include variations in the spacing of the digits, differences in the relative use of different digits, and interlimb differences in paw and digit posture. The rats also display limb preferences in that one paw is used in a supporting function while the other rotates, flips, or pushes the food as is required by the shape of the item. There is a significant correlation between the paw used for manipulation and food items of similar shape but no correlation between the limb used for manipulation and that used for skilled reaching. Small unilateral lesions to the forepaw area of somatic sensorimotor cortex produced impairments in use of the paw contralateral to the lesions. These results: (1) reveal a surprising complexity in the way in which rats use their paws and digits in manipulating food; (2) show that rats have limb preferences in spontaneous food handling; and (3) show that manipulatory dexterity is dependent upon the integrity of the forelimb area of motor cortex. The results are discussed in relation to the evolution of motor skill, the use of rats for investigating questions of motor system organization, neural plasticity, and recovery of function after brain damage.

Morphology of identified corticospinal cells in the rat following motor cortex injury: absence of use-dependent change

After unilateral injury to the forelimb area of the motor cortex, rats are reported to show relatively increased use of the ipsilateral forelimb during exploratory behavior and a concomitant transient increase in dendritic growth in neurons in the undamaged motor cortex. To identify the specific population of cells in motor cortex that undergo such use-dependent change, we examined the morphology of corticospinal pyramidal cells in the intact hemisphere 18 days following unilateral motor cortex damage. Corticospinal neurons in the motor cortex were retrogradely labeled with injections of the fluorescent tracer, DiO, into the cervical enlargement of the spinal cord. Seven days later, the rats received a lesion in the forelimb area of the contralateral motor cortex and 18 days following the lesion, limb use was assessed in two behavioral tests after which the rats were sacrificed. Under fluorescent light, corticospinal cells were visualized and injected with a horseradish peroxidase-fluorescenn conjugate that was then reacted with diaminobenzidine. The labeled cells were reconstructed and the number and centrifugal order of the branches were analyzed. The increased use of the paw contralateral to the intact motor cortex was not associated with an increase in dendritic arborization in corticospinal motor neurons in the intact motor cortex. The results are discussed in light of methodological and theoretical considerations relevant to the study of neural plasticity in the motor system.

Differential spinal projections from the forelimb areas of the rostral and caudal subregions of primary motor cortex in the cat.

We used anterograde transport of WGA-HRP to examine the topography of corticospinal projections from the forelimb areas within the rostral and caudal motor cortex subregions in the cat. We compared the pattern of these projections with those from the somatic sensory cortex. The principal finding of this study was that the laminar distribution of projections to the contralateral gray matter from the two motor cortex subregions was different. The rostral motor cortex projected preferentially to laminae VI-VIII, whereas caudal motor cortex projected primarily to laminae IV-VI. Confirming earlier findings, somatic sensory cortex projected predominantly to laminae I-VI inclusive. We found that only rostral motor cortex projected to territories in the rostral cervical cord containing propriospinal neurons of cervical spinal segments C3-4 and, in the cervical enlargement, to portions presumed to contain Ia inhibitory interneurons. We generated contour maps of labeling probability on averaged segmental distributions of anterograde labeling for all analyzed sections using the same algorithm. For rostral motor cortex, heaviest label in the dorsal part of lamina VII in the contralateral cord was consistently located in separate medial and lateral zones. In contrast, no consistent differences in the mediolateral location of label was noted for caudal motor cortex. To summarize, laminae I-III received input only from the somatic sensory cortex, while laminae IV-V received input from both somatic sensory and caudal motor cortex. Lamina VI received input from all cortical fields examined. Laminae VII-IX received input selectively from the rostral motor cortex. For motor cortex, our findings suggest that projections from the two subregions comprise separate descending pathways that could play distinct functional roles in movement control and sensorimotor integration.

Reversible focal ischemia model in the rat: correlation of somatosensory evoked potentials and cortical neuronal injury.

Somatosensory evoked potentials (SEPs) are valuable in experimental stroke studies, but only a few reports have dealt with small rodent models. Our experiments aimed to reproduce SEP monitoring during reversible middle cerebral artery occlusion (MCAo) in a surgical model of open craniotomy and vessel manipulations in the rat. The changes of median nerve SEPs were correlated to the degree of local cortical neuronal injury in the forelimb area. Based on a pilot group (n = 11) both parameters were examined at 2 h after reperfusion in a subsequent study with increase of MCAo time from 1 (n = 7) to 2 h (n = 7); 5 rats were sham operated. A significantly delayed and incomplete recovery of SEPs, expressed as a percentage of the mean preocclusion values was observed with 2 h compared to 1 h MCAo (20.8% versus 6.0% change in the latency and 59.5% versus 2.2% in the amplitude of the primary cortical response). The functional outcome was found to correlate consistently with the degree of neuronal damage in 1 and 2 h reversible MCAo (2.5% versus 79.2% severely damaged neuronal types III and IV). Our findings suggest, that 2 h MCAo followed by 2 h reperfusion produces a submaximal neuronal injury and partial electrical recovery in the periphery of the occluded vascular territory and could be therefore used as a reliable model for assessment of cerebroprotective drug efficacy.

Unilateral lesions of the forelimb area of rat motor cortex: lack of evidence for use-dependent neural growth in the undamaged hemisphere

Unilateral lesions of the forelimb area of the motor cortex have been reported to produce enhanced dendritic outgrowth in the undamaged hemisphere in response to the behavioral asymmetry produced by the lesions (e.g. Jones, T.A. and Schallert, T., Use-dependent growth of pyramidal neurons after neocortical damage, J. Neurosci., 14 (1994) 2140–2152). We attempted to replicate this result and to determine if there were sex differences in cortical plasticity using the Jones and Schallert model. Animals were given either unilateral aspiration or electrolytic lesions of the forelimb area of the motor cortex or a sham operation. Use of the forelimb ipsilateral to the lesion for postural support was assessed pre- and postsurgery. Eighteen days after surgery the animals were sacrificed and the brains processed for Golgi-Cox staining or a series of other stains for acetylcholine, astrocytes (glial fibrillary acidic protein), and microglia (OX-42). Although the lesions produced significant behavioral asymmetry and enhanced glial response on the lesioned side, there was little evidence for use-dependent neural growth in the undamaged hemisphere in either sex.

Hunger strikes

<h3>SUMMARY</h3> Planning and execution of voluntary movement depend on the contribution of distinct classes of neurons in primary motor and premotor areas. However, the specific functional role of GABAergic cells remains only partly understood. Here, electrophysiological and computational analyses are employed to compare directly the response properties of putative pyramidal (PNs) and fast-spiking, GABAergic neurons (FSNs) during licking and forelimb retraction in mice. Recordings from anterolateral motor cortex and rostral forelimb area, reveal that FSNs fire earlier and for a longer duration than PNs, with the exception of a subset of early-modulated PNs in deep layers. Computational analysis reveals that FSNs carry vastly more information than PNs about the onset of movement. While PNs differently modulate their discharge during distinct motor acts, most FSNs respond with a stereotyped increase in firing rate. Accordingly, the informational redundancy was greater among FSNs than PNs. These data suggest that a global rise of inhibition contributes to early action planning.

Functional recovery of forelimb response capacity after forelimb primary motor cortex damage in the rat is due to the reorganization of adjacent areas of cortex

Functional recovery after brain damage has been described frequently and different mechanisms have been proposed to account for the observed recovery. One possible mechanism involves the capacity of one part of the brain to take over the function of another. A possible area for this to take place is in the cerebral cortex, where a variety of reorganizational processes have been described after different manipulations. We show in the present study that the forelimb force and response capacity of the rat, which becomes highly impaired after the bilateral ablation of the forelimb primary motor cortex, is recovered when the animals receive an electrical stimulation in the ventral tegmental nucleus contingent to each forelimb response in the task. Microstimulation mapping of the cortical areas adjacent to the forelimb primary motor cortex revealed the appearance of an area located caudolaterally to the forelimb primary motor cortex, where forelimb movements could be evoked in recovered animals but to a lesser extent in non-recovered animals. A positive and significant correlation was observed between the size of the reorganized forelimb area and the behavioral performance of the animals. Ablation of the forelimb reorganized area in recovered animals reinstated the forelimb behavioral impairment, while the same lesion in normal animals had no effect on the behavioral performance. The results indicate that recovery after bilateral forelimb primary motor cortex ablation may be due to the organization of specific adjacent areas in the cortex.

Age related change in the vasoactive intestinal polypeptide-immunoreactive neurons in the cerebral cortex of aged rats

Recent studies have explored certain changes in neurons containing neuropeptides with aging. However, until now, the degree of vasoactive intestinal polypeptide (VIP)-containing neuronal cell loss in the aged CNS has not yet been established with certainty, and available data are often contradictory. Changes in the VIP-containing neurons with aging were demonstrated by immunocytochemistry. Major loss of VIP-immunoreactive neurons in the aged rat brain were observed in frontal cortex area 3, parietal cortex area 1, hindlimb area, temporal cortex area 1 and 2, monocular part of occipital cortex area 1, occipital cortex area 2, and retrosplenial cortex. Frontal cortex area 1 and 2, parietal cortex area 2, forelimb area, binocular part of the occipital cortex area 1, and the dentate gyrus were moderately decreased. The axis of the VIP neurons in the aged group showed an irregular orientation tendency, especially in layers II and III. These results indicate the involvement of a neuronal system containing VIP in the aging brain, and provide the first morphological evidence for the loss of VIP neurons in the cerebral cortex of the aged rat.

Spatial distribution of thalamic projections to the supplementary motor area and the primary motor cortex: A retrograde multiple labeling study in the macaque monkey

The exact knowledge on spatial organization of information sources from the thalamus to the supplementary motor area (SMA) and to the primary motor cortex (MI) has not been established. We investigated the distribution of thalamocortical neurons projecting to forelimb representations of the SMA and the MI using a multiple retrograde labeling technique in the monkey. The forelimb area of the SMA, and the distal and proximal forelimb areas of the MI were identified by electrophysiological techniques of intracortical microstimulation and single neuron recording. Injections were made into these three representations with three different dyes in the same animal (horseradish peroxidase conjugated to wheat germ agglutinin, diamidino yellow, and fast blue), and the thalamic neurons were retrogradely labeled. Injections into the SMA densely labeled thalamic neurons in nuclei ventralis lateralis pars oralis (VLo), ventralis lateralis pars medialis (VLm) and ventralis lateralis pars caudalis (VLc), but not in nucleus ventralis posterior lateralis pars oralis (VPLo). Injections into the MI labeled thalamic neurons primarily in VLo, VLc, and VPLo. We found that the distribution of projection neurons to the three areas was largely separate in the thalamus. However, in the middle part of VLo, and in a limited portion of VLc, thalamic neurons projecting to the SMA partially overlapped with those to the distal forelimb area of the MI. They overlapped little with those to the proximal forelimb area of the MI. We noted no overlap between the distributions of thalamic projection neurons to the distal and proximal forelimb areas of the MI. These findings suggest that the SMA and MI receive separate information from the thalamus, while sharing minor sources of common inputs.

Thalamic distribution of projection neurons to the primary motor cortex relative to afferent terminal fields from the globus pallidus in the macaque monkey.

To examine quantitatively the pathway from the internal segment of the globus pallidus to the primary motor cortex through the thalamus, we compared the distribution of thalamocortical neurons projecting to the motor cortex with the distribution of afferent terminal fields from the pallidum in the ventrolateral nuclear group of the thalamus in four Japanese monkeys by using the anterograde and retrograde double-labeling method. In each monkey, different fluorescent retrograde tracers (Fast Blue and Diamidino Yellow) were injected separately into the distal and proximal forelimb areas of the primary motor cortex after physiological mapping with intracortical microstimulation. In the same individual monkeys, an anterograde tracer, wheatgerm agglutinin conjugated to horseradish peroxidase, was injected into the internal segment of the globus pallidus after the forelimb part was identified physiologically. A small group of projection neurons to the distal and proximal representations of the motor cortex were found in the terminal fields from the pallidum, but a majority of the projection neurons were distributed outside the terminal area in the thalamus. These results confirm the existence of the pathway from the pallidum through the thalamus to the primary motor cortex, but also indicate that the primary motor cortex receives its major thalamic inputs from outside of the pallidal projection area, and that the pallidum sends its major outputs to nonprimary motor areas through the thalamus.

Functional relation between corticonuclear input and movements evoked on microstimulation in cerebellar nucleus interpositus anterior in the cat.

The functional relation between receptive fields of climbing fibres projecting to the C1, C3 and Y zones and forelimb movements controlled by nucleus interpositus anterior via the rubrospinal tract were studied in cats decerebrated at the pre-collicular level. Microelectrode tracks were made through the caudal half of nucleus interpositus anterior. This part of the nucleus receives its cerebellar cortical projection from the forelimb areas of these three sagittal zones. The C3 zone has been demonstrated to consist of smaller functional units called microzones. Natural stimulation of the forelimb skin evoked positive field potentials in the nucleus. These potentials have previously been shown to be generated by climbing fibre-activated Purkinje cells and were mapped at each nuclear site, to establish the climbing fibre receptive fields of the afferent microzones. The forelimb movement evoked by microstimulation at the same site was then studied. The movement usually involved more than one limb segment. Shoulder retraction and elbow flexion were frequently evoked, whereas elbow extension was rare and shoulder protraction never observed. In total, movements at the shoulder and/or elbow occurred for 96% of the interpositus sites. At the wrist, flexion and extension movements caused by muscles with radial, central or ulnar insertions on the paw were all relatively common. Pure supination and pronation movements were also observed. Movements of the digits consisted mainly of dorsal flexion of central or ulnar digits. A comparison of climbing fibre receptive fields and associated movements for a total of 110 nuclear sites indicated a general specificity of the input-output relationship of this cerebellar control system. Several findings suggested that the movement evoked from a particular site would act to withdraw the area of the skin corresponding to the climbing fibre receptive field of the afferent microzones. For example, sites with receptive fields on the dorsum of the paw were frequently associated with palmar flexion at the wrist, whereas sites with receptive fields on the ventral side of the paw and forearm were associated with dorsiflexion at the wrist. Correspondingly, receptive fields on the lateral side of the forearm and paw were often associated with flexion at the elbow, whereas sites with receptive fields on the radial side of the forearm were associated with elbow extension. The proximal movements that were frequently observed also for distal receptive fields may serve to produce a general shortening of the limb to enhance efficiency of the withdrawal. It has previously been suggested that the cerebellar control of forelimb movements via the rubrospinal tract has a modular organisation. Each module would consist of a cell group in the nucleus interpositus anterior and its afferent microzones in the C1, C3 and Y zones, characterised by a homogenous set of climbing fibre receptive fields. The results of the present study support this organisational principle, and suggest that the efferent action of a module is to withdraw the receptive field from an external stimulus. Possible functional interpretations of the action of this system during explorative and reaching movements are discussed.

Topographical organization of the cerebellar cortical projection to nucleus interpositus anterior in the cat.

1. A new methodological approach for detailed study of the organization of the cerebellar corticonuclear projection was evaluated in barbiturate-anaesthetized cats. Extracellular field potentials were simultaneously recorded in nucleus interpositus anterior and in the forelimb area of the C3 zone, at the cerebellar surface. On electrical and natural stimulation of the forelimb skin, the evoked positive field potentials in the nucleus and the climbing fibre field potentials in the cerebellar cortex had similar characteristics, indicating that the nuclear potentials were related to climbing fibre activity. 2. Application of a local anaesthetic to the cerebellar surface reversibly diminished the positive field potentials in the nucleus, demonstrating that the potentials were dependent on cerebellar cortical activity. It was thus concluded that the positive field potentials were mainly generated by climbing fibre-activated Purkinje cells and reflected synaptic inhibitory potentials in nuclear neurones. Accordingly, the positive field potentials in the nucleus could be used to reveal the termination area of Purkinje cells activated by a specific climbing fibre input evoked on peripheral stimulation. 3. The topographical organization of the cerebellar cortical projection to the forelimb part of nucleus interpositus anterior was then investigated by systematically mapping the cutaneous tactile and nociceptive 'receptive fields' of the positive field potentials at different sites in the nucleus. Five groups of receptive fields were distinguished and tentatively divided into a total of nineteen subgroups. 4. Each group of receptive fields corresponded to one or two of the previously described receptive field classes of climbing fibres to the C1, C3 and Y zones and was represented in a single area of the nucleus. Within each area there was an orderly representation of different receptive fields. The results suggest that microzones in the C1, C3 and Y zones with similar climbing fibre input project to a common set of neurones in nucleus interpositus anterior. 5. We propose a modular organization for the cerebellar control of forelimb movements through the rubrospinal tract. Each module would consist of a set of neurones in nucleus interpositus anterior and their afferent microzones in the C1, C3 and Y zones. A module would control a specific group of muscles and receive a homogeneous climbing fibre input related to the movement controlled.

Specific patterns of intrinsic connections between representation zones in the rat motor cortex.

The organization of intrinsic connections in rat motor cortex was studied by combining microstimulation and tract-tracing techniques. Maps of forelimb and vibrissal movements were constructed from the distribution of cortical sites from which movements were evoked in response to intracortical microstimulation. Then, a single injection of a fluorescent dextran was placed into either a vibrissal or a wrist representation zone, or into a region bordering these zones, resulting in anterograde labeling of long intrinsic, horizontal axons. Following injection into the vibrissal area, axons were largely restricted to the whisker representation zone and to the border region with the forelimb representation. Injections into a wrist zone labeled projections largely restricted to the forelimb area and to the border with the vibrissal area. Injections into a border region labeled dense projections throughout most of the forelimb and vibrissal areas. These findings indicate that intrinsic axon collaterals in the motor cortex form specific and extensive connections among representation zones related to movements of the same body part. These connections may be involved in the coordination of activity in different representation zones for the execution of complex movement patterns. The projection of axon collaterals into border regions may be the anatomical substrate for the rapid reorganization of motor cortical maps that occurs following various experimental manipulations.

Metabolic Mapping of the Forelimb Motor System in the Rat: Local Cerebral Glucose Utilization Following Execution of Forelimb Movements Mainly Involving Proximal Musculature

The present study was undertaken to establish a metabolic map of forelimb motor pathways under conditions of physiological activation. For that purpose, we used the [14C]2-deoxy-D-glucose (2-DG) method to identify forebrain and midbrain centers showing an increase in 2-DG uptake in animals trained to execute specific lever-pressing movements with the right forelimb. Following repetitive execution of these movements, principally involving proximal (shoulder, elbow, and wrist) muscles, increases in 2-DG uptake were found contralaterally in several neocortical or subcortical centers. The largest left-right differences in local cerebral glucose utilization (LCGU) were found in a central region of the sensorimotor cortex composed of the caudal part of area 3 of the frontal cortex (Fr3; p < 0.01), the intermediate part of area 1 of Fr (Fr1; p < 0.01), and the forelimb cortical area (p < 0.04). Fr3 was the brain center with the highest differences in left-right LCGU. This central region of the sensorimotor cortex seems to correspond closely to the caudal forelimb area of Neafsey et al. (1986). Intermediate left-right differences in LCGU were found (1) in the just-adjoining rostral-medial areas of the motor cortex involving the intermediate part of area 2 of Fr (Fr2; p < 0.01) and the rostral part of Fr1 (p < 0.04), and (2) in the rostral part of area 1 of the parietal cortex (Par1; p < 0.01) and the caudal part of area 2 of Par (Par2; p < 0.05), both corresponding to forelimb representation. Weak (not statistically significant) left-right differences in LCGU were found in the rostral parts of Fr2 and Fr3, in the caudal parts of Fr2 and Fr1, in the hindlimb cortical area, and in the caudal part of Par1 and the rostral part of Par2. In the remaining cortical areas (cingulate; agranular and granular retrosplenial; temporal; and occipital), there was practically no difference in left-right 2-DG uptake. In addition, increased 2-DG uptake was present contralaterally in several subcortical motor-related centers. In those centers in which a somatomotor map has been established (caudate putamen, ventral lateral and ventral posterolateral thalamic nuclei, and red nucleus), increased 2-DG uptake was found in regions corresponding to forelimb representation.(ABSTRACT TRUNCATED AT 250 WORDS)

Synaptic relationships between gaba-immunoreactive boutons and primary afferent terminals in the rat cuneate nucleus

The present study examined the synaptic relation between the primary afferent terminals and intrinsic neuronal elements in the rat cuneate nucleus. For this purpose, experimental degeneration after multiple cervicothoracic dorsal rhizotomies or anterograde transport of wheatgerm agglutinin conjugated to horseradish peroxidase were used to identify the primary afferent terminals, while immunogold postembedding staining was employed to identify the GABA-immunoreactive boutons. The combined procedure allowed us to demonstrate a direct synaptic relationship between the primary afferent terminals and GABA-immunoreactive boutons. At least two types of synaptic relation were observed between the primary afferent terminals, identified by their degenerating features or labeled by wheatgerm agglutinin conjugated to horseradish peroxidase, and the immunogold-labeled GABA-immunoreactive boutons (i) a GABA-immunoreactive bouton making a simple presynaptic contact with the primary afferent terminal; and (ii) a synaptic glomerular complex in which the centrally located primary afferent terminal was postsynaptic to a GABA-immunoreactive bouton and presynaptic to dendrites closely associated with it; both terminals were sometimes presynaptic to a common dendrite. It is speculated from this study that the incoming impulses from the forelimb area are modulated by the GABA-immunoreactive boutons in the cuneate nucleus of the rat.

Differential spinal projections of subregions in the forelimb area of the motor cortex in the cat

Anterograde transport of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) was used to examine the topography of projections from the forelimb area of motor cortex to the cervical spinal cord in the cat. Tracer was injected in sites in the rostrolateral (RL-MCx) and caudolateral (CL-MCx) subregions concerned with the distal forelimb. Whereas both subregions projected throughout the cervical cord, with the greatest density of label present in the cervical enlargement, the dorso-ventral distributions were different for the two injection sites. Injections in RL-MCx produced labeling in the lateral portions of laminae VI, VII, and VIII in the upper cervical segments. This corresponds to the locations of propriospinal neurons that project to forelimb motor nuclei used in reaching [Exp. Brain Res., 42 (1981) 299–318]. In the cervical enlargement, labeling was present in laminae V, VI, VII, and part of VIII. At all levels examined, the density of labeling was greatest in the intermediate zone. After CL-MCx injection, labeling was concentrated in the dorsal horn both in the upper cervical segments and in the cervical enlargement. These findings suggest that the two motor cortical subregions project to different propriospinal and interneuronal systems in the cervical cord and support the idea that the two subregions play different roles in controlling forelimb movements.

Input organization of distal and proximal forelimb areas in the monkey primary motor cortex: A retrograde double labeling study

The present double-labeling study was designed to demonstrate the morphological framework for motor control in coordinated distal and proximal forelimb movements, which may partly, at least, depend on corticocortical and thalamocortical inputs to the forelimb area in the primary motor cortex. After intracortical microstimulation mapping of the forelimb area in the primary motor cortex of four macaque monkeys, a retrograde tracing study with fluorescent dyes was attempted to label simultaneously neurons in cortical and subcortical sites projecting to the distal forelimb representation area and those projecting to the proximal representation area of the primary motor cortex. Neurons projecting to distal and proximal forelimb parts of the primary motor cortex were largely separate in the following areas: the premotor area, primary somatosensory area, secondary somatosensory area, area 5, and thalamus. In contrast, there was no precise topographic organization of labeled projection neurons in the following areas: the supplementary motor area, cingulate motor area, primary motor cortex adjacent to the injection sites, claustrum, and basal nucleus of Meynert. The present study revealed that the forelimb area of the primary motor cortex receives both segregated and intermixed inputs from cortical and subcortical sources. In particular, the fact that the forelimb area of the primary motor cortex receives topographically organized inputs from the premotor area and nontopographically organized inputs from the supplementary motor area and cingulate motor area indicates possible different functional roles of frontal premotor areas in control of coordinated distal and proximal forelimb movements.