Organization of corticospinal neurons in the cat

Interhemispheric differences of pyramidal cells in the primary motor cortices of schizophrenia patients investigated postmortem

Motor disturbances are observed in schizophrenia patients, but the neuroanatomical background is unknown. Our aim was to investigate the pyramidal cells of the primary motor cortex (BA 4) in both hemispheres of postmortem control and schizophrenia subjects-8 subjects in each group-with 2.5-5.5 h postmortem interval. The density and size of the Sternberger monoclonal incorporated antibody 32 (SMI32)-immunostained pyramidal cells in layer 3 and 5 showed no change; however, the proportion of larger pyramidal cells is decreased in layer 5. Giant pyramidal neurons (Betz cells) were investigated distinctively with SMI32- and parvalbumin (PV) immunostainings. In the right hemisphere of schizophrenia subjects, the density of Betz cells was decreased and their PV-immunopositive perisomatic input showed impairment. Part of the Betz cells contained PV in both groups, but the proportion of PV-positive cells has declined with age. The rat model of antipsychotic treatment with haloperidol and olanzapine showed no differences in size and density of SMI32-immunopositive pyramidal cells. Our results suggest that motor impairment of schizophrenia patients may have a morphological basis involving the Betz cells in the right hemisphere. These alterations can have neurodevelopmental and neurodegenerative explanations, but antipsychotic treatment does not explain them.

Origin of Multisynaptic Corticospinal Pathway to Forelimb Segments in Macaques and Its Reorganization After Spinal Cord Injury

Removal of the monosynaptic corticospinal pathway (CSP) terminating within the forelimb segments severely impairs manual dexterity. Functional recovery from the monosynaptic CSP lesion can be achieved through the remaining multisynaptic CSP toward the forelimb segments. In the present study, we applied retrograde transsynaptic labeling with rabies virus to a monkey model of spinal cord injury. By injecting the virus into the spinal forelimb segments immediately after the monosynaptic CSP lesion, we showed that the contralateral primary motor cortex (M1), especially its caudal and bank region (so-called “new” M1), was the principal origin of the CSP linking the motor cortex to the spinal forelimb segments disynaptically (disynaptic CSP). This forms a striking contrast to the architecture of the monosynaptic CSP that involves extensively other motor-related areas, together with M1. Next, the rabies injections were made at the recovery period of 3 months after the monosynaptic CSP lesion. The second-order labeled neurons were located in the ipsilateral as well as in the contralateral “new” M1. This indicates that the disynaptic CSP input from the ipsilateral “new” M1 is recruited during the motor recovery from the monosynaptic CSP lesion. Our results suggest that the disynaptic CSP is reorganized to connect the ipsilateral “new” M1 to the forelimb motoneurons for functional compensation after the monosynaptic CSP lesion.

Signals from posterior parietal area 5 to motor cortex during locomotion

Area 5 of the parietal cortex is part of the "dorsal stream" cortical pathway which processes visual information for action. The signals that area 5 ultimately conveys to motor cortex, the main area providing output to the spinal cord, are unknown. We analyzed area 5 neuronal activity during vision-independent locomotion on a flat surface and vision-dependent locomotion on a horizontal ladder in cats focusing on corticocortical neurons (CCs) projecting to motor cortex from the upper and deeper cortical layers and compared it to that of neighboring unidentified neurons (noIDs). We found that upon transition from vision-independent to vision-dependent locomotion, the low discharge of CCs in layer V doubled and the proportion of cells with 2 bursts per stride tended to increase. In layer V, the group of 2-bursters developed 2 activity peaks that coincided with peaks of gaze shifts along the surface away from the animal, described previously. One-bursters and either subpopulation in supragranular layers did not transmit any clear unified stride-related signal to the motor cortex. Most CC group activities did not mirror those of their noID counterparts. CCs with receptive fields on the shoulder, elbow, or wrist/paw discharged in opposite phases with the respective groups of pyramidal tract neurons of motor cortex, the cortico-spinal cells.

Parcellation of the murine cortical hindlimb area is demonstrated by its subcortical connectivity and cytoarchitecture

Although corticospinal neurons are known to be distributed in both the primary motor and somatosensory cortices (S1), details of the projection pattern of their fibers to the lumbar cord gray matter remain largely uncharacterized, especially in rodents. We previously investigated the cortical area projecting to the gray matter of the fourth lumbar cord segment (L4) (L4 Cx) in mice. In the present study, we injected an anterograde tracer into multiple sites to cover the entire L4 Cx. We found that (1) the rostromedial part of the L4 Cx projects to the intermediate and ventral zones of the lumbar cord gray matter, (2) the lateral part projects to the medial dorsal horn, and (3) the caudal part projects to the lateral dorsal horn. We also found that the border between the rostromedial and caudolateral parts corresponds to the border between the agranular and granular cortex. Analysis of the somatotopic patterns formed by the cortical projection cells and the primary sensory neurons innervating the skin of the hindlimb and its related area suggests that the lateral part corresponds to the S1 hindlimb area and the caudal part to the S1 trunk area. Examination of thalamic innervation by the L4 Cx revealed that the caudolateral L4 Cx focally projects to the ventrobasal complex (VB) and the posterior complex (PO), while the medial L4 Cx widely projects to the PO but little to the VB. These findings suggest that the L4 Cx is parceled into subregions defined by the cytoarchitecture and subcortical projection.

The Cortical Motor System in the Domestic Pig: Origin and Termination of the Corticospinal Tract and Cortico-Brainstem Projections

The anatomy of the cortical motor system and its relationship to motor repertoire in artiodactyls is for the most part unknown. We studied the origin and termination of the corticospinal tract (CST) and cortico-brainstem projections in domestic pigs. Pyramidal neurons were retrogradely labeled by injecting aminostilbamidine in the spinal segment C1. After identifying the dual origin of the porcine CST in the primary motor cortex (M1) and premotor cortex (PM), the axons descending from those regions to the spinal cord and brainstem were anterogradely labeled by unilateral injections of dextran alexa-594 in M1 and dextran alexa-488 in PM. Numerous corticospinal projections from M1 and PM were detected up to T6 spinal segment and showed a similar pattern of decussation and distribution in the white matter funiculi and the gray matter laminae. They terminated mostly on dendrites of the lateral intermediate laminae and the internal basilar nucleus, and some innervated the ventromedial laminae, but were essentially absent in lateral laminae IX. Corticofugal axons terminated predominantly ipsilaterally in the midbrain and bilaterally in the medulla oblongata. Most corticorubral projections arose from M1, whereas the mesencephalic reticular formation, superior colliculus, lateral reticular nucleus, gigantocellular reticular nucleus, and raphe received abundant axonal contacts from both M1 and PM. Our data suggest that the porcine cortical motor system has some common features with that of primates and humans and may control posture and movement through parallel motor descending pathways. However, less cortical regions project to the spinal cord in pigs, and the CST neither seems to reach the lumbar enlargement nor to have a significant direct innervation of cervical, foreleg motoneurons.

Microstimulation of the Premotor Cortex of the Cat Produces Phase-Dependent Changes in Locomotor Activity

To determine the functional organization of premotor areas in the cat pericruciate cortex we applied intracortical microstimulation (ICMS) within multiple cytoarchitectonically identified subregions of areas 4 and 6 in the awake cat, both at rest and during treadmill walking. ICMS in most premotor areas evoked clear twitch responses in the limbs and/or head at rest. During locomotion, these same areas produced phase-dependent modifications of muscle activity. ICMS in the primary motor cortex (area 4γ) produced large phase-dependent responses, mostly restricted to the contralateral forelimb or hindlimb. Stimulation in premotor areas also produced phase-dependent responses that, in some cases, were as large as those evoked from area 4γ. However, responses from premotor areas had more widespread effects on multiple limbs, including the ipsilateral limbs, than did stimulation in 4γ. During locomotion, responses in both forelimb and hindlimb muscles were evoked from cytoarchitectonic areas 4γ, 4δ, 6aα, and 6aγ. However, the prevalence of effects in a given limb varied from one area to another. The results suggest that premotor areas may contribute to the production, modification, and coordination of activity in the limbs during locomotion and may be particularly pertinent during modifications of gait.

Differential innervation within a transverse plane of spinal gray matter by sensorimotor cortices, with special reference to the somatosensory cortices

The corticospinal (CS) neurons projecting to the cervical cord distribute not only in motor-related cortical areas, but also in somatosensory areas, including the primary somatosensory cortex (S1). The exact functions of these widely distributed CS neurons are largely unknown, however. In this study, we injected mice with adeno-associated virus encoding membrane-binding fluorescent proteins to investigate the distribution of axons from CS neurons in different regions within a broad cortical area. We found that CS axons from the primary motor cortex (M1), the rostral part of S1 (S1r), and the caudal part of S1 (S1c) differentially project to specific compartments within the spinal gray matter of the seventh cervical cord segment: (a) M1 projects mainly to intermediate and ventral areas, (b) S1r to the mediodorsal area, and (c) S1c to the dorsolateral area. We also found that the projection from S1r, which corresponds to the forelimb area, largely overlaps the cutaneous afferent terminals from the forepaw (hand) in the dorsal horn, and we detected a similar relation between S1c and the trunk. Our findings suggest the existence of considerably fine somatotopic compartments within the dorsal horn that process somatosensation and descending information, which is provided mainly by S1 CS neurons and contribute to delicate control of sensory information in generation of movement.

More than Just a "Motor": Recent Surprises from the Frontal Cortex

Motor and premotor cortices are crucial for the control of movements. However, we still know little about how these areas contribute to higher-order motor control, such as deciding which movements to make and when to make them. Here we focus on rodent studies and review recent findings, which suggest that-in addition to motor control-neurons in motor cortices play a role in sensory integration, behavioral strategizing, working memory, and decision-making. We suggest that these seemingly disparate functions may subserve an evolutionarily conserved role in sensorimotor cognition and that further study of rodent motor cortices could make a major contribution to our understanding of the evolution and function of the mammalian frontal cortex.

Comparative morphology of gigantopyramidal neurons in primary motor cortex across mammals

Gigantopyramidal neurons, referred to as Betz cells in primates, are characterized by large somata and extensive basilar dendrites. Although there have been morphological descriptions and drawings of gigantopyramidal neurons in a limited number of species, quantitative investigations have typically been limited to measures of soma size. The current study thus employed two separate analytical approaches: a morphological investigation using the Golgi technique to provide qualitative and quantitative somatodendritic measures of gigantopyramidal neurons across 19 mammalian species from 7 orders; and unbiased stereology to compare the soma volume of layer V pyramidal and gigantopyramidal neurons in primary motor cortex between 11 carnivore and 9 primate species. Of the 617 neurons traced in the morphological analysis, 181 were gigantopyramidal neurons, with deep (primarily layer V) pyramidal (n = 203) and superficial (primarily layer III) pyramidal (n = 233) neurons quantified for comparative purposes. Qualitatively, dendritic morphology varied considerably across species, with some (sub)orders (e.g., artiodactyls, perissodactyls, feliforms) exhibiting bifurcating, V-shaped apical dendrites. Basilar dendrites exhibited idiosyncratic geometry across and within taxonomic groups. Quantitatively, most dendritic measures were significantly greater in gigantopyramidal neurons than in superficial and deep pyramidal neurons. Cluster analyses revealed that most taxonomic groups could be discriminated based on somatodendritic morphology for both superficial and gigantopyramidal neurons. Finally, in agreement with Brodmann, gigantopyramidal neurons in both the morphological and stereological analyses were larger in feliforms (especially in the Panthera species) than in other (sub)orders, possibly due to specializations in muscle fiber composition and musculoskeletal systems.

Modality-specific Changes in Motor Cortex Excitability After Visuo-proprioceptive Realignment

Spatial realignment of visual and proprioceptive estimates of hand position is necessary both to keep the estimates in register over time and to compensate for sensory perturbations. Such realignment affects perceived hand position, which the brain must use to plan hand movements. We would therefore expect visuo-proprioceptive realignment to affect the motor system at some level, but the physiological basis of this interaction is unknown. Here, we asked whether activity in primary motor cortex (M1), a well-known substrate of motor control, shows evidence of change after visuo-proprioceptive realignment. In two sessions each, 32 healthy adults experienced spatially misaligned or veridical visual and proprioceptive information about their static left index finger. Participants indicated perceived finger position with no performance feedback or knowledge of results. Using TMS over the M1 representation of the misaligned finger, we found no average difference between sessions. However, regression analysis indicated that, in the misaligned session only, proprioceptive realignment was linked with a decrease in M1 activity and visual realignment was linked with an increase in M1 activity. Proprioceptive and visual realignment were inversely related to each other. These results suggest that visuo-proprioceptive realignment does indeed have a physiological impact on the motor system. The lack of a between-session mean difference in M1 activity suggests that the basis of the effect is not the multisensory realignment computation itself, independent of modality. Rather, the changes in M1 are consistent with a modality-specific neural mechanism, such as modulation of somatosensory cortex or dorsal stream visual areas that impact M1.

MI neuronal responses to peripheral whisker stimulation: relationship to neuronal activity in si barrels and septa

The whisker region in the rodent primary motor (MI) cortex receives dense projections from neurons aligned with the layer IV septa in the whisker region of the primary somatosensory (SI) cortex. To compare whisker-induced responses in MI with respect to the SI responses in the septa and adjoining barrel regions, we used several experimental approaches in anesthetized rats. Reversible inactivation of SI and the surrounding cortex suppressed the magnitude of whisker-induced responses in the MI whisker region by 80%. Subsequent laminar analysis of MI responses to electrical or mechanical stimulation of the whisker pad revealed that the most responsive MI neurons were located >or=1.0 mm below the pia. When layer IV neurons in SI were recorded simultaneously with deep MI neurons during low-frequency (2-Hz) deflections of the whiskers, the neurons in the SI barrels responded 2-6 ms earlier than those in MI. Barrel neurons displayed similar response latencies at all stimulus frequencies, but the response latencies in MI and the SI septa increased significantly when the whiskers were deflected at frequencies of 8 Hz. Finally, cross-correlation analysis of neuronal activity in SI and MI revealed greater amounts of time-locked coordination among septa-MI neuron pairs than among barrel-MI neuron pairs. These results suggest that the somatosensory corticocortical inputs to MI cortex convey information processed by the SI septal circuits.

Cortical reorganization following bimanual training and somatosensory stimulation in cervical spinal cord injury: a case report

BACKGROUND AND PURPOSE

Deficits in upper-extremity function in individuals with tetraplegia are primarily due to the loss of motor pathways. Detrimental cortical reorganization, however, may create further loss of function. The purpose of this case report is to describe the cortical changes associated with a combination intervention using bimanual massed practice training with somatosensory stimulation.

CASE DESCRIPTION

"BR" was a 22-year-old man with C6 tetraplegia and hand impairment who participated in this training intervention for 3 weeks.

OUTCOMES

BR demonstrated improvements in sensory function, strength (the force-generating capacity of muscle), and performance of functional hand skills. Following the training, the cortical motor map of the biceps brachii muscle shifted anteriorly and increased in area and volume.

DISCUSSION

This is the first documented case in which changes in the size and location of the cortical map were associated with an intervention and improvement in function in an individual with tetraplegia. This case suggests that an intensive training intervention may induce both functional and neurophysiological changes.

Three channels of corticothalamic communication during locomotion

We studied the flow of corticothalamic (CT) information from the motor cortex of the cat during two types of locomotion: visually guided (cortex dependent) and unguided. Spike trains of CT neurons in layers V (CT5s) and VI (CT6s) were examined. All CT5s had fast-conducting axons (<2 ms conduction time), and nearly all showed step-phase-related activity (94%), sensory receptive fields (100%), and spontaneous activity (100%). In contrast, conduction times along CT6 axons were much slower, with bimodal peaks occurring at 6 and 32 ms. Remarkably, almost none of the slowest conducting CT6s showed step-related activity, sensory receptive fields, or spontaneous activity. As a group, these enigmatic neurons were all but silent. Some of the CT6s with moderately conducting axons showed step-related behavior (35%), and this response was more precisely timed than that of the CT5s. We propose distinct functional roles for these diverse corticothalamic populations.

Effect of Forelimb Use on Postnatal Development of the Forelimb Motor Representation in Primary Motor Cortex of the Cat

In the cat, the motor representation in motor cortex develops between wk 8 and wk 13. Motor map development is accompanied by a decrease in the current thresholds for evoking movements with a concomitant increase in the number of effective sites, an increase in the distal representation, and the representation of multijoint synergies. In this study we used intracortical microstimulation in anesthetized cats to examine how forelimb motor experiences influence development of map characteristics. To promote skilled movements during wks 8–13, animals were engaged in daily performance of a prehension task. Forelimb movements were prevented by intramuscular botulinum toxin injection or restraint. To determine whether experience-dependent changes were permanent, we examined the map in different animals between 1 wk and 1 yr after cessation of testing. Promoting forelimb use resulted in an increase in the number of sites from which multiple joint effects were produced by stimulation and the number of joints represented at those sites. The effect was maximal at 1 wk after cessation of testing, and became progressively less at 1 mo and at 4 mo. Preventing limb use resulted in a decreased number of effective sites, an increase in current thresholds for evoking responses, and a decreased representation of joints at multijoint sites. Our findings show that the motor map can respond to novel motor demands as it is forming during development but that it reverts back to one with the properties of a map in a control animal if those demands are not maintained in the animal's behavioral repertoire.

Integration of motor and visual information in the parietal area 5 during locomotion

The parietal cortex receives both visual- and motor-related information and is believed to be one of the sites of visuo-motor coordination. This study for the first time characterizes integration of visual and motor information in activity of neurons of parietal area 5 during locomotion under conditions that require visuo-motor coordination. The activity of neurons was recorded in cats during walking on a flat surface-a task with no visuo-motor coordination required (flat locomotion), walking along a horizontal ladder or a series of barriers-a task requiring visuo-motor coordination for an accurate foot placement on surface that is heterogeneous along the direction of progression (ladder and barriers locomotion), and walking along a narrow pathway-a task requiring visuo-motor coordination on surface homogeneous along the direction of progression (narrow locomotion). During flat locomotion, activity of 66% of the neurons was modulated in rhythm of stepping, usually with one peak per cycle. During ladder and barrier locomotion, the proportion of rhythmically active neurons significantly increased, their modulation became stronger, and the majority of neurons had two peaks of activity per cycle. During narrow locomotion, however, the activity of neurons was similar to that during flat locomotion. We concluded that, during locomotion, parietal area 5 integrates two types of information: signals about the activity of basic locomotion mechanisms and signals about heterogeneity of the surface along the direction of progression. We describe here the modes of integration of these two types of information during locomotion.

Origin of corticospinal neurones evoking disynaptic excitation in forelimb motoneurones mediated via C3-C4 propriospinal neurones in the cat

Intracellular recording was made from forelimb motoneurones in the cat (alpha-chloralose anaesthesia) during electrical stimulation of corticospinal neurones (CSNs) and their afferents in the contralateral cortex. Axons of the CSNs were stimulated in the contralateral pyramid. The corticospinal tract was transected at the C5/C6 segmental border in order to restrict transmission through the C3-C4 propriospinal neurones (C3-C4 PNs). Di- and trisynaptic cortical EPSPs could be evoked after transection of the corticospinal fibres in C5/C6 but not after a corresponding transection in C2/C3. Pyramidal stimulation elicited disynaptic EPSPs that were abolished after a C2/C3 transection. Disynaptic pyramidal EPSPs, mediated via C3-C4 propriospinal neurones could be facilitated by a single cortical stimulation. It is concluded that di- and trisynaptic cortical EPSPs and disynaptic pyramidal EPSPs are mediated via the same C3-C4 PNs. Cortical surface stimulation showed that di- and trisynaptic cortical EPSPs could be evoked from distinct spots in the lateral part of the anterior sigmoid gyrus (Sig. a) and/or in the rostral part of the lateral sigmoid gyrus (Sig. l). No cortical EPSPs or facilitation of pyramidal disynaptic EPSPs was evoked from the posterior part of the Sig. l, posterior sigmoid gyrus, coronal gyrus, lateral gyrus, suprasylvian gyrus and ectosylvian gyrus. It is concluded that the CSNs, which issue the command for visually guided target reaching with the forelimb via the C3-C4 PNs, originate in the lateral part of the Sig. a and in the rostral part of the Sig. l. A dual representation of the forelimb in the primary motor cortex of the cat has previously been proposed. The present results show that with respect to one identified interneuronal system like the C3-C4 propriospinal system, the CSNs may have their origin restricted to one region of the primary motor cortex.

Impairments in prehension produced by early postnatal sensory motor cortex activity blockade

This study examined the effects of blocking neural activity in sensory motor cortex during early postnatal development on prehension. We infused muscimol, either unilaterally or bilaterally, into the sensory motor cortex of cats to block activity continuously between postnatal weeks 3-7. After stopping infusion, we trained animals to reach and grasp a cube of meat and tested behavior thereafter. Animals that had not received muscimol infusion (unilateral saline infusion; age-matched) reached for the meat accurately with small end-point errors. They grasped the meat using coordinated digit flexion followed by forearm supination on 82.7% of trials. Performance using either limb did not differ significantly. In animals receiving unilateral muscimol infusion, reaching and grasping using the limb ipsilateral to the infusion were similar to controls. The limb contralateral to infusion showed significant increases in systematic and variable reaching end-point errors, often requiring subsequent corrective movements to contact the meat. Grasping occurred on only 14.8% of trials, replaced on most trials by raking without distal movements. Compensatory adjustments in reach length and angle, to maintain end-point accuracy as movements were started from a more lateral position, were less effective using the contralateral limb than ipsilateral limb. With bilateral inactivations, the form of reaching and grasping impairments was identical to that produced by unilateral inactivation, but the magnitude of the reaching impairments was less. We discuss these results in terms of the differential effects of unilateral and bilateral inactivation on corticospinal tract development. We also investigated the degree to which these prehension impairments after unilateral blockade reflect control by each hemisphere. In animals that had received unilateral blockade between postnatal weeks (PWs) 3 and 7, we silenced on-going activity (after PW 11) during task performance using continuous muscimol infusion. We inactivated the right (previously active) and then the left (previously silenced) sensory motor cortex. Inactivation of the ipsilateral (right) sensory motor cortex produced a further increase in systematic error and less frequent normal grasping. Reinactivation of the contralateral (left) cortex produced larger increases in reaching and grasping impairments than those produced by ipsilateral inactivation. This suggests that the impaired limb receives bilateral sensory motor cortex control but that control by the contralateral (initially silenced) cortex predominates. Our data are consistent with the hypothesis that the normal development of skilled motor behavior requires activity in sensory motor cortex during early postnatal life.

Corticoreticular pathways in the cat. I. Projection patterns and collaterization

This paper summarizes and compares the projection patterns and the receptive fields of cortical neurons in areas 4 and 6 that project to the pontomedullary reticular formation (PMRF). A total of 326 neurons were recorded in area 4 and 129 in area 6 in four awake, unrestrained cats that were chronically implanted with arrays of electrodes in the PMRF and the pyramidal tract (PT). In area 4, 47% of the neurons projected to the caudal PT but not to the PMRF (PTNs); 19% were activated only from the PMRF [corticoreticular neurons (CRNs)], whereas 27% were activated from both the PT and the PMRF (PTN/CRNs). More PTN/CRNs conducted at velocities >20 m/s (82%) than did CRNs (23%). In area 6, only 19% of the neurons were identified as PTNs, 12% were PTN/CRNs and 31% were CRNs; a further 38% could not be activated from either structure. Collateral branches within the PMRF conducted at maximum velocities of 20 m/s (average = 6.5 m/s). No significant differences in the conduction velocities of the collateral branches were found either between fast and slow PTNs or between area 4 and area 6 neurons. A large proportion of neurons in area 4 (85/173, 49%) were activated by passive manipulation of the more distal, contralateral forelimb, with approximately equal numbers being classed as PTNs, PTN/CRNs and CRNs. Most neurons in area 6 for which a receptive field could be found were excited by lightly touching or tapping the face and neck; a receptive field could not be determined for 39% of the area 6 neurons compared with only 5% of those in area 4. Finally, there was evidence that neurons in quite widespread areas of the pericruciate cortex, including both areas 4 and 6 projected onto similar, restricted regions of the PMRF. The fact that the cortical projection from area 4 to the PMRF includes a high percentage of fast PTNs with a receptive field on the distal forelimb is consistent with the view that this projection may serve to integrate movement and the dynamic postural adjustments that accompany them. The fact that the cortical projection from area 6 to the PMRF is primarily from slow PTNs with receptive fields on the face, neck and back is consistent with a role for this cortical area in adjusting the general posture of the animal on which movements are superimposed.

Comparison of the cortical connections of areas 4 gamma and 4 delta in the cat cerebral cortex

Area 4 of the cat cerebral cortex has been subdivided into several regions based on cytoarchitectonic studies: areas 4 gamma, 4 delta, 4sfu, and 4fu (Hassler and Muhs-Clement [1964] J. Hirnforsch. 6:377-420). In a previous study, we found separate representations of contralateral limb movements in areas 4 gamma and 4 delta (Ghosh [1997] J. Comp. Neurol. 380:191-214). To investigate the relationship between these representations, the ipsilateral cortical connections of area 4 gamma and 4 delta were compared by the use of the retrograde neural tracers. After intracortical microstimulation of area 4, tracer was injected into one or two of the following regions: the forelimb regions of the rostral and caudal subdivisions of areas 4 gamma and 4 delta (r4 gamma, r4 delta, c4 gamma, c4 delta, separated by the cruciate sulcus) and the hindlimb regions of c4 gamma and c4 delta. Retrogradely labeled neural profiles were counted in every fourth section of the coronal series and located in cytoarchitectonic areas of the ipsilateral cortex. We found topographically organized reciprocal connections between areas 4 gamma and 4 delta; these connections were part of a rich network of interconnections between the cytoarchitectonic subdivisions of area 4. The forelimb regions of c4 gamma and c4 delta, of r4 gamma and c4 delta, and of r4 gamma and r4 delta were interconnected. These findings support the location of a secondary motor area in 4 delta. No interconnections between the forelimb regions of r4 gamma and c4 gamma, and of r4 delta and c4 gamma, could be found. Area 6 (particularly area 6a gamma) was found to project strongly to the forelimb regions of r4 gamma and r4 delta and relatively weakly to the forelimb region of c4 delta. Retrogradely labeled neurons were also detected in areas 3a, 3b, 1, 2pri, 5, 7, and insula after tracer injections in area 4.

Identification of motor areas of the cat cerebral cortex based on studies of cortical stimulation and corticospinal connections

The location and topography of motor areas in the cat cerebral cortex were studied by electrical stimulation of the cortex in five animals, and by the injection of retrograde tracers into the spinal cord of four animals. Movements evoked by intracortical microstimulation (ICMS) of the anterior, posterior and lateral sigmoid gyri, both banks of the cruciate sulcus and the dorsal bank of the presylvian sulcus were observed in anaesthetized cats. Fluorescent tracers (Fast Blue and/or Diamadino Yellow) were injected into the lateral funiculus in the second cervical segment, into the gray matter of cervical segments C3-T1 and/or into the gray matter of lumbar segments L2-S1. Contraction of the contralateral forelimb, hindlimb or facial muscles was observed following electrical stimulation of several cytoarchitectonic areas: 4 gamma, 4 delta, 6a alpha, 6a gamma, and 3a. These findings suggested representations of contralateral forelimb and hindlimb movements in areas 4 gamma and 4 delta, and of the contralateral forelimb muscles in areas 6a alpha and 6a gamma. Corticospinal neurons were located in all the above cytoarchitectonic areas as well as in areas 3b, 1, 2, 2pri, and 5. Large numbers of neurons were labeled in areas 4 gamma and 4 delta, and moderate labeling was observed in areas 6a gamma and 6a alpha. Corticospinal neurons projecting to cervical and lumbar segments were located in areas 4 gamma and 4 delta, while those projecting only to cervical segments were detected in areas 6a alpha and 6a gamma. Based on these findings it is proposed that within the motor cortex of the cat there are representations of limb movements in several cytoarchitectonic subdivisions. Many of these representations may be candidate secondary motor areas.

Primary motor cortex influences on the descending and ascending systems

The motor cortex plays a crucial role in the co-ordination of movement and posture. This is possible because the pyramidal tract fibres have access both directly and through collateral branches to structures governing eye, head, neck trunk and limb musculature. Pyramidal tract axons also directly reach the dorsal laminae of the spinal cord and the dorsal column nuclei, thus aiding in the selection of the sensory ascendant transmission. No other neurones in the brain besides pyramidal tract cells have such a wide access to different structures within the central nervous system. The majority of the pyramidal tract fibres that originate in the motor cortex and that send collateral branches to multiple supraspinal structures do not reach the spinal cord. Also, the great majority of the corticospinal neurones that emit multiple intracraneal collateral branches terminate at the cervical spinal cord level. The pyramidal tract fibres directed to the dorsal column nuclei that send collateral branches to supraspinal structures also show a clear tendency to terminate at supraspinal and cervical cord levels. These facts suggest that a substantial co-ordination between descending and ascending pathways might be produced by the same motor cortex axons at both supraspinal and cervical spinal cord sites. This may imply that the motor cortex co-ordination will be mostly directed to motor responses involving eye-neck-forelimb muscle synergies. The review makes special emphasis in the available evidence pointing to the role of the motor cortex in co-ordinating the activities of both descending and ascending pathways related to somatomotor integration and control. The motor cortex may function to co-operatively select a unique motor command by selectively filter sensory information and by co-ordinating the activities of the descending systems related to the control of distal and proximal muscles.

Long-lasting potentiation in the secondary somatosensory cortex affects motor control: assessment by H-reflex

We investigated descending projections from the secondary somatosensory cortex to the feline spinal cord and the effects of long-lasting potentiation in secondary somatosensory cortex on the activities of motoneurons of the cat. Electrophysiological examinations revealed that the low-intensity subthreshold secondary somatosensory cortex stimulation could change the H-Reflex induced by radial nerve stimulation. The H-wave amplitudes, recorded in wrist flexor muscles, were enhanced when the intervals from secondary somatosensory cortex to radial nerve stimuli were altered from 0 to 30 ms (initial excitation, 146 +/- 11% (mean +/- S.E.M.) of the control value). In contrast, the H-waves were suppressed with intervals longer than 30 ms (80 +/- 3%). The descending pathways from secondary somatosensory cortex to the spinal cord were assessed using an immunohistochemical technique. c-Fos and Zif268 proteins, induced by stimulation of the hand-represented secondary somatosensory cortex areas, could thus express in activated cervical neurons. The density of labeled cells was significantly higher in the seventh and eighth cervical segments than in other levels. The great majority of positive cells were distributed in the lateral part of the contralateral ventral horn and their somas ranged from 10 to 50 microns in size. Finally, we examined the effects of long-lasting potentiation, induced by high-frequency stimulation of the ventral posterolateral thalamic nucleus, on the activities of spinal motoneurons. Long-lasting potentiation altered the previously observed effects of secondary somatosensory cortex stimulation on the H-wave amplitude. The secondary somatosensory cortex-conditioned initial excitation of the H-reflex was enhanced (from 139 to 175%, P < 0.05), while late suppression was completely blocked (from 74 to 112%, P < 0.01). In conclusion, the descending pathways from secondary somatosensory cortex to the spinal cord modulated the H-reflex, and long-lasting potentiation in secondary somatosensory cortex affected this modulation. We have previously reported that corticocortical inputs from primary to secondary somatosensory cortex is required for induction of long-lasting potentiation in secondary somatosensory cortex. Taken together, the present study suggests that cortical plasticity in secondary somatosensory cortex amplifies somatic inputs from primary somatosensory cortex as a means of adaptive motor control by the sensory 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.

Information processing within the motor cortex. II. Intracortical connections between neurons receiving somatosensory cortical input and motor output neurons of the cortex

Connections between motor cortical neurons receiving somatosensory inputs from area 2 and large pyramidal cells in layer V were examined in the cat via intracellular injection of biocytin and immunohistochemistry of nonphosphorylated neurofilament proteins (npNFP). Biocytin was injected into pyramidal cells in layers II/III of the motor cortex that responded monosynaptically and polysynaptically to microstimulation of the somatosensory cortex and subsequently stained black by the avidin-biotinylated peroxidase complex method with diaminobenzidine (DAB) and nickel. By using a monoclonal antibody SMI-32 and a modified peroxidase-antiperoxidase method with Tris-aminophenyl-methane (TAPM) and p-cresol as a chromogen, pyramidal cells in layers III and V of the motor cortex were stained red for npNFP. In particular, all the large pyramidal cells in layer V, Betz cells, displayed intense npNFP immunoreactivity not only in the perikarya but also in the dendrites. Double staining with DAB/nickel and TAPM/p-cresol showed that biocytin-filled axon varicosities of the pyramidal cells, which were thought to receive monosynaptic inputs from area 2, made contacts with npNFP-positive dendrites in layers I-III around the biocytin-injected cell and in layers V-VI beneath the cell. The present results suggest that the corticocortical input from area 2 to pyramidal cells in layers II/III of the motor cortex is transferred to layer V pyramidal cells, including Betz cells, as well as to neighboring layer II/III pyramidal cells. Since tetanic stimulation of the somatosensory cortex reportedly produces long-term potentiation in layer II/III cells of the motor cortex, it seems reasonable to assume that a given area of the somatosensory cortex can produce a long-lasting change in the activity of a given group of output cells in the motor cortex.

Information processing within the motor cortex. I. Responses of morphologically identified motor cortical cells to stimulation of the somatosensory cortex

Inputs from the somatosensory cortex to the motor cortex have been proposed to function in learning of motor skills. In an attempt to analyze how these somatosensory inputs were processed in the motor cortex, neurons in the superficial layer of the cat motor cortex were classified into three groups on the basis of synaptic responses elicited by intracortical microstimulation (ICMS) of area 2. ICMS was delivered through seven electrodes implanted in area 2. When ICMS through one of the seven sites produced a response that was greater than 50% of the response produced by stimulating the seven sites at a time, the site was called a "dominant" site. Type I cells were those that had a dominant stimulation site and showed a constant response latency when examined by a double shock test. Type II cells were those that had a dominant site but displayed a variable latency. Type III cells had no dominant site and showed a variable latency. Latency of type I responses was 1.2-2.6 milliseconds, which was much shorter than that of type II and type III responses. Seventy-nine neurons in layers II/III of the motor cortex, which responded to ICMS in area 2, were stained by intracellular injection of biocytin. From the presence of an apical dendrite and rich spines on the dendrites, 23 type I, 21 type II, and 15 type III cells were classified as pyramidal cells. Type II pyramidal cells were located more superficially than type I and type III pyramidal cells. On the basis of the absence or sparseness of dendritic spines, three type I and four type II cells in layers II/III were classified as nonpyramidal cells. These cells consisted of five small multipolar cells in layer II and a large multipolar cell and a small bitufted cell in layer III. The remaining 11 cells were not classified because of insufficient staining. Since type I and type II cells are considered to represent monosynaptic and polysynaptic responses to stimulation of area 2, respectively, information flow from type I cells to more superficially located type II cells is presumed in layers II/III of the motor cortex. Type III responses suggest the presence of a convergent flow of impulses inside of and/or between areas 2 and 4.

The mode of cerebellar activation of pyramidal neurons in the cat parietal cortex (areas 5 and 7): an intracellular HRP study

Response properties of 180 pyramidal neurons (11 layer II, 66 layer III, 7 layer IV, 76 layer V and 20 layer VI neurons) in the cat parietal cortex (areas 5 and 7) were examined intracellularly with stimulations of the cerebellar nucleus (CN), the thalamic ventroanterior nucleus (VA) and the lateral posterior nucleus (LP) under pentobarbital anesthesia. Pyramidal neurons were identified morphologically or electrophysiologically with antidromic responses by pontine nuclei (PN) stimulation. CN and VA stimulations evoked EPSPs in layer II, III, V and VI pyramidal neurons, but scarcely induced EPSPs in layer IV pyramidal ones. The mean latencies of CN-EPSPs were nearly the same through layers II to V, giving values of around 4.5 ms, but that of layer VI pyramidal neurons was much longer (5.5 +/- 0.8 ms: mean +/- SD). Those of VA-EPSPs were also nearly the same through all layers (around 2.2-2.8 ms). In case of LP stimulation, EPSPs were scarcely recorded in layer II and IV pyramidal neurons. The mean latency of LP-EPSPs was much shorter in layer III (2.6 +/- 1.0 ms) than those in layers V and VI (3.8-3.9 ms). It was noted that pyramidal neurons in layers II and III responded more frequently to CN and VA stimulations than those in deeper layers, and that the amplitudes of CN- and VA-EPSPs were large and conspicuous in layer III. In layer V, a statistical difference in the latency distributions of CN-, VA-, and LP-EPSPs between fast- and slow-type pyramidal neurons was not detected. The present results suggest that there exist the different modes of thalamocortical (T-C) innervations in the cat parietal cortex; the superficial T-C innervation of CN-VA inputs and the deep T-C innervation of LP inputs.

Origins and conducting pathways of motor evoked potentials elicited by transcranial magnetic stimulation in cats

Spinal cord motor evoked potentials (MEPs-S) were elicited in cats by transcranial magnetic stimulation. The MEPs-S recorded from the epidural electrode at the first lumbar (L1) level consisted of four negative peaks (N1, 2.56; N2, 3.19; N3, 4.06; N4, 4.99 ms) followed by small, multiphasic waves. The mean conduction velocities of N1-N3 of the MEPs-S were calculated to be 90 to 115 m/s, which is consistent with extrapyramidal tract activation. A direct brain stem electrical stimulation and sequential transection of the spinal cord studies showed that N1-N3 of the MEPs-S originated primarily from the brain stem and conducted in the ventral funiculus of the spinal cord, which corresponded to the extrapyramidal tracts (e.g., the reticulospinal and vestibulospinal tracts). We also showed that N1 and N2 of the MEPs-S originated mainly from the lower medulla or upper cervical and that N3 originated from the lower pons or upper medulla (vestibular nucleus). On the other hand, N4 of the MEPs-S had a conduction velocity of 70 m/s, which disappeared by ablation of the sensorimotor cortex and pyramidotomy. A dorsal hemisection of the spinal cord, resulting in the disappearance of N4 of the MEPs-S, indicated that it conducted in the dorsolateral funiculus of the spinal cord. These results suggest that N4 of the MEPs-S originates from the sensorimotor cortex and conducts in the lateral corticospinal tract. MEPs-S by transcranial magnetic stimulation in cats constituted complex responses in both the pyramidal and extrapyramidal tracts.

Recovery of distal skills after neonatal lesion of the sensorimotor cortex in the cat

The cytoarchitectonic cortical areas containing the cells of origin of the pyramidal tract were unilaterally removed from kittens during the first postnatal month. After 5 months, the distal skills of these operated animals were analyzed using a food retrieving task. After an initial deficit, cats achieved the same performance in grasping and wrist movement with the limb contralateral to the lesion as with the opposite limb. The duration of the deficit was dependent on age at the time of lesion. Recovery of distal skills after neonatal lesion is discussed with regard to the low degree of maturity of the corticospinal tract in newborn cat.

Distribution of cortical neurons projecting to dorsal column nuclear complex and spinal cord in the hedgehog tenrec, Echinops telfairi

Using retrograde axonal flow and wheatgerm agglutinin conjugated to horseradish peroxidase, we studied the distribution of cortical neurons giving rise to spinal and dorsal column nuclear projections, and correlated the regions involved in the projections with the cytoarchitectonic areas recently identified in the lesser hedgehog tenrec, Echinops telfairi (Insectivora). Labeled cortical neurons were most numerous following injections of tracer into higher cervical segments, whereas almost none were found following thoracic injections. The cortical labeling appeared more prominent ipsilaterally than contralaterally after spinal injections, although it was more prominent on the contralateral side after injection into the dorsal column nuclear complex. The majority of labeled neurons found in lamina V occupied the neocortex adjacent to the interhemispheric fissure along the rostrocaudal extent of the small corpus callosum. This location corresponded to an intermediate rostrocaudal portion of the hemisphere, and particularly to area 2 of Rehkämper. In some cases, adjacent portions of areas 1 and 3 were also involved, as well as neocortical regions of the lateral hemisphere. The present data did not suggest a somatotopic organization of the projections; likewise, evidence for the presence of more than one somatosensorimotor representation was sparse.

Differential connections by intracortical axon collaterals among pyramidal tract cells in the cat motor cortex

1. Recurrent EPSPs were produced in fast pyramidal tract (PT) cells in the cat motor cortex by stimulation of the medullary pyramid and/or by the glutamate-induced activity of neighbouring PT cells using the spike-triggered averaging (spike-TA) method. 2. In fast PT cells located lateral to the end of the cruciate sulcus, predominantly the motor cortical representation area of the distal forelimb, two components (fast and slow) of recurrent EPSPs were produced by pyramid stimulation. 3. In response to pyramid stimulation, the appearance of the fast and slow components of recurrent EPSPs correlated with the appearance of N1 and N2 field potentials, respectively. 4. The monosynaptic nature of both the fast and slow components of recurrent EPSPs was demonstrated by a double shock test (interstimulus interval less than 5 ms) and high frequency repetitive stimulation (50-100 Hz). 5. The generation of the fast and slow components of recurrent EPSPs was attributed to the synaptic action of recurrent collaterals of fast and slow PT cells, respectively. 6. The amplitude of the slow component of recurrent EPSPs markedly increased with an increase in the stimulus frequency whereas that of the fast component did not, despite the change in stimulus frequency. 7. Selected spike-triggered averaging also revealed frequency facilitation of recurrent individual EPSPs produced in fast PT cells by the activity of single slow PT cells. 8. In fast PT cells located in the anterior and posterior lips of the cruciate sulcus, the motor cortical representation area of the proximal limb or trunk, only the slow component of recurrent EPSPs was produced by pyramid stimulation. 9. It is concluded that the pattern of recurrent connections between neighbouring PT cells differs depending on the motor cortical representation area, and that frequency facilitation of recurrent EPSPs is caused mainly by the input from axon collaterals of slow PT cells.

Bilateral interaction in the second somatosensory area (SII) of the cat and contribution of the corpus callosum

There are indications in the literature that convergent ipsilateral and contralateral input to the second somatosensory area (SII) may interact. Single unit activity of SII bilateral cells was studied to evaluate the impact of simultaneous bilateral stimulation of the receptive fields (RF) on neural discharge. The cellular responses to unilateral ipsilateral and contralateral, as well as to bilateral stimulation were compared. 22% of bilateral cells showed interaction, usually facilitation. Bilaterally evoked responses were found to be as great as 250% of the strongest unilateral response. Only bilateral responses stronger or weaker than the dominant unilateral response by at least 50% were considered as interactive. The great majority of interactive cells had their RF on the forelimb and were responsive to deep stimulation. The corpus callosum appears to be responsible for part of the observed interaction since in callosotomized cats only 5% of bilateral cells were interactive. A non-callosal ipsilateral pathway must be postulated because both bilaterality and bilateral interaction persist to some degree after callosotomy. A putative role for bilateral interaction in sensory-motor integration is discussed.

Identification of neurons producing long-term potentiation in the cat motor cortex: intracellular recordings and labeling

Intracellular, in vivo recordings were used to identify and subsequently to label neurons in the cat motor cortex in which long-term potentiation (LTP) was induced. Thirty-nine motor cortical neurons that produced excitatory postsynaptic potentials (EPSPs) in response to microstimulation in areas 1-2 (SI) or in area 5a (SIII) were studied. Amplitudes of EPSPs produced in response to test stimulation (1 Hz) were recorded before and after tetanic stimulation (200 Hz, 20 seconds). In 25/39 cells (64%), EPSP amplitudes were significantly increased following the tetanic stimulation (65 +/- 51% average increase), and remained at the potentiated level as long as stable recordings could be maintained (20 +/- 18 minutes, maximum = 90 minutes). LTP was induced exclusively in cells that produced monosynaptic EPSPs in response to area 1-2 or area 5a stimulation. Of the 39 analyzed cells, 13 were labeled by intracellular injections of 5% biocytin. Neurons in which LTP was induced included both pyramidal and nonpyramidal cells and were located exclusively in layers II or III of the motor cortex; cells in deeper cortical layers were not potentiated. These findings indicate that various corticocortical inputs can increase the efficacy of synaptic transmission in a subset of motor cortical neurons. We propose that this plasticity in synaptic transmission constitutes one of the bases of motor learning and memory.

Corticospinal projections from areas 4 and 6 in the raccoon

The corticospinal projections from areas 4 and 6 were investigated in the raccoon using the horseradish peroxidase (HRP) retrograde tracing technique. Multiple injections of lectin bound HRP and HRP were made into either the cervical or lumbar cord in 7 anesthetized raccoons. Retrogradely labeled neurons were observed throughout a wide extent of areas 4 and 6a beta. The HRP positive cells were most numerous within the dorsal bank of the cruciate sulcus within area 4 and continued around the fundus to occupy the lateral two-thirds of the ventral bank of the cruciate sulcus within area 6a beta. No labeled cells were observed in the more medially located area 6a alpha. Although the HRP positive cells observed following the lumbar cord injections were situated slightly more medial and caudal to those observed following the cervical cord injections, considerable overlap between the two projections was noted. The corticospinal projections arising from areas 4 and 6a beta in the raccoon largely correspond in location to the regions functionally defined as the primary motor cortex and the supplementary motor area, respectively.

The effects of lesions on autogenetic inhibition in the decerebrate cat

1. The effects of spinal and brain lesions on autogenetic inhibition from contraction receptors were studied in the decerebrate cat. Inhibitory feedback gain was estimated by measuring the effect of tension perturbations on reflex contractions of the soleus muscle. Tendon vibration was used to clamp the firing rate of primary spindle afferents, to prevent spindle unloading from disfacilitating the reflex contraction. In addition, secondary spindle afferents could be selectively excited by stimulating fusimotor fibres during muscle vibration. 2. Following an acute contralateral or bilateral dorsal transection of the spinal cord at L3, the vibration reflex tension fell by between 50 and 74% in three decerebrate animals. This was accompanied by a variable increase in inhibitory feedback, ranging between 180 and 360%. 3. In two animals, selective stimulation of fusimotor fibres supplying soleus muscle was without effect in the presence of muscle vibration both before and after the spinal lesion. In the third animal, a small and variable reduction in tension could be obtained only after the lesion, implying that an inhibitory pathway from homonymous secondary spindle afferents to alpha-motoneurones was released. 4. In a separate series of experiments, contralateral cerebral lesions were made 2-12 months prior to the acute inhibitory feedback measurement. Inhibitory feedback gain was increased, on average twofold in decerebrate animals with chronic cerebral lesions, when compared to control decerebrate animals. 5. Selective stimulation of fusimotor fibres to excite spindle secondary afferents was uniformly without effect in decerebrate animals with chronic cerebral lesions. In one animal spinal transection had only a minor effect on extensor tone and on inhibitory feedback gain, in contrast to the control decerebrate cats. 6. The implications of these findings are discussed in relation to the use of animals with spinal and supraspinal lesions as models of spasticity.

Postnatal redistribution of pericruciate motor cortical projections within the kitten spinal cord

The anatomical distribution of pericruciate cortical axons in the spinal cord was examined using anterograde transport of WGA-HRP from multiple unilateral injections into defined regions of the pericruciate cortex (PC) in 20 time-mated kittens and 3 adult cats. The gray matter and descending white matter tract distributions of WGA-HRP-labelled densities were analyzed using computerized morphometry and 3-dimensional reconstruction. In kittens older than 16 days postnatal (dPN), PC axon densities were found in dorsolateral column tracts corresponding to those of the adult, indicating that the white matter projections from the PC were largely established by this age. However, in kittens less than 38-44 dPN (about 105-109 days gestation), the spinal gray matter PC axon densities were distributed widely (so as to involve dorsal, intermediate and ventral laminae, e.g., I to IX) and bilaterally at all levels of the spinal cord. This contrasted sharply with the adult spinal cord in which the majority of densities was localized to laminae IV-VII on the contralateral side. The proportion of PC densities counted in the ipsilateral grey matter of neonates was found to average 23% of the total gray matter projection, while in adults this value was 9%. In all animals, by about 44 dPN, the terminal fields became effectively restricted to the adult distribution, that is, focused predominantly in the medial portions of laminae IV-VII in the gray matter contralateral to the cortical injection site.

Morphology of layer V pyramidal neurons in the cat somatosensory cortex: an intracellular HRP study

Pyramidal tract (PT) or corticopontine neurons of the cat somatosensory cortex (SI) were identified with antidromic activation on stimulation of the bulbar pyramid or pontine nuclei (PN) and stained intracellularly with HRP after examining the electrophysiological properties. Comparison of the conduction velocity of the stem axons and the soma-dendritic morphology revealed that in the cat SI, there exists two types of layer V pyramidal neurons, i.e. one has smooth apical dendrites with larger soma (51.6 +/- 9.5 x 22.7 +/- 2.8 micron) and the other has richly spinous apical dendrites with smaller soma (34.0 +/- 8.8 x 15.3 +/- 3.3 micron). The former group responded antidromically at latencies shorter than 1 ms by PT stimulation or 1.5 ms by PN stimulation, respectively. These values were consistent with the borderline latencies between two similar groups of layer V pyramidal neurons in the motor (fast and slow PTNs) and parietal (aspiny and spiny layer V corticopontine neurons) cortices in the cat.

Organization of cortical and subcortical projections to area 6m of the cat

By analyzing regional variations of afferent connectivity, we have identified a medial subdivision of feline area 6 (area 6m) which differs from all surrounding sectors of the frontal lobe in its pattern of inputs. Area 6m is located in the ventral bank of the cruciate sulcus and on the adjacent medial face of the frontal lobe and is partially coextensive with the medial frontal eye field as identified previously in electrophysiological experiments. Area 6m is innervated by axons from visual, association, and oculomotor areas and does not receive projections from somesthetic or somatomotor areas. Cortical sources of input to area 6m include several retinotopically organized extrastriate visual areas (AMLS, ALLS, and PLLS), association areas with strong links to the visual system (area 7, granular insula, posterior ectosylvian gyrus, and cingulate gyrus), and a lateral division of area 6 (area 61) with oculomotor functions. Thalamic afferents of area 6m derive from the paralamellar ventral anterior nucleus, from a dorsolateral division of the mediodorsal nucleus, and from the rostral intralaminar nuclei. The claustrum and the basolateral nucleus of the amygdala project to area 6m. Projections from area 7, the posterior cingulate area, the ventral anterior nucleus, and the mediodorsal nucleus are spatially ordered in a pattern such that parts of area 6 close to the fundus of the cruciate sulcus receive input from neurons positioned anteriorly in the cortical areas, dorsolaterally in the ventral anterior nucleus, and ventrolaterally in the mediodorsal nucleus. Our results indicate that area 6m probably is involved in the voluntary control of gaze and attention rather than in skeletomotor functions.

Morphological features of layer V pyramidal neurons in the cat parietal cortex: an intracellular HRP study

Layer V pyramidal neurons in the cat parietal cortex (areas 5 and 7) were investigated with intracellular HRP staining. Antidromic responses were recorded intracellularly as well as extracellularly with pontine stimulation under Nembutal anesthesia. The relationship between the latency of antidromic responses and the morphology of HRP-stained neurons was analyzed. A total of 65 neurons were stained with HRP, and sixteen of these neurons were activated antidromically with pontine stimulation. Two distinct groups of layer V pyramidal neurons were detected morphologically by intracellular HRP staining; i.e., one (F type) consisted of neurons with relatively large somata (58.4 +/- 8.1 micron X 24.5 +/- 5.1 micron, N = 11) and aspiny or sparsely spinous apical dendrites, and the other (S type) consisted of neurons with smaller somata (44.6 +/- 7.6 micron X 19.3 +/- 3.9 micron, N = 22) and richly spinous apical dendrites. These two groups showed different electrophysiological properties; i.e., the former responded antidromically to pontine stimulation at a latency shorter than 1.5 ms (namely, with a conduction velocity faster than 18 m/second) and the latter responded at a latency longer than 1.5 ms. The two neuronal types in the parietal cortex corresponded respectively to fast and slow pyramidal tract neurons (PTNs) investigated in the sensorimotor cortex. Although their morphological features were almost similar to those of PTNs, the branching pattern of apical dendrites of the F-type pyramidal neuron seemed to be different from that of fast PTNs. In the parietal cortex, apical dendrites of F-type neurons showed rather frequent branching in layer I. This was similar to the pattern of branching in slow PTNs. Such a characteristic branching pattern suggested that, in the cat parietal cortex, layer V pyramidal neurons of both types are adapted to receive cerebellar inputs through the ventroanterior (VA) thalamic nucleus to the superficial cortical layers.

The responses of pericruciate cortical neurones to distal forepaw electrical stimulation in the unanaesthetized, unrestrained cat

Experiments were performed to examine the responses of cortical neurons in the pericruciate cortex to cutaneous afferent input from the distal forepaw. Ninety-nine cortical neurons responding to electrical stimulation of the forepaw were recorded from four cats. Their response latencies ranged from 6 to 23 ms. The units had cutaneous receptive fields which ranged in size from those restricted to one digit to those extending over the whole forelimb. They were recorded from area 4 and area 3. Intracortical microstimulation at the recording sites activated either the distal or proximal musculature of the forelimb. When the characteristics obtained from each recording site were examined as a group of features, a uniform population emerged which was significantly different from the rest of the sample. These units had the shortest latency responses to distal forepaw electrical stimulation, the shortest duration of evoked discharge, the smallest distal cutaneous receptive fields. Such units were recorded at the border between areas 3 and 4, at sites which on microstimulation resulted in movements of the distal forepaw musculature.

Distribution of corticospinal motor fibres within the cervical spinal cord with special reference to the phrenic nucleus: a WGA-HRP anterograde transport study in the cat

The anterograde transport of wheat germ agglutinin conjugated with horseradish peroxidase (WGA-HRP) was used to demonstrate the corticospinal fibres which originate in the motor cortex and project to the cervical spinal cord, specifically to the phrenic nucleus, in the cat. Following injections of WGA-HRP into the pericruciate cortex large numbers of fibres were labelled in the contralateral lateral and ventral funiculi and fewer fibres were labelled in the ipsilateral lateral and ventral funiculi. Labelled corticospinal motor fibres entered the gray matter laterally in laminae V and VI and terminated within these two laminae as well as throughout the entire extent of lamina VII. A few labelled fibres were present in medial lamina VIII and also in lamina IX where they were in close association with the phrenic motoneuron pool. Labelling was present in the gray matter at both sides, with a stronger labelling contralaterally. Labelled axons were also seen crossing from each side of the gray matter to the other side. The results suggest that in the cat the corticospinal motor fibres have a wider distribution in the spinal gray matter than has been previously shown, and that corticospinal motor axons may be in direct contact with phrenic motoneurons.

Collaterals of corticospinal and pyramidal fibres to the pontine grey demonstrated by a new application of the fluorescent fibre labelling technique

Selective visualization of collaterals of corticospinal and pyramidal fibres to the pons in cat was obtained by retrograde transport of the fluorescent tracer fast blue (FB) through the stem fibres. Unilateral FB injections in the cervical cord and the pyramidal tract respectively produced soft blue fluorescent labelling of pyramidal fibres and of fibres and structures resembling 'terminals' in the pontine grey: contralateral to the spinal injections and ipsilateral to the pyramidal injections. These labelled elements were concluded to represent collaterals of corticospinal and pyramidal fibres because (a) their distribution corresponded to that of the pericruciate corticopontine fibres, (b) their labelling was prevented when the FB injections were preceded by a transection of either the cerebral peduncle or the pyramidal tract which lesions also prevented the FB labelling of the distal parts of the transected axons. Similar findings were obtained when using wheat germ agglutinin-horseradish peroxidase. In other experiments FB-labelling of pyramidal collaterals was combined with retrograde labelling of pontine neurones projecting to the contralateral anterior lobe of the cerebellum using diamidino yellow dihydrochloride as the second tracer. The distributions of the retrogradely labelled neurones and of the pyramidal collaterals in the pontine grey showed an almost complete overlap indicating that these collaterals mainly establish connections with the cerebellar anterior lobe.