Topography of projections from the motor cortex to rubrospinal units in the cat

Regional (14C) 2-deoxyglucose uptake during forelimb movements evoked by rat motor cortex stimulation: cortex, diencephalon, midbrain

The caudal forelimb region of right "motor" cortex was repetitively stimulated in normal, conscious rats. Left forelimb movements were produced and (14C) 2-deoxyglucose (2DG) was injected. After sacrifice, regions of increased brain (14C) 2DG uptake were mapped autoradiographically. Uptake of 2DG increased about the stimulating electrode in motor (MI) cortex. Columnar activation of primary (SI) and second (SII) somatosensory neocortex occurred. The rostral or second forelimb (MII) region of motor cortex was activated. Many ipsilateral subcortical structures were also activated during forelimb MI stimulation (FLMIS). Rostral dorsolateral caudate-putamen (CP), central globus pallidus (GP), posterior entopeduncular nucleus (EPN), subthalamic nucleus (STN), zona incerta (ZI), and caudal, ventrolateral substantia nigra pars reticulata (SNr) were activated. Thalamic nuclei that increased (14C) 2DG uptake included anterior dorsolateral reticular (R), ventral and central ventrolateral (VL), lateral ventromedial (VM), ventral ventrobasal (VB), dorsolateral posteromedial (POm), and the parafascicular-centre median (Pf-CM) complex. Activated midbrain regions included ventromedial magnocellular red nucleus (RNm), posterior deep layers of the superior colliculus (SCsgp), lateral deep mesencephalic nucleus (DMN), nucleus tegmenti pedunculopontinus (NTPP), and anterior pretectal nucleus (NCU). Monosynaptic connections from MI or SI to SII, MII, CP, STN, ZI, R, VL, VM, VB, POm, Pf-CM, RNm, SCsgp, SNr, and DMN can account for ipsilateral activation of these structures. GP and EPN must be activated polysynaptically, either from MI stimulation or sensory feedback, since there are no known monosynaptic connections from MI and SI to these structures. Most rat brain motor-sensory structures are somatotopically organized. However, the same regions of R, EPN, CM-Pf, DMN, and ZI are activated during FLMIS compared to VMIS (vibrissae MI stimulation). Since these structures are not somatopically organized, this suggests they are involved in motor-sensory processing independent of which body part is moving. VB, SII, and MII are activated during FLMIS but not during VMIS.

Ipsilateral actions from the feline red nucleus on hindlimb motoneurones

The main aim of the study was to investigate whether neurones in the ipsilateral red nucleus (NR) affect hindlimb motoneurones. Intracellular records from motoneurones revealed that both EPSPs and IPSPs were evoked in them via ipsilaterally located premotor interneurones by stimulation of the ipsilateral NR in deeply anaesthetized cats in which only ipsilaterally descending tract fibres were left intact. When only contralaterally descending tract fibres were left intact, EPSPs mediated by excitatory commissural interneurones were evoked by NR stimuli alone while IPSPs mediated by inhibitory commissural interneurones required joint stimulation of the ipsilateral NR and of the medial longitudinal fascicle (MLF, i.e. reticulospinal tract fibres). Control experiments led to the conclusion that if any inadvertently coactivated axons of neurones from the contralateral NR contributed to these PSPs, their effect was minor. Another aim of the study was to investigate whether ipsilateral actions of NR neurones, pyramidal tract (PT) neurones and reticulospinal tract neurones descending in the MLF on hindlimb motoneurones are evoked via common spinal relay neurones. Mutual facilitation of these synaptic actions as well as of synaptic actions from the contralateral NR and contralateral PT neurones showed that they are to a great extent mediated via the same spinal neurones. A more effective activation of these neurones by not only ipsilateral corticospinal and reticulospinal but also rubrospinal tract neurones may thus contribute to the recovery of motor functions after injuries of the contralateral corticospinal tract neurones. No evidence was found for mediation of early PT actions via NR neurones.

Quantitative analysis of the distribution of the motor cortex representations of the fore-and hindlimbs in the red nucleus of the cat

A quantitative analysis of the distribution of corticorubral fibers was performed after precise electrolytic lesioning of the lateral and medial margins of the posterior sigmoid gyrus--the motor representations of the fore-and hindlimbs respectively--in cats. The cortical representations of the forelimbs were found to project to the whole of the rostrocaudal extent of the red nucleus (RN). The number of efferent fibers terminating at the rostral margin of the RN was almost twice that terminating in the caudal third of the RN. Efferent fibers of the cortical representation of the hindlimbs did not project to the rostral two thirds of the RN but ended in its caudal third; the number of projecting corticorubral fibers was the same as the number running from the cortical representation of the forepaws to the caudal third of the RN. The significantly (almost double) greater number of fibers running from the cortical representation of the forelimbs in comparison with the number directed from the representation of the hindlimbs found in the present study is probably evidence of the greater functional importance of corticorubral connections in movement reactions performed by the forelimbs.

Discharge characteristics of neurons in the red nucleus during voluntary gait modifications: a comparison with the motor cortex

We have examined the contribution of the red nucleus to the control of locomotion in the cat. Neuronal activity was recorded from 157 rubral neurons, including identified rubrospinal neurons, in three cats trained to walk on a treadmill and to step over obstacles attached to the moving belt. Of 72 neurons with a receptive field confined to the contralateral forelimb, 66 were phasically active during unobstructed locomotion. The maximal activity of the majority of neurons (59/66) was centered around the swing phase of locomotion. Slightly more than half of the neurons (36/66) were phasically activity during both swing and stance. In addition, some rubral neurons (14/66) showed multiple periods of phasic activity within the swing phase of the locomotor cycle. Periods of phasic discharge temporally coincident with the swing phase of the ipsilateral limb were observed in 7/66 neurons. During voluntary gait modifications, most forelimb-related neurons (70/72) showed a significant increase in their discharge activity when the contralateral limb was the first to step over the obstacle (lead condition). Maximal activity in nearly all cells (63/70) was observed during the swing phase, and 23/63 rubral neurons exhibited multiple increases of activity during the modified swing phase. A number of cells (18/70) showed multiple periods of increased activity during swing and stance. Many of the neurons (35/63, 56%) showed an increase in activity at the end of the swing phase; this period of activity was temporally coincident with the period of activity in wrist dorsiflexors, such as the extensor digitorum communis. A smaller proportion of neurons with receptive fields restricted to the hindlimbs showed similar characteristics to those observed in the population of forelimb-related neurons. The overall characteristics of these rubral neurons are similar to those that we obtained previously from pyramidal tract neurons recorded from the motor cortex during an identical task. However, in contrast to the results obtained in the rubral neurons, most motor cortical neurons showed only one period of increased activity during the step cycle. We suggest that both structures contribute to the modifications of the pattern of EMG activity that are required to produce the change in limb trajectory needed to step over an obstacle. However, the results suggest an additional role for the red nucleus in regulating intra- and interlimb coordination.

Projections from the Cerebral Cortex to the Red Nucleus of the Guinea-pig. A Retrograde Tracing Study

The origin and the topographic distribution of corticorubral (CR) projections in the guinea-pig were studied by using the retrograde axonal transport of a tracer, colloidal gold-labelled, enzymatically inactive horseradish peroxidase conjugated to wheat-germ agglutinin (WGAapoHRP - Au), which was injected in the red nucleus (RN). It was found that the bulk of the CR projections arise from layer V neurons of the agranular frontal cortex in both its medial (Agm) and lateral (Agl) subdivisions; in the Agm labelled neurons are preferentially located in the upper part of layer V, whereas in the Agl they are more concentrated in the central band of the layer. Fewer projections originate from areas of the granular parietal and the agranular cingulate and retrobulbar cortices. CR projections have a bilateral origin, with a large ipsilateral predominance. The pattern of retrograde cortical labelling observed after injection of WGAapoHRP - Au in different portions of the RN indicates that CR projections are distributed throughout the entire rostrocaudal extent of the nucleus, but are slightly more concentrated in the rostral parvocellular area. The morphological arrangement of CR projections in the guinea-pig, as demonstrated in the present study, shows several analogies with other mammals. The functional characteristics of the cortical areas in which CR neurons are located indicate that CR projections may play a significant role in the central organization of movement.

Electrophysiological evidence for formation of new corticorubral synapses associated with classical conditioning in the cat

The present study was performed to clarify whether or not structural plasticity of synaptic connections underlies classical conditioning mediated by the red nucleus (RN) in the cat. Conditioned forelimb flexion is established by pairing electrical conditioned stimuli (CS), applied to corticorubral fibers at the cerebral peduncle (CP), with a forelimb skin shock (the unconditioned stimulus, US), but not by applying the CS alone or by pairing the CS and US at random intervals. In our previous study, it was shown that the firing probability of rubrospinal neurons (RN neurons) in response to the CS was well correlated with acquisition of the conditioned forelimb flexion and that the primary site of neural change underlying establishment of the conditioned forelimb flexion was suggested to be at corticorubral synapses. In the present study, we investigated corticorubral excitatory postsynaptic potentials evoked by CP stimulation (CP-EPSPs), in order to identify the neuronal mechanism underlying establishment of classical conditioning. In normal cats, CP-EPSPs had a typical slow-rising phase, which has been attributed to the distal location of corticorubral synapses on the dendrites of RN neurons. In contrast, in animals that received paired conditioning, subsequent CP stimulation evoked potentials with a fast-rising time course. In control groups of cats that received CS alone, CS randomly paired with the US, or only the same surgical operations as the conditioned animals, most of the CP-EPSPs displayed slow-rising EPSPs that similar to those observed in normal cats. The mean time from onset to peak of the potentials in the conditioned animals was significantly shorter than that seen in other groups. Therefore, the appearance of a fast-rising potential correlates well with acquisition of the conditioned forelimb flexion. The amplitude of the fast-rising potential was gradually changed with stimulus intensity. It had a short onset latency following CP stimulation (0.9 ms), which was similar to that of the slow-rising EPSP in normal cats. It followed high-frequency stimulation up to 100 Hz. These results suggest that the newly appearing, fast-rising potential was a monosynaptically evoked EPSP. Fast-rising EPSPs were also induced by stimulation of the sensorimotor cortex (SM). Since the SM-EPSP was occluded by the CP-EPSP, the SM cortex is, at least in part, a likely source of fast-rising EPSPs. Fast-rising SM-EPSPs were also observed at the unitary level. The SM-EPSPs in the conditioned animals exhibited somatotopical representation in their cortical origin, as has been described in normal cats.(ABSTRACT TRUNCATED AT 400 WORDS)

Lesion-induced establishment of the crossed corticorubral projections in kittens is associated with axonal proliferation and topographic refinement

The aberrant crossed corticorubral projection of the cat, which is very weak compared to the uncrossed one at about 1 month postnatal, becomes pronounced following unilateral lesions of the sensorimotor cortex. In order to determine whether or not terminal proliferation of pre-existing axons underlie this enlargement, the morphological changes of the crossed axons were examined, using the anterograde tracer Phaseolus vulgar- is leukoagglutinin (PHA-L). The crossed corticorubral axons in normal kittens were mostly simple in morphology with infrequent branching and did not often exhibit growth-cone-like axonal endings at 1 month postnatal. Two to 5 days after unilateral lesions of the sensorimotor cortex placed at this age, the axons were as simple as those in normal animals but ended in growth cones more frequently. Seven to 10 days post-lesion, the axons often bore side-branches which ended in growth cones. Two to 3 weeks post-lesion axons with sprays of finger-like fine sprouts occurred throughout the projection zone. There was no clear topography for the crossed projection in normal animals, but at 1-2 weeks post-lesion the axons started to show a certain amount of localization in the regions of the red nucleus which corresponded to the densely innervated region on the ipsilateral side. The topography of the crossed projections roughly mirrors that of the ipsilateral projection at about 1 month post-lesion. Thus, the lesions of the sensorimotor cortex induce substantial growth and proliferation of the crossed corticorubral axons. The post-lesion changes in axonal morphology and topographic refinement are reminiscent of developmental events. It is likely that the lesions permit the crossed axons, which normally fail to develop, to develop like the uncrossed ones.

Postnatal development of crossed and uncrossed corticorubral projections in kitten: a PHA-L study

Morphological changes in individual corticorubral fibers and the pattern of crossed and uncrossed corticorubral projections were studied during the postnatal development of cats in order to understand cellular mechanisms for restriction of corticorubral projections with development. The anterograde tracer Phaseolus vulgaris leucoagglutinin (PHA-L) was injected into restricted areas of the pericruciate cortex in kittens and PHA-L-labeled axons in the red nucleus were examined at postnatal days (PND) 7-73. In accordance with our previous study (Murakami and Higashi, Brain Res. 1988; 447:98-108), a crossed corticorubral projection was observed in addition to the uncrossed one in every experimental animal. During the early period of development (PND 7-8), swellings of irregular shape were observed along the entire course of the axons and they were often interconnected with extremely fine axonal segments. These axons bifurcated only infrequently and often ended as growth cones. These features were common to both uncrossed and crossed corticorubral axons. At later stages of development (PND 28 or later), the total number of swellings decreased and axonal swellings with smooth contours became dominant. A quantitative examination of axonal branches indicated that axons on the ipsilateral side branch occurred more frequently at later stages of development. However, there was no substantial change in branching frequency for the crossed corticorubral fibers during development. In parallel with morphological changes in individual axons, the crossed projection that was initially relatively abundant was reduced during development. Since a PHA-L injection can be confined to a small region of cortex, topographic projections can easily be detected. At PND 7-8 there was no well-defined topographic order in the ipsilateral corticorubral projection. Adult-like topography was first discernible at PND 13. These observations suggest that the unilateral uncrossed corticorubral projection in the adult cat is achieved at least in part by the formation of axonal arbors in the uncrossed projection. This was accompanied by the failure of crossed fibers to form complex arbors. It is possible that a similar mechanism also operates in the formation of topographic maps.

Bilateral pericruciate cortical innervation of the red nucleus in cats with adult or neonatal cerebral hemispherectomy

We studied remodeling of the remaining corticorubral projections in adult cats sustaining a left cerebral hemispherectomy in adulthood or neonatally using cortical injections of [3H]leucine-proline. Injection sites and terminal fields were reconstructed from autoradiography-processed tissue. In all cats, the label filled similar extents of ares 4 gamma and 3a of the right frontal cortex. We used sections at 8 coronal planes throughout the red nucleus (RN) for computer-assisted analysis of visually estimated density and topography of distribution of terminal label, and for calculation of RN cross-sectional area. Additionally, at 3 coronal planes we further quantified terminal label using computerized procedures (number of particles for surface area). In all lesioned cats we found terminal label in the RN contralateral to the injection site with a topographic distribution similar to that of the RN ipsilateral to the injection in normal or lesioned cats and in absence of any significant shrinkage of the nucleus. The difference between the 2 age-at-lesion groups was that in the cats with neonatal ablation the density of contralateral terminal label was about double that seen in adult-lesioned subjects. However, the amount of contralateral labeling in adult-lesioned cats was substantial and represented a significant increase over the minimal labeling seen in normal cats. There were no differences between groups in labeling or size of the RN ipsilateral to the injection site. For reasons discussed, we interpret the label on the side of the hemispherectomy as representative of reinnervation of the cortically deafferented RN by crossing collaterals of fibers arising in the remaining motor cortex and not as lesion-sustained persistent prenatal connections.

Electrophysiological identification of a somaesthetic pathway to the red nucleus

In awake chronically implanted cat, the cells in the red nucleus (RN) can be either activated or inhibited by natural stimulation on periphery. The effective stimuli are touching the fur, rotating the joints and tapping the muscles. A somaesthetic map has been constructed with the face area dorsally, the forelimb more ventrally and the hindlimb lateroventrally in the RN. In acute preparations, after ablation of the motor cortex and the cerebellum and section of the dorsal columns of the spinal cord at cervical level, the RN cells were still reacting to natural stimulation of the skin or electrical stimulation of peripheral nerves. The course of the somaesthetic pathway was systematically mapped by microstimulation of the spinal cord. It was shown that it follows the primary afferents which enter the dorsal columns, where they give off collaterals which relay at segment levels. After decussation the fibres ascend the ventromedial quadrant of the cord. A large portion of the fibres relay a second time in the medulla. The existence of such a pathway can account for the somaesthetic responses recorded in the RN in awake cats.

Electrophysiological peculiarities of cortical inputs to the cat red nucleus

Complex multicomponent EPSPs of the red nucleus rubro-spinal neurons evoked by stimulation of the sensorimotor cortex and associative fields of the parietal cortex were studied in acute pentobarbitalized cats by intracellular recording technique. Complex cortical EPSPs were recorded in 2/3 of the neurons studied. Components of the EPSPs in question were distinguished by using stimulation of various frequency and intensity. The first component of the EPSPs appearing at the lowest threshold was found to have a short and stable latency, stable rising time for depolarization and was able to follow high frequencies of stimulation. The second component was more variable, although in some EPSPs it too had a short latency, was stable enough and, like the first component could be classified as monosynaptic. The complex character of the EPSPs recorded persisted after the removal of the cerebral gray and was observed when stimulating the white matter so excluding its cortical origin. The first two components of the EPSP were evoked by corticofugal impulsation propagating at an average velocity of 18.5 m/s and 7.5 m/s being supposedly the result of activation of the slow-conducting pyramidal and cortico-rubral neurons. In some rubro-spinal neurons they were characterized by a fast rising phase being apparently an electrophysiological manifestation of the activation of axosomatic synapses.

Long term modification of rubral activity after inferior olive destruction

The long term effects of the inferior olive degeneration on red nucleus activity were studied in the rat. The animals were injected with 3-acetylpyridine to produce a pharmacological destruction of the inferior olive and were then used for acute experiments at 1-2, 5-7, 14-18, 29-37, 81-110 and 236-255 days later. After degeneration of the inferior olive, there was an 'initial period' lasting for a few days, characterized by a low discharge frequency of the red nucleus neurones. A 'period of adaptation' followed during the first month, characterized by a slow recovery towards the control firing rates of the rubral units. Nevertheless, the temporal distribution of the discharges was not recovered since the firing became organized in a bursting activity. From 1 up to 8 months, the normal unit activity was not restored. The hypothesis is advanced that the suppression of the inferior olive which increases the cerebellar inhibition, produces a consequent disfacilitation of red nucleus activity which persists for a few days. Then at increasing survival times, a progressive compensation takes place without a real restoration of the initial rubral activity.

Pyramidal and non-pyramidal projection from cortical areas 4 and 6 to the red nucleus in the cat

The pyramidal tract (PT) and the red nucleus (RN) of cats were stimulated electrically to identify by antidromic invasion PT and non-PT neurons projecting from cortical areas 4 and 6 to the RN. A main result was that the input to RN is much stronger from PT neurons than that of non-PT neurons in area 4, and vice versa in area 6.

Evidence for a crossed corticorubral projection in cats with one cerebral hemisphere removed neonatally

Tritiated amino acids were injected into the right pericruciate cortex and the projections to the red nucleus were mapped autoradiographically in 10 intact and 6 adult cats with one cerebral hemisphere (left side) removed neonatally. In the intact cats only projections to the ipsilateral red nuclei were seen. The terminals were distributed along the entire rostrocaudal extent, but the density of the label was greater in the ventral and medial aspects of the nucleus. In the hemispherectomized cats projections to both red nuclei were found. The topography of the terminals was similar to that seen in intact animals. This finding, together with other changes which we have described, suggests that an extensive structural reorganization may occur after removal of one cerebral hemisphere in cats.

Formation of functional synapses in the adult cat red nucleus from the cerebrum following cross-innervating of forelimb flexor and extensor nerves. I. Appearance of new synaptic potentials

We investigated the effects of cross-innervating the peripheral forelimb flexor and extensor nerves of adult cats on the time course of corticorubral EPSPs. Red nucleus neurons were identified by antidromic invasion from C1 or L1 spinal segments as innervating the upper spinal segments (C-cells) or sending axons to the lumbosacral cord (L-cells). In C-cells, a fast-rising component, superimposed on the slow-rising corticorubral EPSPs induced by the cerebral sensorimotor cortex or the cerebral peduncle (CP) stimulation, was noted. The mean time-to-peak of this component in cross-innervated cats operated more than two months earlier was 1.9 +/- 0.9 ms (n = 160), shorter than in normal cats (3.6 +/- 1.4 ms, n = 100). The same value in cats cross-innervated less than two months before was 2.7 +/- 1.0 ms (n = 53). The mean time-to-peak of CP-EPSPs from L-cells was 2.9 +/- 0.9 ms (n = 115). The fast-rising component had a latency of 0.96 +/- 0.19 ms (n = 122), and it was mediated by fibers with conduction velocities of less than 20 m/s. The projective area of the fast-rising component is organized somatotopically. Since it is more sensitive to membrane hyperpolarization than slow rising corticorubral EPSPs, it is mediated by synapses located more proximally than the corticorubral synapses of normal cats. The time course of facilitation by preceding cerebral peduncle stimulation of the nucleus interpositus (IP)-induced RN population responses was measured. It was characterized by a rapid, followed by a slower, rise time in the RN region where C-cells are concentrated. In contrast, the L-cell region was characterized by a slow rise time. In cats subjected to self-union of the peripheral flexor and extensor nerves, the majority of C-cells had CP-EPSPs with a time-to-peak within the normal range. Our results suggest that after cross-innervation sprouting and formation of functional synapses occur on the proximal portion of the soma-dendritic membrane of red nucleus neurons.

Comparative study of the posterior red nucleus in baboons and gibbons

The posterior red nucleus (PRN) was studied in two species of primates by the technique of retrograde degeneration of rubrospinal cells following transection of the spinal cord at different levels. The form of the PRN was reconstructed for both a quadruped monkey (baboon) and an anthropoid with erect posture (gibbon). The PRN contains polymorphic cells characterized by their very chromophilic and granular Nissl substance. These neurons vary in diameter from 25 micrometer to 70 micrometer. Some of them give rise to the rubrospinal tract. Baboon: The approximately 1,300 rubrospinal cells in this species are divided into two equal groups, one related to the contralateral forelimb, with axons ending between the second cervical and third thoracic segment, and the other related to the contralateral hindlimb, projecting caudally beyond T3. Following a high cervical lesion, nondegenerated cells of similar description remain throughout the nucleus. A significantly large group of these cells occurs medially and may be the source of fibers ending in the brain stem or cerebellum. Gibbon: In this species, the number of rubrospinal cells controlling the hindlimb is less than half that found in the baboon. This reduction in the gibbon is much greater for medium-sized cells, but is also significant for the giant cells. These results obtained from primates are compared with those reported for the cat. A possible function for the PRN in the control of limb movements is discussed from the viewpoint of phylogeny.

A search for corticospinal collaterals to thalamus and mesencephalon by means of multiple retrograde fluorescent tracers in cat and rat

An attempt has been made to determine anatomically whether in rat and cat cortical projections to ventrolateral nucleus of thalamus and to mesencephalon are in part composed of corticospinal collaterals. For this purpose two different fluorescent tracers were injected: one in the spinal cord and the other contralaterally in the lateral thalamus and in the mesencephalon respectively. In these experiments Fast Blue and True Blue were used in combination with Nuclear Yellow. Evans Blue was used in combination with Granular Blue. After injections of the tracers into the thalamus and spinal cord two different populations of single retrogradely labeled cortical neurons were found, while after injections in mesencephalon and spinal cord double-labeled cortical neurons occurred. This has lead to the conclusion that in cat and rat corticospinal neurons do not distribute collaterals to specific thalamic nuclei, but do distribute collaterals to mesencephalon. Moreover, the preferential distribution of the double-labeled corticospinal neurons in cat suggest that the corticospinal neurons distributing collaterals to the mesencephalon in part are concentrated in those cortical areas which subserve the steering of movements of the head, neck and trunk.

Development of the brain stem in the rat. V. Thymidine-radiographic study of the time of origin of neurons in the midbrain tegmentum

Groups of pregnant rats were injected with two successive daily doses of 3H-thymidine from gestational day E12 and 13 (E12 j3) until the day before parturition (E21 k2) in order to label in their embryos the proliferating precursors of neurons. At 60 days of age the proportion of neurons generated (no longer labeled) on specific embryonic days was determined quantitatively in 18 regions of the midbrain tegmentum. The neurons of the oculomotor and trochlear nuclei are generated concurrently on days E12 and 13. There was a mirror image cytogenetic gradient in these nuclei and this was interpreted as the dispersal of neurons derived from a common neuroepithelial source to the medial longitudinal fasciculus. Neurons in three other components of the tegmental visual system are produced in rapid succession after the motor nuclei. In the nucleus of Darkschewitsch peak production time was on day E12 and 13, extending to day E15; in the Edinger-Westphal nucleus the time span was the same but with a pronounced between days E13; finally, the neurons of the parabigeminal nucleus were produced between days E13 and E15 with a peak on day E14. The neurons of the periaqueductal gray were generated between days E13 and 17 with a pronounced ventral-to-lateral and lateral-to-dorsal gradient. In the red nucleus the neurons were produced on days E13 and E14 with a caudal-to-rostral gradient: the cells of the magnocellular division preceding slightly but significantly the cells of the parvocellular division. The neurons of the interpeduncular nucleus originated between days E13 and E15; the peak in its ventral portion was on day E13, in its dorsal portion on days E14 and E15. A ventral-to-dorsal gradient was seen also in both the dorsal and the median raphe nuclei in which neuron production occurred between days E13 and E15. The neurons of the pars compacta and pars reticulate of the substantia nigra were both produced between days E13 and E15 with a modified lateral-to-medial gradient. This gradient extended to the ventral tegmental area where neurons of the pars medialis were produced between days E14 and E16. With the exception of the central gray, neuron production was rapid and relatively early in the structures situated ventral to the midbrain tectum. A comparison of the cytogenetic gradients in the raphe nuclei of the lower and upper medulla, the pontine region, and the midbrain suggests that they originate from at least three separate neuroepithelial sources.

Somatotopic localization in cat motor cortex

Punctate intracortical stimulation of the motor cortex (areas 4 and 6), with parallel observation of the induced movements, permits description of a fine somatotopic organization of the motor control areas for different parts of the musculature in freely moving adult cats. The results show that movements produced by electrical stimulation of the motor cortex are always single and non-repetitive, regardless of the duration and intensity of the stimulation. These movements are restricted to a very precise part of the musculature, and experiments show that this localization is related to the exact position of the tip of the stimulating electrode in the motor cortex. Other experimental data show that motor responses which disturb the animal's equilibrium are accompanied by postural adjustments. Stimulation of the cerebral cortex permits the definition of a separate motor control area for each part of the cat musculature, with an individual control area for each of the joints of the forelimb. This was not possible for the hindlimb, which is always activated in its entirety. These results establish a new representation of the somatotopic organization of the cat motor cortex. This diagram shows that area 6 controls the more axial parts of the musculature, while area 4 controls the proximal and distal parts of the limb muscles. This map was compared to numerous previous data on the somatotopic organization in the cat motor cortex, especially to the map of Woolsey.

Input-output relations of the red nucleus in the cat

In unanesthetized cats, microstimulation within the red nucleus produces contraction of single muscles of the contralateral limbs and face. Separate zones may activate different muscles. Forelimb muscles were primarily activated from areas in the dorsomedial quadrants of the red nucleus whereas hindlimb muscles were predominantly activated from the ventorlateral quadrants. With stimulus currents of 10 muA there was considerable overlap in the effective zones activating different muscles. In the majority of cases the minimal threshold was under 10 muA when stimilating with a 50-msec pulse train. Current thresholds for electromypgraphic changes in the muscles varied inversely with pulse frequency and train duration. When long stimulus trains were applied to the red nucleus, the resulting muscle contraction was sustained for the duration of the stimulus. These motor effects did not depend upon the motor cortex or pyramidal tract but were mediated by a tract in contralateral dorsal quadrants of the spinal cord which was likely to be the rubrospinal tract. Units within the red nucleus typically had wide cutaneous receptive fields and responded to deep pressure and joint rotation in one or more limbs. Usually the focus driving the cell most briskly was located in one of the contralateral limbs and corresponded to the limb where muscle contraction was elicited by microstimulation with the same electrode. It is concluded that the red nucleus includes overlapping efferent neuronal colonies controlling individual muscles irrespective of their functional class. This property is shared by the motor cortex and suggests that these two structures may complement each other in the control of movement. The more diffuse activation of rubral than cortical neurons by natural stimuli suggests that rubral activity may not be as tightly linked as that of the motor cortex to specific peripheral input.