Motor cortex vestibular responses in the chloralosed cat

Summary Controlled electrical stimulation of an ampullar nerve in the inner ear was used to demonstrate vestibular projections to the motor cortex of cats anaesthetized with chloralose. Macroelectrode recordings from the cortical surface in the somatomotor region, around the cruciate sulcus, show two kinds of vestibular evoked responses: the latency, shape and distribution of these potentials indicate a different origin for each type, as do also the results of experiments investigating somato-vestibular interactions. A study of the somatic and vestibular inputs to individual pericruciate neurons, and of their possible output into the pyramidal tract, revealed that 1. the vestibular afferents (reaching 40% of the population studied) are principally distributed among the somatic neurons receiving their input from several limbs, and 2. some of the cortical cells receiving vestibular afferents contribute axons to the pyramidal tract. Results are discussed with respect to the possible functional significance of such a vestibular input to the motor area for the adequate performance of movements.Chercheur qualifié FNRS (Belgique).Aspirant FNRS (Belgique).     

Distribution of synapses on an intracellularly labeled small pyramidal neuron in the cat motor cortex

Summary The morphological characteristics and distribution of synapses on a small pyramidal neuron in layer III of the cat motor cortex have been studied by combining intracellular HRP staining and electron microscopic examination. The stained neuron showed spiny apical and basal dendritic profiles under the light microscope, and exhibited the morphological features of a pyramidal neuron. Ultrastructural analysis indicated that about 80% of the presynaptic terminals formed asymmetrical synapses with spines of distal apical and basal dendrites. On proximal apical dendrites, 64% of the synapses were found to make contact with spines, and 16.7% of the synapses were of symmetrical type and formed with dendritic shafts. Two types of terminal could be identified on the soma; they were alternately located and established symmetrical and asymmetrical synaptic contacts respectively. Possible functional implications are discussed.This paper is dedicated to Professor Fred Walberg on the occasion of his 70th birthday.     

Specific features of sensorimotor cerebral cortex activity modulation by dopamine releaser amantadine

The modulatory effects of amantadine (1-adamantanamine) on the activity of sensorimotor cerebral cortex neurones during microiontophoretic application of agonists of glutamatergic and GABA-ergic (γ-aminobutyric acid) transmission were studied. In non-anaesthetised cats, dopamine (DA) released by amantadine application in a small area of the neocortex increased baseline and evoked neuronal activity, providing stabilization and optimum course of both the neuronal and the conditioned responses of the animal. Amantadine eliminates a decrease in the level of neuronal baseline and evoked activity and marked increase in the latency of neuronal activation and conditioned movement mediated by D2 receptor antagonist sulpiride ((S)-5-aminosulfonyl-N-[(1-ethyl-2-pyrrolidinyl) methyl]-2-methoamantadineybenzamide) or GABA. This is reflected by a proportionate decrease in the onset of neuronal impulse reaction and latency of conditioned movement. Combined NMDA (N-methyl-d-aspartate) and amantadine application also caused a considerable increase in baseline and evoked activity, but produced a slightly weaker effect than that evoked by NMDA application alone. A decrease in the baseline and evoked neuronal activity after NMDA withdrawn lasted during next control session (up to 40 min). The ability of DA releaser amantadine to alleviate significant increase in the latency of neuronal responses and conditioned movement induced by sulpiride or GABA suggests that dopamine modulates the activity of GABA-ergic inhibitory fast spike interneurons in the cat sensorimotor cortex during conditioning.     

Role of the sensorimotor cortex in postural adjustments accompanying a conditioned paw lift in the standing cat

Summary The role of the sensorimotor cortex in the postural adjustments associated with conditioned paw lifting movements was investigated in the cat. Cats were trained to stand quietly on four strain gauge equipped platforms and to perform a lift-off movement with one forelimb when a conditioned tone was presented. The parameters recorded were the vertical forces exerted by the paws on each platform, the lateral and antero-posterior displacements of rods implanted on the T2, T12, L5 vertebrae as well as their rotation, and the EMG of triceps and biceps of both forelimbs. Before lesion, the postural adjustment consisted of a nondiagonal pattern where the CG was displaced laterally inside the triangle formed by the three remaining supporting limbs. Here a lateral bending of the thoracic column toward the supporting forelimb could be observed. The associated EMG pattern consisted of an early activation of the triceps lateral head in the moving limb which was probably responsible for the body displacement toward the opposite side, and a late biceps activation associated with the lift. In the supporting forelimb, a coactivation of the biceps and triceps was usually present. After contralateral sensorimotor lesion, the conditioned lifting movements were lost for 4–15 days after the lesion, before being subsequently recovered. The same lateral CG displacement and bending of the back was seen after lesion as before, which indicates that the goal of postural adjustment was preserved. However, the means of reaching it were modified. In most of the intact animals, the CG displacement was achieved in one step, whereas in the animals with lesions, the displacement was made either according to a slow ramp mode or in a discontinuous manner involving several steps. The mechanisms responsible for this disturbance are discussed.     

Task-related coding of stimulus and response in cat motor cortex

Summary In a previous study in the cat, we have reported that motor cortex neurons discharging before the initiation of an aimed forearm response (lead cells) are better timed to movement of a display (stimulus) than to the response. The present study was done to distinguish the coding of stimulus and response features in the discharge patterns of such early activity in motor cortex. Single neurons were recorded in the arm area of motor cortex in three cats performing the same pair of responses (forearm flexion and extension) but to display movements in either of the two directions by changing display polarity. The modulation of lead cell activity was contingent on the occurrence of the learned motor response and timed to the stimulus in all conditions. The majority of lead cells (88%, n = 50) fell into one of two distinct classes. In one class of neurons, force-direction (56%, n = 32), activity was contingent on a single direction of forelimb response (flexion or extension) and was thus independent of the direction of the display stimulus. The only muscles whose patterns matched the activity of this class of response-related neurons were forelimb flexors and extensors. In these neurons, the onset of modulation was timed to one or the other of the two stimuli according to the stimulus direction which elicited the appropriate response. Thus, the display-related input to these neurons varied according to the response required. In the second class of neurons, stimulus-direction (32%, n = 18), modulation was associated with a specific stimulus direction rather than the response direction. The pattern of activity of these neurons was similar to the pattern of EMG signals of shoulder and neck muscles during the different task conditions. The contraction of proximal and axial muscles corresponded to a second response elicited by the stimulus, namely attempts at head rotation towards the moving display and was independent of the conditioned forelimb response in both time of onset and direction. To test the possibility that stimulus-direction neurons participated in the control of head rotation we trained two of the animals to also produce isometric changes in neck torque in the direction of the moving display without making the forelimb response. The activity of stimulus-direction neurons was similarly modulated during performance of the neck task. By contrast, force-direction neurons examined during the neck task were either unmodulated or discharged after the neck response. These data suggest that force-direction neurons participate in response initiation and that their activity is triggered by stimuli specific for the task. The reorganization of the inputs to motor cortex is likely to result from gating mechanisms associated with behavioral set. Such neural gates could provide for the efficient transfer of any member of an array of behaviorally relevant stimuli to restricted sectors of the somatotopically organized motor areas.     

Minimal stimulus parameters and the effects of hyperpolarization on the induction of long-term potentiation in the cat motor cortex

Summary The aim of the research program of which the present work is a part is to understand the neural mechanisms involved in motor learning and memory. One of the mechanisms postulated to be involved in this process is the induction of long-term potentiation (LTP) in the motor cortex. LTP can be induced in motor cortical neurons by tetanic stimulation of their afferents from the somatosensory cortex. In the present study, the effects of different stimulating parameters on the induction of LTP were examined, using in-vivo, intracellular recordings from anesthetized cats. The expression of LTP was documented by measuring the amplitude and rise-time of excitatory postsynaptic potentials (EPSPs) before and after tetanic stimulation. The minimal tetanic stimulation capable of systematically inducing LTP was found to consist of a train of stimuli at 50 Hz, 5 s. Shorter trains of stimulation produced only a short-lasting, transient potentiation. In different cells, identical stimulation parameters resulted in different degrees of potentiation of synaptic responses. Following all the stimulation trains examined, EPSP amplitudes were transiently depressed before reaching potentiated levels. The duration of this depression was directly correlated with the duration and the frequency of the tetanic stimulation. In all the cells in which LTP was induced, the variability in the amplitudes of potentiated EPSP was significantly greater than that of control EPSP amplitudes. Hyperpolarization of the postsynaptic cell, during the delivery of the tetanic stimulation, inhibited the induction of LTP. These phenomena are discussed in relation to the postulated mechanisms of LTP induction in the cortex.     

Segregation of lemniscal inputs and motor cortex outputs in cat ventral thalamic nuclei: application of a novel technique

Summary A double labeling method that permits accurate delineation of the terminals of medial lemniscal fibers was used to determine whether thalamic neurons projecting to motor cortex in the cat are in a position to be contacted by such terminals. Thalamic neurons in the VL nucleus were retrogradely labeled by injections of fluorogold placed in the cytoarchitectonically defined area 4, while lemniscal axons and their terminal boutons were anterogradely labeled, in a Golgi-like manner, from injections of Fast Blue placed under physiological control in different parts of the contralateral dorsal column nuclei. In additional experiments, spinothalamic fibers were similarly labeled by injections of Fast Blue in the spinal cord. The results reveal that there is no significant overlap in the distributions of lemniscal terminals and motor cortex-projecting neurons and that no somata or proximal dendrites of motor cortex-projecting neurons are in a position to receive lemniscal terminals. Spinothalamic terminals, on the other hand, end in clusters around motor cortex-projecting neurons in the VL nucleus as well as in other nuclei and are a more likely route for short latency somatosensory inputs to the motor cortex.Abbreviations AD anterodorsal nucleus - AM anteromedial nucleus - AP area postrema - AV anteroventral nucleus - C cuneate nucleus - CeM central medial nucleus - CL central lateral nucleus - CM centre médian nucleus - EC external cuneate nucleus - G gracile nucleus - L limitans nucleus - LD lateral dorsal nucleus - LP lateral posterior nucleus - MGM magnocellular medial geniculate nucleus - MD mediodorsal nucleus - MTT mamillothalamic tract - MV medioventral nucleus - Pc paracentral nucleus - Pf parafascicular nucleus - Po posterior nuclei - R reticular nucleus - RF fasciculus retroflexus - S solitary nucleus - SG suprageniculate nucleus - T spinal trigeminal nucleus - VA ventral anterior nucleus - VIN vestibular nuclei - VL ventral lateral nucleus - VMb basal ventral medial nucleus - VMp principal ventral medial nucleus - VPL ventral posterior lateral nucleus - VPM ventral posterior medial nucleus - ZI zona incerta - 1,2,3a,3b,4 fields of cerebral cortex - C4, C5, C6 spinal cord segments - 5SP,5ST spinal trigeminal nucleus and tract - 10, 12 vagal and hypoglossal nuclei     

Sensory response properties of pyramidal tract neurons in the precentral motor cortex and postcentral gyrus of the rhesus monkey

Summary Pyramidal tract neurons (PTNs) were identified in precentral motor cortex (MI) and in postcentral cortex (PoC) of a monkey trained to pronate and supinate its forearm. PTN responses to passive, ramp-form displacements of the forearm were examined in relation to the size of the neuron (as reflected by its antidromic latency). Larger PTNs tended to exhibit transient responses to passive limb displacement, whereas smaller PTNs more frequently showed sustained responses. These findings suggest that smaller PTNs, that make up the majority of the total PTN population, receive continuous feedback during posture as well as during the dynamic phase of movement.     

Specialized subregions in the cat motor cortex: Anatomical demonstration of differential projections to rostral and caudal sectors

Summary (1) Ipsilateral cortico-cortical and thalamo-cortical projections to the cat motor cortex were determined from the locations of retrogradely labeled neurons following single small intracortical injections of HRP in area 4. These projections were also examined by studying the distribution of anterogradely transported axonal label following multiple injections of HRP or of tritiated amino acids in areas 1–2 of SI and in area 2pri (SII). (2) The number of retrogradely labeled cells in areas 1–2 and in area 2pri differed markedly between HRP injection sites located in the precruciate (anterior sigmoid gyrus) and postcruciate (posterior sigmoid gyrus) subregions of area 4. These associational projections from primary and secondary somatosensory cortices were dense to postcruciate subrogions but weak to the precruciate subregions. (3) The associational projections from areas 1–2 and from area 2pri to the postcruciate subregion of area 4 were topographically organized, but no clear topographic organization could be demonstrated for the precruciate projection. (4) Anterograde terminal labeling following injection of either HRP or tritiated amino acids into areas 1–2 and area 2pri confirmed the preferential projection of somatosensory cortex to the postcruciate subregion of motor cortex. The projection from somatosensory areas 1–2 was uniform over its terminal field, but that from area 2pri was more patchy and complex. (5) HRP injections in area 4 gave rise to lamellae of labeled neurons in the ventrolateral nucleus of thalamus (VL). A topographic relationship was found between the site of injection and the location of the lamella of labeled neurons. (6) The percentage of retrogradely labeled neurons in the shell zone surrounding the border of the ventrolateral nucleus and the ventrobasal complex (VB) was greater following postcruciate than precruciate injections, whereas fewer retrogradely labeled neurons were found in central lateral nucleus (CL) after postcruciate injections than after precruciate injections. (7) These observations support the hypothesis that differential cortical and thalamic projections to different subregions of area 4 may give rise to the different physiological properties of neurons observed in these subregions (Vicario et al. 1983; Martin et al. 1981).     

Short latency somaesthetic responses in motor cortex,transmitted through the spino-thalamic system,in the cat

Summary Evidence is presented that in the cat, the spinothalamic system contributes to short latency somaesthetic responses in motor cortex efferent cells. Intracellular recordings performed on identified pyramidal tract cells and corticospinal cells show that these cells are still activated and/or inhibited from the periphery after a set of central nervous lesions leaving intact only the ventral half of the spinal cord. The responses were attributed to the spinothalamic system. The ascending system is activated through collaterals of afferent fibres running in the dorsal columns of the spinal cord. This peripheral link to the motor cortex might participate in updating the motor command on the basis of information feedback from the periphery.     

Task-dependent modulation of excitatory and inhibitory functions within the human primary motor cortex

We evaluated motor evoked potentials (MEPs) and duration of the cortical silent period (CSP) from the right first dorsal interosseous (FDI) muscle to transcranial magnetic stimulation (TMS) of the left motor cortex in ten healthy subjects performing different manual tasks. They abducted the index finger alone, pressed a strain gauge with the thumb and index finger in a pincer grip, and squeezed a 4-cm brass cylinder with all digits in a power grip. The level of FDI EMG activity across tasks was kept constant by providing subjects with acoustic-visual feedback of their muscle activity. The TMS elicited larger amplitude FDI MEPs during pincer and power grip than during the index finger abduction task, and larger amplitude MEPs during pincer gripping than during power gripping. The CSP was shorter during pincer and power grip than during the index finger abduction task and shorter during power gripping than during pincer gripping. These results suggest excitatory and inhibitory task-dependent changes in the motor cortex. Complex manual tasks (pincer and power gripping) elicit greater motor cortical excitation than a simple task (index finger abduction) presumably because they activate multiple synergistic muscles thus facilitating corticomotoneurons. The finger abduction task probably yielded greater motor cortical inhibition than the pincer and power tasks because muscles uninvolved in the task activated the cortical inhibitory circuit. Increased cortical excitatory and inhibitory functions during precision tasks (pincer gripping) probably explain why MEPs have larger amplitudes and CSPs have longer durations during pincer gripping than during power gripping. Electronic Publication     

Differential impairments in reaching and grasping produced by local inactivation within the forelimb representation of the motor cortex in the cat

This study analyzed changes in the performance of a reaching task and its adaptive modification produced by reversible inactivation of three sites within the forelimb representation of the motor cortex (MCx, area 4) in five cats by microinjections of muscimol. Two sites were located in the lateral MCx, rostral (RL-MCx) and caudal (CL-MCx) to the end of the cruciate sulcus, where intracortical microstimulation (ICMS) produced contraction of the most distal muscles. The third site was located more medially, in the anterior sigmoid gyrus (RM-MCx) where ICMS primarily produced contraction of more proximal muscles. The task required the animals to reach into a horizontal target well, located in front of them at one of three possible heights, to grasp and retrieve a small piece of food. The height of the reach was primarily achieved by elbow flexion. Grasping consisted primarily of digit flexion, and food retrieval consisted of forearm supination and shoulder extension. In some blocks of trials, an obstacle was placed in the path of the limb to assess the animal's ability to adaptively adjust the kinematic characteristics of their response trajectory. In normal animals, contact with the bar on the first trial triggered a corrective response at short latency that allowed the paw to circumvent the bar. On all subsequent trials, the trajectory was adapted to prevent contact with the obstacle, with a safety margin of about 1 cm. Inactivation at all sites produced a slowing of movement, a protracted and extended forelimb posture, and increased variability of initial limb position. In addition, inactivation of RL-MCx immediately produced systematic reaching errors, consisting of hypermetric movements, as well as impaired grasping and food retrieval. The degree of hypermetria was similar for all target heights and was not associated with alterations in trajectory control. During inactivation, animals did not compensate for the hypermetria by reducing paw path elevation, suggesting a defect in kinematic planning or in adaptive control. This was confirmed by finding that trajectory adaptation to avoid bar contact was impaired during RL-MCx inactivation. The short latency corrective response, triggered by contact of the limb with the obstacle was, however, preserved. Inactivation of CL-MCx did not impair aiming, grasping, or adaptation immediately after injection. However, impairments occurred after about 1 h postinjection, and at that time mimicked the effects of RL-MCx inactivation. This delay suggests that the drug was acting indirectly on the RL-MCx. Inactivation of RM-MCx did not impair the control of distal muscles, but the reaches became hypometric. The hypometria was greater for higher targets, suggesting that it resulted from weakness. Our results suggest that both rostral regions of the forelimb area of MCx play a more important role in the planning and execution of the prehension response than the caudal portion. We hypothesize that (1) the slowing of movement, forelimb postural changes, hypometria, and grasping and food retrieval impairments are due to defective control of muscles represented locally at each site in MCx and that (2) aiming and adaptation defects, which are produced only by RL-MCx inactivation, result from disruption of integrative mechanisms underlying sensorimotor transformations that normally assure movement accuracy.     

Differential effects of local inactivation within motor cortex and red nucleus on performance of an elbow task in the cat

This study examined changes in the performance of a single-joint, elbow task produced by reversible inactivation of local regions within the proximal forelimb representation in area 4 of motor cortex (MCx) and the red nucleus (RN) of the cat. Inactivation was carried out by microinjecting lidocaine, -aminobutyric acid, or muscimol into sites where microstimulation evoked contraction of elbow muscles. Reaction time, amplitude, and speed (velocity or dF/dt) of position and force responses elicited during inactivation were compared to control values obtained immediately prior to inactivation. In addition, we assessed qualitatively the effects of inactivation on reaching, placing reactions, and proprioceptive responses to imposed limb displacement. In the single-joint task, injections in MCx did not increase reaction time (simple or choice) and produced modest and inconsistent reductions in response amplitude (mean-8%) and speed (mean -19%). In contrast, injections of the same amounts of inactivating agents in the forelimb representation of RN consistently increased reaction time (34.4%), and increased the reaction time coefficient of variability (32%). There were small reductions in response amplitude (-4%) and speed (-10%) which were less than those produced by MCx inactivation. During reaching, however, these same injections in MCx and RN produced a substantial loss of accuracy. For MCx, this was due, in part, to systematic hypometria: for RN, inaccuracy resulted from increased variability in paw paths. Placing reactions and corrective responses to imposed limb displacements were also depressed by the cortical and rubral injections. Our results suggest that the forelimb representation in RN plays a role in the initiation of the single-joint, elbow tracking response examined here. The RN may mediate cerebellar regulation of response timing, a function that is likely to be important for interjoint coordination. Although neurons in the forelimb representations of MCx may contribute to force generation in single-joint movements, their contribution to multijoint control appears to be more important and is examined in the subsequent report (Martin and Ghez 1993).     

Formation of new synapses in the cat motor cortex following lesions of the deep cerebellar nuclei

Summary The effects of unilateral lesions of the deep cerebellar nuclei on the corticocortical (CC) projection from the somatosensory to the motor cortex were studied in adult cats, utilizing electrophysiological and electron microscopical methods. Axon terminals in the motor cortex belonging to CC afferents were labeled by degeneration induced by lesions of the somatosensory cortex; neurons in the motor cortex were labeled by the Golgi/EM method. In each cat, data from the motor cortex (MCx) contralateral (experimental) and ipsilateral (control) to the cerebellar lesion were compared. Cerebellar lesions produced marked motor deficits, which receded gradually and disappeared after 30 to 40 days. Subsequent lesions of the somatosensory cortex (area 2) contralateral to the cerebellar lesions resulted in the reappearance of the cerebellar symptoms. The number of CC synapses per unit area in experimental MCx was significantly higher than in control MCx. The increase in the number of CC synapses was apparent throughout layers II–V of the MCx, but was most prominent in layers II/III. The increase in the number of CC synapses in experimental MCx was due mainly to an increase of axon terminals synapsing with dendritic spines belonging to pyramidal neurons. In comparison, the numbers and spatial distribution of CC synapses with aspinous, nonpyramidal neurons from both experimental and control MCx were similar. Field potentials in the experimental MCx, evoked by stimulation of area 2, were altered following cerebellar lesions. In experimental MCx, the polarity of the early component of the field potentials reversed at cortical depths corresponding to layers II–III, whereas this reversal was not observed in control MCx. These findings suggest that lesions of the cerebellar nuclei induced sprouting of axon terminals in the MCx to establish a new function. The results provide the first anatomical evidence for the generation of new synapses in the adult CNS which is not induced by elimination of existing synapses.     

Relations between the ventrolateral nucleus and the motor cortex in the cat

Summary In cats anesthetized with chloralose, a topographic study of the relations between the ventrolateral nucleus and the precruciate cortex has been performed. It has revealed a mediolateral topography inside the ventrolateral nucleus such that the medial neurones project to area 6 and the more lateral ones to area 4. Postsynaptic spikes were evoked in ventrolateral nucleus by stimulation of the precruciate cortex, with the same topography. The postsynaptic spikes are usually preceeded by an antidromic spike. The possible action of the cerebellum on axial musculature by way of the ventrolateral nucleus and the motor cortex is discussed.     

Short latency inputs to phrenic motoneurones from the sensorimotor cortex in the cat

Summary Short latency responses were recorded from C₅ phrenic roots and intracellularly from phrenic motoneurones following stimulation of the pericruciate cortex or medullary pyramids in cats anaesthetized with Nembutal or chloralose-urethane. Focal stimulation of the cortical surface (single pulses, 0.5–2 ms, 0.3–8 mA) during inspiration evoked EPSPs (latency 4.7 ± 1.7 ms, rise time 1.9 ± 1.1 ms, amplitude 0.22 to 3.94 mV) in 42% of motoneurones studied (n = 107). The EPSPs were absent, or on average 60% smaller, following stimulation during expiration. In all but two motoneurones, during both inspiration and expiration, hyperpolarizing potentials were observed either following the initial depolarization or alone. They could be reversed by hyperpolarizing current or chloride injection. Stimulation of the pyramidal tract at mid medullary level (1 to 3 pulses, 0.2 ms) evoked short latency excitation in phrenic motoneurones only with currents of more than 200 A. Smaller stimuli applied to the medial reticular formation above the pyramidal tract evoked excitation (onset latency 1.5–3.2 ms) in which the earliest part was probably monosynaptic. These results show that the corticospinal responses in phrenic motoneurones are both excitatory and inhibitory. They are not transmitted through the pyramidal tract and are at least disynaptic. Excitation evoked from the medullary pyramidal tract can be explained by current spread beyond the pyramidal tract fibres.     

The mode of synaptic activation of pyramidal neurons in the cat primary somatosensory cortex: an intracellular HRP study

Summary A total of 141 pyramidal neurons in the cat primary somatosensory cortex (SI) were recorded intracellularly under Nembutal anesthesia (7 in layer II, 43 in layer III, 8 in layer IV, 58 in layer V and 25 in layer VI). Most neurons were identified by intracellular staining with HRP, though some layer V pyramidal neurons were identified only electrophysiologically with antidromic activation of medullary pyramid (PT) or pontine nuclear (PN) stimulation. Excitatory synaptic potentials (EPSPs) were analyzed with stimulation of the superficial radial nerve (SR), the ventral posterolateral nucleus (VPL) in the thalamus and the thalamic radiation (WM). The pyramidal neurons in layers III and IV received EPSPs at the shortest latency: 9.1±2.1 ms (Mean+S.D.) for SR and 1.6±0.7 ms for VPL stimulation. Layer II pyramidal neurons also responded at a short latency to VPL stimulation (1.7±0.5 ms), though their mean latencies for SR-induced EPSPs were relatively longer (10.6±1.9 ms). The mean latencies were much longer in layers V and VI pyramidal neurons (10.2±2.4 ms and 2.9±1.5 ms in layer V pyramidal neurons and 9.9±2.5 ms and 2.8±1.6 ms in layer VI pyramidal ones, respectively for SR and VPL stimulation). The comparison of the latencies between VPL and WM stimulation indicates that most layer III–IV pyramidal neurons and some pyramidal cells in layers II, V and VI received monosynaptic inputs from VPL. These findings are consistent with morphological data on the laminar distribution of thalamocortical fibers, i.e., thalamocortical fibers terminate mainly in the deeper part of layers III and IV with some collaterals in layers V, VI and II-I. The time-sequences of the latencies of VPL-EPSPs indicate that corticocortical and/or transcallosal neurons (pyramidal neurons in layers II and III) fire first and are followed by firing of the output neurons projecting to the subcortical structures (pyramidal neurons in layers V and VI).     

Connections between pericruciate cortex and the medullary reticulospinal neurons in cat: an electrophysiological study

Summary The connections between the pericruciate cortex and the medullary reticulospinal (RS) neurons were studied in anesthetized cat. Intracellular recordings were made from reticulospinal neurons and the effects of stimulating different areas of the pericruciate cortex were compared. (1) EPSPs were elicited in all the 93 neurons studied which were antidromically activated by spinal stimulation and had an IS-SD notch on the ascending limb of their antidromic spikes. According to the conduction velocity (c.v.) of the axon and the minimal EPSP latency to cortical stimulation, the neurons could be divided into two groups, i.e. fast-conducting RS neurons (FRS neurons, c.v. > 45 m/s) and slow-conducting RS neurons (SRS neurons, c.v. < 45 m/s). The minimal latencies of FRS neurons were equal to or shorter than 2 ms whereas those of SRS neurons were longer than 2 ms. (2) EPSPs with short latency (< 2 ms) could be evoked in FRS neurons by stimulating a relatively wide cortical area including the major part of precruciate area 4 and area 6, with a central area of strongest excitatory effect located in area 4 slighthly medial to the tip of the cruciate sulcus. Stimulation of the postcruciate area 4 only produced long latency EPSPs. (3) By extrapolation from the cortical and peduncular latencies and the conducting distances it was revealed that the earliest part of the minimal latency EPSPs were monosynaptically evoked in FRS neurons and were mediated by fastconducting corticobulbar fibers. (4) FRS neurons could be excited by stimuli applied to both ipsilateral and contralateral pericruciate cortex. The influence from the contralateral cortex was slightly stronger.     

Monocular pattern discrimination in rabbits with unilateral lesions of the motor cortex

Rabbits with unilateral lesions of the motor cortex were trained to discriminate vertical versus horizontal striations. After reaching the training criterion with binocular vision, the performance was tested with each eye separately. It was found that performance with the eye contralateral to the lesion was markedly inferior to that with the other eye.