Synaptic excitation of red nucleus neurones by fibres from interpositus nucleus

Summary Electrophysiological properties of the interpositus-rubral transmission were studied in anaesthetized cats. The axons of interpositus neurones were stimulated either at their origin in the interpositus nucleus or at their terminal in the ventrolateral nucleus of the thalamus. Impulses of the interpositus axons produced in the red nucleus neurones excitatory postsynaptic potentials (EPSPs) monosynaptically. As a unique feature, these EPSPs exhibited very little facilitation or depression during double shock or tetanic stimulation. Correspondingly, the unitary EPSPs evoked by the threshold stimulation showed little failure during many successive trials. The number of the interpositus axons converging onto a single red nucleus cell was about 50, when calculated from the ratio of the maximum rising slopes between the unitary and maximal EPSPs evoked from the interpositus nucleus.     

Nature of the cerebellar influences upon the red nucleus neurones

Summary Large cells in the red nucleus of cats were impaled with glass microelectrodes. Under light Nembutal anesthesia it was found that stimulation of the cerebellar cortex produced hyperpolarization in their membrane. Unlike the inhibitory postsynaptic potentials, this hyperpolarization decreased when the membrane was depolarized by passage of currents through the microelectrode, and it increased during application of hyperpolarizing currents: Hence the hyperpolarization is presumed to be produced by removal of tonically impinging excitatory postsynaptic potentials, in the manner of disfacilitation. In accordance with the above view, spontaneously arising small EPSPs disappeared during the phase of the hyperpolarization. The source of tonic impingement of excitatory impulses onto the red nucleus was found in the interpositus nucleus. The cells in this nucleus were discharging impulses at frequencies of 50–100/sec which were suppressed after the cerebellar stimulation, presumably via Purkinje cell axons, a depression in the excitability of the interpositus neurones being revealed at the same time. Following the depression, the excitability and impulse discharges of the interpositus neurones were enhanced, and correspondingly there was a late depolarization in the red nucleus neurones. During stimulation of the inferior olive and even of the spinal cord, disfacilitation and late facilitation occurred similarly through the interpositus nucleus, though with longer latencies.     

Electrical activity of red nucleus neurones investigated with intracellular microelectrodes

Summary Microelectrodes were inserted into the magnocellular portion of cat's red nucleus (RN), and some basic physiological properties of RN cells were examined by both extra- and intracellular recording. During stimulation of the rubrospinal fibres at the spinal segmental level, the RN cells were invaded antidromically, producing conspicuous field potentials within RN. The somatotopical distribution of RN cells was confirmed by comparing the field potentials induced from C₂ and L₁ levels. When recorded intracellularly, antidromic action potentials showed three-step configuration as those in motoneurones and were followed by a remarkable after-hyperpolarization. The conduction velocity along the rubrospinal fibres ranged from 41–123 m/sec, with the peak frequency at 91–100 m/sec. The membrane properties were examined in some RN cells by intracellular application of current steps. The total membrane resistance was 4 M on the average, and the membrane time constant 6 msec, respectively.Excitatory postsynaptic potentials (EPSPs) were induced monosynaptically in RN cells by stimulation of the nucleus interpositus of the contralateral cerebellum. Their time course was analyzed in comparison with that of the potentials produced by current steps. Stimulation in the ventrolateral nucleus of the thalamus evoked monosynaptic EPSPs via the collaterals of the interpositus axons which innervate RN and thalamus commonly. It was further shown that impulses in cortico-rubral fibres produced EPSPs in RN cells. These cerebral-evoked EPSPs were characterized by much slower time courses than those from the nucleus interpositus.     

The red nucleus of the monkey

Summary The topographic organization of somatosensory input to the primate red nucleus was investigated by studying receptive fields of rubral neurons, and that of the motor output by delivering trains of microstimulating pulses to evoke movements. A receptive field was identified in 191 of 208 rubral neurons. Most neurons (172) responded to passive movement of one or two joints including digits but some (26) had a cutaneous input. Neurons in both the parvocellular (RN_pc) and magnocellular (RN_mc) divisions of the nucleus had receptive fields. Neurons which responded to stimulation of the forelimb were located in the dorsomedial part of the nucleus. Those responsive to stimulation of the hindlimb were in the ventrolateral part. Thin regions on the dorsal and ventrolateral borders of the nuclei, respectively, contained neurons responsive to face and tail stimulation. Within the regions representing each limb, neurons receiving an input from the extremity (hand or foot) formed a core surrounded by neurons with an input from more proximal segments. This core extended uninterrupted throughout the RN_pc and RN_mc.Movements of individual limb segments including digits were readily evoked by microstimulating in the RN_mc with thresholds as low as 3 A. In most cases, movements were evoked in the direction opposite to the passive movement which drove the neurons at the stimulating site, although fibers of passage limited the analysis of the sensory input-motor output organization with stimulation. We conclude that there is topographic localization of somatosensory input and motor output in the macaque red nucleus. Furthermore, the red nucleus of monkeys contributes to the control of independent movements of limb segments including digits, although the number of axons it sends to the spinal cord is less than 1% of the number of corticospinal axons.     

Motor cortical modulation of the macaque red nucleus

Summary The nuclei of the neocerebellum receive inputs from somatosensory receptors and the motor cortex. In cats, the discharge of those nuclear neurons which were driven by passive movement of a limb segment in one direction was suppressed by stimulation of the cortical site from which movement was evoked in the opposite direction (Larsen and Yumiya 1979a). The cortical-evoked suppression of cerebellar neurons resulted in a disfacilitation of red nucleus neurons whose discharge elicited movement in the same direction as the cortical neurons from which the suppression was evoked and which were driven by passive movement in the opposite direction (Larsen and Yumiya 1980a). The purpose of this study was to determine if the cortical modulation of rubral neurons is organized in macaque monkeys in the same way as it is in cats. Red nucleus neurons were characterized by their response to natural stimulation of somatosensory receptors, and their response to cortical microstimulation was examined in peristimulus time histograms (PSTHs). Cortical stimulation evoked a short-latency corticorubral facilitation and a longer latency response which was presumed to be mediated by the cerebellum and which was composed primarily of suppression but was sometimes preceded by a brief facilitation. As was true in cats, over half of the rubral neurons which were driven by passive movement of a limb segment in one direction responded with a facilitation-suppression to stimulation of the agonistic cortical site from which movement was evoked in the opposite direction, but only a few responded to stimulation of the antagonistic cortical sites. Similar responses were evoked in many rubral neurons by stimulation of other cortical sites from which movement was elicited about the same joint in a different plane or at a joint adjacent to that whose passive movement drove the rubral neuron. Responses were found in neurons which received somatosensory input from proximal or distal limb segments and in neurons in the parvocellular or magnocellular divisions of the nucleus, although the corticorubral facilitation was found more frequently in parvocellular neurons. In conclusion, the motor cortical modulation of the red nucleus and cerebellum is similar in monkeys and cats, and is the same for the proximal and distal limb representation.This research was supported in part by NIH grant NS10705     

Stimulation of pre- and postsynaptic elements in the red nucleus

Summary Stimulation of the red nucleus evokes a two-component descending discharge in the contralateral dorsal quadrant of the spinal cord; the first component is caused by direct and the second by synaptic activation of rubrospinal neurones projecting to the lumbosacral cord. Threshold maps for direct and synaptic activation are given. The low threshold focus for direct activation covers not only the hindlimb region of the red nucleus where the antidromic field potential is large but also the medial part of the nucleus where the axons leave. Direct activation can also be evoked from a region extending caudally from the medial part of the red nucleus. Synaptic activation of rubrospinal neurones, evoked at very low threshold from the ventro-medio-caudal border zone of the red nucleus, is due to stimulation of interposito-rubral efferents. The threshold, indicated by unitary EPSPs in rubral cells and antidromic activation of cells in interpositus, is about 1 A, which is 1/10 of the strength required for direct activation of rubrospinal neurones. There is neither evidence that rubral stimuli activate collaterals of corticospinal axons to the lumbosacral cord, nor that antidromic activation of interposito-rubral fibres evokes descending effect via their collaterals to the reticular formation.This work was supported by the Swedish Medical Research Council (Project No. 14X-94-07C).     

Neuronal responses to 5-hydroxytryptamine in the red nucleus of rats

The effects of microiontophoretic 5-hydroxytryptamine (5-HT) on the firing rate of red nucleus (RN) neurons were studied in urethane-anesthetized rats. The background discharge rate of almost all the neurons tested (97%) was modified by 5-HT, and generally increased (89%). Responses were dose dependent. Twenty-three percent of the excitatory responses were preceded by a short inhibitory phase. No significant difference in the effect of 5-HT was found between those RN neurons that project to the spinal cord and those that do not. The excitatory responses to 5-HT were blocked or greatly reduced by the 5-HT antagonists methysergide and ketanserin, and were even reversed in some cases. The 5-HT₂/5-HT_1A antagonist spiperone, in small doses, also blocked the transient inhibitory phases in addition to the excitatory effects. In RN neurons exhibiting a short-lasting inhibition in the response to 5-HT, the 5-HT_1A agonist 8-hydroxy-2(di-n-propyl-amino)tetralin (8-OH-DPAT) induced inhibitory effects. These results support the hypothesis that 5-HT exerts control throughout the RN, mostly by acting on 5-HT₂ receptors. Furthermore, an influence of this amine on the electrical activity of small groups of RN neurons by 5-HT_1A receptors, and eventually by different mechanisms, appears probable. The functional significance of serotoninergic control of RN neuronal activity is discussed.     

Action of γ-aminobutyric acid and glycine on red nucleus neurones

Summary GABA and glycine were applied iontophoretically in the red nucleus. Their effects were tested upon focal potentials in extracellular records, and upon the cell membrane (polarization, conductance, amplitude of spike and synaptic potentials) of rubrospinal neurons in intracellular records. Both GABA and glycine reduced the amplitude of focal potentials, hyperpolarized the membrane and increased its conductance. Quantitatively GABA actions were clearly more pronounced than glycine actions, based on the comparison of iontophoretic currents.Supported by: Deutsche Forschungsgemeinschaft (Grant Br 242/7).     

Motor cortical modulation of feline red nucleus output: Cortico-rubral and cerebellar-mediated responses

Summary The neocerebellum receives an input from the motor cortex, and its output modulates the activity of rubrospinal and corticospinal tract neurons. In a previous study (Larsen and Yumiya, 1979) we examined the organization of the input from the motor cortex to the cerebellum and found that the discharge of cerebellar nuclear neurons, which were driven by passive movement of a limb segment in one direction, was suppressed by stimulation of cortical sites from which movement was evoked in the opposite direction. The purpose of the present study was to examine the organization of the output from the cerebellum to the red nucleus. Red nucleus neurons were characterized by their sensory input with natural stimulation and their motor output with movements evoked by microstimulation in unanesthetized cats. A receptive field was identified in 152 of 184 neurons, 82 of which were driven by passive movement of one or two limb segments. Microstimulation at the rubral recording sites evoked movements in the direction opposite to the passive movement. The response of rubral neurons to motor cortical microstimulation was examined in post-stimulus time histograms. A short-latency facilitation presumably mediated by the corticorubral projection, was termed the early component of the response. A longer latency response presumably mediated by the cerebellum and termed the late component consisted primarily of suppression. Twenty-eight (70%) of 40 of the tested rubral neurons driven by passive movement of a limb segment in one direction responded (with the early and/or late component) to stimulation of the cortical site from which movement was evoked in the opposite direction. By contrast, only three (17%) of 17 tested neurons responded to stimulation of the cortical site from which movement was evoked in the same direction as the passive movement. Therefore, the cerebellar-mediated suppression was found in rubral neurons, which have a target similar to neurons in the cortical region from which the suppression was evoked. Based on this and other studies, a model is proposed in which the cerebellum mediates negative feedback to the motor cortex.The research was supported by NIH Grant NS10705     

Stimulants and lesions of the substantia nigra and red nucleus

The interaction of altered activity levels by stimulants and brainstem lesions was examined. Lesions of the substantia nigra and red nucleus significantly increased activity over control levels in albino rats. The stimulant action of d-amphetamine and methylphenidate was additive with lesion effects. In addition, stimulants disrupted the normal light-dark relationship with activity while the lesions did not. It is suggested that there are two functionally separate systems regulating locomotor activity.     

Effects of amino acids on cat red nucleus neurons in vitro

Summary Intracellular records were obtained from neurons in the region of the red nucleus (RN) of cat brain slices. Both EPSPs and IPSPs were recorded in response to local electrical stimulation and these resembled similar electrophysiological responses observed in experiments conducted in vivo. Monosynaptic and polysynaptic IPSPs were observed, suggesting the existence of inhibitory interneurons near or within the RN region.When added to the bathing solution, L-glutamate and L-aspartate depolarized RN neurons with a decrease in input resistance. -Aminobutyric acid (GABA) and glycine hyperpolarized the cells with a decrease in input resistance. GABA also elicited a depolarizing response. These amino acid actions had direct postsynaptic effects, since the experiments were conducted in a low Ca²⁺/high Mg²⁺ medium which blocked synaptic transmission.     

雌激素对大鼠红核神经元保护作用的体视学定量

目的 观察雌激素（E₂）替代治疗对去卵巢SD大鼠脊髓损伤后红核神经元逆行性损伤的影响。方法 成年雌性SD大鼠80只, 随机分为正常组、红核脊髓束(RST)损伤组、E2替代治疗组、雌激素受体拮抗剂（ICI）治疗组和E₂+ICI联合治疗组。SD大鼠去势后1周采用选择性切断脊髓C3～C4左侧背外侧索制作单侧红核脊髓束（RST）横断损伤模型,给予不同条件治疗后1周、2周和4周对各组大鼠采用前肢支撑探测实验进行行为学评价,用红色荧光金（FR）逆行荧光示踪及体视学定量分析法,观察红核神经元的形态及数目变化。结果 前肢支撑探测实验结果显示,各时间点E2替代治疗组的左前肢使用率均高于除正常组外其他各组,但无统计学意义（P >0.05）;形态学检测结果可见各组大鼠中脑右侧FR阳性红核神经元数目除正常组外均有不同程度减少,尤以4周时最为明显。E2替代治疗组右侧中脑FR阳性红核神经元与RST损伤组、ICI治疗组和E₂+ICI 联合治疗组相比胞体饱满、轮廓清晰、突起长,体视框计数结果显示,E₂替代治疗组红核FR阳性神经元数目明显多于其余各组（P<0.05）,但RST损伤组、ICI治疗组和E₂+ICI联合治疗组之间右侧中脑FR阳性红核神经元数目未见明显差异（P>0.05）。结论 大鼠脊髓横断损伤后中脑红核神经元发生逆行性损伤,E₂替代治疗可减轻脊髓损伤引发的继发性红核神经元损伤。     

Differential effects of stimulation of the cat's red nucleus on lumbar alpha motoneurones and their Renshaw cells

Summary 1. The red nucleus region was stereotaxically stimulated with short trains of high-frequency alternating current pulses in anaesthetized cats. The effects were studied, in contralateral lumbar segments, on the responses of microrecorded individual Renshaw cells (RCs) to antidromic or orthodromic test shocks of ventral root or muscle nerve fibres. Monosynaptic reflexes (MRs) of their motoneurone pools were recorded from one of the cut lumbar ventral roots. Averages of 10–20 replicate test responses of the RC (converted into instantaneous frequency curves, IFCs) and of the MR shapes were computed and graphically displayed. 2. Orthodromic (afferent) test shocks induced simultaneously MRs as well as responses of a RC belonging to the same motor pool. From their paired records at systematically varied shock strengths, whole linkage characteristics of the relation between the two events could be obtained, representing the functional linkage from the motoraxon collaterals to the RC under study. The overall result of rubral conditioning was a change in the course of the characteristic, which indicated a reduction of this linkage (= relative inhibition of the RC against its recurrent input). 3. Sequential trials with test shocks of constant, submaximal strength were performed with 45 individual RCs. The clearest results were obtained with RC responses to antidromic ventral root shocks: 65% of the RCs were partially inhibited by rubral conditioning. Interposed minor facilitory subcomponents could be seen in the course of inhibited IFCs. Mixed sequences of manifest inhibitory/facilitory effects were observed in 11%; reversed sequences (facilitory/inhibitory) did not occur. A pure but weak facilitation was found in only one case, paralleled by an increase of the MR. RCs belonging to either extensor or flexor motor pools were affected about equally. A little over 20% of the tested RCs remained uninfluenced by rubral stimulation. 4. The MRs, induced by constant, submaximal, orthodromic test shocks, were usually enhanced with only few exceptions, by rubral stimulation. The effects on the orthodromic RC responses were mainly inhibitory, but could be more or less masked by the concurrent increase of the MR, providing a stronger recurrent input to the RC. Such inhibition could be uncovered, however, by observing the above described linkage change. 5. Variation of several parameters of rubral conditioning (train duration, timing of train with respect to test shock, strength of train) modified the inhibitory effects on antidromic RC responses to a certain extent without changing their principal character. Higher conditioning strengths frequently induced mass discharges of previously silent motoneurones, but at the same time an increased inhibition of the concurrent RC responses. 6. Spontaneous RC activity (in the absence of test stimuli) occurred infrequently and was weak and interrupted by silent periods. When this persisted long enough for testing repeated rubral stimulation, a strong initial inhibition lasting up to several hundred ms was found, sometimes followed by some oscillations of the average discharge rate. 7. The predominant combination of concurrent effects of the conditioning, namely, inhibition of RCs and facilitation of motoneurones, indicated independent (and mostly divergent) control of the two target neurones by the red nucleus. It is concluded that in this way the RCs can be flexibly and transiently decoupled to some degree from their recurrent motoneuronal input.Dedicated to Prof. Ragnar Granit on behalf of his 85th birthday     

Analysis of potentials induced in red nucleus neurones from the somaesthetic pathway stimulated at the bulbar level

A somaesthetic pathway to the magnocellular red nucleus (RNm) via relays other than corticoor cerebello-rubral relays was previously found to exist in the cat. At the brainstem level, the ascending spinorubral fibres follow the medial lemniscus (LM). The present paper aims at describing in detail and evaluating the quantitative importance of the short-latency responses in RNm cells after microstimulation performed in the LM through a monopolar electrode. The RNm cells, tested intracellularly in cats anaesthetized with -choralose and placed in a stereotaxic device, were identified by their antidromic activation to stimulation of the rubrospinal tract in the cervical cord. It was established that single-shock stimulation below 100 A current delivered to the LM induced short-latency postsynaptic potentials (PSPs) in 87% of all the rubrospinal cells tested. The responses were indeed due to activation of LM fibres, as demonstrated by different tests: the location of the electrode tip in the LM was verified by recording, with the same electrode, the potentials evoked by stimulating the dorsal columns of the cord. The site was later confirmed histologically. The absence of stimulus spread from the LM to the underlying pyramidal tract was systematically checked by simultaneously recording the responses evoked in RNm cells and in the motor cortex. Monosynaptic excitatory responses (EPSPs) were evoked in RNm cells with a minimum stimulating current of less than 20 A in the LM and a mean threshold of 42 A. Disynaptic inhibitory potentials (IPSPs) were evoked in 23% of these cells with single-pulse stimulation. These latter responses showed a temporal facilitation with short trains of three pulses, which indicated that they were transmitted through inhibitory interneurones. Recordings were also performed from presumed LM fibre terminals running inside the RNm. The results therefore confirm the existence of strong lemniscal projections to RNm and demonstrate that they transmit both excitatory and inhibitory messages to rubrospinal cells. As the somaesthetic pathway to the RNm was previously found to come from the spinal cord, where it is located in the ventral portion, the present results also confirm that the LM is composed of fibres originating not only from neurones in the dorsal column nuclei, but also from cells placed at the segmental levels of the cord. The presumed sensorimotor function of this ascending pathway is discussed.     

Task-related coding of stimulus and response in cat red nucleus

Summary In the present study we recorded the activity of single neurons in the forelimb area of red nucleus (RN) during performance of three step-tracking tasks designed to dissociate the coding of stimulus and response variables in the discharge of recorded neurons. In two of these tasks, the standard and stimulus-reversal arm tasks, elbow flexion and extension were elicited by different stimuli enabling us to distinguish activity correlated with the forelimb response from the stimulus eliciting it. The third task (neck task) allowed us to determine whether neuronal modulation was related to an unconditioned orienting response that occurred concurrently with the forelimb response. We have previously reported that these three tasks separate neurons in MCx whose modulation precedes the response (lead cells) into three distinct classes in which task-related activity either is correlated with the direction of the forelimb response, correlated with the stimulus, or not correlated with either (Martin and Ghez 1985). All lead cells, however, remained timed to the stimulus rather than to the response. The present results show that RN lead cells can be subdivided into the same three classes as those in MCx and their discharge was also contingent on the subsequent production of a behavioral response. (1) Force-direction neurons (35%; n = 16) showed changes in activity correlated with the production of forearm force in a particular direction suggesting that they could participate in selecting the appropriate forelimb response. The onset of task-related modulation of activity was better timed to the response, in contrast to force-direction neurons in MCx, which were better timed to the stimulus. (2) Stimulus-direction neurons (18%; n = 8) modulated their activity in relation to a particular stimulus evoking either flexor or extensor responses and during neck task performance. These neurons could be involved in processing stimulus information or in the production of neck torque. The task-related discharge of these lead cells was better timed to the stimulus than to either the forelimb or the neck response. (3) Nondirectional neurons (47%; n = 21) modulated their activity during all tasks examined. Their discharge did not correlate with any specific feature of the stimulus or response, and as a group, was better timed to the stimulus than to the response. Nondirectional neurons may participate in some aspect of motor preparation. To determine the relative contributions of RN and MCx lead cells to response initiation, we compared the amount of response latency variance that could be explained by variation in the latency of the unit modulation to the stimulus for the present data and the data in the earlier MCx study (Martin and Ghez 1985). Between 38% and 53% of response latency variance (for trials examined during performance of the standard arm and stimulus reversal tasks) was accounted for by the latency variations of RN force direction neurons; in contrast, 8% and 11% for MCx force-direction neurons. Variations in timing of stimulus-direction neurons in both RN and MCx account for less than 10% of response latency variance. Our findings suggest that, in the tasks examined, RN force-direction neurons play a more direct role than MCx force-direction neurons in initiating and selecting responses to stimuli. We hypothesized that this subcortical control reflects the high degree of stereotypy of the motor response examined.     

Single-unit activity in red nucleus during the classically conditioned rabbit nictitating membrane response

Previous investigations have suggested that the cerebellum and associated brainstem structures, including the red nucleus, are essential for the expression of the classically conditioned nictitating membrane (NM) response. The present study examined the firing patterns of extracellularly-recorded single units in the red nucleus of the awake rabbit during differential conditioning. Tones were used as conditioned stimulus (CS+ and CS−) and periocular electrostimulation was used as the unconditioned stimulus (US). Most units exhibited one or more changes in firing rate during the presentation of the CS, and increases in firing were much more common than decreases. The onset of some of these changes appeared to be time-locked to the onset of the CS (‘CS-locked’ responses), while other changes were time-locked to the onset of the CR (‘CR-locked’ responses). About one-third of all CS-locked changes were CR-dependent, meaning that the neuronal response was reduced when the CR did not occur. About two-thirds of all CR-locked responses preceded the onset of the CR, and lead times varied considerably across units. Many CR-locked units were located in what has been described as a dorsal face region of the red nucleus. Most units responded to the US, and some of the US responses were CR-dependent: i.e., a smaller US response was evoked when a CR preceded the US than when the CR was absent. Our results support the notion that cerebellum-brainstem circuits are involved in generating NM CRs.     

Spatio-temporal organization of the somaesthetic projections in the red nucleus transmitted through the spino-rubral pathway in the cat

Summary Although it has been known for a long time that in awake cats, natural stimulation of the skin induces short latency responses in rubrospinal cells, the pathway possibly involved has been identified only recently (Padel et al. 1988). This tract, which was described in acute, chloralose anaesthetized cats, ascends in the ventromedial spinal cord and is activated via collaterals of primary afferent fibres running in the dorsal columns of the spinal cord. The present study demonstrates that this newly described spino-rubral tract is able to send detailed somaesthetic information to the red nucleus. After lesions leaving intact only the spino-rubral pathway, excitatory and inhibitory responses to natural peripheral stimulations were recorded in identified rubral efferent cells. The most effective stimuli were touching the skin, passive joint rotation and hair displacement. Each cell was found to possess a particular receptive field. These fields which could be ipsi-, contra-, or bi-lateral were generally located on a single limb, although they could include two or more limbs, or even exceptionally the whole body with or without preferential zones. The topographic organization of receptive fields was arranged somatotopically in the red nucleus and overlapped the motor representation. The somaesthetic inputs transmitted through the spino-rubral pathway to the red nucleus are very similar to those previously observed in the intact cat, which supports the idea that this pathway may play a functional role in motor control. The spino-rubro-spinal loop may provide a fast adaptation of the descending motor command, thus producing a fine and harmonious tuning between the changing surroundings and the animal's movements.     

Afterhyperpolarization modulation in lumbar motoneurons during locomotor-like rhythmic activity in the neonatal rat spinal cord in vitro

Motoneuron afterhyperpolarization (AHP) amplitude and somatic input conductance were monitored during pharmacologically induced, locomotorlike ventral root activity using an isolated neonatal rat spinal cord preparation (transected at the C1 level). Nonspontaneously firing motoneurons were selected for study. Single spikes were evoked at regular intervals by brief depolarizing current pulse injections, while somatic input conductance was monitored by hyperpolarizing current pulses. The induction of rhythmic ventral root activity was associated with tonic depolarization of motoneurons as well as superimposed rhythmically alternating membrane depolarization and hyperpolarization (locomotor drive potentials, LDPs). In 9 of 13 trials (six of eight cells) the peak amplitude of AHPs following current-evoked action potentials was reduced during both the hyperpolarized and the depolarized phases of the LDP, compared with the pre-locomotor condition. The peak AHP amplitude increased during the depolarized phase of the LDP in 4 of 13 trials (three of eight cells); however, in 3 of these 4 trials measurement of the AHP later in the course of its trajectory, using a half decay time (HDt) reference point, demonstrated AHP amplitude reduction during rhythmic activity compared with the prelocomotor condition. In seven of eight motoneurons the induction of rhythmic activity was associated with a decrease in input conductance. The pattern of AHP amplitude and conductance modulation during the two phases of the LDP was consistent for individual trials; however, there was considerable intertrial variation. The results suggest that AHP modulation during locomotor-like activity in this preparation can be mediated independently of supraspinal influences by intrinsic spinal cord mechanisms, and the observed AHP suppression does not appear to be the passive result of an increase in background conductance. The discrepancy between peak and HDt-based AHP amplitude measurements during the depolarized phase of the LDP in some trials may be due to competing effects of passively enhanced potassium currents and a mechanism that actively reduces the calcium-dependent potassium conductance. The possibility that both the AHP amplitude and the input conductance changes observed during locomotor-like activity reflect a regulation of potassium channels is discussed.     

Patterns of firing in cuneate neurones and some effects of flaxedil

Summary In cats under pentobarbitone anaesthesia, single cuneate cells excited directly by microiontophoresis of glutamate (or ATP) reproduce the temporal patterns of firing seen during spontaneous activity or during activity evoked by peripheral stimulation. In particular, the glutamate-evoked discharges of hair or touch cells show their characteristic tendency to fire in high-frequency pairs or bursts of impulses. Since glutamate acts mainly on the post-synaptic cell, the explanation for the multiple discharges must lie in special, repetitive properties of the hair and touch cells. Cuneate neurones are strongly excited by microiontophoretic applications of Flaxedil. The most prominent effect is the appearance of prolonged bursts of spikes at a high frequency. Even systemic Flaxedil can alter the discharge of cuneate hair cells; short intervals occur more frequently, and, in some cases, there is an acceleration in spontaneous firing.NATO Post-doctoral Fellow.     

Somatotopical projections from the supplementary motor area to the red nucleus in the macaque monkey

Direct projections from the supplementary motor area (SMA) to the red nucleus were investigated in the Japanese monkey (Macaca fuscata). The anterograde tracer, horseradish peroxidase conjugated to wheat germ agglutinin (WGA-HRP), was injected into various regions of the SMA after intracortical microstimulation mapping. After WGA-HRP injection into the orofacial, forelimb, or hindlimb region of the SMA, anterogradely labeled axon terminals were found, respectively, in the medial, intermediate, or lateral portion of the parvocellular part of the red nucleus, bilaterally with an ipsilateral predominance. The results indicate the clear somatotopical arrangement of corticorubral projections from the SMA.