The projection from auditory cortex to cochlear nucleus in guinea pigs: an in vivo anatomical and in vitro electrophysiological study.

Previous anatomical experiments have demonstrated the existence of a direct, bilateral projection from the auditory cortex (AC) to the cochlear nucleus (CN). However, the precise relationship between the origin of the projection in the AC and the distribution of axon terminals in the CN is not known. Moreover, the influence of this projection on CN principal cells has not been studied before. The aim of the present study was two-fold. First, to extend the anatomical data by tracing anterogradely the distribution of cortical axons in the CN by means of restricted injections of biotinylated dextran amine (BDA) in physiologically characterized sites in the AC. Second, in an in vitro isolated whole brain preparation (IWB), to assess the effect of electrical stimulation of the AC on CN principal cells from which intracellular recordings were derived. BDA injections in the tonotopically organized primary auditory cortex and dorsocaudal auditory field at high and low best frequency (BF) sites resulted in a consistent axonal labeling in the ipsilateral CN of all injected animals. In addition, fewer labeled terminals were observed in the contralateral CN, but only in the animals subjected to injections in low BF region. The axon terminal fields consisting of boutons en passant or terminaux were found in the superficial granule cell layer and, to a smaller extent, in the three CN subdivisions. No axonal labeling was seen in the CN as result of BDA injection in the secondary auditory area (dorsocaudal belt). In the IWB, the effects of ipsilateral AC stimulation were tested in a population of 52 intracellulary recorded and stained CN principal neurons, distributed in the three CN subdivisions. Stimulation of the AC evoked slow late excitatory postsynaptic potentials (EPSPs) in only two cells located in the dorsal CN. The EPSPs were induced in a giant and a pyramidal cell at latencies of 20 ms and 33 ms, respectively, suggesting involvement of polysynaptic circuits. These findings are consistent with anatomical data showing sparse projections from the AC to the CN and indicate a limited modulatory action of the AC on CN principal cells.

Structural and physiological properties of connections between individual reticulospinal axons and lumbar motoneurons of the frog.

Although the direct, monosynaptic influence of brainstem projections onto motoneurons is well-known, detailed morphological studies on the synaptic contact systems and a correlation with their functional properties are largely lacking. In this work, 43 pairs, each formed by a reticulospinal fiber contacting a lumbar motoneuron, were identified and studied electrophysiologically. Four of these were successfully labeled intracellularly with horseradish peroxidase (HRP) or neurobiotin and reconstructed using a computer-assisted camera lucida with high resolution. The mean amplitude of excitatory post-synaptic potentials (EPSPs) recorded in these four pairs varied from 100 to 730 microV, spanning most of the range obtained for all pairs (70-1,200 microV; mean +/- SD: 400 +/- 250 microV). Between two and four collaterals of reticulospinal axons established 4-19 close appositions with a labeled motoneuron. Mean distance from the origin of each collateral to any bouton on that collateral was 566-817 microm. A presynaptic action potential must pass 11 branch points on average to reach it. Similarly, the boutons presumably contacting motoneurons were on average 558-624 microm (9-11 branch points) from the origin of the collateral. The distributions of diameters of all boutons and those making putative contacts with stained motoneurons were very similar. The dendritic surface of stained motoneurons was symmetrically distributed along the rostrocaudal axis with more than half the surface being more than 500 microm from the soma. However, the contacts from reticulospinal axons were concentrated ventromedially, 262-356 microm (range of average values for four connections) from the motoneuron soma, in some instances on very proximal dendritic segments. Thus, the location and size of putative contacts in relation to axonal collaterals was not distinguishable from location and size of other boutons, but they occupied specific positions on dendrites of lumbar motoneurons. The number of contacts formed by a reticulospinal axon on a motoneuron in a particular location could be described as the product of the available dendritic surface and the total number of presynaptic boutons in this region. Compartmental models of the reconstructed motoneurons were created, and currents with the time course of an alpha function were injected at the sites of these putative contacts. Despite the restricted volume occupied by contacts from a single fiber, a high variability of their contributions to somatic EPSPs owing to electrotonic attenuation was shown: The coefficient of variation of quantal responses was estimated to be between 60% and 120%, comparable to the variability of the path distance between contacts and soma (50-90%).

Floccular modulation of vestibuloocular pathways and cerebellum-related plasticity: An in vitro whole brain study.

The isolated whole brain (IWB) preparation of the guinea pig was used to investigate the floccular modulation of vestibular-evoked responses in abducens and oculomotor nerves and abducens nucleus; for identification of flocculus target neurons (FTNs) in the vestibular nuclei and intracellular study of some of their physiological properties; to search for possible flocculus-dependent plasticity at the FTN level by pairing of vestibular nerve and floccular stimulations; and to study the possibility of induction of long-term depression (LTD) in Purkinje cells by paired stimulation of the inferior olive and vestibular nerve. Stimulation of the flocculus had only effects on responses evoked from the ipsilateral (with respect to the stimulated flocculus) vestibular nerve. Floccular stimulation significantly inhibited the vestibular-evoked discharges in oculomotor nerves on both sides and the inhibitory field potential in the ipsilateral abducens nucleus while the excitatory responses in the contralateral abducens nerve and nucleus were free from such inhibition. Eleven second-order vestibular neurons were found to receive a short-latency monosynaptic inhibitory input from the flocculus and were thus characterized as FTNs. Monosynaptic inhibitory postsynaptic potentials from the flocculus were bicuculline sensitive, suggesting a GABA(A)-ergic transmission from Purkinje cells to FTNs. Two of recorded FTNs could be identified as vestibulospinal neurons by their antidromic activation from the cervical segments of the spinal cord. Several pairing paradigms were investigated in which stimulation of the flocculus could precede, coincide with, or follow the vestibular nerve stimulation. None of them led to long-term modification of responses in the abducens nucleus or oculomotor nerve evoked by activation of vestibular afferents. On the other hand, pairing of the inferior olive and vestibular nerve stimulation resulted in approximately a 30% reduction of excitatory postsynaptic potentials evoked in Purkinje cells by the vestibular nerve stimulation. This reduction was pairing-specific and lasted throughout the entire recording time of the neurons. Thus in the IWB preparation, we were able to induce a LTD in Purkinje cells, but we failed to detect traces of flocculus-dependent plasticity at the level of FTNs in vestibular nuclei. Although these data cannot rule out the possibility of synaptic modifications in FTNs and/or at other brain stem sites under different experimental conditions, they are in favor of the hypothesis that the LTD in the flocculus could be the essential mechanism of cellular plasticity in the vestibuloocular pathways.

Post-lesional plasticity in the central nervous system of the guinea-pig: a "top-down" adaptation process?

Vestibular compensation for the postural and oculomotor deficits following unilateral labyrinthectomy is a model of functional plasticity in the brain of adult vertebrates. The mechanisms involved in this recovery are still controversial. The post-lesional lack of vestibular input might be compensated by changes in the efficacy of the remaining sensory inputs involved in gaze and posture stabilization. However, the compensation process could also rapidly become independent of these external cues, and thus be detectable in vitro in preparations obtained from lesioned animals. In agreement with this hypothesis, we have shown recently that prominent traces of the compensation process appeared three days after the lesion on in vitro isolated brains taken from labyrinthectomized guinea-pigs, where the connectivity of the central vestibular-related networks is preserved. We report here that, one week after the lesion, a slight increase in the intrinsic, spontaneous activity of the deafferented, central vestibular neurons was found in brainstem slices. This increase became stronger in slices taken after one month of compensation, and was associated at this stage with a significant decrease in the intrinsic activity of the vestibular neurons on the contralesional side. Vestibular compensation could thus follow a "top-down" strategy: it would first rely on the external cues given by the intact sensory systems, then on an internal reorganization of the vestibular-related networks, and finally on changes in the intrinsic properties of the vestibular neurons themselves. Similar strategies may be used by the mammalian brain to compensate for other types of deafferentations or environmental changes.

Inhibitory synaptic interactions between cochlear nuclei: evidence from an in vitro whole brain study.

Using guinea-pig isolated whole brain preparation in vitro, synaptic responses to electrical stimulation of auditory nerves were examined in intracellularly recorded and stained neurons of posteroventral and dorsal divisions of the cochlear nucleus. Stimulation of the contralateral auditory nerve evoked exclusively IPSPs in 70% of neurons, with amplitude of 2.3+/-1.2mV. Neurons of all major cell types were inhibited from the contralateral side. In the majority of responding cells (78%) IPSPs were induced at latencies of 3-9 ms suggesting di- and trisynaptic connections from contralateral auditory afferents or, respectively, mono- and disynaptic connections from the contralateral cochlear nucleus. Few cells responded with long-latency IPSPs (13.5-23ms), indicating involvement of polysynaptic pathways. These data demonstrate the existence of functional, direct and indirect inhibitory connections between the cochlear nuclei.

Plastic changes underlying vestibular compensation in the guinea-pig persist in isolated, in vitro whole brain preparations.

Vestibular compensation for the postural and oculomotor deficits induced by unilateral labyrinthectomy is a model of post-lesional plasticity in the central nervous system. Just after the removal of one labyrinth, the deafferented, ipsilateral vestibular nucleus neurons are almost silent, and the discharge of the contralateral vestibular nucleus neurons is increased. The associated static disorders disappear in a few days, as normal activity is restored in both vestibular nuclei. In this study, we searched for traces of vestibular compensation in isolated whole brains taken from adult guinea-pigs. The electrophysiological responses evoked in control brains were compared to those evoked in brains taken from animals that had previously been labyrinthectomized. Guinea-pigs compensated for an initial labyrinthectomy within three days. In vivo, subsequent deafferentation of vestibular nucleus neurons on the intact side triggered "Bechterew's phenomenon": a new postural and oculomotor syndrome appeared, similar to the one induced by the first lesion, but directed to the newly deafferented side. These disturbances would be caused by the new imbalance between the discharges of neurons in the two vestibular nuclei triggered by the second deafferentation. Experiments were designed to search for a similar imbalance in vitro in brains taken from labyrinthectomized animals, where the intact vestibular nerve is cut during the dissection. Isolated whole brains were obtained from young guinea-pigs at various times (one to seven days) following an initial labyrinthectomy. An imbalance between the resting activities of medial vestibular nucleus neurons on both sides of the brainstem was revealed in brains taken more than three days after the lesion: their discharge was higher on the compensated, initially lesioned side than on the newly deafferented side. In some cases, an oscillatory pattern of discharge, reminiscent of the spontaneous nystagmus associated in vivo with Bechterew's syndrome, appeared in both abducens nerves. These data demonstrate that most of the changes underlying vestibular compensation persist, and can thus be investigated in the isolated whole brain preparation. Brains removed only one day after the lesion displayed normal commissural responses and symmetric spinal inputs to vestibular nucleus neurons. However, an unusually large proportion of the neurons recorded on both sides of the preparation had very irregular spontaneous discharge rates. These data suggest that the first stages of vestibular compensation might be associated with transient changes in the membrane properties of vestibular nucleus neurons. Brains taken from compensated animals displayed a significant, bilateral decrease of the inhibitory commissural responses evoked in the medial vestibular nucleus by single-shock stimulation of the contralateral vestibular nerve. The sensitivity of abducens motoneurons on the initially lesioned, compensated side to synaptic activation from the contralesional vestibular nucleus neurons was also decreased. Both changes may explain the long-term, bilateral decrease of vestibular-related reflexes observed following unilateral labyrinthectomy. Spinal inputs to vestibular nucleus neurons became progressively asymmetric: their efficacy was increased on the lesioned side and decreased on the intact one. This last modification may support a functional substitution of the deficient, vestibular-related synergies involved in gaze and posture stabilization by neck-related reflexes.

Neural activity of supplementary and primary motor areas in monkeys and its relation to bimanual and unimanual movement sequences.

A chronic single-unit study of motor cortical activity was undertaken in two monkeys trained to perform a bimanually coordinated task. The hypothesis was tested that the supplementary motor area plays a specific role in coordinating the two hands for common goal-oriented actions. With this objective, a special search was made for neurons that might exhibit properties exclusively related to bimanual task performance. Monkeys learned to reach for and to pull open a spring-loaded drawer with one hand, while the other hand reached out to grasp food from the drawer recess. The two hands were precisely coordinated for achievement of this goal. Monkeys also performed, in separate blocks of trials, only the pulling or grasping movements, using the same hands as in the bimanual task. Task-related activity of 348 neurons from the supplementary motor area and 341 neurons from the primary motor area, each examined in the bimanual and in both unimanual tasks, was recorded in the two hemispheres. Most neurons from the supplementary motor area were recorded within its caudal microexcitable portion. Contrary to expectation, the proportion of neurons with activity patterns related exclusively to the bimanual task was small, but somewhat higher in the supplementary motor area (5%) than in the primary motor cortex (2%). Another group of neurons that were equally modulated during the bimanual as well as to both unimanual task components might also contribute in controlling bimanual actions. Such "task-dependent" rather than "effector-dependent" activity patterns were more common in neurons of the supplementary motor area (19%) than of the primary motor cortex (5%). Bilateral receptive fields were also more numerous among the supplementary motor area neurons. However, a large majority of neurons from primary and supplementary motor areas had activity profiles clearly related only to contralateral hand movements (65% in the primary motor and 51% in the supplementary motor area). A similar group of neurons showed an additional slight modulation with ipsilateral movements; they were equally common in the two areas (14% and 16%, respectively) and their significance for bimanual coordination is questionable. Summed activity profiles of all neurons recorded in the primary and supplementary motor areas of the same hemisphere were compared. The modulations of the three histograms, corresponding to the two unimanual and the bimanual tasks, were similar for the two motor areas, i.e. prominent with bimanual and contralateral movements and weak with ipsilateral movements. It is concluded that the supplementary motor area is likely to contribute to bimanual coordination, perhaps more than the primary motor cortex, but that it is not a defining function for the former cortical area. Instead, it is suggested that the supplementary motor area is part of a callosally interconnected and distributed network of frontal and parietal cortical areas that together orchestrate bimanual coordination.

Central vestibular networks in the guinea-pig: functional characterization in the isolated whole brain in vitro.

The isolated, in vitro whole brain of guinea-pig was used to assess some of the main physiological and pharmacological properties of the vestibulo-ocular pathways in this species. Extracellular and intracellular recordings were obtained from the vestibular, abducens and oculomotor nuclei, as well as from the abducens and oculomotor nerves, while inputs from the vestibular afferents, the visual pathways and the spinal cord were activated. The three main types of medial vestibular nucleus neurons (A, B and B+LTS), previously described on slices, were also identified in the isolated brain. They had similar membrane properties in both preparations. Eighty-five per cent of cells recorded in the vestibular nucleus responded with monosynaptic, excitatory postsynaptic potentials (latency 1.05-1.9 ms) to stimulation of the ipsilateral vestibular nerve, and were thus identified as second-order vestibular neurons. In addition, stimulation of the contralateral vestibular afferents revealed in most cases a disynaptic or trisynaptic, commissural inhibition. Second-order vestibular neurons displayed in the isolated brain a high degree of variability of their spontaneous activity, as in alert guinea-pigs. Type A neurons always exhibited a regular firing, while type B and B+LTS cells could have very irregular patterns of spontaneous discharge. Thus, type A and type B neurons might correspond, respectively, to the tonic and phasic vestibular neurons described in vivo. The regularity of spontaneous discharge was positively correlated with the amplitude of spike after hyperpolarization, and there was a trend for irregular neurons to be excited from ipsilateral vestibular afferents at shorter latencies than regular units. Synaptic activation could trigger subthreshold plateau potentials and low-threshold spikes in some of the second-order vestibular neurons. As a second step, the pharmacology of the synaptic transmission between primary vestibular afferents and second-order neurons was assessed using specific antagonists of the glutamatergic receptors. Both the synaptic field potentials and excitatory postsynaptic potentials elicited in the medial vestibular nucleus by single shock stimulation of the ipsilateral vestibular nerve were largely or, sometimes, totally blocked by 6-cyano-7-nitroquinoxaline-2,3-dione, indicating a dominating role of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor-mediated glutamatergic transmission. The remaining component of the responses was completely or partially suppressed by DL-2-amino-5-phosphonovaleric acid in 35% of the cases, suggesting a concomitant, moderate involvement of N-methyl-D-asparate receptors. In addition, a synaptic response resistant to both antagonists, but sensitive to a zero Ca2+/high Mg(2+)-containing solution, was often observed. Finally, recordings from abducens and oculomotor complexes confirmed the existence in the guinea-pig of strong bilateral, disynaptic excitatory and inhibitory inputs from vestibular afferents to motoneurons of extraocular muscles, which contribute to generation of the vestibulo-ocular reflex. The functional integrity of vestibular-related pathways in the isolated brain was additionally checked by stimulation of the spinal cord and optic tract. Stimulation of the spinal cord evoked, in addition to antidromic responses in the vestibular nucleus, short-latency synaptic responses in both the vestibular nucleus and abducens motoneurons, suggesting possible recruitment of spinal afferents. Activation of visual pathways at the level of the optic chiasm often induced long latency responses in the various structures under study. These results demonstrate that the in vitro isolated brain can be readily used for detailed, functional studies of the neuronal networks underlying gaze and posture control.

The vestibular system as a model of sensorimotor transformations. A combined in vivo and in vitro approach to study the cellular mechanisms of gaze and posture stabilization in mammals.

To understand the cellular mechanisms underlying behaviours in mammals, the respective contributions of the individual properties characterizing each neuron, as opposed to the properties emerging from the organization of these neurons in functional networks, have to be evaluated. This requires the use, in the same species, of various in vivo and in vitro experimental preparations. The present review is meant to illustrate how such a combined in vivo in vitro approach can be used to investigate the vestibular-related neuronal networks involved in gaze and posture stabilization, together with their plasticity, in the adult guinea-pig. Following first a general introduction on the vestibular system, the second section describes various in vivo experiments aimed at characterizing gaze and posture stabilization in that species. The third and fourth parts of the review deal with the combined in vivo-in vitro investigations undertaken to unravel the physiological and pharmacological properties of vestibulo-ocular and vestibulo-spinal networks, together with their functional implications. In particular, we have tried to use the central vestibular neurons as examples to illustrate how the preparation of isolated whole brain can be used to bridge the gap between the results obtained through in vitro, intracellular recordings on slices and those collected in vivo, in the behaving animal.

NMDA receptors of the vestibular nuclei neurones.

Cloning and pharmacological studies have shown that glutamatergic receptors can be divided in two classes (refer to Table 1): ionotropic receptors including N-methyl-D-aspartate (NMDA) and non-NMDA subtypes, and the G-protein-coupled metabotropic receptors (glutamate metabotropic receptor). There are two types of non-NMDA receptors: the AMPA/low-affinity kainate receptor type (the AMPA receptors) activated by a specific agonist, the alpha-amino-3-hydroxy-5-methyl-4-iso-xalone propionate (AMPA), and the high affinity kainate receptors. The vestibular nuclei neurones are endowed with all these types of glutamatergic receptors, which fits well with the fact that various afferents, including the primary vestibular afferents, most probably use glutamate or aspartate as a neurotransmitter. This article is aimed at summarising several past studies of our group and some more recent data obtained in the in vitro whole-brain preparation concerning the NMDA receptors of the central vestibular neurones. In that process, we will detail also many valuable studies of other groups that had been devoted to the same topic.

Cerebellothalamocortical and pallidothalamocortical projections to the primary and supplementary motor cortical areas: a multiple tracing study in macaque monkeys.

The goal of the present study was to clarify whether the primary motor cortex (M1) and the supplementary motor cortex (SMA) both receive, via the motor thalamus, input from cerebellar and basal ganglia output nuclei. This is the first investigation that explores the problem by direct comparison, in the same animal, of thalamic zones that 1) project to M1 and SMA and 2) receive cerebellar-nuclear (CN) and pallidal (GP) afferents. These four zones were mapped in two monkeys by means of two retrograde tracers for M1 and SMA injections and of two anterograde tracers for CN and GP injections. All injections were performed under electrophysiological control (microstimulation and multiunit recordings). Injections in cortical areas were restricted to the hand/arm representation; in the SMA, the tracer deposit was within the "SMA-proper" (or "area F3") and did not include its rostral extension ("pre-SMA" or "area F6"). It was found that zones of all four types formed a number of highly complex patches of labeling that were usually not confined to one cytoarchitectonically defined thalamic nucleus. The overlap of clusters of labeled terminals and perikarya was evaluated morphometrically (area measurements) on a number of coronal sections along the anteroposterior extent of the motor thalamus. In line with previous studies, the thalamic territories innervated by CN and GP afferents rarely overlapped. However, zones projecting to M1 and/or to SMA included thalamic regions receiving CN as well as GP projections, providing the first evidence of such overlap from individual animals. The present observations support the previous conclusion from this laboratory (based on transsynaptic labeling) that the SMA receives, apart from its strong pallidal transthalamic input, a CN transthalamic input. These present findings that both M1 and SMA are recipients of transthalamic inputs from GP and CN thus support the concept that a mixed subcortical input consisting of weighted contributions from cerebellum, basal ganglia, substantia nigra, and spinothalamic tract is directed to each functional component of the sensorimotor cortex.

Transcallosal connections of the distal forelimb representations of the primary and supplementary motor cortical areas in macaque monkeys.

The goal of the present neuroanatomical study in macaque monkeys was twofold: (1) to clarify whether the hand representation of the primary motor cortex (M1) has a transcallosal projection to M1 of the opposite hemisphere; (2) to compare the topography and density of transcallosal connections for the hand representations of M1 and the supplementary motor area (SMA). The hand areas of M1 and the SMA were identified by intracortical microstimulation and then injected either with retrograde tracer substances in order to label the neurons of origin in the contralateral motor cortical areas (four monkeys) or, with an anterograde tracer, to establish the regional distribution and density of terminal fields in the opposite motor cortical areas (two monkeys). The main results were: (1) The hand representation of M1 exhibited a modest homotopic callosal projection, as judged by the small number of labeled neurons within the region corresponding to the contralateral injection. A modest heterotopic callosal projection originated from the opposite supplementary, premotor, and cingulate motor areas. (2) In contrast, the SMA hand representation showed a dense callosal projection to the opposite SMA. The SMA was found to receive also dense heterotopic callosal projections from the contralateral rostral and caudal cingulate motor areas, moderate projections from the lateral premotor cortex, and sparse projections from M1. (3) After injection of an anterograde tracer (biotinylated dextran amine) in the hand representation of M1, only a few small patches of axonal label were found in the corresponding region of M1, as well as in the lateral premotor cortex; virtually no label was found in the SMA or in cingulate motor areas. Injections of the same anterograde tracer in the hand representation of the SMA, however, resulted in dense and widely distributed axonal terminal fields in the opposite SMA, premotor cortex, and cingulate motor areas, while labeled terminals were clearly less dense in M1. It is concluded that the hand representations of the SMA and M1 strongly differ with respect to the strength and distribution of callosal connectivity with the former having more powerful and widespread callosal connections with a number of motor fields of the opposite cortex than the latter. These anatomical results support the proposition of the SMA being a bilaterally organized system, possibly contributing to bimanual coordination.

Cortical influences on cervical motoneurons in the rat: recordings of synaptic responses from motoneurons and compound action potential from corticospinal axons.

The synaptic responses of cervical motoneurons to intracortical stimulation (ICS) of the motor cortex were studied in the rat by means of intracellular recordings. Motoneurons (n = 80) were identified either by their antidromic response to peripheral nerve electrical stimulation and/or by intracellular staining with biocytin. As a result of ICS (0.6-1.5 mA) of the contralateral motor cortex, the vast majority of motoneurons responded with EPSPs (77 out of 80), while only three motoneurons exhibited IPSPs. For increasing ICS intensities, the amplitude of the EPSPs in a given motoneuron increased, whereas their latency was not substantially affected. For the whole population of motoneurons, identified mainly by their antidromic response, the latency of the EPSPs was on average 8.45 ms (SD 1.6 ms), ranging from 4.7 to 12.6 ms. A very comparable latency distribution was obtained from the subpopulation of biocytin stained motoneurons (n = 23). In 7 of 19 tested motoneurons EPSPs could follow high frequencies (50-100 Hz) of stimulation without change of latency. The compound action potential (descending volley) travelling along corticospinal fibers reached the level of intracellular recording with a minimal latency estimated to be about 3 ms after ICS. The conduction velocity of corticospinal axons contributing to the descending volley was calculated to range from 9 to 19.7 m/s, based on morphometric measurements of conduction distance from the motor cortex and duration of the compound action potential. The time delay between the latency of descending volley and the latency of early EPSPs on the one hand, and frequency following properties of EPSPs on the other hand, suggest that some cervical motoneurons receive secure, most likely, indirect (presumably disynaptic) inputs from fast conducting corticospinal axons or direct contacts from slower conducting corticospinal fibers. The biocytin labeled cervical motoneurons exhibited extraordinary long dendritic trees, extending both laterally in the white matter near the edge of the spinal cord and medially in the gray matter as far as the midline of the spinal cord. The motoneurons were also characterized by the presence of one or several recurrent axon collaterals, ramifying profusely in the neuropil, with numerous boutons en passant and terminaux contacting most likely neighboring cervical neurons.

Structure of recurrent axon collaterals of frog lumbar motoneurons as revealed by intracellular HRP labelling.

Iontophoretical injection of horseradish peroxidase (HRP) into lumbar motoneurons of the isolated and perfused frog spinal cord allowed to reveal recurrent axon collaterals of motoneurons and to study their structural organization. The axons of about 50% of stained motoneurons gave rise to recurrent collaterals. Motoneurons emitted usually one and in a few cases two recurrent axon collaterals. Recurrent collaterals exhibited a complex ramification pattern with numerous swellings presumably corresponding to synaptic boutons both en passant and terminaux. Swellings of recurrent axon collaterals were observed exclusively in the gray matter. Most of the boutons were located in the neuropil, while some of them were found in close apposition to the soma of ventral horn neurons, in particular, motoneurons. The present data provide direct evidence for structural basis involved in recurrent actions of motoneuronal activity in the amphibian spinal cord.

[A light microscopic study of the axonal collaterals of the lumbar motoneurons in the spinal cord of the frog Rana ridibunda]. / Svetoopticheskoe issledovanie aksonnykh kollateralei poiasnichnykh motoneironov spinnogo mozga liagushki Rana ridibunda.

By using intracellular injection of horseradish peroxidase into the lumbar motoneurones of the isolated spinal cord of the frog Rana ridibunda the structure of axon collaterals was studied. It was shown that about 50% of the HRP-stained cells had mainly one axon collateral of the 1st order. The subsequent branching patterns of the collaterals showed considerable variations. The number of swellings in various collaterals was from 10 up 100. The mean diameter of swellings varied from 0.8 up 10.0 microns. It is believed that the axon collateral swellings from contacts on the dendrites of the nerve cells mainly. Apparent axosomatic contacts were revealed on the motoneurons and small nerve cells. As in the cat, collateral swellings were found on the dendrites of the parent motoneuron. Obtained morphological data the structure of motor axon collaterals in the frog are compared with those in the cat. Functional significance of the axon collateral in the frog is discussed.

[Electrophysiologic properties of red nucleus neurons in the rat brain slices]. / Elektrofiziologicheskia svoistva neironov krasnogo iadra na perezhivaiushchikh srezakh mozga krysy.

Intracellular recordings from the red nucleus (RN) neurons were made in experiments on the rat brain slices. Passive membrane properties (input resistance and membrane time constant) of the RN neurons were evaluated. Phenomena of potential-dependent rebound depolarization and time-dependent inward rectification were revealed by means of passing hyperpolarizing current pulses through the recorded cells. Injections of depolarizing currents caused repetitive firing of neurons with frequencies directly depending on the intensity of injected currents. Repetitive firing was also characterized by a fast frequency adaptation during injections of currents of different intensities. Stimulation of a region of slices presumably corresponding to the decussation of brachium conjunctivum evoked mainly monosynaptic EPSPs with a "fast"-rise time in the RN neurons, which suggests activation of the synaptic input from the cerebellar nucleus interpositus. Stimulation of the same region sometimes evoked EPSP-IPSP mixtures or pure IPSPs in the RN neurons.

Morphophysiological characteristics of connexions between single ventrolateral tract fibres and individual motoneurones in the frog spinal cord.

In experiments on the isolated frog spinal cord, simultaneous intracellular recordings were performed from synaptically connected individual ventrolateral tract fibres and lumbar motoneurones. Statistical parameters of amplitude fluctuations of unitary excitatory postsynaptic potentials evoked by direct activation of presynaptic axons were compared with the number of synaptic contacts at the same connexions revealed by staining both pre- and postsynaptic elements with horseradish peroxidase. Close correspondence between the number of contacting boutons and that of release elements (n) determined from binomial relations was observed.