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PubMed1984

Medial septal projections to the dentate gyrus of the rat: electrophysiological analysis of distribution and plasticity.

Previous electrophysiological experiments in rabbits have suggested that medial septal stimulation activates dentate granule cells and evokes an associated negative field response at the granule cell layer, without an associated "dendritic" response. Anatomical studies have suggested that septal input to the granule cells may be to stratum moleculare, or close to the cell layer, or may not exist at all. The present experiments confirmed in rats anaesthetised with urethane that medial septal stimulation elicits single action potentials from cells in the granule layer. The associated negative field potential was maximal in the granule cell layer and there was no sign of a separate dendritic potential. The fibres responsible for this potential travel to the dorsal hippocampus in the fornix superior rather than the fimbria, taking the same course as the fibres which contribute to the dense cholinesterase staining just above the granule cell layer. Stimulation at 100 Hz for 1 s of either medial septal, or perforant path, input to the dentate granule cell layer produced long term potentiation of the subsequent evoked field responses to the stimulated pathway. The responses to the non-stimulated pathway were unchanged. Paired pulse stimulation produced both homosynaptic and heterosynaptic potentiation. These data suggest that medial septal input synapses close to granule cell bodies and produces a negative field potential which is a combination of dendritic and population spike potentials. Medial septal input also appeared to produce direct activation of hilar neurones, some of which may be basket cells or other interneurones. The data also show that long term potentiation is specific to this input, perhaps dependent on presynaptic mechanisms. Paired pulse potentiation, at least in the heterosynaptic case appears to depend on postsynaptic mechanisms.

Animals↗
PubMed1987

Reflex pathways from group II muscle afferents. 1. Distribution and linkage of reflex actions to alpha-motoneurones.

The interneuronally mediated reflex actions evoked by electrical stimulation of group II muscle afferents in low spinal cats have been reinvestigated with intracellular recording with motoneurones to knee flexors and ankle extensors. The results of Eccles and Lundberg (1959) have been confirmed and extended. There was wide convergence from flexors and extensors of group II excitation to flexor and group II inhibition to extensor motoneurones. Some quantitative differences in the effect from the different nerves are described. Latency measurements suggest that the minimal linkage is disynaptic in the excitatory interneuronal pathways and trisynaptic in the inhibitory pathways. Disynaptic group II EPSPs were found in 14% of the ankle extensor motoneurones but were much more common in unanaesthetized high spinal cats (Wilson and Kato 1965). From these results and corresponding ones on flexors (Holmqvist and Lundberg 1961) it is postulated that secondary afferents in addition to the weak monosynaptic connexions (Kirkwood and Sears 1975) have disynaptic excitatory pathways and trisynaptic inhibitory pathways to both flexor and extensor motoneurones. It is proposed that the group II actions of the flexor reflex pattern characterizing the anaesthetized low spinal cat are due to suppression of the inhibitory pathway to flexor motoneurones and the excitatory pathway to extensor motoneurones. In some ankle extensor motoneurones the disynaptic group II EPSPs occurred in combination with IPSPs from the FRA (including group II and III muscle afferents). The possibility is considered that these group II EPSPs are mediated by an interneuronal group II pathway with little or no input from group III muscle afferents but probably from extramuscular receptors. In other ankle extensor motoneurones group II EPSPs were combined with EPSPs from group III muscle afferents, cutaneous afferents and joint afferents. It is postulated that these group II EPSPs are mediated by an interneuronal pathway from the FRA which also supply interneuronal pathways giving inhibition to extensor or/and flexor motoneurones and excitation to flexors as postulated by Eccles and Lundberg (1959) and Holmqvist and Lundberg (1961).

Animals↗
PubMed1987

Reflex pathways from group II muscle afferents. 3. Secondary spindle afferents and the FRA: a new hypothesis.

A hypothesis is forwarded regarding the role of secondary spindle afferents and the FRA (flexor reflex afferents) in motor control. The hypothesis is based on evidence (cf. Lundberg et al. 1987a, b) summarized in 9 introductory paragraphs. Group II excitation. It is postulated that subsets of excitatory group II interneurones (transmitting disynaptic group II excitation to motoneurones) may be used by the brain to mediate motor commands. It is assumed that the brain selects subsets of interneurones with convergence of secondary afferents from muscles whose activity is required for the movement. During movements depending on coactivation of static gamma-motoneurones impulses in secondary afferents may servo-control transmission to alpha-motoneurones at an interneuronal level. The large group II unitary EPSPs in interneurones are taken to indicate that, given an adequate interneuronal excitability, impulses in single secondary afferents may fire the interneurone and produce EPSPs in motoneurones; interneuronal transmission would then be equivalent to that in a monosynaptic pathway but with impulses from different muscles combining into one line. It is postulated that impulses in the FRA are evoked by the active movements and that the role of the multisensory convergence from the FRA onto the group II interneurones is to provide the high background excitability which allows the secondary spindle afferents to operate as outlined above. The working hypothesis is put forward that a movement governed by the excitatory group II interneurones is initiated by descending activation of these interneurones, but is maintained in a later phase by the combined effect of FRA activity evoked by the movement and by spindle secondaries activated by descending activation of static gamma-motoneurones. As in the original "follow up length servo" hypothesis (Rossi 1927; Merton 1953), we assume that a movement at least in a certain phase can be governed from the brain solely or mainly via static gamma-motoneurones. However, our hypothesis implies that the excitatory group II reflex connexions have a strength which does not allow transmission to motoneurones at rest and that the increase in the gain of transmission during an active movement is supplied by the movement itself. Group II inhibition. It is suggested that the inhibitory reflex pathways like the excitatory ones have subsets of interneurones with limited group II convergence. When higher centres utilize a subset of excitatory group II interneurones to evoke a given movement, there may mobilize inhibitory subsets to inhibit muscles not required in the movement.(ABSTRACT TRUNCATED AT 400 WORDS)

Afferent Pathways↗
PubMed1980

Autogenetic effects of muscle contraction on extensor gamma motoneurones in the cat.

The effect of isometric twitch contractions of hind limb muscles on the discharge of gamma motoneurones has been studied in decerebrated cats with thoracic spinal cord section. Contractions of flexor digitorum longus (FDL) or gastrocnemiussoleus (GS) strongly inhibited the background discharge of their homonymous gamma motoneurones. In contrast, contraction of FDL generally caused less inhibition, or did not affect the discharge, of the synergist GS gamma motoneurones. Both the autogenetic and synergist inhibition were considerably weaker in decerebrated cats with intact spinal cords. The inhibition lasted 20 to 50 ms and occurred principally during the rising phase of contraction. It could be followed or terminated by a weaker period of facilitation during relaxation of the muscle. Shortening the muscle so that no active tension developed during contraction either abolished or substantially curtailed the inhibition. We discuss the probability that the inhibition is a spinal segmental reflex brought about by impulses generated in Ib afferents of tendon organs. The inhibition is under tonic inhibitory control in the decerebrated cat when the spinal cord is intact. Unlike the Ib inhibition of alpha motoneurones that of gamma motoneurones has a particularly strong autogenetic component.

Animals↗
PubMed1983

Cerebral cortical areas of origin of excitation and inhibition of rubrospinal cells in the cat.

The relations between the cerebral cortex and the red nucleus have been studied in acute, chloralose anaesthetized cats using intracellular recording techniques. Stimulation of the cerebral cortex induces in rubrospinal cells a short latency excitation followed by a long lasting silent period. The evidence is presented that at least a great part of the latter is due to genuine IPSP evoked in these cells. Three populations of rubrospinal neurones have been distinguished according to the cortical origin of their afferents: one group receives projections from the forelimb cortical area. These cells project to the cervical spinal cord and thus should control the forelimb. The second group receives projections from the hindlimb cortical area. These cells project to the lumbar spinal cord and should control the hindlimb. The third group of rubrospinal neurones receives convergent projections from both forelimb and hindlimb cortical areas. If these cells have collateralized axons terminating in both rostral and caudal spinal cord, they could contribute to the coordination of fore- and hindlimb movements. The projections originate in cytoarchitectonic areas 1-5 i.e. in the primary motor and sensory areas and in the rostral portion of the parietal area. No projection has been found from area 6 (premotor) or from area 7 (caudal parietal). The projection upon single rubrospinal cells has been found to originate from large cortical regions with a large overlap between those with excitatory and inhibitory actions. This could indicate the intermingling of cortical cells transmitting both effects.

Animals↗
PubMed1988

Centrifugal regulation of neuronal activity in the olfactory bulb of the waking rabbit as revealed by reversible cryogenic blockade.

The influences of centrifugal projections to the olfactory bulb were examined on the bulbar EEG and mitral-tufted cell activity in waking rabbits. Each of 6 rabbits was implanted, under surgical anesthesia, with fine wire electrodes for recording of the EEG and mitral-tufted cell unit activity and for stimulating the lateral olfactory tract. Two cooling probes, for reversible cryogenic blockade, were implanted on either side of the left olfactory peduncle. Records of EEG and unit activity were taken for 200 s before, during and after cooling of the probes to 3 degrees centigrade. Antidromic evoked potentials were used to assess the efficacy of the blockade. During the cryogenic blockade bursts of EEG activity, evoked in the bulb by inspiration through the nose, were augmented in amplitude and reduced in frequency. Mitral-tufted cell unit activity was reduced in rate but was more highly correlated with the phase and amplitude of the EEG bursts. Analysis of individual EEG bursts revealed that the variance in frequency of bulbar activity was significantly reduced in the isolated state. The data demonstrate that oscillatory bursting activity in the olfactory bulb is intrinsically maintained within a relatively fixed frequency range during receptor input and does not depend on centrifugal projections for its electrogenesis. Changes in EEG frequency, amplitude and correlation with unit activity support the hypothesis that centrifugal projections act in part to inhibit mitral-tufted cell output by direct excitation of granule cells. These findings are supported by a theoretical model in which distributed feedback to the granule cells from more central olfactory structures acts to regulate the coherency of bulbar activity.

Action Potentials↗
PubMed1989

Single medullary reticulospinal neurons exert postsynaptic inhibitory effects via inhibitory interneurons upon alpha-motoneurons innervating cat hindlimb muscles.

This study was aimed at elucidating the brainstem-spinal mechanisms of postural suppression evoked by stimulating the dorsal portion of the caudal tegmental field (DTF) in the pons. For this purpose, we first sampled a group of reticulospinal neurons located in the medial part of medullary reticular formation, which were activated orthodromically and antidromically by stimulating the DTF area and the first lumbar spinal segment, respectively (DTF-RS neurons; N = 26). These DTF-RS neurons were located within the nucleus reticularis gigantocellularis (NRGc) and projected their descending axons to the lumbar spinal cord through the ventrolateral funiculus. The postsynaptic inhibitory effects of single DTF-RS neurons upon hindlimb alpha-MNs intracellularly recorded (N = 78) were then studied with spike-triggered averaging. Twelve DTF-RS neurons evoked IPSPs in 21 hindlimb alpha-MNs. Five DTF-RS neurons exerted postsynaptic inhibitory effects upon more than one alpha-MNs. These alpha-MNs were located from L5 to S1 segments of the spinal cord. A mean latency of IPSPs which was measured from the onset of the trigger spike was 5.1 ms with time to peak of 1.8 ms, and the mean segmental delay of the IPSPs was 1.5 ms, which was measured from the onset of the descending axonal volley recorded extracellularly adjacent to alpha-MNs. Amplitudes of the IPSPs were augmented with an increase in the firing frequencies of the DTF-RS neurons, the increase being produced by iontophoretic application of glutamate. These characteristics of the IPSPs suggest that reticular effects are mediated at least by a single spinal inhibitory interneuron. These results suggest that the DTF-NRGc system participates in generalized motor inhibition.

Action Potentials↗
PubMed1979

Auxiliary spinal networks for signal focussing in the segmental stretch reflex system.

In continuation of a previous paper, the auxiliary signal focussing properties of more complicated spinal neuronal networks are considered here. Special emphasis is put on the distributive function of the recurrent feedback system of alpha-motoneurones, but also the inhomogeneous distribution of excitatory and inhibitor input to motoneurones is taken into account as an essential prerequisite for signal focussing. Simple hypothetical calculations for steady-state conditions yield a more vivid insight into the interaction of the two types of neuronal circuitry contributing to signal focussing.

Feedback↗
PubMed1980

A spatio-temporal filter approach to synchronous brain activities.

This paper presents a mathematical mechanism for neuronal synchronization in oscillatory brain activities on the basis of the layer structures with recurrent inhibition. To begin with, a linear theory reveals that the recurrent inhibition tends to cause a synchronous uniform oscillation if the loop delay increases, and that an oscillating neuron recruits neighboring neurons by delivering synchronous inputs through the recurrent inhibition loop if the frequency is that of the selfexcitatory oscillation. Then, a quasilinearized dual wave model (DWM), employing the two-sinusoids plus bias input describing functions (TSBDF), shows the competitive relationship between the synchronous oscillation and a spatial wave that is introduced to represent normal brain activity patterns. Results of computer simulations conform well to the predictions of the DWM. Thus, synchronous brain activities are suggested to be the result of the spatio-temporal filter characteristics of the brain layer structures, modified by the neural nonlinearity.

Brain↗
PubMed1978

An inference upon the neural network finding binocular correspondence.

Previously, the authors proposed a model of neural network extracting binocular parallax (Hirai and Fukushima, 1975). It is a multilayered network whose final layers consist of neural elements corresponding to "binocular depth neurons" found in monkey's visual cortex. The binocular depth neuron is selectively sensitive to a binocular stimulus with a specific amount of binocular parallax and does not respond to a monocular one. As described in the last chapter of the previous article (Hirai and Fukushima, 1975), when a binocular pair of input patterns consist of, for example, many vertical bars placed very closely to each other, the binocular depth neurons might respond not only to correct binocular pairs, but also to incorrect ones. Our present study is concentrated upon how the visual system finds correct binocular pairs or binocular correspondence. It is assumed that some neural network is cascaded after the binocular depth neurons and finds out correct binocular correspondence by eliminating the incorrect binocular pairs. In this article a model of such neural network is proposed. The performance of the model has been simulated on a digital computer. The results of the computer simulation show that this model finds binocular correspondence satisfactorily. It has been demonstrated by the computer simulation that this model also explains the mechanism of the hysteresis in the binocular depth perception reported by Fender and Julesz (1967).

Computers↗