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At least 19 recordsLinked to original sources

PubMed1990

Transneuronal transport of wheat germ agglutinin conjugated horseradish peroxidase into last order spinal interneurones projecting to acromio- and spinodeltoideus motoneurones in the cat. 1. Location of labelled interneurones and influence of synaptic activity on the transneuronal transport.

Transneuronal transport of wheat germ agglutinin conjugated horseradish peroxidase (WGA-HRP) was used to define the location of last order spinal interneurones projecting to deltoideus motoneurones in C5-C8 of the cat. Labelled interneurones were found bilaterally from rostral C1 to caudal Th5 and from L3 to the L4/5 border. Ipsilaterally they were located in laminae V-IX, while contralaterally they were confined to lamina VIII except for a few cells in laminae VII and IX. To estimate the degree to which interneuronal activity facilitates the transneuronal transport from deltoideus motoneurones, the numbers of labelled interneurones were compared under different experimental conditions after WGA-HRP injection. The number of labelled last order interneurones was larger in one awake and active cat than in one awake but inactive cat and also larger in six anaesthetized animals in which spinal pathways were stimulated to evoke antidromic and synaptic activation of the interneurones, than in two anaesthetized animals without stimulation. It is concluded that the transneuronal transport of WGA-HRP is considerably facilitated by increased activity in the last order interneurones. An overall tendency was observed for a positive correlation between the number of labelled interneurones and the number of primarily stained deltoideus motoneurones. In order to reach a detectable concentration of WGA-HRP in the last order interneurones a certain number of motoneurones has to be labelled to the extent that they appear homogenously black.

Action Potentials↗

PubMed1976

Convergence on interneurones mediating the reciprocal Ia inhibition of motoneurones. I. Disynaptic Ia inhibition of Ia inhibitory interneurones.

Interneurones identified as mediating the disynaptic reciprocal Ia inhibition of motoneurones (referred to as "Ia inhibitory interneurones") were recorded in the lumbar spinal cord of the cat. It was revealed that the Ia inhibitory interneurones themselves receive disynaptic Ia inhibition. The muscles from which this inhibition is evoked are strictly antagonistic to those supplying their Ia excitation. Similar to the Ia inhibition in motoneurones the Ia inhibition in the Ia inhibitory interneurones is decreased when preceded by an antidromic stimulation of ventral roots. Furthermore, transmission of Ia inhibition to the Ia inhibitory interneurones is facilitated from ipsilateral and contralateral primary afferents as well as several supraspinal pathways analogous to earlier findings for the Ia inhibition of motoneurones. The pattern and control of the Ia inhibition of motoneurones and of Ia inhibitory interneurones display so striking similarities that it is suggested that identical interneurones are responsible. The conclusion thus emerges that "opposite" Ia inhibitory interneurones (i.e. interneurones monosynaptically connected to antagonistic muscles) are mutually inhibiting each other. The functional significance of this organization is discussed.

Afferent Pathways↗

PubMed1990

Transneuronal transport of wheat germ agglutinin conjugated horseradish peroxidase into last order spinal interneurones projecting to acromio- and spinodeltoideus motoneurones in the cat. 2. Differential labelling of interneurones depending on movement type.

Transneuronal transport of wheat germ agglutinin conjugated horseradish peroxidase was used to define the location of last order spinal interneurones projecting to deltoideus motoneurones during voluntary target-reaching and/or in unrestricted walking on the ground. Labelled interneurones were found bilaterally from C2 to Th1 in target-reaching cats and almost exclusively in the C5-Th1 segments in walking cats, although the total number of labelled interneurones in these cats was considerably higher than in the target-reaching cats. These results confirm the previous finding that propriospinal neurones in the C3-C4 segments can mediate the descending command for target-reaching movements with the forelimb. In both groups of cats labelled interneurones were found ipsilaterally in laminae V-IX, while contralaterally they were mainly restricted to lamina VIII. In the forelimb segments there was a larger number of labelled interneurones in the walking cats in the lateral part of laminae V-VII and in laminae VIII and IX. There was a positive, almost linear correlation between the total number of labelled interneurones and motoneurones in all cats. The results suggest that both excitatory and inhibitory last order interneurones can be transneuronally labelled. It is concluded that this method can be used for functional identification of last order interneurones active during the preparation and/or execution of different movements.

Animals↗

PubMed1976

Anatomy of the ocellar interneurons of acridid grasshoppers. I. The large interneurons.

The anatomy of the large ocellar interneurons in the brain of five species of acridid grasshoppers of two different subfamilies (Schistocerca vaga, S. gregaria, Gastrimargus africanus, Trimerotropis pallidipennis, and Arphia conspersa) was revealed by cobalt-filling of the three ocellar nerves and subsequent reconstructions from silver-intensified (Timm's method) serial sections. Conflicts in the literature are reviewed (Tables 1, 2) and differences in the number of cells, anatomical descriptions of these cells, and nomenclature are resolved by demonstration of an identical number of large ocellar identical number of large ocellar interneurons in all five species examined (Fig. 1). There are 17 large 1st-order ocellar interneurons (Figs. 2, 3). Each of the three ocellar nerves contains the axons of seven large interneurons; four of these interneurons have axons in two ocellar nerves. The anatomy of three pairs of 2nd-order ocellar interneurons (with branches in the ocellar tracts within the brain and axons in the circum-esophageal connectives) is reconsidered in light of recent conflicts in the literature. Previous accounts by Williams (1975) of interneurons O2, O3, and PI(2):5 are corroborated and new details added (Fig. 7) by the use of a cobalt method that appears to stain these 2nd-order interneurons transsynaptically (Fig. 6).

Animals↗

PubMed1976

Anatomy of the ocellar interneurons of acridid grasshoppers. II. The small interneurons.

The anatomy of the small ocellar interneurons in the brain of the acridid grasshopper Schistocerca vaga was revealed by cobalt-filling the three ocellar nerves and subsequent reconstructions from silver-intensified (Timm's method) serial sections. In total, 61 small ocellar interneurons were repeatedly identified with arborizations in many areas of the brain and optic lobe, including in particular the posterior neuropil, ocellar tracts, protocerebral bridge, lobula, ventral bridge and tritocerebral crotch, calyces, and antenno-glomerular tracts. Each ocellar nerve contains the axons of small cells that arborize in the other two ocellar tracts; these tracts are sites of ocellar integration. Direct interactions between the ocelli and compound eyes are suggested by the projections of small ocellar interneurons into the proximal lobula. Small cell arborizations from all three ocelli are distributed actoss much of the protocerebral bridge, implying a role for the bridge as an ocellar neuropil within the brain. Four of the small interneurons could be seen in whole-mount preparations and are demonstrated to be identical in five species of acridid grasshoppers of two different subfamilies: Schistocera vaga, S. gregaria, Gastrimargus africanus, Trimerotropis pallidipennis, and Arphia conspersa.

Animals↗

PubMed1992

Evidence that synaptic transmission between giant interneurons and identified thoracic interneurons in the cockroach is cholinergic.

In the cockroach, a population of thoracic interneurons (TIs) receives direct inputs from a population of ventral giant interneurons (vGIs). Synaptic potentials in type-A TIs (TIAs) follow vGI action potentials with constant, short latencies at frequencies up to 200 Hz. These connections are important in the integration of directional wind information involved in determining an oriented escape response. The physiological and biochemical properties of these connections that underlie this decision-making process were examined. Injection of hyperpolarizing or depolarizing current into the postsynaptic TIAs resulted in alterations in the amplitude of the post-synaptic potential (PSP) appropriate for a chemical connection. In addition, bathing cells in zero-calcium, high-magnesium saline resulted in a gradual decrement of the PSP, and ultimately blocked synaptic transmission, reversibly. Single-cell choline acetyltransferase (ChAT) assays of vGI somata were performed. These assays indicated that the vGIs can synthesize acetylcholine. Furthermore, the pharmacological specificity of transmission at the vGI to TIA connections was similar to that previously reported for nicotinic, cholinergic synapses in insects, suggesting that the transmitter released by vGIs at these synapses is acetylcholine.

Acetylcholine↗

PubMed1992

Biogenic amines modulate synaptic transmission between identified giant interneurons and thoracic interneurons in the escape system of the cockroach.

In the escape system of the cockroach, Periplaneta americana, a population of uniquely identifiable thoracic interneurons (type A or TIAs) receive information about wind via chemical synapses from a population of ventral giant interneurons (vGIs). The TIAs are involved in the integration of sensory information necessary for orienting the animal during escape. It is likely that there are times in an animal's life when it is advantageous to modify the effectiveness of synaptic transmission between the vGIs and the TIAs. Given the central position of the TIAs in the escape system, this would greatly alter associated motor outputs. We tested the ability of octopamine, serotonin, and dopamine to modulate synaptic transmission between vGIs and TIAs. Both octopamine and dopamine significantly increased the amplitude of vGI-evoked excitatory postsynaptic potentials (EPSPs) in TIAs at 10(-4)-10(-2) M, and 10(-3) M, respectively. On the other hand, serotonin significantly decreased the vGI-evoked EPSPs in TIAs at 10(-4)-10(-3) M. These results indicate that octopamine, serotonin, and dopamine are capable of modulating the efficacy of transmission of important neural connections within this circuit.

Animals↗

PubMed1992

Cholinergic interneurons in the feeding system of the pond snail Lymnaea stagnalis. II. N1 interneurons make cholinergic synapses with feeding motoneurons.

The N1 neurons are a population of interneurons active during the protraction phase of the feeding rhythm. All the N1 neurons are coupled by electrical synapses which persist in a high Mg/low Ca saline which blocks chemical synapses. Individual N1 spikes produce discrete electrotonic postsynaptic potentials (PSPS) in other N1 cells, but the coupling is not strong enough to ensure 1:1 firing. Bursts of N1 spikes generate compound PSPS in the feeding motoneurons. The sign (excitation or inhibition) of the N1 input corresponds with the synaptic barrage recorded during the protraction phase. Discrete PSPS are only resolved in a Hi-Di saline. Their variation in latency and number can be explained by variation in electrotonic propagation within the electrically coupled network of N1 cells. The excitatory postsynaptic potentials (ESPS) in the 1 cell are reduced by 0.5 mM antagonists hexamethonium (HMT), atropine (ATR), curare (d-TC) and by methylxylocholine (MeXCh), all of which block the excitatory cholinergic receptor (Elliott et al. (Phil. Trans. R. Soc. Lond. 336, 157-166 (Preceding paper.) (1992)). The 1 cell EPSPS were transiently blocked by phenyltrimethylammonium (PTMA), which is both an agonist and antagonist at the 1 cell excitatory acetylcholine (ACh) receptor (Elliott et al. 1992). The inhibitory postsynaptic potential (IPSP) in the 3 cell is blocked by bath applications of MeXCh and PTMA, which both abolish the response of the 3 cell to ACh (Elliott et. al. 1992). The effects of the cholinergic antagonists on the response of 4 cluster and 5 cells to N1 stimulation matches their response to ACh (Elliott et al. 1992). It is concluded that the population of N1 cells are multiaction, premotor cholinergic interneurons.

Acetylcholine↗

PubMed1992

Analysis and modeling of the multisegmental coordination of shortening behavior in the medicinal leech. II. Role of identified interneurons.

1. Mechanical stimulation of the leech, Hirudo medicinalis, elicits withdrawal behavior that has two components: local bending in the segment stimulated and shortening in outlying segments. Local bending is characterized by excitation of longitudinal muscle on one side of the segment and inhibition on the other side. In shortening, all longitudinal muscles are excited. We wished to understand how these distinct motor patterns are produced by a nervous system with segmentally iterated neurons, a configuration that places some limitations on the complexity of connection patterns. 2. We searched for neurons in the segmental nervous system that subserved shortening behavior, expecting to find at least one interneuron in each segment that was involved in shortening behavior exclusively. We found instead that all interneurons involved in shortening are also involved in local bending, and no individual interneuron can completely account for shortening. 3. The motor output caused by individual identified interneurons is not entirely consistent with the shortening motor output pattern. For instance, one interneuron, cell 115, has the same pattern of motor effects from segment to segment, causing excitation of dorsal excitatory motor neurons and inhibition of ventral excitatory motor neurons. These effects would cause dorsal local bending, not shortening, in a few segments. Only one interneuron, cell 125, has motor effects that would cause shortening. 4. Individual interneurons were hyperpolarized while single sensory cells were stimulated, to quantify the contributions of individual interneurons to the observed motor pattern. Interneurons 115 and 125, and the inhibitory motor neuron, cell 1, were found to have significant roles in producing the shortening motor output. 5. A quantitative estimate of the role of each interneuron type showed that the identified interneurons account for most of the excitation of dorsal motor neurons, but for very little of the excitation of ventral motor neurons. This predicts that at least one additional interneuron type remains to be identified, one that would provide excitation to ventral motor neurons in several segments. 6. A back-propagation trained neural network model was constructed to predict the connections of the as yet unidentified interneurons. To match the known properties of interneurons, it was necessary to include a segmental similarity constraint in the training algorithm for segmentally iterated model neurons. The modeled networks predicted that there are at least two kinds of interneurons yet to be found. Also, the modeling showed that interneurons can have input and output patterns that differ very little from segment to segment but yet produce major differences in the motor output.

Action Potentials↗

PubMed1977

Interconnections of identified multiaction interneurons in buccal ganglia of Aplysia.

1. The 26 identified neurons of Aplysia buccal ganglia include 4 interneurons and their follower cells. Each interneuron makes cholinergic synaptic connections on eight identified ipsilateral follower neurons. Each interneuronal action potential also produces a zero-latency, Mg-intensitive electrotonic coupling potential in one cholinergic and electrotonic input from the interneurons. Electrotonic connections are bidirectional and nonrectifying. 2. Ipsilateral pairs of interneurons receive extensive common synaptic input from several unidentified neurons: each interneuron also receives some input not received by the other. These pairs are linked by bidirectional nonrectifying electronic coupling which is insensitive to high Mg. As a consequence of this organization, ipsilateral interneuron pairs can fire a) independently, or b) synchronously, or c) one active interneuron can depolarize the other. 3. Each follower receiving synaptic input from one ipsilateral interneuron also receives similar input from the other interneuron. Common follower cells thus receive a) asynchronous PSPs, or b) large summated PSPs, or c) an increased number of PSPs from each interneuron. The latter two modes constitute feed-forward summation of interneuronal action. 4. Interneuronal output is confined to ipsilateral neurons. Symmetric pairs of interneurons are coordinated by common inputs and are not directly interconnected by either chemical or electrotonic synapses. Synchrony of firing of symmetric pairs is, therefore, looser than that of ipsilateral pairs.

Action Potentials↗

PubMed1975

Nonspiking interneurons in walking system of the cockroach.

Intracellular recordings were made from the neurites of interneurons and motoneurons in the metathoracic ganglion of the cockroach, Periplaneta americana. Many neurons were penetrated which failed to produce action potentials on the application of large depolarizing currents. Nevertheless, some of them strongly excited and/or inhibited slow motoneurons innervating leg musculature, even with weak depolariziing musculature, even with weak depolarizing currents. Cobalt-sulfide-straining of these nonspiking neurons showed them to be interneurons with their neurites contained entirely within the metathoracic ganglion. Two further characteristics of these interneurons were rapid spontaneous fluctuations in membrane potential and a low resting membrane potential. One nonspiking neuron, interneuron I, when depolarized caused a strong excitation of the set of slow levator motoneurons which discharge in bursts during stepping movements of the metathoracic leg. During rhythmic leg movements the membrane potential of interneuron I oscillated with the depolarizing phases occurring at the same time as bursts of activity in the levator motorneurons. No spiking or any other nonspiking neuron was penetrated which could excite these levator motoneurons. From all these observations we conclude that oscillations in the membrane potential of interneuron I are entirely responsible for producing the levator bursts, and thus for producing stepping movements in a walking animal. During rhythmic leg movements, bursts of activity in levator and depressor motoneurons are initiated by slow graded depolarizations. The similarity of the synaptic activity in these two types of motoneurons suggests that burst activity in the depressor motoneurons is also produced by rhythmic activity in nonspiking interneurons. The fact that no spiking neuron was found to excite the depressor motoneurons supports this conclusion. Interneuron I is also an element of the rhythm-generating system, since short depolarizing pulses applied to it during rhythmic activity could reset the thythm. Long-duration current pulses applied to interneuron I in a quiescent animal did not produce rhythmic activity. This observation, together with the finding that during rhythmic activity the slow depolarizations in interneuron I are usually terminated by IPSPs, suggests that interneuron I alone does not generate the rhythm. No spiking interneurons have yet been enccountered which influence the activity in levator motoneurons. Thus, we conclude that the rhythm is generated in a network of nonspiking interneurons. The cellular mechanisms for generating the oscillations in this network are unknown. Continued.

Action Potentials↗

PubMed1979

The morphology of local non-spiking interneurones in the metathoracic ganglion of the locust.

The morphology is described of a number of non-spiking interneurones in the metathoracic ganglion of the locust that control motor neurones innervating muscles in the coxa and femur of a hind leg. The non-spiking interneurones are penetrated with microelectrodes, physiologically characterized, injected with cobalt, and the stain subsequently intensified with silver. The interneurones have diverse shapes but all are local, intraganglionic interneurones. Their cell bodies are 10-20 micrometer in diameter and lie in either the ventral or dorsal layers of cell bodies that form a cortex around the ganglion. The branches of the interneurones are profuse and overlap those of the motor neurones that they affect. On interneurone may have branches in both the most ventral and the most dorsal areas of the neuropile. Most interneurones have branches only in one half of the ganglion, but one interneurone has extensive and asymmetrical regions of branches in both halves of the ganglion (fig. 4). Similar physiological effects can be mediated by interneurones with distinct morphologies. For example, the single slow extensor motor neurone is excited by six distinct morphological types of interneurones (figs. 10-13). It is suggested that as many as 65% of the neurones within a ganglion may be local interneurones, many of which in turn may be non-spiking.

Action Potentials↗

PubMed1992

Reliability and effectiveness of transmission from exteroceptive sensory neurons to spiking local interneurons in the locust.

Mechanosensory information from exteroceptive hairs on the legs of a locust is first processed in a segmental ganglion by a midline population of spiking local interneurons for use in adjustments of posture and locomotion. Each interneuron receives excitatory inputs from a characteristic array of these receptors so that the surface of a leg is mapped onto the whole population of interneurons as a series of overlapping receptive fields. The properties of this first synaptic connection, and the contributions of individual afferents forming the receptive fields of the interneurons are examined. The gain of the excitatory synaptic connection between the hair afferents and the interneurons is often high, so that a single afferent spike can lead directly to a spike in the interneuron. Repetitive spikes in a hair afferent evoke EPSPs in an interneuron that decline in amplitude but that may summate. The first EPSP in any sequence is always the largest. The high frequencies of afferent spikes that are evoked by a normal deflection of a hair saturate the synaptic connection so that the amplitude of depolarization is no greater than to a single spike. The EPSPs from two hairs in a receptive field can summate but lead to no heterosynaptic facilitation. High-frequency bursts of spikes in one afferent can reduce the postsynaptic effect of another afferent. The amplitude of the EPSPs and the gain of the synaptic connections differ markedly between the hairs that comprise the receptive field of an interneuron. There are gradients of effectiveness, generally according to the axes of the leg, with one group of adjacent hairs producing the largest-amplitude EPSPs and having the highest gains. Individual hairs may contribute to the receptive field of more than one interneuron, and the gain of these connections may differ. The complexity of a receptive field is further accentuated by the specificity of connections made by the different physiological types of hair receptors. High-threshold hairs may make synaptic connections with an interneuron, but adjacent low-threshold hairs may not. This organization of the receptive fields means that the interneurons are sensitive to certain inputs and can reliably pass on a signal from one hair. It also implies that greater weighting is given to inputs from certain regions.

Animals↗

PubMed1977

Organization of visual inputs to interneurons of lateral geniculate nucleus of the cat.

1. Two groups of interneurons that are involved in the organization of the lateral geniculate nucleus (LGN) are described. The cell bodies of one group lie within the LGN; these units are referred to as intrageniculate. The cell bodies of the other group are found immediately above the LGN at its border with the perigeniculate nucleus; these units are referred to as perigeniculate. 2. Intrageniculate interneurons have center-surround receptive fields that resemble those of relay (principal) cells. They can be subdivided into brisk or sluggish and sustained or transient categories. They are stimulated transsynaptically from the visual cortex and have a characteristic variation in the latency of their spike response to such stimulation both at threshold and for suprathreshold stimuli. The pathway for this stimulation appears to be via cortical efferents to the LGN. Intrageniculate interneurons receive direct, monosynaptic retinal inputs, as determined by recording simultaneously from such interneurons and from the ganglion cells which provide excitatory input to them. Similar to relay cells, they are shown to have one or two major ganglion cell inputs. 3. Perigeniculate interneurons are generally binocularly innervated and give on-off responses to small spot stimuli throughout their receptive field. They respond well to rapid movement of large targets. They respond to electrical stimulation of the retina with a spike latency that falls between that of brisk transient and brisk sustained relay cells. This latency is one synaptic delay longer than that of brisk transient relay cell activation and suggests that they are excited by axon collaterals of these relay cells. Electrical stimulation of the visual cortex is also consistent with this model; the latency of the response of perigeniculate interneurons is approximately one synaptic delay longer than the latency of the response of brisk transient relay cells. 4. The interneuronal pathways described are consistent with proposed circuits that subserve the generation of IPSPs that arise in response to optic nerve and visual cortical stimulation. We now show that such inhibition has feed-forward (intrageniculate) and feed-back (perigeniculate) components that are mediated by two different classes of geniculate interneurons. It is suggested that the intrageniculate interneurons are involved in precise, spatially organized inhibition and that the perigeniculate interneurons are part of a more general, diffuse inhibitory system that modulates LGN excitability.

Animals↗

PubMed1979

Graded synaptic interactions between local premotor interneurons of the locust.

1. Graded synaptic interactions are revealed between pairs of nonspiking, local interneurons in the metathroracic ganglion of the locust. These interneurons drive motor neurons innervating muscles of a hindleg. 2. All the interactions found between the interneurons are inhibitory and one way. Synaptic transmission is effected by the graded release of chemical transmitter. Some of the connections are apparently direct. One local interneuron can, therefore, exert a graded control over the membrane potential of another local interneuron. 3. There are inhibitory connections between local interneurons that excite the same motor neuron, between local interneurons that excite antagonistic motor neurons, and between local interneurons that excite motor neurons to muscles moving different joints of a hindleg. 4. Other pairs of interneurons, which are not connected, may be driven by common synaptic inputs. Their outputs add together at the level of the motor neurons to produce effects that are greater than the sum of their individual effects. 5. It is proposed that graded interactions between these local interneurons are an essential element in the generation of motor patterns.

Electrophysiology↗

PubMed1992

Embryonic development of synapses on spiking local interneurones in locust.

The development of synapses on an identified population of spiking local interneurones in the thoracic ganglia of embryonic locusts was examined by means of intracellular horseradish peroxidase injection and electron microscopy. In adult locusts, spiking local interneurones of the midline group receive direct inputs onto a ventral field of branches from leg mechanosensory afferents and in turn make output synapses, mainly from a dorsal field of branches, directly upon leg motor neurones, nonspiking local interneurones, and intersegmental interneurones. The aim of this study is to examine the development of these connections. These interneurones are born relatively late in embryogenesis and are not identifiable until approximately 55% of development. At this time (55-60%) only simple filopodial contacts or punctate contacts are evident between the stained interneurones and other neurones. By 65-70% embryogenesis, vesicles are found adjacent to regions where apposed membranes are symmetrically thickened with amorphous electron-dense material. These symmetrical contacts lack distinct presynaptic bar-shaped densities and therefore, are not considered to be synapses. At this stage, the interneurones do not produce action potentials upon intracellular injection of depolarising current. Morphologically identifiable synapses, with vesicles, a presynaptic bar, and relatively little postsynaptic density, are first evident at 70-75%, coincident with the time of arrival of the majority of leg mechanosensory afferents into the central nervous system. At this stage, action potentials and synaptic potentials are also recorded for the first time. The midline spiking interneurones thus become electrically excitable when synapses are first recognisable, at approximately 70% embryogenesis. Most of the synapses found on the interneurones are outputs. The ratio of outputs to inputs on ventral branches is 7.5:1 which contrasts markedly to the adult ratio of 1:2. By 85-90%, output synapses still predominate on the ventral branches, but the ratio of outputs to inputs is reduced to almost 2:1. Dorsal branches have predominantly output synapses throughout embryogenesis. The ratio of dorsal outputs to inputs at 85-90% is 8.5:1 which compares with the adult ratio of 6.5:1. At this stage, action potentials and synaptic activity are always recorded.

Action Potentials↗

PubMed1976

Convergence on interneurones mediating the reciprocal Ia inhibition of motoneurones. II. Effects from segmental flexor reflex pathways.

Interneurones identified as mediating the disynaptic reciprocal Ia inhibition of motoneurones (referred to as "Ia inhibitory interneurones") were recorded in the lumbar spinal cord of the cat. Volleys in ipsilateral and contralateral high threshold muscle afferents, cutaneous and high threshold joint afferents evoked a mixture of polysynaptic excitation and inhibition. These effects were ascribed to pathways activated by flexor reflex afferents (FRA) and in addition a specific ipsilateral low threshold cutaneous pathway. Ia inhibitory interneurones excited monosynaptically from flexor nerves received stronger net excitation by volleys in ipsilateral FRA than did extensor coupled interneurones, while the opposite pattern was seen from the contralateral FRA. These patterns are similar to those found in flexor and extensor motoneurones respectivey. The FRA inhibition in Ia inhibitory interneurones was partly mediated by "opposite" Ia inhibitory interneurones, i.e. those which are mediating the Ia inhibition of Ia inhibitory interneurones. The extent to which the FRA inhibition is transmitted by Ia inhibitory interneurones was roughly estimated by its susceptibility to recurrent depression by antidromic ventral root stimulation. The main conclusion is that most segmental pathways seem to evoke their effects in parallel to motoneurones and Ia inhibitory interneurones which are monosynaptically linked to the same muscle. The functional importance of this conclusion is discussed in a following report.

Animals↗

PubMed1976

Convergence on interneurones mediating the reciprocal Ia inhibition of motoneurones. III. Effects from supraspinal pathways.

Supraspinal effects were investigated in interneurones identified as mediating the disynaptic reciprocal Ia inhibition of motoneurones (referred to as Ia inhibitory interneurones). It was revealed that volleys in the vestibulospinal tract may evoke mono- and disynaptic EPSPs in interneurones monosynaptically excited from extensor muscles, i.e. extensor coupled Ia inhibitory interneurones. Flexor coupled interneurones instead received disynaptic inhibition. Volleys in the rubrospinal tract evoked a dominating polysynaptic excitation, usually mixed with inhibition, in flexor as well as extensor coupled interneurones. Disynaptic rubrospinal EPSPs and IPSPs were also revealed. The pyramidal tract also gives rise to a dominating polysynaptic excitation, usually mixed with inhibition, in flexor as well as extensor coupled Ia inhibitory interneurones. Rubrospinal and pyramidal volleys were shown to facilitate transmission in various segmental reflex pathways to the Ia inhibitory interneurones. A detailed comparison reveals a striking parallelism of segmental and supraspinal effects on alpha-motoneurones and Ia inhibitory interneurones connected to the same muscles. This considerably strengthens the hypothesis of an "alpha-gamma-linkage in the reciprocal inhibition".

Animals↗