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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↗

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↗

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↗

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↗

PubMed1978

Graded synaptic transmission between local interneurones and motor neurones in the metathoracic ganglion of the locust.

1. In the metathoracic ganglion of the locust some neurones can effect changes in the membrane potential of identified post-synaptic motor neurones without themselves spiking. 2. These 'non-spiking' neurones have processes only within the metathoracic ganglion, and therefore are local intraganglionic interneurones. 3. The absence of spikes in the interneurones reflects their normal physiological state and is not due to the experimental conditions. 4. When the interneurones are depolarized by the injection of current pulses lasting several hundred milliseconds, post-synaptic motor neurones are either depolarized, or hyperpolarized, for the duration of the pulse. 5. The magnitude of the change in post-synaptic voltage is graded according to the amount of presynaptic current. 6. A number of physiological tests indicate that the graded effects upon motor neurones are mediated by chemical synaptic transmission. For example, an evoked hyperpolarization of a motor neurone can be reversed in polarity by simultaneously hyperpolarizing the motor neurone with injected current. 7. At their resting potential some interneurones tonically release sufficient transmitter to have a measurable post-synaptic effect. The injection of depolarizing and hyperpolarizing currents into these interneurones effects opposite changes in post-synaptic potential. 8. Other interneurones must be depolarized from resting potential before a post-synaptic effect is observed, and hyperpolarizing currents have no post-synaptic effect. In these interneurones it is estimated that a depolarization of only 2 mV is sufficient to effect the release of transmitter. 9. The membrane potentials of non-spiking interneurones can fluctuate by as much as 15 mV during active movements of the hind legs and individual p.s.p.s as large as 5 mV can be recorded. Therefore, summed p.s.p.s or even single ones are expected to be the electrophysiological signals effecting transmitter release from these interneurones.

Action Potentials↗

PubMed1992

Single local interneurons in the locust make central synapses with different properties of transmitter release on distinct postsynaptic neurons.

Quantal analysis has been applied to the inhibitory synapses made by single spiking local interneurons onto several nonspiking local interneurons (and motorneurons) in the locust CNS. Transmission at these synapses appears to be mediated by GABA. The apparent reversal potential of the IPSP and inhibitory postsynaptic current were -80 to -85 mV, a value similar to that of the potential evoked by pressure-applied GABA. This reversal potential was 25-30 mV more negative than the resting potential of the nonspiking interneurons in the experimental conditions. The statistical properties of release at these synapses were studied by recording simultaneously from pre- and postsynaptic interneurons with intracellular electrodes. The distribution of postsynaptic potential amplitudes could be described by a simple binomial model, implying uniformity of binomial p (probability of release at a single release site) for each synapse. The mean quantal amplitude was 290 +/- 110 microV, and the mean quantal content m of the IPSPs was 6.25 +/- 2.83. The mean values of binomial n (average size of the releasable pool) and p were 13.11 +/- 2.8 and 0.45 +/- 0.16, respectively. Numerical simulations of statistical experiments were performed to test whether the IPSP amplitude distribution histograms might be misleadingly indicative of quantal release. These simulations showed that such a hypothesis was very unlikely. When a spiking local interneuron was impaled, several of its target interneurons could sometimes be successively sampled. Quantal analysis was then performed with the different IPSPs evoked, in identical conditions, by a same presynaptic interneuron, and the quantal parameters were compared between the synapses. It was found that binomials n and p and their product m generally differed between the synapses made by a given spiking interneuron onto different target neurons. These results show that quantal contents can vary for the many synapses made centrally by one interneuron, and suggest that this variability may arise from differences in release probabilities between the sites associated with different synapses.

Animals↗

PubMed1992

The paired domain-containing nuclear factor pax[b] is expressed in specific commissural interneurons in zebrafish embryos.

The zebrafish paired box (Pax) genes are expressed in the early neural tube and are thought to be transcription factors that regulate the differentiation of cells in the central nervous system (CNS). The protein product of one of these Pax genes, pax[b], is detectable as a nuclear antigen in all the regions of the embryo that transcribe the gene including the posterior midbrain, the nephritic primordium, the Wolffian duct, the optic stalk, and, in specific neurons, in the hindbrain and spinal cord. The timing and pattern of axonal outgrowth by the early pax[b]-positive neurons suggest that they are the commissural secondary ascending (CoSA) interneurons in the spinal cord; the primary commissural interneurons (MiD2c and MiD3c) in hindbrain rhombomeres mi2 and mi3; and a previously unclassified set of commissural interneurons that we termed the commissural caudalrhombomere ascending (CoCaA) interneurons in the caudal hindbrain. In contrast, the Mauthner interneurons do not express pax[b] early in development. Shortly after the appearance of the first pax[b]-positive interneurons, additional nuclei adjacent to the first pax[b]-positive interneurons become pax[b] positive. This pattern of expression suggests that the pax[b] protein may be involved in determining the identity of specific commissural interneurons.

3,3'-Diaminobenzidine↗

PubMed1976

Types of interneurons and their participation in the neuronal network of the medial geniculate body.

Three different types of interneurons can be separated in the Golgi picture, and many of their details can be identified under the electron microscope, in the medial geniculate body (MGB) of the cat: (1) typical short axon Golgi II. cells of the thalamic type, (2) somewhat larger Golgi type II cells with medium range axon, and (3) spidery neurogliform short axon cells. The most distinctive features of the two first types (1) and (2) are their irregular drumstick shape appendages, increasing in number as well as in length and irregularity of their stalks towards the periphery of the dendrites. These appendages form the vast majority of synaptic profiles in the aggregations of synaptic neuropil (glomeruli) of the nuclei, and they are both presynaptic and postsynaptic by the usual standards applied for the evaluation of the polarity of synapses. The characteristic beaded dendrites of the (3) neurogliform cell type can be recognised particulary easily in the electron microscopic picture. They are both presynaptic and postsynaptic in structural polarity. All identified process profiles of interneurons contain flattened (F-type) or pleomorphic synaptic vesicles. Membrane contacts, in which the interneurons appear to be presynaptic are either of the symmetric (Gray type II) or of an intermediate type. The membrane contacts of postsynaptic portions of the interneurons are usually of the asymmetric type (Gray type I) and the presynaptic profiles contain round (R-type) vesicles. The larger one have been shown already earlier to be derived from specific sensory (inferior collicular) afferents, while many of the smaller ones could be identified in the present study as being derived from cortico-geniculate descending pathways, arising from the auditory areas. Some of the synaptic contacts of the interneurons are apparently derived from other interneurons, the presynaptic profiles being often equivocal or more likely of axonal origin (all interneurons have clear axons in the Golgi picture). The occurrence of three distinct types of interneurons--probably all of inhibitory nature--the complexity in synaptic arrangement, and more particularly in the dendritic linkage of numerous synaptic sites does not favour such simple explanations as surround inhibition by forward or by backward inhibition, but suggests more sophisticated modes of impulse processing in the MGB.

Animals↗