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G M Shepherd

Publications and source records attributed to G M Shepherd.

At least 127 records · Page 7Linked to original sources
PubMed1982

Impulse activity in presynaptic dendrites: analysis of mitral cells in the isolated turtle olfactory bulb.

Impulse activity has been reported in neuronal dendrites in several regions of the central nervous system, where it is believed to assist in boosting transmission of signals from remote dendritic sites to the cell body. We have studied this activity in the dendrites of mitral cells in an isolated preparation of the turtle olfactory bulb. Intracellular recordings have been obtained from mitral cells responding to single volleys in the olfactory nerves or lateral olfactory tract. In addition to the large somatic spike, a small fast prepotential (FPP) was present in nearly all cells in response to an orthodromic volley in the olfactory nerves, but it was never seen in antidromic responses from the lateral olfactory tract. Collision tests using antidromic and orthodromic volleys showed that the EPP does not propagate into the axon. Hyperpolarizing current injections caused delay and blocking of the soma spike with little effect on the FPP response. These and other tests provided evidence to localize the EPP in the dendrites and to distinguish it from injury potentials and from spikes in the axon hillock or axonal initial segment. These results suggest that one function of the impulse in mitral cell dendrites is the classical one of boosting transmission of synaptic responses from the glomerular tuft to the cell body. In addition, it si well established that mitral cell dendrites are presynaptic to the dendrites of interneurons within the bulb and that these connections provide pathways for recurrent inhibition of the mitral cells. It therefore appears that the dendritic impulse in mitral cells acts as a booster for local dendritic synaptic output. These results provide further evidence for the multiple state-dependent input-output functions of cells with presynaptic dendrites.

Action Potentials↗
PubMed1981

Topographical and laminar localization of 2-deoxyglucose uptake in rat olfactory bulb induced by electrical stimulation of olfactory nerves.

Experiments were carried out to examine the topographical projection of the olfactory nerves to the olfactory bulb in the rat, using the Sokoloff [14C]2-deoxyglucose (2-DG) technique. Electrical stimulation of a medially located bundle of olfactory nerves produced a discrete zone of 2-DG uptake at the rostral pole of the bulb. Increasing stimulus strength yielded a slightly larger focus at this site. In contrast, electrical stimulation of laterally situated bundles of olfactory nerves resulted in a broad zone of activity extending along the lateral wall of the bulb, and increasing stimulus intensity produced a more extensive area of uptake. Laminar analyses provided information on the relation between activity in the glomerular layer, where the olfactory nerves terminate, and activity in deeper layers. The results support previous studies of the topographical projections of the olfactory nerves to the olfactory bulb. They also support the hypothesis that odor-induced 2-DG uptake in the olfactory bulb represents activation of groups of receptors in the olfactory epithelium whose axons terminate in activated glomerular regions in the olfactory bulb.

Animals↗
PubMed1981

Localization of synaptic responses in the in vitro turtle olfactory bulb using the [14C]2-deoxyglucose method.

Studies were carried out on the in vitro turtle olfactory bulb preparation with a modification of the [14C]-2-deoxyglucose (2-DG) technique. Electrical stimulation of either the dorsal or ventral division of the olfactory nerve produced broad bands of activity, spreading across laminae, in the corresponding half of the olfactory bulb. Pretreatment of the isolated preparation in a low calcium bath eliminated focal 2-DG uptake induced by electrical stimulation of the nerve. In contrast, pretreatment with the GABAergic antagonist, bicuculline, produced a notable increase in 2-DG accumulation. These findings are discussed in the light of the topographical innervation of the olfactory bulb by the olfactory nerve as well as the known synaptic organization of the olfactory bulb and the putative neurotransmitters operative in local circuits.

Animals↗
PubMed1981

Evoked potential and single unit responses to olfactory nerve volleys in the isolated turtle olfactory bulb.

An in vitro preparation of the whole olfactory bulb of the turtle has been investigated using extracellular evoked potentials and unitary spikes. The compound action potential has a slow conduction velocity (0.15 mm/msec), brief relatively refractory period (10 msec) and prolonged supernormality (several hundred msec). The evoked potentials undergo prolonged periods of partial and complete suppression, as tested by the responses to paired volleys in the olfactory nerves. The suppression has a complex temporal pattern, and lasts more than 4 sec. Similar periods were revealed by the unitary spike responses of presumed mitral cells to olfactory nerve volleys. The results are similar to those reported in in vivo studies of turtle and other species. They suggest that the basic properties of mitral cells and their synaptic interactions with local interneurons are maintained in this isolated olfactory bulb preparation.

Animals↗
PubMed1981

Electrophysiological analysis of mitral cells in the isolated turtle olfactory bulb.

1. An in vitro preparation of the turtle olfactory bulb has been developed. Electrophysiological properties of mitral cells in the isolated bulb have been analysed with intracellular recordings. 2. Mitral cells have been driven antidromically from the lateral olfactory tract, or activated directly by current injection. Intracellular injections of horseradish peroxidase (HRP) show that turtle mitral cells have long secondary dendrites that extend up to 1800 micrometer from the cell body and reach around half of the bulbar circumference. There are characteristically two primary dendrites, each supplying separate olfactory glomeruli. 3. Using intracellular current pulses, the whole-neurone resistance was found to range from 33 to 107 M omega. The whole-neurone charging transient had a slow time course. The membrane time constant was estimated to be 24-93 msec by the methods of Rall. The electrotonic length of the mitral cell equivalent cylinder was estimated by Rall's methods to be 0.9-1.9. 4. The spikes generated by turtle mitral cells were only partially blocked by tetrodotoxin (TTX) in the bathing medium. The TTX-resistant spikes were enhanced in the presence of tetraethylammonium (TEA), and blocked completely by cobalt. 5. The implications of the electrical properties for impulse generation in turtle mitral cells are discussed. The mitral cells have dendrodendritic synapses onto granule cells, and the TTX-resistant spikes may therefore play an important role in presynaptic transmitter release at these synapses.

Animals↗
PubMed1981

Analysis of synaptic potentials in mitral cells in the isolated turtle olfactory bulb.

1. The synaptic responses of mitral cells have been analysed in intracellular recordings from the isolated olfactory bulb of the turtle. 2. The response of a mitral cell to a single volley in the lateral olfactory tract consisted of and antidromic impulse and a complex hyperpolarizing potential that had the properties of an inhibitory post-synaptic potential. The inhibitory response consisted of two successive components I1 and I2, followed by a prolonged hyperpolarization. 3. High-gain recordings revealed miniature hyperpolarizing potentials during the I1 and I2 responses. Both the miniature potentials and the I1 and I2 responses were increased in amplitude by depolarizing injected currents, and decreased and reversed in polarity by hyperpolarizing currents. The input conductance was increased during the I2 component. In some cells the I1 and I2 components, recorded with micropipettes filled with potassium acetate or potassium citrate, were depolarizing. 4 A single orthodromic volley in the olfactory nerves elicited a complex depolarizing-hyperpolarizing potential in mitral cells. The depolarization consisted of two successive components, E1 and E2. The hyperpolarization consisted of two successive components, I1 and I2, followed by a prolonged hyperpolarization. 5. The depolarizing components had the properties of excitatory post-synaptic potentials. They decreased in amplitude with depolarizing current injection and increased with hyperpolarizing injection. The hyperpolarizing components resembled the I1 and I2 components of the tract-evoked responses in their timing and properties. 6. It is postulated that the E1 component reflects the initial excitation by olfactory nerve terminals of the mitral cell dendritic tufts in the olfactory glomeruli. The I1 component is postulated to arise from dendrodendritic synaptic input mediated by interneurones, mainly granule cells. The E2 and I2 components are likely to arise mainly from intrinsic synaptic circuits within the olfactory bulb.

Animals↗
PubMed1981

Analysis of a long-duration inhibitory potential in mitral cells in the isolated turtle olfactory bulb.

1. An inhibitory potential of long duration has been analysed in intracellular recordings from mitral cells in the isolated turtle olfactory bulb preparation. 2. A single volley in the olfactory nerves or lateral olfactory tract elicited synaptic inhibition in mitral cells, followed by a long-lasting hyperpolarization of the cell. This slow potential has been termed the Is component, to distinguish it from the earlier I1 and I2 components to the inhibitory post-synaptic potentials (i.p.s.p.s). 3. The slow Is component was inhibitory, as shown by the interruption of spontaneous discharges, and the blockage of responses to injected current and incoming volleys. The duration of the hyperpolarization following a single volley usually lasted up to 5 sec, and occasionally longer. 4. The Is response decreased in amplitude with either depolarizing or hyperpolarizing current injection, without showing a reversal potential. This contrasted with the earlier I1 and I2 components, which showed reversal potentials characteristic of i.p.s.p.s. 5. Tests of membrane conductance showed that the conductance, which was increased during the preceding i.p.s.p., decayed to control values during the early part of the Is potential. 6. The Is response shares some characteristics with slow potentials reported in certain other vertebrate neurones. This type of response may be involved in longer-term neuromodulatory control of the excitability of the mitral cell.

Animals↗
PubMed1979

Functional organization of rat olfactory bulb analysed by the 2-deoxyglucose method.

The spatial patterns of activity elicited in the rat olfactory bulb under different odor conditions have been analysed using the 2-deoxyglucose (2DG) technique. Rats were injected with 14C-2DG, exposed to controlled environments of amyl acetate, camphor, cage air, dimethyl disulfide, and pure air and autoradiographs prepared by the method of Sokoloff. Amyl acetate was associated with regions of glomerular layer densities in the anterolateral and mid- to posteromedial parts of the bulbar circumference, as previously reported. The extents of the densities increased with increasing concentration. Camphor odor was associated with regions of increased density in the anterodorsal and mid- to posteromedial parts of the bulb. Exposure to cage air produced scattered densities in the posteromedial and posterolateral bulb. Exposure to dimethyl disulfide gave variable results. Pure air was associated with a minimal number of small dense foci. The results with amyl acetate, camphor and cage air suggest that patterns for different odors are distinguishably different but overlapping. The regions of activity are greatest in extent and density with the highest odor concentrations. These define the regions within which more restricted and isolated foci appear at lower concentrations. The results thus provide evidence for the specific role of spatial factors in the neural processing of odor quality and odor concentration.

Acetates↗
PubMed1978

Responses of olfactory receptor cells to step pulses of odour at different concentrations in the salamander.

1. The response properties of single olfactory receptor cells in the salamander have been analysed in unitary recordings obtained with platinum-black metal-filled micro-electrodes. 2. Stimulation has been carried out using an apparatus which delivers odour pulses of abrupt onset, steady plateau and abrupt termination. The pulses have been monitored near the site of stimulation on the olfactory epithelium during the experiments. 3. The main type of response was a discharge of impulses that was time locked to the stimulus pulse. The pattern of the responses consisted of a relatively brief latency of onset, a rapid rise in impulse frequency, a continuation of impulse firing during the plateau of the pulse, and an abrupt termination of the discharge correlated with the termination of the pulse. 4. There was a clear relationship between the receptor responses and odour concentration. In general, impulse firing frequency increased with increasing odour concentration. The firing frequency ranged from approximately 1--3 impulses/sec at threshold, up to 20 impulses/sec at the highest concentration. 5. Two types of reduced impulse activity were observed. One occurred after the termination of the pulse and lasted 1--3 sec; this was a common occurrence. The other type was seen during a pulse as a reduction of impulse activity compared to the background level; this type was rarely observed. 6. The receptor responses resembled those of mitral cells in the olfactory bulb to odour pulses in their sensitivity to odour concentration. They differed in that mitral cells show primary response categories consisting of brief excitation followed by suppression, and pure suppression, that are rarely seen at the receptor level. These differences may be ascribed to synaptic interactions in the olfactory bulb. 7. It is concluded that the majority of receptor cells have a stereotyped discharge response pattern and a systematic relation to odour concentration. These properties appear to reflect the simple time course of the odour pulses used in these experiments. This represents an initial step toward analysing olfactory coding at the receptor level using stimuli controlled in a manner similar to that used in other sensory systems.

Action Potentials↗
PubMed1978

Adaptive properties of olfactory receptors analysed with odour pulses of varying durations.

1. The adaptive properties of salamander of olfactory receptors have been analysed in extracellular unit recordings. Stimulation has been by step pulses of odour of varying durations for 1--10 sec. 2. The most common response was a prolonged impulse discharge that continued throughout the duration of the pulse and terminated abruptly within 1 sec of the end of the pulse. The interval for termination was relatively independent of the pulse duration. Pulses were frequently followed by a period of impulse inactivity lasting 1--3 sec, usually independent of previous pulse duration. 3. The impulse discharges were typically slowly adapting. Initially, during the first 1--2 sec, the frequency rose to 5--10 impulses/sec, at threshold concentration. In some cases, the initial level was maintained throughout the pulse, with little or no adaptation. More commonly, there was a distinct initial phasic peak, followed by decay to a lower level of 4--8 impulses/sec, which was maintained during the pulse. It was concluded that most olfactory receptors are slowly adapting, with variable phasic responsiveness dependent on odour concentration and other factors. 4. Reductions in impulse activity, compared with background, during a pulse were rarely seen. Methods for increasing the level of background activity and the use of very long duration pulses were necessary in order to bring out this type of response. Uniformly reduced activity throughout a pulse was seen clearly in only one case. A pattern consisting of a waning and then recovery of impulse frequency during a pulse was also observed in rare cases. 5. The results have shown that olfactory receptor discharges characteristically have a relatively precise relation to step pulses of odour of varying duration. The properties of the response have implications for the steps involved in the overall processes of activation and inactivation of receptor mechanisms at the olfactory mucosa.

Action Potentials↗