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Biomedical subjects

G M Shepherd

Publications and source records attributed to G M Shepherd.

At least 145 records · Page 8Linked to original sources
PubMed1977

Dopamine-sensitive adenylate cyclase within laminae of the olfactory tubercle.

There are three histological laminae within the rat olfactory tubercle: plexiform, pyramidal and polymorphic. We have assayed dopamine-sensitive adenylate cyclase in homogenates of frozen sections cut parallel to these laminae. Consecutive sections were cut of alternating thickness, 16 micron and 100 micron. The 16 micron sections were stained with toluidine blue to ascertain the depth and orientation of each section. The 100 micron sections were homogenized and assayed for dopamine-sensitive adenylate cyclase. Substantial levels of dopamine-sensitive adenylate cyclase were found within all three laminae. The results suggest that the enzyme occurs in cell processes, including dendrites, of the plexiform layer, and they are consistent with the localization of dopamine-sensitive adenylate cyclase in the pyramidal cells.

Adenylyl Cyclases↗
PubMed1977

Analysis of the onset phase of olfactory bulb unit responses to odour pulses in the salamander.

1. A method for delivering odour pulses of controlled onset, steady plateau and abrupt termination, has been developed and applied to a single unit study of mitral cell responses in the olfactory bulb of the salamander. The pulses have been monitored during the experiments near the site of stimulation on the olfactory mucosa.2. Responses have been categorized as excitatory or suppressive based on the initial response to the odour pulse.3. Initially excitatory responses had sustained discharges near threshold. With increasing concentration, the discharge changed to a brief burst followed by suppression. The briefest latency of a unit response was 120 msec, using stimulation of medium concentration, after the start of the pulse; the majority of units appeared to be excited within 200-300 msec. Ramp stimuli gave increasing periods of excitation as the rise time of the odour front became less abrupt.4. Initially suppressive responses showed suppression at all levels of concentration. The majority of units appeared to have an onset of suppression about 300-400 msec after the start of the pulse.5. These basic responses, involving suppression or excitatory-suppressive sequences, can be correlated with some basic properties of the synaptic circuits in the olfactory bulb. The time courses of the initial responses appear to be within the time periods of the inhalation cycle of the salamander, and therefore may reflect mechanisms of processing of natural olfactory stimuli.

Action Potentials↗
PubMed1975

Synaptic actions on mitral and tufted cells elicited by olfactory nerve volleys in the rabbit.

1. A unitary study has been carried out of mitral and tufted cell responses to olfactory nerve volleys in the olfactory bulb of rabbits lightly anaesthetized with urethane-chloralose. 2. With volleys of different strengths, some mitral cells responded with a spike whose latency decreased considerably as the strength increased (elastic response); other cells responded at an invariant latency (inelastic response). The former may reflect diffuse olfactory nerve inputs to the dendritic tufts in the olfactory glomeruli, while tha latter may reflect input from discrete bundles of fibres. 3. The shortest spike latencies are consistent with monosynaptic excitation by the olfactory nerves; longer latencies may be due to longer pathways through the nerves, or polysynaptic pathways within the glomerular layer. 4. Facilitation, in terms of lower threshold and shorter spike latency, was found when testing with paired volleys of weak intensity at relatively short intervals (less than 40 msec). Suppression, in terms of raised threshold, longer latency and briefer repetitive discharges, was found at intervals up to several hundred msec. The facilitation and suppression are consistent with the hypothesis of synaptic excitation and inhibition, respectively, mediated through interneurones in the olfactory bulb. 5. Presumed tufted cells were similar in response properties to identified mitral cells. 6. Intracellular recordings revealed long-lasting hyperpolarization and in some cases, an initial depolarization leading to spike initiation, in response to an olfactory nerve volley.

Action Potentials↗
PubMed1975

Short-axon cells in the olfactory bulb: dendrodendritic synaptic interactions.

1. In the rabbit olfactory bulb, analysis has been carried out of extracellular unitary responses in the glomerular layer to olfactory nerve volleys. 2. Units in the glomerular layer responded to single volleys with single, double, triple or longer repetitive spike discharges. The shortest initial latencies are consistent with monosynaptic excitation from the olfactory nerves; longer latencies may reflect longer nerve pathways or polysynaptic connexions in the glomerular layer. 3. Like mitral and tufted cells, some glomerular layer units gave evidence of activation by discrete nerve bundles. This correlates with recent anatomical evidence for projections of discrete olfactory nerve bundles to the glomeruli. 4. Facilitation of glomerular layer units took the form of lower spike thresholds and shorter latencies, when testing with paired olfactory nerve volleys of weak strength at relatively short intervals (less than 40 msec). Supression took the form of raised thresholds, longer latencies and briefer repetitive discharges; this was particularly evident with strong volleys at long testing intervals. 5. The early period of facilitation and later period of suppression did not correlate with the recovery cycle of the olfactory nerves; the nerves had an absolute refractory period of approximately 3 msec, relative refractory period of 15-30 msec, and a small supernormal period of several hundred msec or more. 6. The evidence that the facilitation and suppression are mediated by dendrodendritic pathways through the periglomerular short-axon cells is discussed in relation to recent electronmicroscopical studies. The results have implications for similar pathways through short-axon cell dendrites in other parts of the nervous system.

Action Potentials↗
PubMed1975

Axons, dendrites and synapses.

Classical concepts of neuronal organization have been based on the motoneuron as a model. Recent work on the mitral cell of the olfactory bulb has revealed synaptic connections of dendrites and cell bodies that are not present in the motoneuron. Similar findings in many other parts of the nervous system suggest the need to revise our concept of synaptic relations and functions properties of axons and dendrites.--SHEPHERD, G. M. Axons, dendrites and synapses.

Axons↗