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

Abstract

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.