Analysis of potentials induced in red nucleus neurones from the somaesthetic pathway stimulated at the bulbar level

Abstract

A somaesthetic pathway to the magnocellular red nucleus (RNm) via relays other than cortico- or cerebello-rubral relays was previously found to exist in the cat. At the brainstem level, the ascending spinorubral fibres follow the medial lemniscus (LM). The present paper aims at describing in detail and evaluating the quantitative importance of the short-latency responses in RNm cells after microstimulation performed in the LM through a monopolar electrode. The RNm cells, tested intracellularly in cats anaesthetized with alpha-choralose and placed in a stereotaxic device, were identified by their antidromic activation to stimulation of the rubrospinal tract in the cervical cord. It was established that single-shock stimulation below 100 miroA current delivered to the LM induced short-lantency postsynaptic potentials (PSPs) in 87% of all the rubrospinal cells tested. The responses were indeed due to activation of LM fibres, as demonstrated by different tests: the location of the electrode tip in the LM was verified by recording, with the same electrode, the potentials evoked by stimulating the dorsal columns of the cord. The site was later confirmed histologically. The absence of stimulus spread from the LM to the underlying pyramidal tract was systematically checked by simultaneously recording the responses evoked in RNm cells and in the motor cortex. Monosynaptic excitatory responses (EPSPs) were evoked in RNm cells with a minimum stimulating current of less than 20 microA in the LM and a mean threshold of 42 microA. Disynaptic inhibitory potentials (IPSPs) were evoked in 23% of these cells with single-pulse stimulation. These latter responses showed a temporal facilitation with short trains of three pulses, which indicated that they were transmitted through inhibitory interneurones. Recordings were also performed from presumed LM fibre terminals running inside the RNm. The results therefore confirm the existence of strong lemniscal projections to RNm and demonstrate that they transmit both excitatory and inhibitory messages to rubrospinal cells. As the somaesthetic pathway to the RNm was previously found to come from the spinal cord, where it is located in the ventral portion, the present results also confirm that the LM is composed of fibres originating not only from neurones in the dorsal column nuclei, but also from cells placed at the segmental levels of the cord. The presumed sensorimotor function of this ascending pathway is discussed.