Saywell, Shane Anthony; (2001) The thoracic respiratory interneurone: Physiology, morphology and pharmacology, with reference to the transmission of respiratory drive. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The central respiratory drive generated in the medulla is transmitted to the motoneurones of the cervical and thoracic cord that innervate the muscles that produce respiratory movements. This respiratory drive is thought to be transmitted to the motoneurones monosynaptically from the bulbospinal neurones and polysynaptically via interneurones. In this study in the anaesthetized paralysed cat, the strength of the bulbospinal input to thoracic interneurones was investigated using intracellular spike-triggered averaging. Similarly, spike-triggered averaging was performed in motoneurones to identify any bulbospinal input for comparative purposes and to re-assess the strength of this input. No input to the interneurones was identified. However, the input to the motoneurones was found to be more abundant than previously thought. Some of the intracellularly penetrated interneurones were injected with Neurobiotin to allow morphological characterisation. Interneurones with somata located in laminae VII to X of the spinal cord were labelled and had a varied morphology. These neurones had extensive dendritic projections throughout the ventral horn with a predominant rostrocaudal orientation. The majority of the interneurones had axons that projected to the contralateral cord where the axons descend in the medial and ventral funiculi. Interneurones with ipsilateral and rostral projections were also identified. The axons were found to give off up to four collateral branches with the collaterals projecting within the ventral horn. The collaterals tended to be fine possessing both en passant and terminaux boutons. Renshaw cells that exhibited respiratory phasing of their discharge have also been characterised by intracellular labelling in the thoracic cord. These cells were shown to have similar morphology and axonal projections to those seen in the lumbar cord. The thoracic motoneurones receive a stronger monosynaptic drive than previously thought, but an input to the interneurones has not been identified. The interneurones represent a diverse morphological population that predominantly have a crossed descending axon. These interneurones have fine collateral projections with generally small terminal fields and they may be inhibitory or excitatory. The fine nature of the collaterals is supported by Renshaw cell collaterals being larger and stained at long distances from the injection site. The thoracic Renshaw cells are similar in character to those of the lumbar cord.

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