Mammalian afferent nerve fibres

The original research work described in this thesis falls into three categories:- a) a study of mammalian myelinated and non-myelinated afferent nerve fibres and their endings, this forms the principal part, b) an examination of some properties of nerve-cells in the mammalian spinal cord and c) a minor section on the sympathetic nervous system.a) Afferent nerve fibres. The major advance described is the development of techniques which allow individual active afferent nerve fibres to be identified, and their conduction velocities to be measured, when they are in nerve strands containing many axons. By these means the diameters of the active fibres may be assessed. These techniques, combined with improved dissection methods, have made the non-myelinated axons in peripheral nerves accessible as functional single units. The results of applying the techniques first to visceral nerves, then to cutaneous nerves and finally to somatic muscle nerves, establish that the nerves to the abdominal viscera and to the skin contain an unexpected variety of non¬ myelinated afferent axons, as judged by the responses of the nerve-endings to quantitative mechanical, thermal and chemical stimuli. In the stomach there were both mechanically-sensitive and pH-sensitive units located in different layers of the stomach wall. In the skin there were mechanoreceptors and thermoreceptors of highly selective sensitivity. Some of these latter nerveendings and their afferent fibres could function as 'modality-specific' cutaneous transducers, with very considerable and unexplored implications in terms of central nervous function. The muscle afferent C fibres appear to be nociceptors. A detailed examination of single myelinated cutaneous afferent nerve-fibres also supports the thesis that the afferent fibres and their receptors have a selective sensitivity. A relation between the structure of afferent nerve-endings and the selective sensitivity of the afferent unit was clearly established for one particular nerve-ending - a 'cutaneous touch corpuscle' innervated by a thick myelinated axon. The dependence of the structure and sensitivity of the 'touch corpuscle' on its innervation was also established.b) Spinal cord neurones. The pattern of 'recurrent inhibitory' connections from motor axon collaterals to small interneurones (Renshaw cells) in the ventral horn of the lumbo-sacral spinal cord and from these interneurones to motorneurones displayed no reflex pattern. In general, there was feedback from motoraxons to adjacent Renshaw cells, which in turn fed back to the motoneuronal nucleus from which the motor-axons arose and also to other adjacent motor-neurones. There was a striking correlation between the intensity of recurrent inhibition and the function of the motor-neurone; those motor-neurones innervating slowly - contracting extensor muscles were most strongly inhibited.c) Sympathetic nervous system. Depletion of the stores of catecholamines and 5 -hydroxytryptamine in the brain of cats by reserpine treatment did not abolish sympathetic preganglionic efferent activity, so that the dramatic depressant action of reserpine on an animal is unlikely to be due to this depletion. The depressant effect of intravenously injected catecholamines on sympathetic preganglionic disch arge was shown to be exerted principally, if not entirely, by reflex mechanism not by a direct action on the central nervous system