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.