Action of non-myelinated afferent nerve fibres on transmission through the spino-cervical tract

Abstract

The work presented is an investigation in the synaptic processing in the spinal cord of nervous activity transmitted through cutaneous C-fibres. Microelectrode recordings were made from axons in the spino-cervical tract of the cat after electrical stimulation of cutaneous nerves and after natural stimulation of the skin.SECTION I The relevant literature has been reviewed. (Reference is made to the composition of peripheral nerves and to cutaneous receptors innervated, to methods of selective block of conduction in myelinated nerve fibres, to the anatomy of the dorsal horn, to the modes of processing afferent activity in the dorsal horn and to the properties of the spino-cervical tract.SECTION II Electrical stimulation of C-fibres in peripheral nerves also excites A-fibres. With a selective block of A-fibres a pure C-volley can be produced. In this section experiments are described determining the properties of nerve fibres under polarisation. A new type of polarising electrode was developed allowing non-traumatic polarisation of peripheral nerves over minutes. Unitary recordings after electrical stimulation were made from small strands dissected from cutaneous nerves, and primary afferent fibres recorded from included C-fibres.Between stimulating and recording sites the nerves were subjected to increasing polarising currents. The effect of polarisation on conduction of single impulses and short trains of impulses was investigated. It was possible to produce a selective block of myelinated fibres leaving the C-fibres conducting. But the method was not precise enough to block all alpha fibres leaving only the delta fibres conducting. Polarisation reduced the conduction velocity of C-fibres and it impaired the transmission of repetitive activity. However, trains of impulses with frequencies between 20 and 50 imp / s passed the polarisation site when myelinated fibres had ceased to conduct.SECTION III A classification of feline spino-cervical tract units is given relating to input from cutaneous C-fibres and to responsiveness to gross movement of hairs and to pressure applied to the skin. Recordings were made from spino-cervical tract axons in decerebrate cats made spinal. About twenty percent of all units responded to hair movement and to electrical stimulation of primary afferent A-fibres alone. Eighty percent received additional input from pressure receptors and from cutaneous C-fibres as well. The excitatory pattern did not depend on whether A+C or pure C-volleys were used.The receptive fields of a few spino-cervical tract units were heated to temperatures over 45 °C and hair only units showed a marked increase in their discharge to this stimulus.SECTION IV The experiments presented are an investigation of segmental and descending inhibition influencing activity in spino-cervical tract units evoked by cutaneous C-fibre stimulation. The basic preparation was the same as in Section III. Additionally the contralateral dorsal columns and dorsolateral funiculi and both ventral quadrants in the cervical parts were given short repetitive stimuli for conditioning. The dorsal columns were cut at Th , which prevented antidromic -L O excitation of primary afferent fibres. Inhibition of discharges in spino-cervical tract units following activation of cutaneous A and C fibres was observed after conditioning stimuli given to A-fibres in non-excitatory nerves and after stimulation of the spinal cord in the three places mentioned above. Primary afferent fibres providing inhibition frequently supplied areas of skin on the contralateral leg. Conditioning with activity in cutaneous C-fibres was without effect. The time course of the inhibition was long and it was considered to be presynaptic. The significance of segmental and descending inhibition is discussed.SECTION V This section contains general conclusions. The role of the spino-cervical tract as an early alerting system and the role of descending inhibition effecting a switch in modalities transmitted through the tract are discussed. Existing theories about segmental inhibition are considered in the light of the results obtained from the experiments described in the previous sections.