Physiological studies on the postsynaptic dorsal column system

Abstract

This thesis has made an electrophysiological study of segmental and descending influences on identified neurones of the postsynaptic dorsal column (p.s.d.c.) system and has examined aspects of the relationship between the neurones of this projection and the spinocervical tract (s.c.t.). Extracellular single unit microelectrode recordings were made from the axons of p.s.d.c. neurones ascending the dorsal columns of cats anaesthetised with chloralose.1) The response properties and organisation of the cutaneous receptive fields of p.s.d.c. neurones were investigated using light tactile, and noxious mechanical and thermal stimuli.The receptive fields of units with input from glabrous skin had a complex organisation and many were discontinuous. These units could be inhibited by both light tactile and noxious cutaneous stimuli. In contrast, units with receptive fields confined to hairy skin of the proximal limb often had a concentric receptive field organisation in which the high threshold excitatory component extended beyond the low threshold area. These units could be inhibited only by light tactile stimuli and their inhibitory receptive fields generally covered an extensive area of skin virtually surrounding the excitatory componentsThese observations are contrasted with the (ii) relatively simple receptive field organisation of s.c.t. neurones and are discussed in relation to previous observations of the morphology and ultrastructure of neurones of the p.s.d.c. and s.c.t. systems.2) A study has been made of the influence of systems descending from the brain on the response properties and receptive field organisation of p.s.d.c. neurones. The cutaneous receptive fields of p.s.d.c. units were investigated both before and during a block of conduction in descending fibres produced by cooling a region of cord rostral to the recording site. The results indicate that both the responsiveness of p.s.d.c. neurones to noxious mechanical and thermal stimuli and the area of skin from which such stimuli may effectively excite these cells are powerfully suppressed by inhibitory controls descending from the brain. The possible functions of these descending actions are discussed.3) The relationship between neurones of the p.s.d.c. and s.c.t. systems has been investigated. Contrary to recent reports in the literature, it was established that p.s.d.c. and s.c.t. projections arise in substantial part, if not entirely, from separate populations of neurones in the dorsal horn. There is, however, a close relationship between the two systems at the level of the dorsal horn. Evidence was obtained to support the suggestion that some s.c.t. cells make effective excitatory collateral connections with p.s.d.c. neurones.