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 components
These observations are contrasted with the
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.