Connexions between hair follicle afferent fibres and spinocervical tract neurones in the cat: the synthesis of receptive fields.

Abstract

Relationships between the terminal arborizations of hair follicle afferent fibers and dendritic trees of spinocervical tract (sct) neurons were studied using intra-axonal and intracellular injections of horseradish peroxidase in chloralose-anesthetized, paralyzed cats. Afferent-neuron pairs (17) were successfully stained and their receptive fields determined. Of the pairs, 10 had sct neurons with a field containing that of the hair follicle afferent and 7 pairs had separate fields on the hind limb. Where the afferent fiber''s field was outside the neuron''s field there were no indications of synaptic contacts between the 2 neuronal elements. Synaptic contacts were always observed (at the light microscope level) for the 10 pairs with the hair afferent''s receptive field contained within the sct cell''s field. Contacts were always made by the branches of only a single collateral from the hair follicle afferent fiber. The numbers and locations of synaptic contacts were related to the relative positions of the receptive field; where the hair follicle afferent''s field was centrally placed there were many (40-60) contacts on proximal dendrites: where the hair follicle afferent''s field was peripherally placed in the sct cells receptive field there were few contacts (2-13) and these were peripherally placed on the dendritic tree. Where the primary afferent fiber had a centrally placed, field contacts upon dendritic spines were observed. Results are discussed in terms of the synthesis of receptive fields and the organization of neuronal connections within the mammalian spinal cord.