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

1. Relationships between the terminal arborizations of hair follicle afferent fibres and dendritic trees of spinocervical tract (s.c.t.) neurones were studied using intra-axonal and intracellular injections of horseradish peroxidase in chloralose-anaesthetized, paralysed cats. 2. Seventeen afferent-neurone pairs were successfully stained and their receptive fields determined. Ten of the pairs had s.c.t. neurones with a field containing that of the hair follicle afferent and seven pairs had separate fields on the hind limb. 3. Where the afferent fibre's field was outside the neurone's field there were no indications of synaptic contacts between the two neuronal elements. 4. Synaptic contacts were always observed (at the light microscope level) for the ten pairs with the hair afferent's receptive field contained within the s.c.t. cell's field. Contacts were always made by the branches of only a single collateral from the hair follicle afferent fibre. 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 (forty to sixty) contacts on proximal dendrites; where the hair follicle afferent's field was peripherally placed in the s.c.t. cell's receptive field there were few contacts (two to thirteen) and these were peripherally placed on the dendritic tree. Where the primary afferent fibre had a centrally placed field contacts upon dendritic spines were observed. 5. The results are discussed in terms of the synthesis of receptive fields and the organization of neuronal connexions within the mammalian spinal cord.