Somatotopically inappropriate projections of single hair follicle afferent fibres to the cat spinal cord.

In the L7 segment of cats anaesthetized with alpha-chloralose, single hair follicle afferent (h.f.a.) fibres were injected intra-axonally with horseradish peroxidase (HRP). In the same animal light tactile stimuli were used to plot the receptive fields of laminae III and IV dorsal horn neurons (d.h.n.s) at various rostrocaudal levels below the stained segment of axon. The results showed that eight of the ten injected axons gave rise to collaterals in somatotopically inappropriate regions of the dorsal horn, i.e. those regions in which the receptive fields of d.h.n.s did not encompass the receptive field of the stained h.f.a. Over-all the stained h.f.a.s gave rise to fifty-three collaterals, twenty-two (42%) of which projected to somatotopically inappropriate regions of the dorsal horn. Collateral arborizations in the somatotopically inappropriate region were always located in the same mediolateral region of the dorsal horn as those arborizations in the appropriate region. Collateral arborizations more than 600 microns from the somatotopically appropriate region had few branches and gave rise to very few boutons compared with those within the appropriate region.