Individual sensory neurones responding to touch (T) and to noxious (N) stimuli applied to the skin of the leech were killed by injecting pronase into their cell bodies, situated within the C.N.S. This procedure destroys one neurone in its entirety without damaging the cells. 2. When three out of four N cells within a ganglion have been killed, the receptive field of the remaining N sensory cell expands to cover the denervated area of skin. Similarly the field of the touch cell that innervates dorsal skin spreads across the mid line to innervate contralateral skin after the three touch cells on that side have been deleted. 3. The spread is graded and develops with time. The earliest effects appear within 4 weeks and the full spread develops by 3 months. 4. No detectable spread of receptive fields occurs if only two N cells, one on each side, are killed. 5. Following deletion of N cells, the receptive fields of T and pressure sensory cells are unaffected. Similarly, if T cells have been killed, the fields of N cells or pressure cells do not become enlarged. 6. These results represent a modality-specific mechanism by which one sensory cell can be influenced to extend the territory it supplies in the periphery in response to a minimal lesion without its own terminals having been damaged.
