New growth elicited in adult leech mechanosensory neurones by peripheral axon damage.

1. New growth in cutaneous mechanosensory neurones elicited by axotomy or axon crush was studied using intracellular injection of horseradish peroxidase at different times after the lesion, ranging from a few days to over a year. 2. Cutting or crushing major, large-calibre axon branches of mechanosensory neurones elicits sprouting of new processes, either centrally within the ganglion neuropile or at the site of the lesion in the peripheral nerve. In contrast, cutting or crushing fine-calibre axon branches supplying accessory parts of the receptive field does not elicit sprouting of the main arbor or main axon branches. 3. Different modalities of mechanosensory neurone respond differently to lesions of their axons. Cutting the axons of high-threshold units responding to noxious stimulation of the skin elicits sprouting of additional processes from the axon hillock region within the central nervous system (CNS), whereas cutting or crushing the axons of low-threshold cells responding to light touch of the skin elicits sprouting at the site of the lesion only, and not within the CNS. 4. In addition to the new growth directed into the peripheral nerve, damaged nociceptive neurones also form new processes that wrap the somata of particular cells within the ganglion. 5. Sprouted processes of axotomized neurones are retained for long periods after the lesion (up to 425 days). 6. The electrical properties of touch and nociceptive cells were studied between 1 and 60 days after axotomy, by intracellular recording from the centrally located cell bodies. The amplitude, width and maximum dV/dt of the action potential and after-hyperpolarization, as well as the resting potential and input resistance, did not change significantly after axotomy, despite the considerable process sprouting known to occur during this time.