Neural plasticity: Part 3. Responses to lesions in the peripheral nervous system.

This paper describes some of the remarkable recuperative responses that neurons of the peripheral nervous system have to injury of their axons. The initial response is a sealing of the cut ends of the axons, followed by orderly degenerative and phagocytic events in the segments lying distal and proximal to the lesion. The chromatolytic changes occurring in the cell body are adaptive responses involving the morphological, physiological, and biochemical properties of the neuron. The cell loses dendrites and synapses, its electrophysiological properties are altered, and its biochemistry is altered to prepare the neuron for synthesizing proteins needed for formation of growth cones and regeneration of the axon. During this period, the denervated muscle undergoes dramatic atrophic changes that not only modify the electrical nd chemical properties of its membrane but also reduce the diameter and weight of its fibers. This denervation atrophy of muscle is reversible if reinnervation occurs before muscle material has been replaced by connective tissue. Responses to injury of sensory axons depend on the site of the lesion. Cutting a dorsal root causes Wallerian degeneration of the primary afferents' central processes. Collateral sprouts from neighboring uncut root fibers are thought to take over the vacant synapses on the second-order neurons in the dorsal horn. Cutting a sensory axon peripherally eliminates the receptive field of the primary afferent initially, but in time, collaterals from neighboring skin afferents provide the second-order neurons with a new receptive field. Synaptic reclamation and receptive field reorganization are but two dramatic examples of neuronal plasticity that lead to "rewired" neural circuitry.