Regeneration of the axons of the peripheral nervous system has always been accepted, but comparable structural regrowth has also been observed in the brain and spinal cord (Veraa and Grafstein, 1981). For example, injury to catecholamine-containing fibers in the medial forebrain bundle leads to regrowth of axonal portions connected to nerve cells. Dendrites in the brain may also grow back following injury. Figure 1 illustrates one form of regrowth after injury: collateral sprouting.

Figure 1  Collateral Sprouting of a Brain Neuron
(a) The septal nucleus is normally innervated by the fimbria and the medial forebrain bundle. (b) After the medial forebrain bundle has been severed, a fimbria axon develops sprouts that occupy synaptic sites formerly occupied by axon terminals from the medial forebrain bundle. (After Raisman, 1978.)

In the peripheral nervous system the story goes like this: If a peripheral sensory or motor fiber is injured, the terminal portions degenerate, and sensory or motor function in the affected region is immediately lost. Nerve fibers adjacent to the injured fibers recognize this injury (perhaps by a chemical signal delivered from the injured site), and they respond by developing sprouts or branches from intact axons. In time, usually weeks, these sprouts connect to denervated skin or muscle and acquire functional control of these regions on the periphery of the body (Diamond et al., 1976).

This mechanism seems to provide functional compensation for a loss of neuronal connections. The injured nerve fiber (axon) slowly regrows, and as it approaches the skin or muscles to which it had been connected, the sprouts retract. Again, chemical signals from the regrowing original fiber probably produce this change.

Demonstrations of collateral sprouting in the brain and spinal cord, once rarely observed, are now reported with regularity. A growing view is that injury of the nervous system might release neurotrophic factors that stimulate collateral sprouting (Nieto-Sampedro and Cotman, 1985).

References

Diamond, J., Cooper, E., Turner, C., and Macintyre, L. (1976). Trophic regulation of nerve sprouting. Science 193: 371–377.

Nieto-Sampedro, M., and Cotman, C. W. (1985). Growth factor induction and temporal order in central nervous system repair. In C. W. Cotman (Ed.), Synaptic plasticity (pp. 407–457). New York, NY: Guilford Press.

Raisman, G. (1978). What hope for repair of the brain? Annals of Neurology 3: 101–106.

Veraa, R. P., and Grafstein, B. (1981). Cellular mechanisms for recovery from nervous system injury: A conference report. Experimental Neurology 71: 6–75.