Regeneration of an identified central neuron in the cricket. I. Control of sprouting from soma, dendrites, and axon.

When the axon of the medial giant interneuron (MGI) of the cricket is axotomized close to the cell body, the normally stable, characteristic dendritic arborization is induced to sprout supernumerary neurites. The origin of the induced dendritic sprouts is not random; they emerge preferentially from the dentritic tips and branches close to the exit of the axon from the terminal ganglion. If any growth also occurred from the axon, there was a reciprocal relationship between the extent of dendritic and axonal sprouting. On the other hand, axotomy distant to the cell body induces sprouting only from the axon and does not alter the dendritic structure of the MGI. After crushing the ventral nerve cord distant to the cell body, the MGI sprouted neurites from the proximal axonal stump which crossed the site of lesion and continued growing within the distal cord. After a distant cut of the cord, however, the axonal neurites formed a neuroma in the proximal cord stump at the site of lesion and stopped elongating after 1 month. At this time, supernumerary sprouts first began to emerge from the normally smooth, rounded contours of the cell body. Based on these observations, we propose that axotomized neurons produce membrane at a constant rate. This newly synthesized membrane is preferentially inserted into neurites emerging from the proximal axonal stump. When axonal neurites stop growing in a neuroma following a distant cut, then this new membrane appears as supernumerary neurites from the soma. After a close cut, the axon often dies back into the ganglion and appears unable to receive the full complement of sprouting membrane. In such cases, the balance of the newly synthesized membrane is inserted into the dendrites and the characteristic structure of the arborization is significantly altered.