Maintenance of specificity by sprouting and regenerating peripheral nerves. II. Variability after lesions.

In previous studies we showed that collateral sprouting in cat tibialis anterior (TA) muscle was elicited by selective peripheral spinal nerve section sparing L7. After chronic (3 week) section of L5, L6, S1 and S2 spinal nerves in the present study, two different reflex patterns were observed. In some cats, with presumed prefixation of the lumbosacral plexus, the TA tendon reflex was weakened initially and became stronger beginning 2-3 days postoperative. In other cats, with presumed postfixation of the plexus, the TA tendon reflex was abolished for 7-9 days and then returned. The TA muscles were injected with HRP and labeled motor neurons plotted. In the 'prefixed' first group, the number, location and size of motor neurons projecting to TA through the spared L7 nerve were symmetrical when acute and chronic sides were compared. In the 'postfixed' group (reflex abolished then returned) the acute and chronic sides were asymmetrical: the chronic side displayed a significant increase in number of labeled cells and an increase in the rostocaudal extent of the cell column within the L7 segment. These results are consistent with two types of collateral sprouting: homonymous, in which the sprouts arise from nerves within the muscle, and heteronymous, in which the sprouts arise from nerves in adjacent muscles. In animals with very chronic (up to 2 years) spinal nerve section (L5, L6, S1 and S2) regeneration of the cut nerves was superimposed on the spared L7 innervation. Topography was completely disrupted except in the L7 segment. Thus, there appears to be a difference in specificity of motor neurons for target sites depending upon degree and location of denervation. Homonymous sprouting displays strict specificity, regeneration does not and heteronymous sprouting represents an intermediate form in which cells are recruited from adjacent motor neuron pools in the segment of the spared innervation.