Maintenance of specificity by sprouting and regenerating peripheral nerves. I. Normal variability.

Inter-animal variability in the spinal representation of a single hindlimb muscle, tibialis anterior (TA), in the cat, was examined by retrograde transport of intramuscularly injected HRP, dissection of the lumbosacral plexus and reflex testing after acute section of spinal nerves L5, L6, S1 and S2 sparing L7. No variability between the two sides of the same animal was seen. The transverse position of the TA motor nucleus and the number of labeled cells was constant between the two sides in each animal. Inter-animal variability was considerable, however, in that the number of motor neurons and rostrocaudal extent of the motor neuron column supplying TA varied considerably from animal to animal. According to the relationship between the position of the lumbosacral plexus and the distribution of spinal nerves, 3 classes of representation of the plexus were found: prefixed, postfixed and intermediate. In animals in which the lumbosacral plexus was prefixed, more than one half of labeled cells were rostral to the L7 segment; in those with postfixed plexus more than half the cells were caudal to L7. Section of L5, L6, S1 and S2 spinal nerves weakened the tibialis anterior tendon reflex in 'prefixed plexus' animals but abolished that reflex in 'postfixed' plexus animals, in spite of the presence of labeled motor neurons projecting through the spared L7 nerve. This suggests that some of the afferents and efferents comprising the TA tendon reflex may travel in different spinal roots or that a particular distribution of motor axons within a muscle is required for the maintenance of this particular reflex activity.