Development of the topographical projection of motor neurons to a rat muscle accompanies loss of polyneuronal innervation.

The rat lateral gastrocnemius muscle receives a topographical projection from lumbar segmental nerves L4 and L5. A study has been made of the development of this projection during the period when polyneuronal innervation is eliminated. The tetanic contraction due to stimulation of each nerve was compared with that due to stimulation of both nerves simultaneously. This percentage of contraction declined from about 90% to 70% for L4 from birth to 2 postnatal weeks; it declined from about 90% to 30% for L5 over the same period. The innervation of about 60% of the cells by both L4 and L5 is therefore eliminated during 2 postnatal weeks. Tetanic and twitch contraction due to stimulation of ventral rootlets of segmental level L4 and L5 was compared to stimulation of the whole muscle directly. There was a general reduction in the size of the motor units from both L4 and L5. A selective reduction in the number of L5 motor units was observed during the first 5 postnatal days. The distribution of L4 and L5 terminals on the dorsal surface of the muscle was determined by intracellular impalement of muscle cells and was determined by recording endplate potentials (EPPs) due to stimulation of L4 and L5. Polyneuronal innervation of the lateral gastrocnemius is eliminated by about 2 postnatal weeks. The percentage of muscle cells innervated by L4 or L5 in each of six equal-size muscle sectors was ascertained during this period. This percentage of innervation of muscle cells by L5 declined in all sectors between 3 days and 2 weeks postnatal.(ABSTRACT TRUNCATED AT 250 WORDS)