Ability of motoneurons to regulate quantal release and terminal growth after reduction in motor unit size.

Experiments were performed to test the hypothesis that motoneurons are programmed to produce fixed amounts of growth and transmitter release substrates and are unable to regulate production to match the number of nerve terminals they support. The size of the average motor unit in mouse tensor fascia latae muscles was approximately halved by cutting out the distal half of the muscle. Surviving terminals were assessed 100-110 days later for level of quantal output and percentage of terminals with sprouts. Both quantal output and sprout percentage were the same as in contralateral control muscles. Both parameters have been reported to be elevated 1-9 days after a similar operation. We conclude that mouse motoneurons can down-regulate their production of growth and release substrates but that the process takes more than 9 days to become effective.