EMBRYONIC EXPRESSION OF MOTONEURON TOPOGRAPHY IN THE RAT DIAPHRAGM MUSCLE

The phrenic motoneuron pool of the rat projects onto the diaphragm mus cle with a distinct rostrocaudal bias. This bias is detectable at birt h and is reestablished following denervation. In an effort to define t he mechanisms underlying this topographic bias, we asked whether growi ng phrenic motoneurons select their muscle contacts initially upon fir st contact or whether the initial neuromuscular distribution is random , to be specified later through synaptic rearrangement. The onset of n eurotransmission in embryonic diaphragm muscles aged E-14 to E-18 was studied using focal extracellular microelectrodes. Two important pheno mena were observed. First, motoneurons from all three cervical ventral roots (C-4, C-5, and C-6) establish functional innervation at the sam e time. Second, already at E-15, when the earliest synaptic potentials could be recorded, a distinct rostrocaudal bias was detected. This bi as was amplified as innervation progressed to rostral and caudal secto rs during E-16 to E-18. These results suggest that growing phrenic mot oneurons make topographic choices as they navigate toward their muscle targets. Moreover, the results indicate that further research into th e mechanisms for topographic selectivity should focus on initial nerve -muscle contacts in the embryo, rather than secondary processes of err or correction. (C) 1994 Academic Press, Inc.