ORGANIZATION OF MOTOR POOLS SUPPLYING AXIAL MUSCLES IN THE CHICKEN

Intramuscular injections of the retrograde tracers -dioctadecyl-3,3,3' ,3'-tetramethylindocarbocyanine perchlorate (Dil) and horseradish pero xidase (HRP) were used to map the motor pools innervating axial muscle s in the cervical and thoracic regions of the chicken. We found that m otor pool position is well correlated with the muscle's embryonic orig in, and not necessarily with its position. Muscles of myotomal (exclus ively somitic) origin were innervated by medially positioned motoneuro ns in the median motor column, and the motor pools supplying these mus cles were somatotopically organized. Muscles having a dual embryonic o rigin, from the somites and lateral plate, were innervated by motoneur ons positioned further laterally within the median motor column. The r elationship between motor pool position and embryonic origin of the mu scle may be a general principle of ventral horn organization.