Organization of motor pools supplying axial muscles in the chicken.

Intramuscular injections of the retrograde tracers 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) and horseradish peroxidase (HRP) were used to map the motor pools innervating axial muscles in the cervical and thoracic regions of the chicken. We found that motor pool position is well correlated with the muscle's embryonic origin, and not necessarily with its position. Muscles of myotomal (exclusively somitic) origin were innervated by medially positioned motoneurons in the median motor column, and the motor pools supplying these muscles were somatotopically organized. Muscles having a dual embryonic origin, from the somites and lateral plate, were innervated by motoneurons positioned further laterally within the median motor column. The relationship between motor pool position and embryonic origin of the muscle may be a general principle of ventral horn organization.