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.