Location of motor pools innervating chick wing.

Intramuscular injections of HRP were used to map the spinal cord location of motoneurons innervating wing and shoulder girdle muscles in newly hatched chicks. Motor pools are grouped in the lateral motor column in relationship to embryonic origin of the muscles: muscles derived from the ventral muscle mass are innervated by medially lying motor pools, while muscles derived from the dorsal mass are innervated by lateral pools. Motor pool position is also well correlated with nerve supply. Muscles innervated by nerves diverging from common nerve trunks are innervated by neighboring motor pools. The rostro-caudal organization of the motor pools reflects both proximo-distal and antero-posterior axes of the limb with proximal and anterior muscles innervated from rostral motor pools.