Organization of the motor innervation of craniocervical muscles in the Mallard, Anas platyrhynchos L.

The organization of the motorsystem supplying the craniocervical muscles in the mallard was investigated. Firstly, the distribution of peripheral nerves supplying these muscles was macroscopically examined. Each muscle was found to be innervated by several spinal nerves, and each nerve to contact various muscles. Secondly, the position of neck motoneurons was studied by means of retrograde tracer transport. Labeled motoneurons were found in the supraspinal nucleus of the lower brainstem and in the ventral horn of the upper cervical cord. Each of the craniocervical muscles appeared to be innervated by a separate motorpool, and pools supplying different muscles were found to overlap considerably. Despite this overlap, some organization pattern could be recognized: the most ventral muscles are innervated by motoneurons that occupy dorsomedial positions, while more dorsal muscles are innervated by neurons at ventro-lateral positions. Thirdly, the arrangement of motoneurons innervating different parts of three craniocervical muscles was investigated by means of retrograde tracer transport. This indicated that each muscle part is also innervated by a separate motor column, and that columns supplying parts of the same muscle have similar positions in transverse sections but are shifted with respect to each other in the longitudinal direction. The separate columns of one muscle mingle to form one main motorpool. These results are discussed with respect to observations in other animals.