Innervation of the complexus ("hatching") muscle of the chick.

The complexus muscle of avians, also known as the "hatching" muscle, is notable for the dramatic, transient pseudohypertrophy which it undergoes around the time of hatching. The muscle is believed to be involved in specific dorsal and lateral head movements used for hatching. Innervation of the complexus muscle was studied in the hatchling chick by using horseradish peroxidase (HRP). HRP injections of the muscle showed that motor innervation arose, as expected, from the cervical motor column (C1-C6). However, additional innervation was also discovered; the spinal accessory nucleus (column of von Lenhossek), the nucleus supraspinalis, and the dorsal and ventral facial motor nuclei all contributed efferents to the hatching muscle. This observation constitutes the first description of dual innervation of a neck muscle by nuclei in both the brain and spinal cord. In addition, transganglionic transport of HRP revealed labelled primary afferent fibers from the hatching muscle ascending in the dorsal columns and terminating extensively within the vestibular complex, especially on the principal cells of the tangential nucleus. The tangential nucleus itself undergoes synaptic changes at the time of hatching. Possible functional relations between the tangential nucleus and the hatching muscle are discussed.