Anatomy of the ansa cervicalis: nerve fiber analysis.

Ansa cervicalis complex, which innervates the infrahyoid muscles, is formed by the union of two roots derived from the anterior primary rami of the first three or four cervical spinal nerves. According to its relationship with the internal jugular vein, there are three ansa types: medial, lateral, or mixed. Nerve fiber microscopy was carried out in 18 ansa cervicalis complexes after staining with Sudan Black, in order to study composition and structural arrangement in detail. Fibers from both first and second cervical nerves (C1, C2) joined the hypoglossal nerve and later left it partially in the superior root of the ansa, which also contained ascending fibers derived from the inferior root components; usually the second and third cervical nerves. Those ascending fibers, together with the C1 and C2 fibers remaining in the hypoglossal nerve, innervate thyrohyoid and geniohyoid muscles. Cervical nerve fibers could be observed within the hypoglossal nerve even after the ramification of those branches. Further, cervical nerve fibers were continuously issued to the peripheral layers of the hypoglossal nerve where the fibers of the two nerves intermingled, forming a complex structure. The general arrangement of the ansa did not change according to its type or segmental composition. Although the hypoglossal nerve does not make any significant contribution to the formation of the ansa, the close relationship between the two nerves observed in the present study calls for the use of a more descriptive term such as ansa hypoglosso-cervicalis. The pattern of innervation of the infrahyoid muscles is also discussed.