Balance and equilibrium, II: The retrovestibular neural pathway.

In this article, we will consider the anatomy of the retrovestibular pathways: the superior and inferior vestibular nerves and related intraaxial structures. The reader is encouraged to review previous installments if questions arise regarding terminology. The internal auditory canal (IAC) is highly variable in size, shape, and orientation (1). As a general rule, there is symmetry between the right and left canals in the same person. The IAC is defined as having a porus (meatus) medially along the petrous surface and a fundus laterally abutting the labyrinth. At the fundus, a transverse (falciform) crest separates the canal into superior and inferior compartments. Furthermore, a vertical crest (Bill’s bar) subdivides the superior compartment into anterior and posterior portions (Figs 1 and 2). The facial nerve is located in the anterosuperior compartment. The cochlear nerve is anteroinferior to the facial nerve. These structures will be discussed further in subsequent installments. Impulses from the utricular macula and the ampullae of the superior and lateral semicircular canals travel via the superior vestibular nerve in the posterosuperior compartment of the IAC (2, 3) (Figs 3 and 4). Impulses from the saccular macula and posterior semicircular canal travel primarily via the inferior vestibular nerve within the posteroinferior compartment of the internal auditory canal (the saccule is actually innervated by all three segments of the vestibulocochlear nerve (Fig 5). The nerve of the posterior semicircular canal (singular nerve) travels separate from the inferior vestibular