Middle Ear Muscles and Nerves

The middle ear has two muscles associated with the auditory ossicles that help modulate auditory transduction and a third muscle group that controls patency of the auditory tube. The tensor tympani muscle originates rostrally and medially from the bony recess in the petrous portion of the temporal bone and makes its tendinous insertion onto the muscular process of the neck of the malleus (Fig. 20-16; see Fig. 20-13, A). It receives its innervation via a motor branch of the trigeminal nerve. Contraction of the tensor tympani muscle pulls the tympanic membrane medially and rostrally, placing greater tension on the auditory ossicle chain, which results in an increased resonant frequency of the auditory sound conduction system.

The stapedius muscle originates in the stapedius muscule fossa located dorsomedial to and obscured by the facial nerve as it courses through the facial (Fallopian) canal of the temporal bone (Fig. 20-17). The stapedial branch of the facial nerve innervates this muscle as it converges into a thin tendon that inserts onto the muscular process of the short crus close to the head of the stapes. In the cat the displacement variance of the stapes in its vestibular window is approximately 0.2 µm, whereas the maximal contraction of the stapedius muscle leads to dorsal and caudal stapedial bone displacement of 40 to 60 µm. This displacement, which is perpendicular to the normal movement of the stapes, maximally attenuates sound transmission up to 30 decibels. Contraction of the stapedius and tensor tympani muscles are integral parts of the acoustic reflex, defined as consensual, reflexive contraction of the muscles in response to stimuli (typically high-energy sound pressure) that leads to attenuated acoustic transmission.

The tensor veli palatini muscle arises from a groove in the petrous portion of the temporal bone medial and ventral to the tensor tympani muscle. Along with its nerve, the tensor veli palatini nerve, a branch of the trigeminal nerve, this long, slender muscle extends rostrally from the tympanic cavity parallel to the auditory tube. In concert with the levator veli palatini muscle, which is innervated by the facial nerve, coordinated contraction of these muscles opens the pharyngeal orifice of the auditory tube.

Two cranial nerves provide motor branches to the muscles of the middle ear. Branches of the trigeminal nerve named for their respective muscles innervate the tensor tympani and tensor veli palatini muscles within the tympanic cavity. The facial nerve initially leaves the cranial cavity through the internal acoustic meatus (see Fig. 20-17), along with the vestibulocochlear cranial nerve, and then courses through the facial canal of the petrous portion of the temporal bone in close proximity to the vestibular window (Fig. 20-18). Several millimeters medial and lateral to the tendon of the stapedius muscle, the bony casing of the facial canal is incomplete, forming a facial canal foramen. This opening allows an unimpeded connection between the stapedius tendon and the stapes. It also allows direct communication between the epineurial connective tissue of the facial nerve and the tympanic cavity (see Fig. 20-18). This proximity and exposure of the facial nerve to the tympanic cavity potentially explains why middle ear disease can manifest as facial nerve dysfunction. The facial nerve emerges from the middle ear through the stylomastoid foramen immediately caudal to the external acoustic meatus.