Auditory brainstem circuits that mediate the middle ear muscle reflex

Abstract

The middle ear muscle (MEM) reflex is one of two major descending systems to the auditory periphery. There are two middle ear muscles (MEMs): the stapedius and the tensor tympani. In man, the stapedius contracts in response to intense low frequency acoustic stimuli, exerting forces perpendicular to the stapes superstructure, increasing middle ear impedance and attenuating the intensity of sound energy reaching the inner ear (cochlea). The tensor tympani is believed to contract in response to self-generated noise (chewing, swallowing) and non-auditory stimuli. The MEM reflex pathways begin with sound presented to the ear. Transduction of sound occurs in the cochlea, resulting in an action potential that is transmitted along the auditory nerve to the cochlear nucleus in the brainstem (the first relay station for all ascending sound information originating in the ear). Unknown interneurons in the ventral cochlear nucleus project either directly or indirectly to MEM motoneurons located elsewhere in the brainstem. Motoneurons provide efferent innervation to the MEMs. Although the ascending and descending limbs of these reflex pathways have been well characterized, the identity of the reflex interneurons is not known, as are the source of modulatory inputs to these pathways. The aim of this article is to (a) provide an overview of MEM reflex anatomy and physiology, (b) present new data on MEM reflex anatomy and physiology from our laboratory and others, and (c) describe the clinical implications of our research.

Figure 1.

Stapedial reflex pathways

Figure 2.

Tensor tympani (TT) reflex pathways

Figure 3.

Middle ear muscles in man

Figure 4.

Horseradish peroxidase (HRP) labeled stapedius motoneuron (SMN) in a rat model

Figure 5.

Electron micrographs of the major terminal types seen on the stapedial motoneuron (SMN) from Figure 4.

Figure 6.

Fluoro-gold (FG)-labeled tensor tympani motoneurons (TTMNs) in a mouse model

Figure 7.

Tensor tympani motoneuron (TTMN) subtypes Row A: Micrographs of three fluoro-gold labeled TTMN subtypes found in mice; “octopus-like” (red), “fusiform” (blue) and “stellate” (green). Scale bars: 10 μm. Row B: Left graph shows the total percentage of each TTMN subtype; Most subtypes were octopus-like (52%) and least were stellate (7%). Fusiform were 41%. Middle graph shows the average minor axis diameter (light color) and major axis diameter (dark color) of each TTMN subtype; octopus-like TTMNs were the largest whereas stellate TTMNs were the smallest. Right graph shows the average number of branch points in the first 100 μm of primary dendritic length for each TTMN subtype; stellate TTMNs branched the most.

Figure 8.

Fluoro-gold (FG)-labeled tensor tympani motoneuron (TTMN)

Figure 9.

Polar plot of tensor tympani motoneuron (TTMN) dendritic length and orientation

Figure 10.

Electron micrographs of major round vesicle terminal types seen on tensor tympani motoneurons (TTMNs; D. J. Lee et al., 2008) in a rat model

Figure 11.

Camera Lucida drawings of ventral cochlear nucleus (VCN) labeling

Figure 12.

Pseudo-rabies virus (PRV)–labeled ventral cochlear nucleus (VCN) neurons following injection into the tensor tympani muscle

Figure 13.

Middle ear muscle reflex interneurons labeled with pseudo-rabies virus (PRV) found in the posteroventral and anteroventral cochlear nucleus (PVCN and AVCN, respectively; Windsor et al., 2007) after 62 hours

Figure 14.

Bilateral superior olivary complex (SOC) labeling following pseudorabies virus (PRV) injection into the left tensor tympani (Windsor et al., 2007)