Inner Hair Cell

Structure and/or Key Feature(s)

The hair cells of the cochlea are mechanosensory cells which are found on the superior aspect of the basilar membrane. There are two types, inner and outer hair cells, which possess common features. They are typically elongated cells that contain stereocilia on their apical pole and are associated with a synaptic complex at their basal pole. The inner and outer hair cells are also arranged in rows and the apices of the hair cells inclining in toward each other, at the same angle as the neighboring inner and outer pillar cells. The structural inclination of the hair cells is supported by the closely related supporting cells. These include the border cells and the phalangeal cells. The apices of the hair cells and the supporting cells are linked by tight junctions, adherens junctions, and desmosomes. This forms an impermeable barrier, called the reticular lamina, for the passage of ions. Endolymph (high potassium and low sodium concentrations) engulfs the apical surfaces of the hair cells and the supporting cells, while perilymph (high sodium and low potassium) bathes their lateral and basal aspects. This separation generates an electrochemical gradient along the lamina, forming an endocochlear potential which is necessary for the depolarization of hair cells (Standring, 2016).

Some differences in the outer and inner hair cells structure separate the distinctive roles both cell types have is sound transduction. There are around 3,500 inner hair cells in the human cochlea, arranged in a single row. This row of inner hair cells is positioned internal to the inner pillar cells and the tunnel of Corti. Individually, the inner hair cells possess an uneven pear shape; they are typically broadest at the basal pole and become narrower toward the apical pole.

At the apical surface, fine wispy projections known as the stereocilia emerge. There are anywhere between 50 and 60 stereocilia per inner hair cell. They possess many direct connections with each other via lateral and tip links. These stereocilia also ascend in height, within the tallest stereocilia located on the outer aspect of the cell surface. The base of each inner hair cell makes 10–20 synaptic contacts with afferent nerve ending (Goutman, Elgoyhen and Gómez-Casati, 2015).

Anatomical Relations

The inner hair cells are located on the superior aspect of the basilar membrane and are the closer of the two hair cell types to the internal sulcus of the spiral lamina. They are inferior and closely related to the underside of the tectorial membrane. They have a lot of epithelial structures neighboring them, providing support to the inclination of the inner hair cell. External to the inner hair cells are the inner and outer pillar cells. The tunnel of Corti is formed by the angling of these pillar cells against each other. Internal to the inner hair cells are the inner phalangeal and inner border cells.

Function

The hair cells function to transduce the vibrations of the sound energy coming from the middle ear into the cochlea into nerve impulses. The hair cells are extremely sensitive and can detect the frequency and amplification of these sounds.

Sound waves result in disruption of the surrounding cochlear fluid, which creates deflections of the stereocilia at their base from their original position, thus resulting in mechanotransduction. This is facilitated by the opening of the mechanotransduction cation channels, resulting in a graded depolarization due to ionic transfer between the surrounding endolymph. This information is carried through the afferent synapses and all the way to the auditory processing centers. The inner hair cell nerve synapses provide information about the acoustic environment, rather than sound amplification, which has a stronger association with outer hair cells.

The structural incline of the inner hair cells occurs at an angle similar to that of the incline of the neighboring inner pillar cells. This delicate arrangement is firmly correlated to the sensory performance of the cochlea (Goutman, Elgoyhen and Gómez-Casati, 2015).

List of Clinical Correlates

—Sensorineural hearing loss

References

Dorland, W. (2011) Dorland's Illustrated Medical Dictionary. 32nd edn. Philadelphia, USA: Elsevier Saunders.

Goutman, J. D., Elgoyhen, A. B. & Gómez-Casati, M. E. (2015) Cochlear hair cells: The sound-sensing machines. FEBS letters, 589(22), 3354-3361.

Standring, S. (2016) Gray's Anatomy: The Anatomical Basis of Clinical Practice., 41st edition. Elsevier Limited.