Outer 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 roughly 12,000 outer hair cells in the human cochlea, arranged in anywhere from three to five rows. The rows are positioned between the outer pillar cells and the columnar external supporting cells. Each outer hair cell has a lengthy, uniform cylindrical appearance throughout. However, the outer hair cells are much longer than the inner hair cells, reaching almost twice their length. One important feature is the variance in size of these outer hair cells which is correlated with their respective position within the cochlea. The outer row of the outer hair cells is the longest of the rows, as well as that hair cells in the apex of the cochlea are taller than hair cells in the base.

The outer hair cells also have a different arrangement than the single line of inner hair cells. Regarding the three rows of outer hair cells, the innermost and outermost rows run parallel to each other, while the middle row cells are positioned in a slightly oblique position. This is due to the alignment of the underlying phalangeal epithelial cells. Phalangeal processes from the phalangeal epithelial cells extend toward the apices of the outer hair cells, giving a chessboard type appearance when viewed from above.

At the apical surface of each outer hair cell, fine wispy projections known as the stereocilia emerge. There are usually 100 stereocilia per outer hair cell. They are organized into three rows of graded height and the rows form a “V” or “W” pattern, depending on the region in the cochlea, with the longest stereocilia situated at the outer aspect of each apical surface.

The bases of the outer hair cells do not rest on the basilar membrane, but rather sit in the cup-shaped apical surface of the external phalangeal epithelial cells, and so, are anchored to the basilar membrane via the bases of these supporting cells.

At the base of each hair cell, there is a cluster of nerve fibers that form a synaptic complex. Unlike the inner hair cell, efferent nerve fibers directly synapse at the base of each outer hair cell. These terminations at the base of each outer hair cell are from efferent axons that originate from cells within the brain. In contrast to the inner hair cell nerve fibers, only a single efferent nerve fiber innervates multiple outer hair cells.

Anatomical Relations

The outer hair cells are located on the superior aspect of the basilar membrane and are the closer of the two hair cell types to the outer spiral sulcus. The apical aspects of the outer hair cells and the apical processes of the supporting cells form a mosaic-like structure known as the reticular lamina. The hair cells are inferior and closely related to the tectorial membrane; their projected stereocilia are embedded in a thickened ridge on the underside of the tectorial membrane.

The hair cells are arranged in three rows and have a variety of anatomical neighbors. The inner row of outer hair cells is positioned external to the outer pillar cells and the middle row is interjacent to the innermost and outermost row of the outer hair cells. The outer row is interjacent to the middle row of hair cells and the columnar external supporting cells, the latter of which have numerous rows leading to the outer spiral sulcus.

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.

Given the efferent supply to each outer hair cell, the nerve synapses provide an amplification ability. The outer hair cells can increase auditory sensitivity at specific frequencies by mechanically amplification of low-level sound that enters the cochlea by quickly oscillating in length. This results in a greater amplification of quiet sounds over louder sounds. This differs from the inner hair cell function, as they mainly provide information about the acoustic environment itself and relay it back to the auditory cortex (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.