Stereocilia

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Structures and types of simple epithelia.

Stereocilia are cylindrical immotile microvilli found on the apical surfaces of epithelial cells. They are found in the epithelium of the epididymis, the proximal part of the ductus deferens and the sensory hair cells of the inner ear.

Their molecular structure includes tightly packed actin filaments that are cross-linked with actin-bundling proteins. This cross-linked structure makes them rigid and stiff. When they are deflected they pivot at their proximal ends like stiff rods.

The stereocilia contain mechanoelectric transducer channels in their distal ends, which means they convert mechanical energy into electrical energy. Each stereocilium has a mechanosensitive tip link. This mean that the bending of the tip of the stereocilium causes the opening of voltage-gated ion channels. This allows an influx of potassium (K+), which leads to the cell's depolarisation and generates an action potential. Neighboring stereocilia are connected via these tip links.

Within the epididymis and vas deferens, the stereocilia are long processes that facilitate the absorption of excess fluid from the lumen of the ducts. The voltage-gated ion channels are activated by passive movement of the fluid. 

Stereocilia are also found in the spiral organ (of Corti), where they convert auditory stimuli into electrical stimuli. They are embedded in a gel-like structure called the tectorial membrane arranged in three rows, increasing in length.

As sound waves move through the cochlear they cause vibration of the inner ear's endolymph. This causes bending of the stereocilia and this mechanical energy is converted into electrical energy. This energy is then transmitted via the cochlear nerve, a division of the vestibulocochlear nerve (CNVIII), to the brain.