Module 4 of 12

Hair Cells & Mechanoelectric Transduction

Type I and Type II hair cells, stereocilia and kinocilium, MET channels, and cosine tuning.

Vestibular hair cells are the elementary sensory receptors that convert the mechanical deflection of head movement into an electrical signal. Everything the vestibular system reports begins here.

Each hair cell carries a tuft of fine hairs called stereocilia, with one taller hair, the kinocilium, at one edge. Bending the tuft towards the kinocilium switches the cell on — it sends more signals to the brain. Bending it the other way switches it down. At rest the cell already fires steadily, so it can report movement in both directions.

Hair cells come in two types. Flask-shaped Type I cells sit in the central zones of the cristae and maculae and are enclosed by a single afferent calyx; cylindrical Type II cells lie more peripherally and synapse with multiple bouton terminals⁶. Each bundle holds roughly 40–200 stereocilia of graded height alongside one kinocilium, which defines the cell’s axis of directional sensitivity. Deflection towards the kinocilium opens mechanically-gated channels at the stereocilia tips; potassium flows in from the K⁺-rich endolymph, the cell depolarizes, and afferent firing increases. Deflection away closes the channels and the cell hyperpolarizes ⁷.

Directional sensitivity follows a cosine tuning curve: response amplitude scales with the cosine of the angle between the stimulus direction and the cell’s polarity axis. The receptor potential reaches roughly 20 mV, and transduction current can peak near 200 pA at maximal deflection, giving high-resolution encoding of both angular and linear acceleration depending on hair-cell location⁸. Type I cells, with their calyceal synapse and predominantly irregular afferents, support the fast, high-frequency channel that clinical tests such as VEMP and the video head impulse test depend upon ¹⁶.

Two morphologies. Type I cells, flask-shaped, are enclosed by a single calyceal afferent and dominate the irregular, dynamically sensitive channel. Type II cells are cylindrical and contact multiple bouton terminals, driving the regular, tonic channel.

Resting. Deflection towards the kinocilium opens mechanically-gated channels; K⁺ influx depolarizes the cell and increases afferent firing. Deflection away closes them.

Mechanoelectric transduction

Transduction is fast and graded rather than all-or-none. Because the cell holds a resting discharge, the afferent can encode the direction of head motion as an increase or decrease from baseline — the foundation of the bidirectional, push-pull signalling seen across the labyrinth¹.