Hair Cells

Stereocilia are hair-like sensory receptors that protrude from the top of hair cells in the cochlea. They are arranged by height from shortest to tallest and connected by thin filaments called tip links. These tip links attach to stretch-activated cation channels, enabling hair cells to detect and respond to sound vibrations.

Sound waves vibrate the basilar membrane, creating a shearing force that moves stereocilia side to side. When cilia move toward the tallest cilium, tip links stretch and open cation channels, allowing potassium ions to flow in and depolarize the cell. This triggers voltage-gated calcium channels to open, releasing neurotransmitters onto the auditory nerve.

When stereocilia move toward the shortest cilium, tip links relax and cation channels close. The cell becomes hyperpolarized, making the membrane potential more negative. This decreased depolarization reduces neurotransmitter release onto the auditory nerve, encoding directional information about sound waves.

A very high concentration of potassium exists in the fluid surrounding stereocilia, creating a strong electrochemical gradient. When cation channels open, this gradient drives potassium ions into the cell, rapidly depolarizing it and triggering the calcium influx needed for neurotransmitter release.

Hair cells encode sound frequency through the pattern of their activation in response to vibrations. Different frequencies cause stereocilia at different locations along the cochlea to move at different rates, creating distinct patterns of depolarization and neurotransmitter release that the brain interprets as specific pitches.

Calcium ions entering through voltage-gated channels trigger a signaling cascade that causes synaptic vesicles containing neurotransmitters to fuse with the cell membrane. This release of excitatory neurotransmitters excites the postsynaptic auditory nerve cells, increasing action potential transmission to the brain for sound interpretation.

Hair cells are located in the organ of Corti within the cochlea of the inner ear, positioned between the basilar and tectorial membranes. As sensory receptors of the auditory system, they transduce mechanical sound waves into electrical signals that the nervous system can process and interpret.