19.6:
Hair Cells
The cochlea of the inner ear contains hair cells, sensory receptors that transduce sound waves into neural signals that can be interpreted by the brain.
Each sell has hair like stereocilia on top, arranged from shortest to tallest and attached to each other through thin tip links. Sound waves vibrate the basilar membrane beneath the hair cells, causing the cilia to move from side to side.
When the cilia move towards the tallest cilium, the tip lengths stretch and pull open attached cat ion channels. Potassium ions flow into the cell, depolarizing it, making it more positively charged inside.
The increase in voltage causes voltage-gated calcium channels to open and the resulting influx of calcium ions triggers the release of neurotransmitters onto the post-synaptic auditory nerve that carries information to the brain.
When the cilia move toward the shortest cilium, the tip lengths are compressed and the cation channels close hyperpolarizing the cell, making it more negative inside, decreasing neurotransmitter release.
In this way, hair cells are able to encode the characteristics of sound waves, such as frequency, into neural signals, allowing for perception.
19.6:
Hair Cells
Hair cells are the sensory receptors of the auditory system—they transduce mechanical sound waves into electrical energy that the nervous system can understand. Hair cells are located in the organ of Corti within the cochlea of the inner ear, between the basilar and tectorial membranes. The actual sensory receptors are called inner hair cells. The outer hair cells serve other functions, such as sound amplification in the cochlea, and are not discussed in detail here.
Hair cells are named after the hair-like stereocilia that protrude from their tops and touch the tectorial membrane. The stereocilia are arranged by height and are attached by thin filaments called tip links. The tip links are connected to stretch-activated cation channels on the tips of the stereocilia.
When a sound wave vibrates the basilar membrane, it creates a shearing force between the basilar and tectorial membranes that moves the hair cell stereocilia from side to side. When the cilia are displaced towards the tallest cilium, the tip links stretch, opening the cation channels. Potassium (K+) then flows into the cell, because there is a very high concentration of K+ in the fluid outside of the stereocilia. This large voltage difference creates an electrochemical gradient that causes an influx of K+ once the channels are opened.
This influx of positive charge depolarizes the cell, increasing the voltage across the membrane. This causes voltage-gated calcium (Ca2+) channels in the cell body to open, and Ca2+ flows into the cell. Ca2+ triggers a signaling cascade that causes synaptic vesicles containing excitatory neurotransmitter molecules to fuse to the cell membrane and be released, exciting the postsynaptic auditory nerve cell and increasing the transmission of action potentials to the brain. When the stereocilia are pushed in the opposite direction, towards the shortest stereocilia, the tip links relax, the cation channels close, and the cell becomes hyperpolarized (i.e., the membrane potential is more negative) compared to its resting state.
Characteristics of the sound wave, such as frequency, are encoded in the pattern of hair cell activation and, consequently, auditory nerve cell activation. This information is then sent to the brain for interpretation.
Suggested Reading
Schwander, Martin, Bechara Kachar, and Ulrich Müller. “The Cell Biology of Hearing.” The Journal of Cell Biology 190, no. 1 (July 12, 2010): 9–20. [Source]
Goutman, Juan D., A. Belén Elgoyhen, and María Eugenia Gómez-Casati. “Cochlear Hair Cells: The Sound-Sensing Machines.” FEBS Letters 589, no. 22 (November 14, 2015): 3354–61. [Source]