Question

How do hair cells work and what role do they play in hearing? What role does the basilar membrane play in frequency diserimination? 

Answer 1

The pressure waves in the endolymph cause the basilar membrane to vibrate, which moves the hair cells of the spiral organ against the tectorial membrane. This leads to bending of the stereocilia and ultimately to the generation of nerve impulses in first-order neurons in cochlear nerve fibers.

• The hair cells transduce mechanical vibrations into electrical signals. As the basilar membrane vibrates, the hair bundles at the apex of the hair cell bend back and forth and slide against one another.
• A tip link protein connects the tip of each stereocilium to a mechanically gated ion channel called the transduction channel in its taller stereocilium neighbor.
• As the stereocilia bend in the direction of the taller stereocilia, the tip links tug on the transduction channels and open them.
• These channels allow cations in the endolymph, primarily K⁺, to enter the hair cell cytosol.
• As cations enter, they produce a depolarizing receptor potential.
• Depolarization quickly spreads along the plasma membrane and opens voltage-gated Ca²⁺ channels in the base of the hair cell.
• The resulting inflow of Ca²⁺ triggers exocytosis of synaptic vesicles containing a neurotransmitter, which is probably glutamate.
• As more neurotransmitter is released, the frequency of nerve impulses in the first-order sensory neurons that synapse with the base of the hair cell increases.
• Bending of the stereocilia in the opposite direction closes the transduction channels, allows hyperpolarization to occur, and reduces neurotransmitter release from the hair cells.
• This decreases the frequency of nerve impulses in the sensory neurons.

Sound waves of various frequencies cause certain regions of the basilar membrane to vibrate more intensely than other regions. Each segment of the basilar membrane is “tuned” for a particular pitch. Because the membrane is narrower and stiffer at the base of the cochlea (closer to the oval window), high-frequency (high-pitched) sounds induce maximal vibrations in this region. Toward the apex of the cochlea, the basilar membrane is wider and more flexible; low-frequency (low-pitched) sounds cause maximal vibration of the basilar membrane there. Loudness is determined by the intensity of sound waves. High-intensity sound waves cause larger vibrations of the basilar membrane, which leads to a higher frequency of nerve impulses reaching the brain. Louder sounds also may stimulate a larger number of hair cells.