Summary of Step-through Tranduction Mechanism
1. In the external ear, the pinna gathers sound energy and focuses it through the auditory canal on the tympanic membrane (middle ear).
2. Sound waves vibrate the ear drum, which then move the three tiny bones (the malleus, incus and stapes) in the middle ear.
3. The stapes act like a piston by pushing the cochlea fluids at the oval window, hence transmitting amplified sound energy.
4. In the inner ear, perilymph is moved within the cochlea from base at the oval window toward the apex, back to the base at the round window below through a small hole (helicotrema).
5. The fluid movement causes the deflection of the basilar membrane.
6. Due to the structure of the basilar membrane (wider at the apex and stiffer at the base), sounds of high frequency only displace the base, whereas low frequencies travel further along the membrane and maximally displace the apex. This gives rise to a topographical mapping of frequency.
7. Displacement of the basilar membrane causes the bending of the stereocilia of hair cells within the organ of Corti, which lies on the basilar membrane.
8. Depending on the bending direction of the stereocilia, the tip links either open or close the transduction channels (TRPA1 channels) in the stereocilia in response to the mechanical force.
9. Opening and closure of TRPA1 channels control the potassium conductance in hair cells. The hair cells hyperpolarize if the channels are closed, and depolarize if the channels are open due to potassium entry.
10. The depolarization, in turn, activates voltage-gated calcium channels and causes calcium influx. The calcium triggers neurotransmitter release, which stimulates the afferent nerve fibers that form part of the auditory nerve. Hence, the signal is passed along to the brain.