Neural encoding of sound
The neural encoding of sound is the representation of auditory sensation and perception in the nervous system.
[citation needed] Given the simple physics of sound, the anatomy and physiology of hearing can be studied in greater detail.
Resonances of the external ear selectively boost sound pressure with frequency in the range 2–5 kHz.
The vertical asymmetry of the pinna selectively amplifies sounds of higher frequency from high elevation thereby providing spatial information by virtue of its mechanical design.
[2] The three small bones that are responsible for this complex process are the malleus, the incus, and the stapes, collectively known as the ear ossicles.
[4] Furthermore, the ossicles are arranged in such a manner as to resonate at 700–800 Hz while at the same time protecting the inner ear from excessive energy.
These two muscles can restrain the ossicles so as to reduce the amount of energy that is transmitted into the inner ear in loud surroundings.
[3][4] The cochlea of the inner ear, a marvel of physiological engineering, acts as both a frequency analyzer and nonlinear acoustic amplifier.
Each hair bundle contains approximately 300 fine projections known as stereocilia, formed by actin cytoskeletal elements.
In addition to the stereocilia, a true ciliary structure known as the kinocilium exists and is believed to play a role in hair cell degeneration that is caused by exposure to high frequencies.
The actin filaments that form the core of a stereocilium are highly interlinked and cross linked with fibrin, and are therefore stiff and inflexible at positions other than the base.
The influx of cations, particularly potassium, through the open MET channels causes the membrane potential of the hair cell to depolarize.
This results in an increase in the calcium concentration, which triggers the exocytosis of neurotransmitter vesicles at ribbon synapses at the basolateral surface of the hair cell.
The release of neurotransmitter at a ribbon synapse, in turn, generates an action potential in the connected auditory-nerve fiber.
[2] Mechanotransduction by stereocilia is highly sensitive and able to detect perturbations as small as fluid fluctuations of 0.3 nanometers, and can convert this mechanical stimulation into an electrical nerve impulse in about 10 microseconds.
The ratio of innervation that is seen between Type I neurons and inner hair cells is 1:1 which results in high signal transmission fidelity and resolution.
Primary auditory neurons carry action potentials from the cochlea into the transmission pathway shown in the adjacent image.
