A tube {Eustachian tube}| goes from middle ear to pharynx, to equalize pressure inside and outside eardrums. Pharynx valves close tube when talking but open tube when swallowing or yawning or when outside air pressure changes.
Area {belt area} adjacent to area-A1 primary auditory cortex can receive from area A1 and respond to complex sound features.
Area {parabelt area} laterally adjacent to belt area can receive from belt area and respond to complex sounds and multisensory features.
Cortical frequency-sensitive auditory neurons align from low to high frequency {tonotopic organization}.
Hearing neurons {auditory neuron} receive input from 10 to 30 hair-cell receptors.
frequency
Auditory neurons respond to one frequency, within several percent. Frequencies are between 20 Hz and 20,000 Hz.
intensity
Auditory neurons respond to low, medium, or high intensity. Low-spontaneous-firing-rate neurons {low-spontaneous fiber} are for high-intensity sound and have narrow-band frequency tuning. With no stimulation, their firing rate is less than 10/s. Firing rate rises with intensity {rate-intensity function, neuron}.
High-spontaneous-firing-rate neurons {high-spontaneous fiber} are for low-intensity sound and have broad-band frequency tuning. With no stimulation, their firing rate is greater than 30/s. Firing rate rises with intensity to maximum at low intensity.
Mid-spontaneous fibers are for intermediate-intensity sound. With no stimulation, firing rate is greater than 10/s and less than 30/s.
Free intracellular calcium ions modulate cricket hearing interneurons {omega interneuron} [Huber and Thorson, 1985] [Sobel and Tank, 1994].
Human hearing organs {ear} have outer ear to catch sounds, middle ear to concentrate sounds, and inner ear to analyze sound frequency and intensity.
Pinna and ear canal {outer ear}| gather and focus sound on eardrum.
Only mammal ears have a cartilage flap {pinna}| {pinnae}, to catch sounds.
A 2.5-centimeter tube {auditory canal}| {ear canal}, from outside pinna to inside tympanic membrane, protects tympanic membrane from objects and loud sounds.
Auditory canal has wax {earwax}|. Perhaps, earwax keeps ear canal moist and/or sticks to insects.
Thin connective-tissue membrane {tympanic membrane} {eardrum}| is across ear-canal inner end. Tympanic membrane is 18 times larger than oval window.
Eardrum connects to air cavity {middle ear}|.
Middle ear has three small bones {ossicles}|: hammer, anvil, and stirrup. Two middle ear bones evolved from reptile lower jawbones [Ramachandran, 2004].
Eardrum connects to middle-ear bone {hammer bone}| {malleus}, which connects to anvil.
Hammer bone connects to middle-ear bone {anvil bone}| {incus}, which connects to stirrup. Anvil bone is smaller than hammer bone to concentrate sound pressure.
Anvil bone connects to middle-ear bone {stirrup bone}| {stapes}, which connects to oval window. Stirrup bone is smaller than anvil bone to concentrate sound pressure.
Muscles {tensor tympani muscle} attached to malleus can tense to dampen loud vibration.
Muscles {stapedius muscle} attached to stapes can tense to dampen loud vibration.
A coiled trumpet-shaped fluid-filled organ {inner ear} {cochlea}|, 4 mm diameter and 35 mm long, is in temporal bone.
Inner ear, nearer auditory nerve, has one straight row of 3500 inner hair cells {hair cell, cochlea} and has three S-curved rows with 3500 outer hair cells each (10,500 total). Outer-hair-cell cilia poke through tectorial membrane. Hairs have long part, medium part, and short part, linked by hairs from small tip to medium middle and from medium tip to large middle. Cochlea hair-cell receptors microscopic fibers and microscopic cross-fibers cause resonance between frequencies.
Oval-window movement makes pressure waves, down vestibular canal, which cause middle-canal vertical movement, which slides tectorial-membrane gel horizontally over upright cilia. If pushed one way, hair-cell-membrane potential increases from resting potential. If pushed other way, potential decreases. Inner hair cells send to 10 to 30 auditory neurons.
Outer hair cells can receive brain signals to extend cilia, to stiffen cochlear partition and dampen sound. This reduces signal-to-noise ratio, lowers required input intensity to sharpen tuning, or sends secondary signals to inner hair cells.
Stapes connects to membrane across opening {oval window, hearing}| at cochlea beginning. Oval window is 18 times smaller than tympanic membrane, to concentrate sound pressure.
At base, tympanic canal has soft tissue {round window} that absorbs high pressure.
Cochlea outside has a canal {tympanic canal} {scala tympani}. Tympanic membrane is over tympanic-canal end. Round window is over tympanic-canal base.
Cochlea outside has a canal {vestibular canal} {scala vestibuli}.
Tympanic and vestibular canals join at cochlea point {helicotrema}.
Cochlea middle has a canal {middle canal} {scala media}.
Cochlea inside has a canal {cochlear canal}.
Membrane {Reissner's membrane} separates middle canal and vestibular canal.
In cochlear canal, a coil {basilar membrane} also separates middle canal and tympanic canal. Close to oval window {base, basilar membrane}, basilar membrane is stiff and narrow. At other end {apex, basilar membrane}, basilar membrane is wider and less stiff.
Basilar-membrane structures {organ of Corti} have 30,000 hair-cell receptors, with stereocilia and fibers. Organ-of-Corti base detects high frequencies, and organ-of-Corti apex detects low frequencies (place code).
Gel membrane {tectorial membrane} attaches to end of, and floats in, middle canal and touches outer hair cells.
Basilar membrane, tectorial membrane, and organ of Corti together {cochlear partition} detect sounds. Cochlear partition is in middle canal.
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Date Modified: 2022.0225