Senses: Hearing

What does each feature of a wave convey?

amplitude - loudness

frequency - pitch

Why do your ears pop on the plane?

because there is a rapid pressure decrease as the plane rises.

the tympanic membrane (eardrum) is surround by air (i.e., air in the ear canal and air in the middle ear which is connected to the back of the nose and throat via the eustachian tube). The popping is when the eardrum "pops" back into normal orientation when you open the eustachian tubes. As you enter lower pressure environments rapidly, there is less pressure outside of the TM than inside, so the eardrum bulges.

How are sound waves converted to fluid movement?

There are three regions in the ear to convert sound waves into electrical signal: external, middle, and inner ear.

When a sound wave enters the ear canal, it rhythmically "push/pulls" the tympanic membrane and the connected ossicles in the air pouch (malleus, incus, stapes), which converts the wave energy to mechanical energy (movement).

The associated stapes movement causes the stapes itself to push in or retract from the oval window in the cochlea scala vestibuli. This then causes movement of the fluid inside the scala vestibuli.

Sound dissipation pathway

1. Sound wave enters the ear canal

2. It rhythmically pushes/pulls the tympanic membrane, converting wave energy to mechanical energy.

3. The associated stapes movement causes the stapes to push in or retract from the oval window of the cochlea scala vestibuli, causing movement o fluid inside the scala vestibuli

4. The fluid movement all around the scala vestibuli (through the helicotrema and through the scala tympani) causes the round window to vibrate

Sound reception pathway

1. Sound wave enters the ear canal

2. It rhythmically pushes/pulls the tympanic membrane, converting wave energy to mechanical energy.

3. The associated stapes movement causes the stapes to push in or retract from the oval window of the cochlea scala vestibuli, causing movement of fluid inside the scala vestibuli

4. The scala vestibuli fluid movement against the vestibular membrane displaces/moves fluid inside the scala media which causes the displacement of the basilar membrane (it moves up and down)

5. This movement moves the hair cells which lies on top of the basilar membrane. The movement of the BM and hair cells results in sound

Auditory pathway

1. Sound waves enter pinna and through the ear canal. The sound waves rhythmically push/pull against the tympanic membrane, converted wave energy into mechanical energy. The associated stapes movement causes the stapes itself to push in or retract from the oval window of the cochlea scala vestibuli, causing movement of fluid inside the scala vestibuli. The scala vestibuli fluid movement against the vesicular membrane displaces fluid inside the scala media which then causes displacement of the basilar membrane. Hair cells (auditory receptor cells), located on basilar membrane, are displaced as they detect sensory stimulus. This movement causes depolarization or depolarization. When the BM moves upward, stereocilia on the hair cell bend toward the longest stereocilium. This stretches open the mechanically-gated K+ channels, K+ from the endolymph moves into the hair cell which causes depolarization, which opens VG-Ca2+ channels to increase the release of vesicles with Glu to promote an AP in the postsynaptic neuron. When the BM bends toward the shortest stereocilia, the mechanically-gated K+ channels close, so no K+ influx, no opening of the VG-Ca2+ channels, no Glu release, no AP in the postsynaptic cell. (usually not ALL hair cells are "on" or "off" though)

2. Inner hair cells synapse onto many spiral ganglion cells. The axons of the spiral ganglion cells compose the auditory nerve.

3. The fibers from each ear go to both sides of the brain where the axons synapse onto the auditory cortex on the temporal lobe and onto the median geniculate nucleus (MGN) of the thalamus. The auditory nerve also carries axons to synapse at the cochlear nuclei in the brainstem

Compare inner and outer hair cells in terms of abundance and function

inner - number = way less than outer, function = convey sound to the brain; specific ones convey information about pitch of the sound

outer - number = 3x more than inner, function = do not convey sound to the brain; instead they amplify motion of the BM, thus enhancing stimulation of inner hair cells. Each one can convey loudness (amplitude) via how much stereocilia bend against the tectorial membrane

How does the location of hair cells within the inner ear determine the loudness and pitch?

only hair cells where the BM is deflected move and the location of this deflection conveys the pitch.

Deflection near the oval window conveys high frequencies; whereas, deflection near the helicotrema conveys low frequencies.

Describe the structure of hair cells, including receptor types, and the fluid that surrounds them

have stereocilia, tip links, VG-Ca2+ channels, and are surrounded by endolymph and perilymph

Endolymph

K+ rich fluid located in the scala media which is above the hair cells in the organ of corti

Perilymph

Na+ rich fluid located in the scala vestibuli (upper compartment) and scala tympani (lower compartment)

Conductive deafness

Anything that blocks sound waves from entering through the external and middle ear (i.e., build-up of earwax, ruptured eardrum, ear infection, fluid in outer ear)

No internal fluid movement

What anatomical structures are affected in conductive deafness and is it reversible?

external and middle ear

reversible

Sensorineural deafness

Damage to any portion of the auditory pathway in the brain, including the auditory cortex, auditory nerve, organ of corti

What anatomical structures are affected in Sensorineural deafness and is it reversible?

Auditory cortex, Auditory nerve, Organ of corti (results in loss of hair cells)

No