Hearing Flashcards
Outer Ear
funnels sound waves to the eardrum
Pinna
Collects sound
Tympanic membrane or eardrum
Separates outer and middle ear; vibrates in response to sound wave
Middle ear
Air-filled cavity; connected to the nasopharynx by the auditory
(Eustachian) tube
Equalizes pressure
Contains ossicles (malleus, incus, stapes)
malleus, incus, stapes
The smallest bones in the body
The ossicles conduct vibrations of the tympanic membrane through the middle ear to the cochlea (the inner ear).
Cochlear
Is coiled like snail shell
Contains:
Basilar and Reissner’s membranes
Scala vestibuli
Scala media
Scala tympani
Basilar and Reissner’s membrane divide the tube into
Divide the tube (Cochlear) into 3 chambers called Scalae:
Scala vestibuli
Scala media
Scala tympani
Atop the basilar membrane lies the hair cell receptor organ for hearing (Organ of Corti)
Scala vestibuli (dorsal)
Is filled with perilymph and connects at the large end to the oval window
Scala media (middle, flexible)
The cochlear duct; contains endolymph
Scala (ventral)
Contains perilymph and connects at its large end to the round window
Number of cochlear turns
Guinea pig 4.0
Cow3.5
Dog 3.25
Cat 3.0
Human 2.75
Apex
At the apex, the two perilymphs communicate via a small opening, the helicotrema
Apex
At the apex, the two perilymphs communicate via a small opening, the helicotrema
Organ of Corti
4 rows of hair cells (1 inner, 3 outer) throughout entire scala media
Most (~95%) of cochlear nerve endings synapse with inner cells (more important functionally than the outer cells)
The stereovilli of hair cells project into a collagen-containing tectorial membrane that is tightly attached to the basilar membrane on one edge
Stria vascularis
Produces endolymph
Transduction
The organ of Corti’s hair cells mediate transduction of sound waves into action potentials.
The hair cell receptors are similar in structure and function to those in the vestibular system.
The cells synapse on sensory neurons that form the cochlear portion of CN-VIII. The cell bodies of these neurons reside in the spiral ganglion.
Sound-induced bending of cilia changes the frequency of action potentials in CN-VIII fibers (N.B. the high [K+] in endolymph, role of K-channels in initiating depolarization)
Displacement of the stapes & the oval window moves fluid in the scala vestibuli, causing opposite fluid movement in the s. tympani and thus an opposite displacement of the r. window; the deflected pressure pushes the stapes/r.window towards the middle ear, t. vestibuli pressure falls and hence b. membrane is pushed up
Steps in Transduction
1.Upward bowing of basilar membrane: tilts hair bundles toward longer cilia, opening channels.
2.Depolarization of inner hair cells causes enhanced transmitter release.
- In outer hair cells, depolarization causes contraction of prestin, a motor protein
- Downward movement of b. membrane tilts cilia away from longer ones, closing channels.
Sound Transmission
A, Ossicular movement displaces the oval window of the fluid-filled inner ear, resulting in a traveling wave on the basilar memb
B, Basilar membrane is narrow and stiff at the base and wider and more flexible farther from the base.
C, As the frequency of a sound decreases, the region of maximum displacement of the basilar membrane, produced by the traveling wave, is located progressively farther from the base.
A region near the base (oval window) is significantly displaced by a high-frequency sound, and as the sound frequency decreases, the region of significant displacement is located progressively closer to the apex.
High frequency
High frequency sounds are most likely to affect hair cells and nerves near the base
Low frequency
Low frequency activates hair cells and neurons that are located progressively closer to the apex.
Tonotopic Organization
Tonotopic organization refers to the orderly relationship between the frequency of a sound and the region of the cochlea that is activated by that frequency
Auditory pathways
Action potentials travel along cochlear branch of CN-VIII to cochlear nuclei (medulla).
Then to the superior olivary nucleus (pontomedullary border), inferior colliculus (the midbrain), medial geniculate nucleus (thalamus), and finally to the auditory cortex (temporal lobe).
Conscious perception of sound occurs in the cerebral cortex.
Deafness
Conduction deafness
Nerve deafness
In animals, deafness is usually congenital; some drugs are also ototoxic.
Conduction deafness
Loss of sound transmission in outer or inner ear
Nerve deafness
Malfunction of the cochlear hair cells or CN-VIII fibers
Cochlear implants for problems distal to CN-VIII (animals as models)
In 2009, 25,000 US children received implants
