38 - Hearing Flashcards
Basic manner in which sound is percieved
1
2
3
1) Tympanic membrane moves ossicles.
2) Ossicles move oval window
3) Cochlear transforms physical motion of the oval window into a neural response.
Function of ossicles
Normally, when sound passes form air to fluid, most is deflected.
Match the impedance of the air to impedance of fluid in the inner ear.
Three chambers of the cochlea
Scala vestibuli
Scala media
Scala tympani
Basilar membrane 1 2 3 4 5 6
Lies between scala tympani and scala media. Responds to sounds. Wider at apex than base Stiffer at base Base responds to high frequencies Apex responds to low frequencies
Organ containing hair cells
Organ of Corti
Organ of Corti
1
2
3
Structure containing basilar membrane, tectorial membrane
1) Auditory receptors are hair cells:
2) Sandwiched between basilar membrane and reticular lamina.
3) Inner and outer hair cells.
Hair cells
Each has about 100 stereocilia.
Bending of stereocilia causes neuronal signalling
To what do inner and outer hair cells attach?
Basilar membrane.
Stereocilia insert into tectorial membrane
How do cochlear hair bundles lead to neuronal impulses? 1 2 3 4
1) Potassium channels are partially open.
2) Deflection of hair bundle towards tallest stereocilium causes opening of channels, and depolarization.
3) Increased K+ causes depolarization and opening of voltage-gated Ca2+ channels, which leads to release of neurotransmitter glutamate.
4) Deflection of hair cells in any direction alters membrane potential: towards tallest stereocilia leads to depolarisation (opening of cation channels). Towards smallest stereocilia leads to hyperpolarisation (closure of cation channels).
K+ gradient in cochlear
High K+ in endolymph in scala media
Low K+ in scala tympani perilymph
Two types of hair cells
1) Inner hair cells:
* 95% of nerves that project to brainstem nuclei
2) Outer hair cells
* Efferent inputs from the superior olivary complex.
Role of outer hair cells 1 2 3 4
1) Change length during low intensity stimuli.
2) Change in length accentuates movement of basilar membrane - IE amplification of signal that is received by inner hair cells
3) Depolarization causes cell contraction
4) Hyperpolarization causes cell elongation.
Effect of loss of outer hair cells
Basilar membrane movement is ~100 times less
Auditory pathway 1 2 3 4 5
1) Ear
- Hair cells (within cochlear
- CNVIII (auditory-vestobuar nerve)
- Spiral ganglion
2) Brainstem
- Cochlear nucleus (medulla)
- Superior olive
- Lateral lemniscus
3) Midbrain
- Inferior colliculus
4) Thalamus
- Medial geniculate nucleus
5) Temporal lobe
- Auditory cortex
Projections from cochlear nuclei
Auditory nerve from cochlea projects to neurons in the cochlear nuclei in rostral medulla.
These project directly to inferior colliculus (on both sides), or to neurons in both superior olives, which then project to inferior colliculi.
Relay nuclei in brainstem which mediate sound locatlisation
Anteroventral cochlear nucleus projection to superior olivary complex
Superior olivary complex
1
2
3
Involved in localisation of sound Consists of: 1) Lateral superior olive 2) Medial superior olive 3) Trapezoid body.
MSO: Localisation of sound by measuring time delay.
LSO: localisation of sound by sensing intensity differences
Medial superior olive role
Localisation of sound by measuring time delay
Lateral superior olive role
Localisation of sound by sensing intensity differences
How is sound thought to be localised in the horizontal plane?
Duplex theory:
Difference in the time for sound to arrive at each ear (more for low-frequency).
Difference in intensity between ears (more for high-frequency).
Auditory cortex location
Below lateral sulcus.
Organisation of auditory cortex 1 2 3 4
1) Neurons are sharply tuned for sound frequency.
2) Columnar organization: cells in same column are tuned to same frequency.
3) Alternating regions of input from both ears
4) Excitatory input from one ear and inhibitory input from the other ear.
Lateralisation of speech sounds
Left hemisphere
Lateralisation of environmental sounds
Both hemispheres
Lateralisation of music
Right hemisphere
