Auditory System Flashcards
What is the range of human hearing?
20Hz - 20kHz
Outer ear (air)
Pinna-cartillagenous structure
Formed from pharyngeal arches 1 & 2
Forms between 10-18th week
Directs soundwaves towards ear canal
Folds are unique to each individual, specifically direct sound to not cause doubled waves
Ear canal is 1/3 cartilage and 2/3 bone
Tympanic membrane
What is in the Middle ear?
Has bones - malleus, incus and stapes
Muscles - tensor Tympani (it tenses the tympanic membrane) and Stapedius
Tubes - Eustachian Tube
What does the Middle Ear do?
Sound amplifier.
Acoustic impedance match, between air and fluid-filled inner ear. The air that hits your ear drum has energy. 97% of energy is dissipated at air - fluid interface. Mechanism to move fluid effectively.
Amplifying the movement makes it louder.
Ratio Area of TM: Stapes is 14:1
Lever action of ossicles lead to gain (making it louder).
200 fold increase boost in pressure from TM to inner ear.
What do the muscles do in the middle ear?
Protection from acoustic trauma, stiffens ossicular chain.
Strapedius stimulated acoustically, reflex arc: 3 or 4 neurons. Also deadens the sound when you’re chewing.
What does the eustachian tube do?
Ventilation of middle ear space, drainage of secretions. This is what makes your ear pop eg when on plane.
The inner ear: vestibulocochlear apparatus
A set of fluid filled sacs, encased in bone
Cochlear-responsible for hearing
Labyrinth - responsible for balance
Innervation: vestibulocochlear nerve
Cochlear fluids
Scala media contains endolymph has high K+
Perilymph is like ECF and CSF, it is Na+ rich
Gradients maintained by Na,K-ATPase & NKCC1 CIC-K chlorine channels
Ion channel abnormalities - deafness.
The Cochlea
Oval window causes a wave through the scala vestibuli and down the scala tympani due to the elasticity in the round window. Basilar membrane is raised due to pressure.
Hair cell in contact with tectorial membrane.
Basilar membrane
Narrow at base, wide at apex, stiff at base and floppy at apex. High frequencies detected at base, low frequency at apex.
Displacement of basilar membrane cuases movement of hair cells (ie specialized mechanical transducing cells)
Suspension system.
Basilar membrane is pushed up, hair cells waft back. Movement of the stereocillia has mechaninally gated K+ channels opened causing depolarisation (K+ rich endolymph)
Depolarisation results in opening of voltage gated Calcium channels.
Release of neuro-transmitter-glutamate
Repolarisation through K+ efflux (into K+ poor perilymph)
Each nerve responds maximally at a specific frequency.
Outer hair cells can alter the stiffness of the basilar membrane to ensure maximal stimulation at one site and dampened response at another.
Increased resolution.
From neuron to brain
Auditory fibre –> spiral ganglion –> cochlear nerve (VIII) –> central auditory pathway.
The central auditory pathway
Auditory processing disorder when sound is physically going in, not being processed properly.
How do we localize sound?
Left and right ears are separated but need to hear one sound at the same time.
Coincident detectors.
