Anatomy of ear + physiology of hearing Flashcards
Describe the structure of the 2 components of the external ear and function
Pinna/auricle: concave, cartilaginous structure which is made of elastic cartilage. It collects and directs sound waves to the auditory canal.
Auditory canal/external acoustic meatus: ends at the tympanic membrane.
Lined with mucous membrane which contains sebaceous glands and ceruminous glands Their secretions combine to form cerumen (earwax).
Earwax/cerumen prevents entrance of foreign particles and protects from infection.
Outer 1/3 is cartilage and inner 2/3 is embedded in temporal bone.
Function: the canal directs sound waves to the tympanic membrane
Describe the tympanic membrane and tis function
Tympanic membrane separates the external ear form the middle ear.
tympanic membrane is a thin, semi-transparent, concave sheet made of collagen and epithelium.
It is fixed into a ring in the temporal bone
Function: it vibrates in response to sound waves and transmits these vibrations to the malleus
describe the tympanic cavity, what does it contain, what re the lateral and medial walls?
Tympanic cavity is connected to the nasopharynx via the eustachian tube.
It contains the 3 ossicles.
Lateral wall: tympanic membrane
Medial wall: round and oval window
what are the ossicles, what is their function? Describe each of them
ossicles: 3 tiny bones (malleus, incus, stapes)
Function: transmit and amplify sounds from the tympanic membrane and across the tympanic cavity to the OVAL window
Malleus: largest, most lateral, attached to the tympanic membrane and articulates with the incus
Incus: middle ossicle, articulates with the stapes
Stapes: smallest, most medial, articulates with the oval window.
What do the round and oval windows open into and how are they positioned in relation to one another ?
Round window: opens up into vestibule of inner ear
Oval window: opens up into the cochlear portion of the inner ear
OVAL is higher than the ROUND
What does the eustachian the connect and what is its function ?
Connects the middle ear to the nasopharynx
It equalises the pressure between the middle ear and the outer ear
Describe the cochlea in the inner ear. What is it composed of, what is inside them?
Cochlea is composed of 3 fluid filled chambers that extend the length of the structure.
2 outer chambers (top and bottom): Scala vestibuli (upper) Scala tympani (lower) - they both contain perilymph
Middle chamber is called the Scala media/cochlear duct and is filled with endolymph.
what is the function of the fluid filled chambers?
Scala tympani and vestibuli: transmission of pressure
Scala media: houses the sensitive Organ of Corti which detects pressure impulses and responds with electrical impulses which travel along the (vestibulocochlear) auditory nerve to the brain
where and what is the organ of Corti found? what can you find there?
Organ of Corti is in the Scala media and rests on the basilar membrane. It is the sensory epithelium for hearing.
It contains 2 types of hair cells: Inner and outer hair cells.
On the basement membrane there are 3 rows of outer hair cells and 1 row of inner hair cells. Each hair cell is connected to a nerve fibre that relays various impulses to the COCHLEAR branch of the 8th cranial nerve.
Aside from the cochlea what other structure is found in the inner ear?
Describe it and its function
Vestibular system - important for balance and spatial orientation
3 semi-circular canals: for balance, vestibular portion of the vestibulocochlear nerve
vestibule: central part of boney labyrinth
Aside from the cochlea what other structure is found in the inner ear?
Describe it and its function
Vestibular system - important for balance and spatial orientation
3 semi-circular canals: for balance, vestibular portion of the vestibulocochlear nerve
Vestibule: central part of bony labyrinth
describe the physiology of hearing
External sound waves are collected and directed by the auricle through the auditory canal, to the tympanic membrane.
The sound waves cause the TM to vibrate and, as the TM articulates with the Malleus, the vibrations are passed on by the ossicles.
the ossicles transfer the sound waves from a solid medium to a liquid medium via the Stapes. The footplate of the stapes articulates with the oval window.
the stapes moves in and out of the oval window which creates motion/vibration of the perilymph fluid of the Scala vestibuli. These vibrations are transmitted to the perilymph in Scala tympani and reaches the round window.
Vibrations of the perilymph are transmitted across the vestibule/Reissner’s membrane (membrane between Scala vestibuli and Scala media) to the endolymph in the Scala media.
This causes the Basilar membrane to vibrate/ripple, causing movement/vibration of hair cells against the tectorial membrane of the organ of corti to generate. This causes bending of stereo cilia and generates a nerve impulse which is transmitted via the cochlear fibre of the 8th cranial nerve.
what are the mediums that sound waves pass through along the auditory system?
outer ear: air
middle ear: mechanical
inner ear: liquid
brain:neural
what are the mediums that sound waves pass through along the auditory system?
outer ear: air
middle ear: mechanical
inner ear: liquid
brain: neural
describe how the bending of hair cells can generate a nerve impulse
Stereocilia of hair cells are connected by tip links.
Vibration of basil membrane causes movement of hair cells against the tectorial membrane.
Bending of the hair cells in the direction of taller stereocilia causes channels to open to allow entry of K ions into the cell.
This causes depolarisation of the cell membrane which causes calcium channels to open and an influx of calcium into cell.
Vesicles containing NT fuse with the membrane and there is exocytosis of NT. Impulse is transmitted to afferent nerve fibres and conducted.
