The Auditory System Flashcards
What is the function of the muscles in the inner ear?
To dampen the movements of the auditory ossicles, thereby decreasing sensitivity of the auditory apparatus.
This is thought to be a protective mechanism against sounds that can be anticipated (like a continuous loud noise)
What is the function of the ossicles?
To match the low impedence form the air filled outer ear to the high impedence of the fluid filled inner ear
What is the function of the pharyngeotympanic tube?
To equalize pressure between the middle ear and the atmosphere, allowing for free movement of the tympanic membrane
Inner ear - Bony Labyrinth
1) Name
2) Structure
1) Cochlea - in the petrous portion of the temporal bone
2) It begins at the vestibule, and consists of 3 coils of progressively diminishing diameter stacked in a conical structure; it contains 3 fluid filled tubes, wound around a conical bony core called the MODIOLUS
Cochlea - Scala Vestibuli
Structure
It is the UPPERMOST fluid filled chamber in the cochlea.
It is filled with PERILYMPH.
It begins at the oval window, coils around the modiulus, and is continuous with the scala tympani at the helicotrema at the apex. (uppermost is therefore continuous with lowermost)
Cochlea - Scala Tympani
Structure
It is the lowermost chamber
It is filled with PERILYMPH
It begins where it is continuous with the scala vestibuli at the helicotrema at the apex and ends at the ROUND WINDOW, which is located in the vestibule and is separated from the middle ear by a membrane.
Cochlea - Scala Media
Structure
This is the MEMBRANOUS LABYRINTH, also called the COCHLEAR DUCT
It is between the other 2 layers and is filled with ENDOLYMPH. It does not extend all the way to the apex
It is suspended in the bony labyrinth, where it is bounded by elastic partitions:
VESTIBULAR MEMBRANE –> separates media from vestibuli
BASILAR MEMBRANE –> separates media from tympani; the shape and mechanical properties of this membrane vary along its length. It is narrow and stiffest at its base and broader and thinner nearer the apex. It is the location of the receptor region called the ORGAN OF CORTI
THIS IS WHERE TRANSDUCTION OCCURS
What is happening within the bony labyrinth?
1) the stapes moves the oval window in response to air pressure changes.
2) movement at the oval window causes movement of the perilymph in the scala vestibuli
3) movement in the scala vestibuli causes movement of the scala media
4) fluid in the scala tympani beings to move as well in response to movement in the scala media and the scala vestibuli (with which it is continuous)
5) Movement in the scala tympani causes movement in the round window in the middle ear
Organ of Corti
Where is it located? What is its function?
It sits on top of the basilar membrane in the scala media in the cochlea.
It is the site of TRANSDUCTION
Organ of Corti: Hair Cells
Structure
At their apical surface, they have 50 to 150 stereocilia, which are arranged in curving rows that are graded in height.
They are connected to their neighbors by tip links.
Organ of Corti: Hair Cells
Types (What are the 2 types and how are they organized?)
Inner hair cells are SENSORY RECEPTORS. They form a single row of 3500 cells
Outer hair cells are SONUND AMPLIFIERS. They form 3 rows totaling 12000 cells
Inner and outer cells are separated by the TUNNEL OF CORTI
Organ of Corti: Tectorial Membrane
Structure
it is a gelatinous shelf extending outward over the organ of corti from the modiolus.
its lower surface is in contact with the tips of the tallest stereocillia
What are the steps in the transduction of a sound wave?
1) When a sound wave reaches oval window and moves the perilymph within the scala vestibuli,
2) The movement of the perilymph causes a vertical displacement of the basilar membrane
3) this movement is translated into a SHEARING FORCE between the tectorial membrane and the steroecilia of the hair cells.
4) When the hair bundle is defelcted TOWARD the tallest stereocilium, ion channels near the tips of the stereocilia open, allowing K+ to flow into the cell down its concentration gradient, and causing the cell to DEPOLARIZE
5) Depolarization opens Ca channels and Ca comes into the cell, facilitating the release of neurotransmitter
What is the place theory of cochlear tuning?
The basilar membrane and associated hair cell separate out frequencies from complex sounds
Because of the changing properties of the basilar membrane along its length, different points along the membrane will move maximally at different frequencies. Points that respond to higher frequencies are at the base and points that respond to lower frequencies are at the apex.
Hair cells exhibit the strongest response when the associated part of the basilar membrane is maximally displaced so that every hair cells is most sensitive to stimulation at a specific frequency. This gives rise to a TOPOGRAPHICAL MAPPING of frequency.
At high amplitude, more hair cells are stimulated
Processing Auditory Information - Afferent cells
What are they called?
Where are the cell bodies?
What is their distribution amongst the receptor cells?
Where does hearing predominatly arise?
These are called the cochlear (spiral) ganglion cells.
Their cell bodies are located in the modiolus
90% of the terminate on the inner hair cells. Each axon innervates only one hair cell, but hair cells send signals to multiple axons.
10% terminate on outer hair cells. Each axon supplies several outer hair cells.
Information from which HEARING arises originates almost entirely at the inner hair cells.
