At the tympanic membrane, after the auditory canal.
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What is connected to the medial surface of the tympanic membrane?
Ossicles - specifically the malleus
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What do the ossicles transfer the tympanic membrane movements through?
Oval window - a whole in the bone of the skull
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What follows after the oval window has been moved?
Fluid movement in the cochlea
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What is the output for the auditory nerve?
the medial geniculate nucleus
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What shape is the external auditory meatus and tympanic membrane? What nerves supply sensory fibres?
S shaped, with the tympanic membrane at an oblique angle. Vagus and trigemina;
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The three ossicles, in order
malleus, incus, stapes.
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What is important about the flootplate?
It is smaller than the area of the tympanic membrane, so there is increased pressure at the oval window. We need this because the fluid wouldn't transfer sound well on it's own
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What is the eustachian tube? How can this help us when on an aeroplane?
It is a tube that is continous with the nasal cavities, but is covered by a valve. When air pressure changes, opening the valve by yawning equalises the pressure.
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Where does the eustachian tube open?
into the nasopharynx at the level f the inferior concha
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What is the attenuation reflex?
At the onset of a loud sound, the tensor tympani and stapedius muscles contract and make the ossicles more rigid, so sound conduction is diminished.
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Draw the cross section of the cochlear duct
Scala vestibuli, scala media, scala tympani. Reissners membrane. stria vascularis, organ of corti, tectoral membrane
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What is the hole in the membranes where the tympani and vestibuli becomes continuous called?
the heliotroma
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What is the fluid in the SV and ST.
Perilymph.
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What fluid is in the SM and how is it different?
Endolymph. It has a high K+ and a low Na+, similar to intracellular fluid.
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What is the function of the stria vascularis
reabsorbs sodium and secretes potassium against their gradients, mainting the unusal concentrations in the scala media.
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What is the non-rigid structure in the cochlear. Where does sound pressure end up?
The basilar membrane. Sound pressure ends up at the round window;
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Two features of the basilar membrane that allow it to respond to sound.
It's wider at the apex, and the base is much stiffer. like a swimming flipper
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What does the distance travelled along the BM depend on?
The frequency of the sound - High = not far. Low freq.= far
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What does the organ of corti consist of? Draw it.
hair cells, rods of corti and supporting cells.
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What are hair cells sandwhiched between?
The basilar membrane and a thin sheet of tissue called the reticular lamina
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What do the rods of corti do?
Span these two membranes and provide structural support.
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Where do the stereocillia of the outer hair cells extend to?
above the reticular lamina and into the endolymph
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Where are the cell bodies of the neurons of the auditory nerve?
In the spiral ganglion in the modulus
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Why do sterocillia bend only at the base?
They contain rigid actin filaments
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What happens when the sound wave causes the stereocilla to bend back and forth
they alternatively hyperpolarise and depolarise from their resting potential of -70
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What channel is on the tip of the cillia?
TRPA1 channel - a mechanically gated cation channel.
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What feature allows the cells to move together and open their channels?
a tip link
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Why does K+ entering cause a depolarisation?
the unusal endolymph means that k+ has a eq pot of 0mv.
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Where does most of the neural output come from?
from the inner hair cells, even though there are much fewer
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What do outerhair cells do?
Enhance basement membrane motion and frequency sensitivity
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What allows them to do this?
motor protien prestin in the membranes, they can change the length of the hair cells in response to sound.
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