auditory system Flashcards
what is the human hearing range (Hz)?
20 - 20,000 Hz
describe the anatomy of the ear areas involved in the auditory system
very outer ear = pinna, lots of folds assist in locating whether sound is coming from above or below
external auditory canal leads to tympanic membrane/ear drum
next is the middle ear with the three ear bones - ossicles
at the end of the middle ear are two membranes the oval and round window
after this is the inner ear, the cochlea
how does the middle ear transduce sound to the inner ear? include the benefit of this process
and why the round window is needed
three ossicles are the malleus/hammer, connected rigidly to the incus/anvil, which is connected flexibly to the stapes/stirrup
the compression part of a sound wave pushes the tympanic membrane, moving the malleus and therefore the incus and the stapes in, pushing the oval window too
this is a way of concentrating the force of the sound wave in order to actually move the fluid in the cochlea
The tympanic membrane is pulled back in the rarefaction phase of the sound wave, moving the malleus etc.. in pulling the oval window out
While the oval window is going in and out, the round window moves in the opposite direction to keep pressure balanced so as not to harm cells
describe the structure of the cochlea
three compartments, leading on from the oval window is the scala vestibuli, filled with perilymph that gets pushed due to sound waves, this movement continues in the second compartment the scala tympani (the two are separated by the helicotrema)
in the middles is the scala media, filled with endolymph which is very high in K+
between the media and tympani is the organ of corti, with its inner and outer hair cells
how does the cochlea transduce sound into electrical signals?
movement of perilymph in the scala vestibuli and tympani cause the upwards and downwards movement of the basilar and tectorial membranes, between which the organ of corti rests
this results in the sideways movement of the hair bundles of the inner hair cells on the organ of corti
these hairs known as stereocilia, are connected by tip links
movement of the hair bundles towards the tallest one (there is a short, medium and tall layer) moves the tip links so that non-selective cation channels are exposed, allowing K+ to diffuse in and depolarise the cell
if moved in the other direction, the tip links completely block these channels, hyperpolarising the cell
what is the basilar membrane’s role in hearing different frequencies?
the basilar membrane is narrower and stiffer at one end, and wider and thinner at the apical end
higher frequencies are heard at the stiffer end (takes a bit more to make something stiffer move) while lower frequencies make the wider end move
this means multiple frequencies can be heard at the same time
what is the function of the vestibular system?
it tells us which way is up, what direction we are walking in, keeps eyes still as we move, maintains upright posture, its part of our ability to perceive our own movement in space
describe the gross anatomy of the vestibular system and the function of each part
you’ve got the otolithic organs, the utricle and the saccule, with a gelatinous otolithic membrane these are involved in detecting linear acceleration and orientation, e.g. looking down
then the three semi-circular canals filled with endolymph, each with its own ampulla where the hair cells are found, horizontal, anterior and posterior are involved in detecting rotation - nodding or shaking the head, spinning round, ear to shoulder motion etc…
how do the hair cells work in the vestibular system and how does this differ from the auditory system?
hair cells tilt to the taller end of the bundle = depolarisation due to tip links opening, K+ moves in, glutamate is released
if the hairs move the other way hyperpolarisation occurs
key difference is these hair bundles keep their kinocilium after development - this is like the other stereocilia but central, taller and stiffer
what are the two kinds of vestibular hair cell?
literally the same in most ways, the difference is in the way they contact the afferent neuron - one is regular the other has the afferent dendrite forming a kind of horseshoe around the base of the cell called a calyx
how do the otolithic organs work?
hair cells project their stereocilia up into the gelatinous otolithic membrane, the gelatinous nature and the otoliths (calcium carbonate crystals) of the membrane make it heavy, so when you lean your head forward, gravity means this gelatinous membrane lags back, pulling the hairs back too, causing hyperpolarisation or depolarisation depending on which way the hair cells are pulled
what is the striola in the otolithic organs?
its like a ridge that acts as a mirror line for the hair cells down the middle of the otolithic organs, so the hair cells on either side have opposite arrangements , the taller end of the hair bundle faces the striola
Also means the same movement has opposite effect on hair cells either side of the striola???
again, opposite effects in the two ears
what areas of the brain do the vestibular organs communicate with?
the organs directly link to the vestibular nuclei (medulla and pons) and the cerebellum
from the vestibular nuclei info is passed to the spinal cord (for motor output) and the cerebral cortex for conscious perception
also links to oculomotor nuclei (midbrain) so eyes can correct position and to the reticular formation for visceral reactions e.g. feeling sick when you are dizzy
does the vestibular system work alone?
No, info from things like the visual system assist, as does proprioception
how do the semi-circular canals and ampulla work? do he ears work alone?
each loop is responsible for detecting rotation within a certain plane
inside the ampulla is the gelatinous cupula
within the cupula are the hair cells, surrounding the cupula and filling the canals is endolymph
If you turn your head right, the endolymph fluid moves to the left due to inertia, pushing the cupula, bending the hair cells, resulting in depolarisation or hyperpolarisation (it’s depolarisation if the hair bundles bend toward the taller kinocilium) this travels along sensory fibres to vestibular-cochlea nerve/ CN VIII
The two canals in either ear work together - focusing on the horizontal, if turn your head one way it results in excitation in one ear, and inhibition in the other