Week 10 - hearing and equilibrium Flashcards
whats the beyond the eardrum (middle ear)
an air filled space connected to the pharynx by the eustachian tube
external ear
pinna and the ear canal, sealed at its end by the tympanic membrane or eardrum.
inner ear
cochlea for hearing and the
vestibular apparatus for equilibrium.
Sound as a wave
At the peaks of the waves, the molecules are crowded together
and the pressure is high; at the troughs the molecules are far apart
and the pressure is low.
What do we percieve frequency as
pitch
low frequency as low pitched and high frequency as high pitched
frequency
number of waves per second
Hertz
human sound range
Humans hear sounds in the range 16–20,000 Hz — ~10 octaves.
where is acuity the highest for humans when percieveing sound
Acuity is highest 1000–3000 Hz.
amplitude
pressure difference between peak and trough
what does loudness depend on
the larger the amplitude, the louder the sound
Loudness depends on frequency as well, e.g. a sound of 30,000
Hz is beyond the range of human hearing, so it won’t be loud no
matter how large its amplitude.
what separates outer ear from inner ear
eardrum
what does the sound waves vibrate
eardrum
how do bones convey vibrations to inner ear
The eardrum vibrates the malleus bone, which moves
the incus , which moves the stapes , which pushes like a piston against the oval window, a membrane between middle and inner ear.
ossicles
maleus, incus and stapes
smallest bone sin the body, act as a lever system carrying vibrations from eardrum to much smaller oval window
what oval window lead into
cochlea, which contains the receptor cells
ducts in cochlea
vestibular, tympanic and cochlear ducts
what do the vestibular and tympanic ducts contain and where do they communicate
perilymph (like plasma)
they communicate at helicotrema
what the does the cochlear duct contain
endolymph (like intracellular fluid)
how does wave wave energy enter and exit
Wave energy enters the cochlea at the oval window and exits,
eventually, back into the middle ear through another membrane
called the round window.
En route, the waves shake the cochlear duct, which contains the
auditory receptor cells (hair cells),
what does the cochlear duct contain
organ of corti
position of organ of corti
The organ of Corti sits on the basilar membrane and under the tectorial membrane
what does organ of corti contain
auditory receptor cells — mech-
anoreceptors called hair cells. They are epithelial cells, not neurons,
and number ~20,000 per cochlea.
sterocilia
Each hair cell has 50–100 stiff “hairs” called stereocilia, which extend into the tectorial membrane. They bend when waves in the perilymph deform the basilar and tectorial membranes.
what happens when cilia bend towards longest cilium
hair cell excites its neuron
The hair cell depolarizes and releases transmitter, activating a primary sensory neuron.
Axons of these neurons form the auditory nerve (also called the cochlear nerve), a branch of cranial nerve VIII
what happens when cilia bends away
The hair cell hyperpolarizes, so it releases less transmitter and
doesn’t excite its neuron as much.
The basilar membrane responds to different frequencies
at different points
The membrane is narrow and stiff near the round and oval windows, wider and more flexible at its other end.
High-frequency waves maximally displace the membrane at the oval-window end; low-frequency waves maximally displace the other end. So the brain can deduce the frequency by noting which hair cells
are most active
what reveals pitch to the brain
pattern of mebrane motion
where is primary auditory cortex
temporal lobe
The brain localizes sounds based on loudness
and timing
If a sound is louder in the right ear than in the left then it is coming from the right side of the head. Loudness is conveyed by firing
frequency, i.e. louder sounds make auditory sensory neurons fire at a faster rate.
If the sound reaches the right ear before the left then it is coming from the right side of the head.
conductive hearing loss
sound can’t be transmitted through the
external or middle ear.
sensorineuronal hearing loss
there is damage to the hair cells or
elsewhere in the inner ear. Mammals can’t replace dead hair cells,
90% of hearing loss in the elderly (presbycusis) is
sensorineural.
presbycusis
hearing loss in elderly - sensorineuronal
central hearing loss
there is damage to the cortex or the path-
ways from cochlea to cortex. Typically the patient’s trouble is in
recognizing and interpreting sounds, rather than in detecting them.
Rinne test
you hold a tuning fork against the mastoid bone
and then beside the ear, and ask when the sound is louder. Normally
it is louder through the ear canal. If it is louder through the bone, there
is conductive loss.
Weber test
you hold the tuning fork to the patient’s forehead,
in the midline, and ask in which ear the sound is louder. With
sensorineural loss, it is louder in the good ear. With conductive loss, it
is louder in the bad ear, because it doesn’t have to compete with
sounds heard through that ear canal.
what senses head position and motion
vestibular apparatus (utricule and saccule)
what do utricule and saccule do
contain hair cells that are activated when
the head tilts relative to gravity
what do semicircular canals do
fluid-filled hoops that detect head
rotation, e.g. when your head turns rightward, the fluid in the tubes
sloshes leftward, activating hair cells.
Equilibrium pathways project mainly to
the cerebellum
Vestibular hair cells activate primary sensory neurons of the vestibular nerve, which is a branch of cranial nerve VIII.
These neurons may either pass directly to cerebellum or synapse in the medulla, whence they proceed to the cerebellum or up through thalamus to cortex.
Your brain uses vestibular information to infer your own position and motion, and keep you upright.
