ICL 9.1: Peripheral & Central Auditory Systems Flashcards
what are sound waves?
patterns of alternating compressions and rarefaction
what are the 3 properties of sound waves?
- amplitude
- frequency
- pure tone
what is amplitude?
the magnitude of alternation
it is perceived as loudness
it’s measured as sound pressure level (SPL) and it’s rated on logarithmic scale (decibels dB = 20 log10(P/Po) )
what is frequency?
the temporal spacing of alternation
it’s perceived as pitch!
it’s measured in cycles per second (Hertz)
the adult human hearing range ~ 100 – 160,000 Hz but the most useful range is 4,000-6,000 –> animals can hear higher and lower frequencies
what is pure tone?
the sound of a single frequency or a range of frequencies
vs. complex sound which contains mixed frequencies and amplitudes
what are the components of the peripheral auditory system?
- outer ear
- middle ear
- inner ear (cochlea and organ of Corti)
- auditory (spiral) ganglion and cochlear nerve
what are the 5 auditory centers?
- cochlear nuclei (the primary center!)
- superior olivary nuclei
- nuclei of lateral leminiscus
- inferior colliculus
- auditory cortex
what are the auditory reflexes and pathways?
- acoustic reflex
2. sound localization reflex
what are the components of the outer ear?
the outer ear and the external acoustic meatus funnel sounds to vibrate the tympanic membrane
- external ear (auricle) = pinna can filter sound and assists vertical localization
- external acoustic meatus
channels sound to tympanic membrane
it also has hair follicles and gland (cerumen) which warm the ear and prevent things from entering
- tympanic membrane
what are the components of the middle ear?
- ossicles = malleus, incus and stapes
- stabilizing ligaments
- muscles: tensor tympani m. (CN V) and stapedius m. (CN VII)
- eustachian tube
what are the eustachian tubes?
it’s the part of the middle ear that connects middle ear with nasopharynx
this is what helps balance the pressure in the ear
so clinically if this tube is blocked, the pressure will build up
how do the oss
the tympanic membrane receives pressure from the external auditory canal and the the ossicles amplify the sound!
the tympanic membrane surface area is much larger than the oval window which is what helps amplify the sound
the oval window is at the end of the middle ear and the beginning of the inner ear and it connects the tiny bones of the middle ear to the scala vestibuli, which is the upper part of the cochlea
where is the inner ear located?
inside the petrous portion of the temporal bone!!
what are the components of the inner ear?
- vestibule
- semicircular canals
- cochlea
what is the structure of the cochlea?
part of the inner ear that is a snail-shaped tunnel about 2 ¾ turns
it’s a bony tube with 3 parallel membranous ducts inside; the cochlear duct is in the middle and it’s surrounded by the scala tympani and scala vestibuli ducts as they all curl around into a snail shape
what are the 3 parallel membraneous ducts that make up the cochlea?
- scala vestibuli
- scala tympani
- scala media (cochlear duct)
the scala vestibuli and tympani are connected at the apex of the cochlea
the scala vestibuli connects the oval window of the middle ear to the helicotrema so it’s like the top layer of the coil in the cochlea while the scala tympani connects the helicotrema to the round window –> they both contain perilymph which has high Na+ and low K+
the cochlear duct contains endolymph which has low Na+ and high K+ and it also contains the tectorial membrane and organ of corti
what is the important structure inside the cochlear duct?
the organ of corti which generates electrical signals from the mechnical stimulation
it’s composed of:
1. outer hair cells (3 rows)
- inner hair cells (1 row)
- supporting cells
- inner tunnel
there is also a basilar membrane which is an array of hair cell supporting cells at the bottom of the organ of corti –> so think of it like a sandwich; the hair cells are in the middle while the tectorial membrane is on the top and basilar membrane is on the bottom (the basilar membrane is NOT part of the organ of corti)
what are the characteristics of the hair cells of the organ of corti?
the organ of corti is the part of the inner ear which generates electrical signals from the mechanical stimulation
they have 3 rows of cilia arranged by height which are embedded in tectorial membrane!!!
the hair cells are polar so there is an apical and basolateral surface –> the apical surface has cilia which exposed to endolymph in the cochlear duct while the basolateral surface is exposed to the perilymph in the scala tympani duct
what is the function of the outer vs. inner hair cells of the organ of corti?
outer hair cells
are targets of efferent modulation (olivocochlear reflex) and they improve frequency selectivity
inner hair cells are a major source of auditory afferent fibers and input to the auditory nerve
what’s the difference between the composition of endolymph vs. periplymph?
perilymph has high Na+ and low K+
endolymph has low Na+ and high K+
what is mechanosensitive transduction?
when the oval window is compressed by the ossicles, alternating compressions and rarefaction causes endolymph fluid waves and vertical displacement of basilar membrane
the vibration of the basal membrane will cause a shearing of the tectorial membrane and the embedded cilia
displacement of cilia bundles towards the kinocilia (the tallest sterocilia) stretches open the mechanosensitive K+ channels –> the endolymph is high in K+ which will move into the hair cell and depolarizes it; the fact that the perilymph has low K+ also helps the K+ move into the hair cells
once K+ has depolarized the cell, Ca+2 gated channels will open and Ca+2 will also enter the hair cells –> this causes NTs to be released from the basolateral membrane which go to the auditory nerve and then to the brain (usually the NT is glutamate)
what are the 2 types of afferent nerve fibers in the auditory nerve?
so these nerve fibers are receiving NTs released from the hair cells after they’ve been depolarized
- type 1 afferent:
myelinated, each fiber synapses with a few inner hair cells, and they contribute to >90% of auditory input
- type 2 afferent fibers are unmyelinated, each fiber synapses with only like 20 outer hair cells, and their function is unknown – they might modulate frequency selection
auditory nerve projects to neurons in cochlear nucleus (primary auditory center)
what is the function of the efferent fibers of the auditory nerve?
they are inhibitory fibers from superior olivary nucleus
they protects hair cells from loud sound damage
where are high vs. low frequency sounds registered on the basilar membrane?
high frequency stuff is closer to the oval window and in the narrow part of the basilar membrane
low frequency stuff is towards the apex of the cochlea and in the wide part of the basilar membrane