Themes 1 and 5: Audiology and child development Flashcards
what structures are in the outer ear and what are they functions?
the pinna- funnels sound into auditory canal, externalisation of sound, localisation of sounds in the vertical plane (sound waves interact with ridges of pinna
external auditory meatus (ear canal)- canal leading down to tympanic membrane, protects sensitive machinery of the inner ear contained in the petrous portion of the temporal bone, walls secrete cerumen which captures dust and foreign bodies
where is the cone of light in a normal tympanic membrane?
5 o’clock on right
7 o’clock on left
what is the area where the malleus attaches to the tympanic membrane called?
Umbo
what are the names of the ossicles of the middle ear?
malleus, incus, stapes
what is the function of the middle ear?
it acts as an impedance transformer helping to convert energy from one medium to another- from air to fluid. without the middle ear most sound waves will be reflected by the fluid filled inner ear
how does the middle ear conduct sound to the inner ear?
- the area of the tympanic membrane is larger than the footplate of the stapes. this means that the force on the tympanic membrane acts over a smaller area at the stapes thus producing a greater pressure so more sound travels into the cochlea
- ossicles act as leavers- the malleus is longer than the incus so in a given distance that the malleus moves the incus moves a shorter distance but with more force.
overall there is a total 25x increase in pressure from the tympanum to the stapes
what are the muscles of the inner ear and their innervation?
tensor tympani- attaches to malleus, innervated by a branch of the mandibular nerve
strapedius- starts at the pyramidal eminence and attaches to the stapes, innervated by a branch of the facial nerve
what is the middle ear reflex?
it protects against loud sounds. contracting the muscles of the middle ear causes stiffening of the ossicular chain and this restricts the amount of sound that moves into the cochlea. activated when talking. can’t protect against loud sudden bried sounds e.g. gun shot
what are each of the compartments of the cochlea filled with?
the scala vestibuli and scala tympani are filled with perilymph (high Na), the Scala media is filled with endolymph (high K)
how are sound frequencies separated in the cochlea?
it causes the basilar membrane to vibrate which sets up a travelling wave on the basilar membrane which travels from base to apex. the position of maximal displacement of the wave corresponds to the frequency of the sound. high frequency near base, low frequency near apex. the hair cells are most stimulated at the point of maximal displacement
how are the hair cells of the cochlea activated?
in both inner and outer hair cells the movement of the basilar membrane causes the steriocilia of hair cells to interact with the tectorial membrane and bend. bending towards the tallest sterocilia causes mechanically gets K channels to open and K flows in causing depolarisation. bending towards the shortest sterocilia closes K channels which leads to hyperpolarisation
how are nerve impulses produced in inner hair cells?
in only inner hair cells, depolarisation causes voltage gated Ca channels to open. inflow of Ca causes the release of a neurotransmitter which activates the cochlear nerve fibres. hyperpolarisation causes neurotransmitter to stop being released. the pattern of cochlear nerve firing corresponds to the pattern of sound frequencies
what are the function of the outer hair cells?
they act as cellular motors. the hair cells change in length when the sterocilia bend in response to sound stimulation. this is driven by a motor protein in the cell membrane called Prestin which injects energy and amplifies the movement of the basilar membrane allowing hearing to be extremely sensitive.
what are the two types auditory nerve fibres and their differences?
type I fibres- 95% of auditory fibres. they innervate inner hair cells. each IHC makes contact with around 10 type I fibres but each fibre is only associated with 1 cell. they carry most of the sound
type II fibres- make contact with multiple outer hair cells. their function is not fully understood.
what are the mechanisms for encoding sound frequency?
- place code- the auditory system keeps track of where the information initiated on the basilar membrane throughout the auditory pathway by mapping sound frequency in the processing centres and on the surface of the auditory cortex e.g high frequency fibres from the base of the cochlea will synapse on the anterior part of the cochlear nucleus.
- time code- the firing of action potentials in auditory nerve fibres synchronise with the peaks of the sound waveform (phase locking). so the time between action potentials tells us about the frequency of the sound. only happens at low frequencies
what are the key processing stages in the auditory pathway?
the cochlear nucleus is the first synapse in the auditory pathway. from here information travels to the superior olivary complex (information from both ears interacts here- important for localising sound) here most fibres cross to the conralateral side before travelling in the medial lemniscus to to the inferior colliculus in the midbrain auditory centre. then moves to the medial geniculate body (in the thalamic auditory nucleus) before ending in the auditory cortex in the temporal lobe.
how is sound localised?
sounds are heard slightly differently by each ear as they are separated by the mass of the head. this will lead to differences in the intensity and timing of the sound between the ears. the brain extracts these differences (inter-aural differences) to localise sound.
where is the vestibular apparatus found?
it is within the inner ear in the bony otic capsule in the lateral part of the petrous bone
what is the structure of of the vestibular apparatus?
there are 3 semicircular canals the lateral (lies parallel to base of skull), the superior and posterior. they all lie perpendicular to one another
what is the structure and function of the otolith organs?
the utricle and saccule are otolith organs and they make up the vestibule. they are connected by the urico-saccular duct.
the otolith organs are comprised of macula-sensory epithelium which contain hair cells arranged horizontally in the utricle and vertical in the saccule. the hair cells are covered by a gelatinous cap overplayed by otoliths (calcium carbonate fragments). as the head moves gravity acts on the otoliths and the pull on the gelatinous cap causing stereocilia to bend (bending towards longest depolarises). this allows the otolith organs to detect changes in head angle and linear acceleration.
what is the function of the semicircular canals?
they detect rotational acceleration. the superior, posterior and lateral semicircular canals detect rotation in different planes.
what is the structure of the semicircular canals and how do they detect rotational acceleration
hair cells are embedded in a gelatinous cupula in the ampulla of the semicircular canal. the canal is filled with fluid. when rotation begins the head and canal accelerate, but the fluid initially remains stationary due to its inertia. this causes the cupula and steriocilia to bend causing depolarisation of the vestibular hair cell and and the nerve fibre increases it firing rate. at constant velocity eventually the fluid will catch up and the firing rate will decrease to the background rate. when rotation stops the canal will stop but the fluid will remain rotating causing the cupula and sterocilia to bend in the opposite direction causing hyperpolariation and decreased nerve firing.
how are signals from the vestibular system carried to the brain?
signals are carried by the vestibular portion of the cranial nerve VIII and they synapse in two regions. fibres from the otolith organs synapse in the lateral vestibular nucleus and project to the cerebellum and limb motor neurones. fibres from the semicircular canal synapse in the medial vestibular nucleus and send projections to the neck motor neurones and the extra ocular motor neurones in nuclei III, IV, VI
what are the role of the central vestibular pathway?
important for controlling head and eye movement and maintaining balance. lateral vestibular nucleus projects to spinal cord via vestibulospinal tract to control limbs and maintain balance.
