Neurobiology of hearing Flashcards
LOs
- Describe the components and functions of the external, middle and inner ear
- Explain the roles of the tympanic membrane, the auditory ossicles (malleus, incus and stapes) and scala vestibule in sound transmission
- Describe the way that movements of molecules in the air are converted into impulses generated in the cochlea
- Explain how pitch, loudness and timbre are coded in the auditory pathways
- Describe the components of the auditory pathway from the cochlear hair cells to the cerebral cortex
- Compare causes of conductive and sensorineural hearing loss and tests used to distinguish between them
Ears let us to: (2)
detect sounds
maintain balance
via receptors for eharing and equilibrium in the ear
The anatomy of the human ears is divided into:
outer ear
middle ear
inner ear
which 3 structures are associated w hearing
which 3 structrues are associated w equillibrium
external ear, middle ear and cochlea
the external ear is composed of: (3)
- auricle (pinna) that captures sound waves
- external auditory meatus (ear canal)
- tympanic membrane
what is the stated hearing range for humans
over which range is hearing most clear
20 –> 20,000 Hz
2,000 –> 5,000
what is the auricle/ pinna
function?
paired structure on either side of the head
external ear
fucntion: gather and focus sound energy through the external auditory meatus to the tympanic membrane
lobule
the one part of the auricle that has no cartilage
aka ear lobe
concha
shell shaped structure of the cavity of the external ear
microtia- what is it and what are the grading systems
a congenital deformity where the pinna is underdeveloped
complete underdevelopment of the pinna is anotia
graded 1–> 4 with increasing severity
grade 1 severity- small ecternal ear and a small but present ear canal.
grade 2- partially developed ear (usually the top portion is underdeveloped) with a closed external ear canal (atresia) producing a conductive hearing loss.
grade 3- most common form with an absent external ear and small peanut-like vestige structure and canal atresia.
grade 4- complete absence of the external ear with canal atresia
thr concha and external auditory canal act as a resonator
effectively enchance the intensity of sound that reaches the tympanic membrane by about 10-15 dB
what is the ear canal/ external auditory meatus
Functions as an entryway for sound waves, which get propelled toward the tympanic membrane known as the eardrum.
3 major components of middle ear
general function of middle ear 2
- tympanic membrane (eardrum)
- auditory ossicles
- eustachian/ auditory tube
overall function:
- amplify the vibrations of the tympanic membrane to the oval window
- impedence matching
tympanic membrane
thin and pliable, therefore a sound, consisting of compressions and rarefractions of air particles, pulls and pushes at the membrane moving it inwards and outwards at the same frequency as the incoming sound wave.
It is this vibration that ultimately leads to the perception of sound
- greater the amplitude the greater the deflection of the membrane.
- higher the frequency of the sound, the faster the membrane vibrates.
auditory ossicles
what are the 3
3 of the smallest bones in the body.
they transfer vibrations of the tympanic membrane to the cochlea
- malleus (hammer)- forms a rigid connection with the incus
- incus (anvil)- forms a flexible connection with the stapes
- stapes (stirrups)- connects the oval window
how do the ossicles work to transmit sound to the cochlea
- The inward-outward movement of the tympanum displaces the malleus and incus
- the action of these two bones alternatively drives the stapes deeper into the oval window and retracts it
- resulting in a cyclical movement of fluid within the inner ear.
eaustachian tube
helps to ventilate the middle ear and maintain equal air pressure on noth sides of the tympanic membrane, inside the ,middle ear and outside the body, via nasopharynx (the nasal part of ther pharynx, lying behind the nose and above the level of the soft palate).
what is the vestibular system?
which 3 structures make it up?
the sensory apparatus of the inner ear that helps the body maintain its postural equilibrium.
Info furnished by the vestibular system is also essential for co-ordinating the position of the head and movement of the eyes.
Engages a number of reflex pathways that are responsible for making compensatory movements and adjustments in body position.
this info provideed is proprioceptive (inside)
1) semi-circular canals
2+3) the utricle and saccule
the semi-circular canals
- respond to rotational movements (angular acceleration)
- made of 3 pairs of ducts oriented at roughly 90* to each other for maximum ability to detect angular rotation
- canals don’t contribute to sense- the fleshy- fluid filled ducts inside
- mediate interactions between the vestibular system and the eye muscles via the cranial nerve- smooth eye movement left and right keeping visual fields stable when moving
Otolith organs
consist of the saccule and utricle
which are membranous sacs perpendicular to each other
AKA gravity receptors
both contain a maccula
both organs monitor the position of the head relative to vertical
cochlea
The cochlea is the part of the inner ear involved in hearing. It is a spiral-shaped cavity in the bony labyrinth, in humans making 2.75 turns around its axis, the modiolus.
raissner’s membrane
together with the basilar membrane creates a compartment in the cochlea filled with endolymph, which is important for the function of the spiral organ of Corti.
Thought to play an important role in otoacoustics as a wave on Reissner’s membrane can propagate along the whole extent of the cochlea
basilar membrane
forms divisions between the cochelar duct (scala media) and the tympanic membrane, allowing different frequencies to reach max amplitudes in different positions
Performs frequency selectivity by its filter bank so is effectively a continuous array of filters which decompose a complex waveform into its constituent frequency components.
sound transduction
sound is a mechanical wave that needs a medium to propagate itself and is created by something that vibrates
vibration creates air particle movement due to change of air pressure.
- The sound wave travels thorugh air to the human ear via the pinna.
- sound waves (vibrations) are converted into action potentials by the inner row of hair cells in the cochlea (The apical end of the hair cell contains the sereocilia and they are arranged in order of ascending lengths from one side of the cell to the other. The membranes of the stereocilia contain mechanically gated cation channels. Extending from the gate of the ion channel to the adjacent, taller, stereocilium is a fibrous protein called a tip link.)
- When the stereocilia bend toward the longest stereocilium the tension in the tip link increases, pulling the gates on the ion channels open, and when they bend in the opposite direction the tension decreases and the gate closes.
- Stereocilia are bathed in endolymph of the cochlear duct (high in K+). When the gate on the cation channel opens, K+ rushes into the cellm], depolarising the membrane and causing voltage gated Ca2+ channels on the basal membrane of the hair cell to open, allowing Ca2+ to enter.
- Influx of Ca2+ stimulates the release of a NT by the hair cell, triggering an ap IN THE NEURONE THAT Synapses with the hair cell. The axons of these neurones form the cochlear nerve that transmits the AP’s to the auditory cortex of the brain.
In hair cells at rest, ~10% of the K+ ion channels are open resulting in a low frequency of AP’s travelling to the brain whrn it is perfectly quiet.
This allows for both an increase ein AP frequency when hair cells bend toward the longest stereocilium, and a decrease in frequency of action potentials when the hair cells bend the other way.
sensorineural hearing loss
Sensorineural hearing loss (deafness) is the most common type of hearing loss, it is usually due to the loss of cochlear hair cells but can be resultant from damage to the 8th cranial nerve or within the central auditory pathways.
It often impairs the ability to hear certain pitches while others are unaffected.
conductive hearing loss
refers to impaired sound transmission in the external or middle ear and impacts all sound frequencies.
Among the causes of conduction hearing loss are:
- plugging the external auditory canals with wax (cerumen) or foreign bodies
- otitis externa (inflammation of the outer ear “swimmers ear”)
- oitisis media (inflammation of the middle ear) causing fluid accumulation or scarring, or perforation of the eardrum.
Severe conductive deafness can result from otosclerosis in which bone is resorbed and replaced with sclerotic bone that grows over the oval window.
