Microanatomy Flashcards
what is the auditory system
It is a specialised sensory system that detects changes in sound wave pressure.
what does the auditory system comprise of
cochlea, its nerve and the pathway to the auditory cortex in the temporal lobe
what frequencies can the auditory system detect
20Hz-20Khz
how are sounds detected
- they are detected by both ears
- in order to locate the sound the sound complex inhibitory circuit is used between the two ears in order to magnify differences in timing and intensity of the sounds
what are the 3 main regions of the ear
- outer
- middle
- inner ear
what does the outer ear do
- it collects sound waves and channels them to the middle ear
what does the middle ear do
- converts the sound pressure waves to vibration of the fluid in the inner ear by movement of the oval window.
- This is accomplished by 3 small bones.
what does the inner ear do
- houses the hearing and balance receptors
what do the different parts of the ear do
- outer - it collects sound waves and channels them to the middle ear
- inner - houses the hearing and balance receptors
- middle – converts the sound pressure waves to vibration of the fluid in the inner ear by movement of the oval window.
where are the hearing and balance receptors
- the hearing receptors - located within the cochlea
- balance receptors are located in the semicircular canals and vestibule
what is the membranous labyrinth
it is a delicate continuous membrane suspended within the bony labyrinth, creating a second chamber within the first in the cochlear
what are the three fluid spaces in the cochlea
- scala vestibuli
- scala tympani
- cochlea duct
describe the scala vestiboli and scala tympani
- enclosed in bone
- contain perilymph
- continous with each other at the apex which is known as the helicotrema
what does the cochlea duct contain
- contains the endolymph and is part of the membranous labyrinth
what does the vestibular membrane do
A delicate Vestibular (Reissner) membrane separates scala vestibuli from the cochlea duct
what does the basilar membrane do
Basilar membrane separates scala tympani from the cochlea duct.
what does the spiral organ consist of
- consists of inner and outer hair cells and supporting cells
describe inner hair cells
- arranged in a single row and have a 1:1 relationship with auditory neurones and play a major role in auditory discrimination
describe the structure of outer hair cells
- increase from 3 rows at the base to 5 rows at the apex,
- receive synapses from a large number of auditory afferents - this helps sharpen the frequency tuning of the cochlea
what do hair cells have
sterocillia
what do sterocilia do
- they have there tips embedded in the overlying tectorial membrane
- when the basilar membrane is in motion the sterocilia bend causing changes in the membrane potential and cause action potential firing
describe how sound transduction takes place
- sound waves are collected by the outer ear and channeled to the tympanic membrane
- When sound waves are transmitted from the tympanic membrane along the middle ear ossicles it causes the stapes to vibrate the oval window
- The stapes vibrates in response to vibrations of the eardrum, setting the perilymph fluid of the inner ear in motion
- The round window serves as a pressure valve, bulging outward as pressure rises in the inner ear. When this moves inwards, the round window moves outwards & vice versa
- Because of the size (area) differential between the tympanic memnbrane and oval window, sound is amplified four fold in the middle ear
- Soundwaves are converted into perilymphatic pressure, which is transmitted through the vestibular membrane to the basilar membrane
- Because the cochlear duct sits on the basilar membrane, it too is set into motion, stimulating auditory receptors located on this membrane
where does the tonotpic map of sound frequency occur
occurs along the basilar membrane
how does the size of the basilar membrane increase
- the width of the membrane increases in size in the direction from the base to the apex
- ## narrow end is taught while the wider end is more flexible
how does the basilar membrane respond to pitch
- the highest frequencies set the narrow base end resonating while the lower frequencies set apex in motion
how is the loudness of the sound determined by
- by the intensity of the sound waves
greater intensity causes…
greater vibration of the basilar membrane and hence greater activation of spiral ganglion cells
where are the cell bodies of the cochlear nerve found
- found in the spiral ganglion
where do the peripheral process synapse with
- it synapses with the hair cells and their central process travels in the cochlea nerve which terminates in both the dorsal and ventral cochlea nuclei in the medulla
what does the ventral cochlea nucleus do
- it encodes intensity information
what does the dorsal cochlea nucleus do
- it encodes pitch information and analyses the quality of sound
where do the axons from the dorsal and ventral cochlear nuclei go
Axons from the dorsal & ventral cochlear nuclei cross the midline to form the lateral lemniscus (LL) which ascends through the pons to the inferior colliculus in the midbrain.
where do ventral nucleus fibre enter the lateral leminscus (LL)
- they enter via the superior olivary nucleus (SON) and trapezoid body
where do dorsal nucleus fibre enter the lateral leminscus (LL)
it goes directly to contralateral LL
where do the auditory fibres travel from the IC
- the fibres travel to synapse in the main auditory nucleus of the thalamus - the medial geniculate nucleus (via inferior brachium)
- then thalamocortical ions travel to the primary auditory cortex in the temporal lobe (Heschi’s Gyrus) via auditory radiation of internal capsule
- auditory cortical cells receive input from both ears but are preferentially response to input from the contralateral ear
what are tonotpically organised
IC, MGN & AI are all tonotopically organised; (in AI, high tones posteromedially, low tones anterolaterally)
why are central auditory pathways unlike other ascending pathways
Presence of accessory nuclei that modulate the input.
Bilateral representation of auditory impulses on each side.
what does the superior olivary nucleus do
- It receives input from both ears & gives rise to fibres in both the ipsilateral & contralateral LL.
- Ipsilateral inputs are excitatory, contralateral ones inhibitory (effect mediated by TB).
- Plays a key role in localisation of sounds in space as it is responsive to differences in the intensity & timing between sounds entering the ears simultaneously
what does the trapezoid body do
Helps locate the spatial direction of the sound by exaggerating differences through crossed inhibition of SON cells.
where is the lateral lemniscus nuclei located
- located in and adjacent to LL
what does the lateral meniscus nucleic do
- Send axons to both the ipsilateral and contralateral LL.
- Participate in acoustic reflexes.
what does the inferior colliculus do
- Integrates spatial information from SON, intensity information from Ventral cochlea nucleus & pitch information from dorsal cochlea nucleus
- Projects to MGN & also to contralateral IC (inhibitory
what does auditory modulation do
- Reciprocal connections between AI & MGN & IC.
- Also IC, SON & LL send fibres back to the dorsal and ventral cochlear nuclei.
- Fibres (from SON & TB) & RF fibres also terminate on hair cells of Organ of Corti.
- Provides feedback mechanism for regulating selective attention to certain sound
what is the startle reflex
- it is when you have loud unexpected sounds that cause flinching
- the reflex is mediated by reticulospinal fibres and fibres to the VII motor nucleus
what do fibres entering the V and VII motor nuclei do
- fibres enter the V and VII motor nuclei to link with motorneurnes that supply the tenor tympani and stapedius VII muscles in the middle ear
- the muscle exert a daring action on the middle ear ossciles
- tensor tympani is activated by ones own voice and stapedius by external sounds
what is the blood supply to the auditory tube
iddle ear branches of external carotid artery – ascending pharyngeal a. and branches of the maxillary artery - middle meningeal and pterygoid canal arteries.
what is the blood supply to the brainstem pathway from the cochlear nucleus to the inferior colliculus
supplied by the posterior inferior cerebellar artery PICA.
what is the blood supply to the medial geniculate nucleus
supplied by the posterior cerebral artery
what is the blood supply to the auditory cortex
- middle cerebral arteries
what id sound intensity measured in
decibles
a 10 db increase in sound represents
a ten-fold increase in intensity
when does sound become painful
Sound becomes uncomfortable to the ear at around 120db and frankly painful above 140dB.
what does prolonged loud sound do
- it kills the hair cells
- it causes deafness or hearing deficit and the louder the sound the quicker the loss
- the higher frequencies are the first to go
what lobe is the language centres in
left
what happens when the language areas are damage
aphasia
describe how the language centres work
- When a person hears a sentence this is transmitted via the auditory apparatus to the primary auditory cortex.
- This then connects to Wernicke’s area (in the temporal lobe), which decodes the language into meaning.
- If the sentence is to be repeated the information has to be transmitted forward to Broca’s area (expressive speech) in the posterior inferior part of the frontal lobe (via the arcuate tract).
- This area then produces speech via the motor programs of the motor cortex that activates the tongue and laryngeal muscles.
what are the two types of deafness
conduction deafeness
nerve deafness
what does conduction deafness result from
Results from any interference with the passage of sound waves through the external or middle ear e.g. wax build up in outer ear or otis media (inflammation).
describe conduction deafness
- bone conduction through cranial bones can still occur
- conduction deafness is nerve complete or total
what does nerve deafness result from
- results from damage t the receptor cells of the organ of Corti or the cochlear nerve
describe nerve deafness
- The defect is in the portion of the auditory mechanism common to both air and bone conduction and thus hearing failure by both routes occurs.
- Amount of loss depends on degree of damage to organ or nerve
how can you distinguish between nerve deafness and conduction deafness
- rhine test
- weber tuning fork test
why can you tell the difference between nerve deafness and conduction deafness
- there are differences in air conduction and bone conduction of sound
- sound transmitted by air conduction depends on the integrity of the middle ear while bone conduction can bypass the middle ear and activate the basilar membrane of the inner ear direclty
how does the Rhine test work
- Compares conduction by air & bone and determines their relative sensitivity.
- Vibrating tuning fork is held near ear & moved away until it is no longer heard.
- Then the stem of the fork is placed in contact with the mastoid bone.
- Normally the sound is heard louder & longer by air conduction.
what is the outcome for nerve deafness and conduction deafness in the Rhine test
- In perception deafness, the sound is heard better in the unaffected ear
- In conduction deafness, sound appears to arise from the deaf side because of the improved efficiency of bone conduction in the presence of middle ear damage.
how does the weber tuning fork test work
- Place tuning fork in middle of forehead & ask patient in which ear tone is heard. If hearing is equal then the sound seems to come from inside the head.
- Patient with unilateral nerve deafness hears tone in unaffected ear because it is more sensitive.
- Patient with unilateral conduction deafness hears the tone louder in the affected ear
what does the weber tuning fork test test for
for sound lateralisation
what is presbyacusis
when old people progressively lose hearing
- high frequencies are lost first
what are the common causes of deafness
- prebyacusi
- otosclerosis
- persistant loud noises
- infection
- viruses
- middle ear infection
what is otosclerosis
- most common cause of adult hearing loss
- autosomal dominant genetic characteristic - there is fusion of stapes to oval window which causes difficulty of movement and then eventually cessation of ear ossicle movement
- can be treated by sugary
what are examples of viruses that cause deafness
- mumps
- German measles during pregnancy can cause complete destruction of cochlear nerve in fetus
what are middle ear infections that can cause deafness
- Otitis media – bacterial infection causing swelling and outward bulging of tympanic membrane, pain and pus collection in middle ear.
- Most common in children as eustachian tubes are not fully formed and thus don’t drain well into nasopharynx
