Hearing Flashcards
Name the 4 divisions of the auditory system
Outer ear, middle ear, inner ear and central auditory nervous system
What is the overall function of the outer ear?
Takes in air vibrations and works to protect the air alongside localising and amplifying sound
What is the overall function of the middle ear?
Takes in mechanical vibration and is involved in impedance matching, pressure equalisation and inner ear stimulation
What is the overall function of the inner ear?
It takes in mechanical, hydrodynamic and electrochemical stimulation and is involved in sound filtering and signal transduction
What is the overall function of the central auditory nervous system?
It takes in electrochemical stimulation and is involved in information processing
What is involved in impedance matching?
It involves the Eustachian tube where the differences in area between the eardrum (tympanic membrane) and the oval window are compensated for, between the air and cochlear fluid
Which component of the inner ear is involved in amplification of sound signals?
Outer hair cells
How is transduction facilitated in the inner ear?
The mechanical signals are converted into electrical signals by the opening (depolarisation) or closing (hyperpolarisation) of potassium channels, depending on the direction of hair movement
Describe the flow of sound/vibration in the inner ear
Stapes (bone) –> oval window –>perilymphatic fluid in the cochlear (vibrates) –> round window
What is the difference in composition between the endolymph and the perilymph?
Endolymph is high in potassium whereas perilymph is high in sodium
How many canals are there in the cochlea?
Three
Name the three canals within the cochlea
Scala vestibuli, Scala tympani and Scala media
What type of fluid is in the scala vestibuli and scala tympani?
Perilymph (high in sodium)
What type of fluid is in the scala media?
Endolymph, as it contains the organ of Conti
Where is the Organ of Conti located?
In the scala media of the cochlea
Describe the structure of the organ of Conti
There is the upper membrane (tectorial membrane) and the lower (basilar membrane) and there are hair cells inbetween these
Describe the structure and function of inner hair cells in the organ of Conti
The inner hair cells are the inner one layer of cells in the organ of Conti and these cells form 95% of the cochlear nerve (auditory nerve) and are involved in auditory discrimination
Describe the structure and function of outer hair cells
These cells consist of the 3 outer rows (nearer to basal side) which have cilia embedded in the tectorial membrane and are involved in frequency tuning (amplify the basilar membrane vibrations, so enhance sensitivity and frequency selection)
How do we differentiate sounds at different frequencies?
Movement from vibration in perilymph (cochlea) reaches organ of Conti –> vibration of basilar and tectorial membranes which stimulates certain hair cells –> these cells depolarise and stimulate cochlear aspect of CN VIII, and brain will relay pitch based on the hair cell that is stimulated
How do we localise sound (direction)?
We use timing and loudness - the superior olivary nucleus uses interaural time differences and interaural level differences in order to localise sound cues
Which nucleus is involved in the localisation of sound direction?
Superior olivary nucleus
What is meant by pitch/frequency?
The number of oscillations per second
What is meant by intensity/loudness?
Height of the sound wave
What is loudness/intensity measured in?
Decibels (dB)
Name the 5 main brain structures involved in the auditory pathway
Cochlear nucleus, superior olivary nucleus, inferior colliculus, medial geniculate nucleus and primary auditory cortex
Describe the auditory pathway
The cochlear primary nerve afferent travels through internal auditory meatus to cochlear nucleus –> some transmitted to superior olivary nucleus –> rest transmitted to inferior colliculus –> transmitted to medial geniculate nucleus –> primary auditory cortex
Where is the superior olivary nucleus found, and what is its function?
In the pons and it is involved in spatial localisation
Where is the cochlear nucleus located, and what is its function?
It’s located in the medulla, and the ventral aspect is involved in intensity and location of sound whereas the dorsal aspect is involved in pitch and quality
Where is the inferior colliculus located?
Midbrain
Where is the medial geniculate nucleus located?
Midbrain
Where is the primary auditory cortex located?
Cerebrum
Which limbic areas are activated by sound?
Medial pre-frontal cortex, nucleus accumbens, orbitofrontal, amygdala and midbrain
Where does CN VIII get it’s auditory information from?
Cochlea
Where does CN VIII get its vestibular information from?
Semicircular canals, utricle and saccule
Describe the ventral cochlear nucleus
This is very fast and precise and projections go to the superior olivary nucleus and stellate cells code intensity whilst bushy cells encode timing
Describe the dorsal cochlear nucleus
Complex responses are facilitated here, where axons project to the lateral lemniscus and encode quality of sound and analyse pitch
What is meant by conduction deafness?
When air conduction is impeded such as is the case with damage to the ear itself
What is meant by sensorimotor/nerve deafness?
A type of deafness whereby there is nerve damage (brain or cochlear nerve etc.) or if you damage the cochlea itself
Describe how you would conduct Rinne’s test
Place a vibrating tuning fork on the mastoid process (just behind the ear) and then when they can no longer hear it put it next to their ear and see if they can hear it now
What is a normal/non-pathological result for Rinne’s test?
Sound is heard louder and longer by air conduction (>15 seconds)
What would suggest conduction deafness in Rinne’s test?
If the bone conduction is better than the air conduction on the affected side
What would suggest sensorimotor deafness in Rinne’s test?
If air conduction is better than bone conduction in the affected ear but sound is loudest in the unaffected ear
Name the two major tests used to diagnose deafness
Rinne’s test and Weber’s test
Describe how you would conduct Weber’s test
Place a vibrating tuning fork in the middle of the forehead and ask the patient if the sound is the same in both ears
Describe a normal result for Weber’s test
Sound is heard equally well on both sides
Explain how conduction deafness may present from Weber’s test
The sound is usually heard louder in the affected ear due to the fact the vibrations reach cochlea by both air and bone, and in affected ear these two are out of tune so interfere more in the sensitive ear
Describe how sensorimotor deafness would present from Weber’s test
The sound would be heard more loudly in the unaffected ear
Give some examples of middle ear disorders
Perforation of tympanic membrane, glue ear (otitis media) or otosclerosis
What can middle ear disorders lead to?
Conduction deafness
What are the two most common tumour types of the internal auditory canal, cerebellopontine angle and skull base?
Schwannomas and meningiomas
Describe schwannomas
Benign neoplasms which arise from the sheaths of cranial nerves
Name some of the symptoms of a vestibular schwannoma
Unilateral hearing loss, tinnitus and disequilibrium
Where do meningiomas most commonly occur?
Cerebellopontine angle
How do meningiomas develop?
They develop from the arachnoid layer of the meninges
What are epidermoids?
Type of rare intracranial mass where patients become symptomatic in a gradual manner; symptoms are related to mass location in the cranium
What is the function of Wernicke’s area?
Involved in fluent and meaningless speech and therefore any damage causes severely impaired speech understanding
What is the function of Broca’s area?
Involved in abbreviated and ungrammatical meaningful speech and therefore damage causes impaired speech understanding where the syntax conveys the meaning
What is the arcuate fasciculus?
Tissue that connects Broca’s and Wernicke’s areas and therefore damage to this will result in global aphasia
What may the result be of damage to the angular gyrus?
Can lead to alexia (can’t read) with agraphia (can’t write) but the individual can comprehend speech and talk themselves
Which part of the brain is abnormal in dyslexics?
Angular gyrus
What is the purpose of auditory reflexes?
Prevent damage to the eardrum and distinguish sounds from the background
Describe what is meant by the ‘attenuation reflex’
This is activated by your own voice/loud sounds and causes contraction of muscles in the ear in order to dampen the transmission from middle to inner ear to prevent damage. This reflex is also stimulated by low frequencies so that higher frequencies can be discerned
What is a weakness of the attenuation reflex?
There is a delay of 50-100ms after the stimulus and therefore doesn’t offer much protection from very sudden loud sounds
What happens if there is damage to the attenuation reflex pathway?
Hyperacuisis results
Which parts of the central nervous system does the blink/startle reflex involve?
Facial nerve and reticular formation
Describe the function of the blink reflex
Works to protect the back of the neck (whole body startle) or the eye (blink), and this is a rapid reflex
Where is the auditory cortex located?
Temporal lobe in the superior temporal gyrus
What are the functions of the auditory cortex?
Identify complex auditory sounds, detect changes in auditory environment, learn about behaviour-related sound, integrate attention with auditory processing, and higher functions such as language and musical interpretation
Describe the main functions of the auditory system
Hearing - distinguishing different sounds and localisation of sounds
Language - production and comprehension of language
