When using an audiogram to assess normal hearing what does the 0dB line represent?
Softest sound that a person with normal hearing ability would be able to detect at least 50% of the time
What is the range for normal hearing?
-10 to 20dB
What is the range for classifying:
a) Mild hearing loss
b) Moderate hearing loss
c) Severe hearing loss
d) Profound hearing loss
a) 20 - 40dB
b) 40 - 70dB
c) 70 - 100dB
What is the ratio of the tympanic membrane surface area to the oval window, and why is this the case?
More pressure must be concentrated in a smaller area as the oval window oscillates against the perilymph fluid which requires more force (versus air) to move
How can the Eustachian tube be opened?
- Tensor veli palatini muscle
- Levator palatine muscle
What is the purpose of opening the Eustachian tube?
Equalises pressure between the middle ear and external environment
Dysfunction in the Eustachian tube may lead to what?
Middle ear negative pressure
(and many conditions such as glue ear may follow)
What are the two openings in the cochlea which can be accessed via the middle ear?
- Oval window
- Round window
Which three chambers can be found within the cochlea?
- Vestibular duct (scala vestibuli)
- Tympanic duct (scala tympani)
- Cochlear duct (scala media)
Name the chambers A, B and C within the cochlear cross section
A - Scala vestibuli
B - Scala media
C - Scala tympani
Which chamber within the cochlea borders the oval window?
Vestibular duct (scala vestibuli)
Which chamber within the cochlea borders the round window?
Tympanic duct (scala tympani)
Which type of fluid do the following chambers contain?
a) Vestibular chamber (scala vestibuli)
b) Tympanic chamber (scala tympani)
c) Cochlear duct (scala media)
a) Perilymph - Vestibular chamber (scala vestibuli)
b) Perilymph - Tympanic chamber (scala tympani)
c) Endolymph - Cochlear duct (scala media)
What is special about the endolymph which can be found in the cochlear duct (scala media)?
It has a very high potassium concentration
Hair cells project into the cochlear duct
How does the perilymph differ from the endolymph in terms of its biochemical components?
Perilymph - High Na+ Low K+
Endolymph - High K+ Low Na+
Which organ lies on the basilar membrane of the ear?
Organ of Corti
What is the modiolus?
Conical shaped central axis in the cochlea
Consists of spongy bone
(cochlea turns approximately 2.5 times around it)
In the ear, what does the term transduction refer to?
Conversion of mechanical (bending of hair cells) energy to electrical impulse
How are the stereocilia arranged?
In height order
What is the significance of the stereocilia being arranged in height order?
Depolarisation will occur when stereocilia become deflected towards the longest
Hyperpolarisation will occur when the sterocilia become deflected away from the longest
After depolarisation of the Organ of Corti occurs, what happens next?
- CN VIII stimulated
- Central pathways carry impulse
- Activity occurs in superior temporal gyrus where the primary auditory cortex is found
What is Oscillopsia?
A condition causing the whole visual field to oscillate making focussing and balance very difficult
Nausea and vomiting is common
It is often due to the vestibulo-ocular reflex being lost
Which central pathways contribute to a sense of balance?
- Vestibulospinal tract
- Vestibulo-ocular reflex
What is the overall structure of the inner ear called?
What are the otolith organs in the ear?
Saccule and utricle
What are the 5 vestibular end organs in the inner ear?
Otolith organs (saccule and utricle)
Ampullae of lateral, posterior and superior semicircular canals
What is the otoconia?
Small crystals of calcium carbonate found in the saccule and utricle
When impacted in a certain direction, they stimulate the hair cells by their movement relative to the gelatinous supporting substrate (contains hair cells)
Which organs have stereocilia pointing into the gelatinous matric with otoconia?
Saccule and utricle
What causes movement to be detected?
Otoconia move with gravity
This impacts on the gelatinous matrix which in turn bends the stereocilia (act like tree branches in heavy snow)
This causes depolarisation or hyperpolarisation when the head is tilted in a certain direction
When will firing from the otolith organs stop?
(it always continues with a head tilt, but stops in the neutral position)