NB9-4 - Auditory System and DLA Flashcards
C
Why are the middle ear bones necessary? Why cant the sound waves just directly vibrate the endolymph of the cochlea?
Up to 98% of acoustic energy is lost in transfer from air to liquid. The middle ear bones amplify sound through leverage and a reduction in surface area. Because of this, up to 67% (instead of 2%) of acoustic energy gets transmitted to the endolymph
There are also muscles attached to the middle ear bones that when contracted can protect the inner ear from damage from sustained lound sounds.
List the muscles of the sound attenuation reflex, what they are attached to, and which nerve innervates them.
Tensor Tympani - attached to the malleus and innervated by CN V
Stapedius - attached to the stapes and innervated by CN VII
List the aspects of the auditory pathway that we need to know.
- CN VIII carries sensory information from the spiral ganglion to the chochlear nuclei in the medulla
- From there many different nuclei become involved in the pons and midbrain but the signal is eventually delivered to the contralateral medial geniculate nucleus in the thalamus. Decussatoin does occur in the pons
- From the thalamus the signal is carried to the transverse temporal gyrus, aka Herschl’s Gyrus or BA 41/42, in the cerebral cortex
T/F - a central lesion does not typically cause unilateral hearing loss. Explain. What about a peripheral lesion?
True
Due to the very complex nature of the auditory pathway in the brain stem, a signal is almost always able to make it to at least one auditory cortex no matter where the central lesion is. Because only one auditory cortex is really needed to process the signal, bilateral hearing is maintained.
A peripheral lesion can easily coss unilateral hearing loss
Lesions to which structures are most likely to cause unilateral hearing loss?
- Structures of the ear
- CN VIII
- Cochlear Nuclei
Where are high and low frequencies heard within the cochlea?
High frequencies are sensed closer to the round window while lower frequencies are sensed closer to the helicotrema.
Which regions of the primary auditory cortex (A1) interpret high and low frequency sound?
The more medial portions of A1 interpret high frequency while the more lateral portions interpret low frequencies.
Where in the brain is Heschl’s Gyrus located?
What is the technical term for deafness?
Anacusia
What ear structures can be involved with conductive hearing loss?
Structures of the outer and middle ear
How is the Rinne test performed and what does it assess? How should the results be interpreted.
The Rinne Test tests for conductive hearing loss.
The stem of a vibrating 512Hz tuning fork is held against the mastoid process and you ask the patient to tell you when they can no longer hear the sound. You then hold the tuning fork in a perpendicular orientation about 1cm from their ear and ask if they can hear it again. If they can then there is no conductive hearing loss.
Normal - air conduction (AC) > bone conduction (BC)
Conductive hearing loss - BC > AC
B
B
How is the Weber test performed and what does it assess? How should the results be interpreted.
The Weber test assesses bone conduction
The stem of a vibrating 256Hz tuning fork is placed at the apex of the head (or central forehead) and the patient is asked if they hear the sound and if it’s equal in both ears.
Normal - both sides equal
Sensorineural Hearing Loss - sound louder in unaffected ear
Conductive Hearing Loss - sound louder in affected ear
A
C
A
B
What is an audiogram and what does a normal and abnormal one look like?
An audiogram is a read out of a test where a patient is asked if they can hear varying different frequencies and amplitudes of sound. Refer to image, the top left one is normal
