Lecture 22/23: Auditory System Flashcards
Parts of the ear
- External
- Middle
- External and middle separated by ear drum
- Inner
- Fluid filled
- Cochlea which has hearing receptors; hair cells
What is the external ear innervated by?
Cranial nerves V, VII, and X
Major parts and border of the external ear
- Pinna or auricle
- External auditory meatus
- Ends at tympanic membrane or ear drum
What ear are you looking at if the light reflex is on the lower right?
RIGHT ear
Middle ear structure
- Air filled cavity behind drum
- 3 bones transfer vibrations to oval window with fluid behind
- Opens to outside world via eustasian tube
Inner ear structure
- Fluid filled sacs within bony sacs behind an oval window
- Has sensory hair cells for hearing and balance
What innervates hair cells in the inner ear?
Cranial nerve VIII
Embryonic origin of the ear
- Branchial cleft 1
- External acoustic meatus
- Branchial pouch 1
- Middle ear space and eustacian tube
- Membrane between cleft and pouch 1 is ear drum
- Branchial Arch 1
- Middle ear bones malleus and incus
- Branchial Arch 2
- Stapes
- Otic placode
- Otic vesicle –> Hair cells and CN VIII cells
Where do sensory axons of the ear (CN VII and X) terminate?
Chief nucleus of 5 and trigeminal nucleus
Innervation of ear drum
- Outer surface
- V3, VII, X
- Inner surface
- IX
Purpose of middle ear bones
Amplify and transfer sound pressure waves
Two duct systems of the inner ear
Vestibular bony duct and Cochlear bony duct
Three spaces of the cochlear bony duct
- Scala vestibuli
- Scala tympani
- Scala media (membranous labyrinth)
- Has the hair cells
Reissner’s membrane
Separates scala media from vestibuli
Stria vascularis
Pasted to the bony wall and secretes endolymph
Part of the membranous labyrinth
Basilar membrane
On which the auditory hair cells sit, separates scala media from tympani
Tectorial membrane
Sits over the hair cells and helps to activate them
Path of sound wave in inner ear
- Scala vestibuli
- Helicotrema
- Scala tympani
- Creates pressure difference in scala media and vibrates it
- Responds to highest frequencies at the base, lowest at top
Stereocilia in inner ear
When the cilia bend towards the tallest one (kinocilium), ion channel is pulled open and excites auditory nerve fibers
Where do auditory nerves terminate?
Cochlear nucleus in rostral medulla
How many rows of hair cells are there along the basilar membrane?
3 outer, 1 inner
What are outer hair cells thought to do?
- Change length when near threshold amplitudes are detected
- The hairs will change length and push on the membrane to make it vibrate more and cause more signal
What kills outer hair cells?
Aminoglycoside antibiotics
What nerve innervates the middle ear surface and inner surface of ear drum?
CN IX
What muscles help protect the ear against loud sound?
- Tensor tympani
- CN V
- Stapedius muscle
- CN VII
Main function of the middle ear
Amplify the sound waves coming in because water has a much higher density than air
How is endolymph composition special?
High in potassium low in calcium
Otoacoustic emissions
Vibrations of the ear drum received from the inner ear; caused by the outer hair cells changing length. Can be used to test ear function when the person can’t tell you anything.
How are nerves arranged in the internal acoustic meatus?
Facial nerve top, cochlear nerve bottom
Where do auditory nerve fibers terminate?
Dorsal and ventral cochlear nuclei at pons/medulla border
Where are the signals from both ears integrated?
Superior olive nuclei in the pons via the trapezoid body pathway
How do cells in the cochlear nucleus bypass the superior olive?
They use the lateral lemniscus to go straight to the inferior colliculus
Where do outputs from the superior olive go?
Inferior colliculus
Where do outputs from the inferior colliculus go?
Auditory thalamus or across the midline to the other IC
Auditory thalamus
- Also called the medial geniculate
- Responds to sounds in both ears
- Projects to ipsilateral auditory cortex
Tonotopic organization
Neurons are organized by frequency at every point along the auditory pathway
Wernicke’s Area
Language comprehension and word selection
Arcuate fasciculus
Communication between Broca’s and Wernicke’s areas
Output via motor cortex to LMNs
Lesion of Wernicke’s Area
Can’t understand questions
Speech has normal rhythm but is meaningless
Lesion of Broca’s Area
Can understand questions but can’t produce any speech
Lesion of arcuate fasciculus
Can understand language and produce speech but can’t repeat heard phrases
Conductive hearing loss
Sound physically can’t reach the inner ear. Obstruction or damage to structures like ear drum or ossicles
Sensorineural hearing loss
Abnormality in the cochlea or cochlear nerve (rarely cochlear nucleus)
Commonly due to loss of hair cells
Weber Test Results
- Louder in bad ear
- Conductive hearing loss in bad ear
- Possibly hair cells protected from outside interference by obstruction
- Lets the vibration be received better
- Louder in good ear
- Sensorineural hearing loss
- Hair cells are not being stimulated in bad ear
Rinne Test Results
- Easier to hear via air than by bone
- Tuning fork placed on mastoid bone
- Taken off when the pt can’t hear it anymore
- Asked if they hear it by air
- A normal ear will hear the sound again
- Conductive hearing loss won’t hear the sound again
- Doesn’t work for sensorineural hearing loss
Presbycusis
Age related sensorineural hearing loss
Auditory brainstem response
Can test infants’ hearing by presenting sound and recording brain waves
Tinnitus
- Perception of sound in the ear when none is present
- Causes
- Wax
- Otosclerosis
- HBP
- Allergies
- Diabetes
- Thyroid problems
- NSAIDs, antibiotics, sedatives, antidepressants
- Increased spontaneous hair cell firing
Objective tinnitus
Hearing sound from turbulent blood flow
Meniere’s Disease
- Rupture of Reissner’s membrane causing mixing of endolymph and perilymph
- Affects one ear usually
- Causes vertigo, imbalance, nausea, tinnitus, ear pressure, hearing loss
Mechanism of cochlear implants
String of electrodes inserted into cochlea. Electrodes are activated corresponding to different frequencies.
