Auditory System Flashcards
Three compartments of the ear
Outer: external auditory meatus which magnifies sound by passive resistance. Pinna which is for localizing sound in vertical plane.
Middle: Malleus Incus Stapes, tensor typani (which improves high frequency hearing), stapedius (which pulls stapes off oval window
Inner: Cochlea, semicircular canals, otolith organs
Structure of cochlea
Bony cochlea: filled with perilymph (high Na, low K)
Membranous cochlea: filled with endolymph, more aqueous (high K, low Na)
Input/output of waves at cochlea
Input at scala vestibuli, output at scala tympani. Scala media in middle filled with endolymph
Organ of corti
One row of inner hair cells at basal membrane (95% of auditory nerve), three rows of outer hair cells.
Inner vs outer hair cells
Inner, actual hearing. Outer hair cells receive input from the superior olive via olivocochlear bundle, sharpen the sound frequency resolution by modulating the position of the tectorial membrane.
Outer hair cells receive input from?
Superior olive
Hair bundle displacement towards kinocilium? Away?
Towards = depolarization. Away = hyperpolarization
Structure of the basilar membrane
Wider and thinner as it winds in cochlea. Lower frequencies at end (helicotrema).
Where do aminoglycosides act?
Poison outer hair cells
Otoacoustic emissions
Vibration of outer hair cells
Cell bodies for auditory nerve located where?
In the modialus in spiral ganglia.
Do hair cells make synaptic contact with auditory nerve?
Yes
Auditory pathway
Cochlea inputs to cochlear nucleus, which projects bilaterally to two superior olives. Which project up to inferior colliculus via the lateral lemniscus, which project to MGN, which project to A1 in the temporal lobe (Heschel’s Gyri).
Is hearing binaural?
Yes of course, once you get past the cochlear nucleus. This serves to ensure that sound can be localized temporally and by intensity
Two ways to localize sound in superior olive
By intensity (high freq) and by timing (low freq).
Olivocochlear bundle
Projections from superior olive to outer hair cells to control auditory attention and noise masking
Lateral lemniscus
Projections from superior olive to inferior colliculus
heschel’s gyri
Receive input from MGN, projections duck under internal capsule. Also tonotopically encoded.
Semicircular canals sense…?
Where am I going?
Otolith organs sense…?
Which way is up?
Vestibulothalamocortical projections
Conscious perception of equilibrium
Vestibuloocular reflex
Maintains stable gaze
Vestibulospinal projections
Maintains postural equilibrium, head stability during body movements. Includes vestibulocolic.
Vestibuloautonomic reflex
Compensatory changes in BP, respiration, and digestion with changes.
Peripheral sensory apparatus
Semicircular canals and otolith organs, project to four vestibular nuclei
Three semicircular canals
Two otolith organs
Anterior, posterior, lateral. Utricle, saccule
Structure of semicircular canals
Bony structures that are filled with perilymph. Contain membranous sacs that are filled with endolymph.
Crista ampullaris
Sensory sheet within the ampulla that contains hair cells and supporting cells. These hair cells are lodged in a gelatinous cupula.
Bilateral coupling of semicircular canals
LARP
RALP
Horizontals together
Saccule detection
Vertical plane (which way is up?)
Utricle detection
Translation
Maculae
Sensory sheets of otolith organs that are covered in otoconia, which make the membrane heavier.
Scarpa’s Ganglion
Where the cell bodies of the vestibular nerve are located.
The names of the four vestibular nuclei?
Superior
Inferior
Medial
Lateral
Vestibulothalamocortical projections
Vestibular nerve projects to superior and lateral vestibular nuclei. This projects to VP of thalamus, which projects to the vestibular cortex located in the posterior parietal lobe.
Vestibuloocular Reflex
More to come on this!
Vestibulospinal tract
Vestibular nerve projects to lateral vestibular nucleus, which projects to all levels of the ipsilateral spinal cord via lateral vestibulospinal tract.
What uses medial vestibulospinal tract?
Vestibulocolic pathway, which maintains head stability during body movement. Medial vestibular nuclei project BILATERALLY to the cervical spinal cord (via MLF)
Vestibular system connections with the cerebellum
Very important.
Weber and Rinne
W: tuning fork on top of head. Lateralizes towards a conductive hearing loss, away from a sensorineural hearing loss.
R: Tuning fork behind the ear, AC>BC, normal or sensorineural. BC>AC, conductive hearing loss.
Noise induced hearing loss
audiogram shows sharp decline at 4K HZ
Meniere’s Syndrome
Episodic vertigo, lasts hours at a time. Created by too much fluid in semicircular canals, also causes hearing loss
Benign Paroxysmal Positional Vertigo
otoconia dislodge and drift into posterior semicircular canal. Diagnose with Dix Hallpike maneuver, correct with epley.
