4.3 Hearing and Balance Flashcards
What property of a sound wave determines loudness?
Amplitude
What property of a sound wave determines pitch?
Frequency
What is the Confusionist name for earwax?
Cerumen
Describe the two sections of the external acoustic meatus
- Cartilaginous (outer 1/3)
- Bony (Inner 2/3)
Where is cerumen produced?
Cartilaginous component of external acoustic meatus
Function of cerumen
Stop foreign bodies/bacteria entering into middle and inner ears
What is the boundary between the external and middle ear?
Tympanic membrane
What is the purpose of the middle ear? Why is this important?
- Amplify the sound waves before they enter the fluid-filled cochlea
- Sound is dampened by fluid, so must be amplified to compensate
What is the hollow portion of the middle ear called?
Tympanic cavity
What are the three bones of the middle ear (and their more useful names)?
- Malleus (Hammer)
- Incus (Anvil)
- Stapes (Stirrup)
Which two muscles are associated with which two ossicles, and which nerves innervate them?
- Tensor Tympani (Malleus) CN V3
- Stapedius (Stapes) CN VII
What are the components of the bony labyrinth of the inner ear?
- Cochlea
- Vestibule
- Semicircular canals
What are the components of the membranous labyrinth of the inner ear?
- Cochlear duct
- Utricle
- Saccule
- Semicircular ducts
What fluids are located in the membranous/bony labyrinth?
Bony: Perilymph
Membranous: Endolymph
Describe the flow of sound through the different windows/chambers of the cochlea
- Stapes transmits sound through oval window into perilymph of scala vestibuli
- Flows to apex (i.e., centre), and then flows out through scala tympani
Why does sound travel from the apex back through the scala tympani and out of the oval window? Why couldn’t it just stay where it is?
We need to relieve the buildup of pressure, otherwise your cochleas will explode (which is slightly suboptimal).
What are the three membranes of the scala media?
- Basilar (which is embedded with hair cells)
- Tectorial (which the stereocilia come into contact with)
- Vetibular
What fluid is in the scala media (hint: is the scala media located in the membranous or bony labyrinth?)
Endolymph
What is the organ of Corti made of, and where is it?
It is made up of the hair cells that are embedded in the basilar membrane of the scala media of the cochlea of the ear.
Describe how stereocilia movement causes neurotransmitter release, with reference to endolymph.
- Endolymph is rich in K+
- Contact with tectorial membrane opens mechanically gated potassium channels on tips of stereocilia
- K+ rushes in, opening voltage-gated Ca2+ channels in base of hair cells
- Triggers neurotransmitter release
What’s the deal with auditory afferents, the spiral ganglion, and the cochlear nerve?
- Sensory neurons have axons that branch up to base of hair cells in organ of Corti
- Their cell bodies are located in a nearby spiral ganglion
- Since they are bipolar, the second part of their axons go on top form the cochlear nerve, which then joins CN VIII
What is the dorsal/ventral cochlear stream important for?
Dorsal: quality of sound (like bat vs bet vs debt)
Ventral: timing
Outline the ventral cochlear stream pathway
- Ventral cochlear nuclei
- Some decussate at trapezoid body
- Superior olivary nucleus
- To Inf. Colliculi (via lateral lemniscus)
- To MGN (via brachium of inf. colliculus)
- To A1 (via corona radiata and internal capsule)
Outline the dorsal cochlear stream pathway
- Dorsal cochlear nuclei
- Decussates at pontine tegmentum
- Inf. Colliculi (via lateral lemniscus)
- To MGN (via brachium of inf. colliculi)
- To A1 (via corona radiata and internal capsule)
Where must a neurological lesion be in order to cause unilateral hearing loss? Why?
Must be proximal to cochlear nuclei - after that point, everything is bilateral
In which Gyrus is the primary auditory cortex located?
Herschl’s gyrus (superior temporal gyrus)
True or false: the primary auditory cortex is tonotopically organised.
True. Like the basilar membrane.
Where is the auditory association cortex? What does it do?
It surrounds A1, cortically. It is responsible for recognition of sounds (e.g., based on past experience)
What fasciculus connects Wernicke’s and Broca’s?
The Arcuate Fasciculus
Using a mnemonic, recall the passage of nervous auditory signals
E: Eighth cranial nerve
C: Cochlear nucleus
O: Olivary nucleus (superior)
L: Lateral lemniscus
I: Inferior colliculus
M: MGN
A: Auditory nucleus
(ECOLIMA)
What are the two kinds of acceleration tracked by the vestibular system? What tracks them?
- Angular acceleration (rotation): anterior, posterior, and horizontal semicircular canals
- Linear acceleration (along a line): saccule (up/down), and utricle (forward/backward)
To which part of the vestibulocochlear apparatus does the stapes transmit force?
The vestibule
Describe how the semicircular canals detect rotational acceleration
- Endolymph sloshes when movement is in same plane as canal
- Sloshes against cupula
- Pushes hair cells one way = excitation, another way = inhibition
- Therefore, the brain can interpret the pattern of excitation/inhibition as vestibular information
Describe how otolith organs detect gravity and other causes of acceleration
- Each organ contains otoliths (calcium carbonate crystals), and hair cells, separated by a gelatinous layer
- In response to acceleration, the crystals are pulled on, displacing gelatinous layer and activating hair cells
What are the two sets of vestibular nuclei in the brainstem? What tracts do they form?
- Lateral vestibular nuclei (form lateral vestibulospinal tract)
- Medial vestibular nuclei (form medial vestibulospinal tract)
What is the medial vestibulospinal tract important for?
Controlling head and neck position
What is the lateral vestibulospinal tract important for?
Maintaining balance and extensor tone
Where does the medial vestibulospinal tract terminate?
Cervical spine
