NEUR 0010 - Chapter 11 Flashcards
What is the outer ear?
Pinna and auditory canal
What is the middle ear?
TM and ossicles
What is the inner ear?
Oval window and structures medial to it; cochlea
What is the thalamic relay center for audition (like the LGN)?
MGN
Where is the primary auditory cortex located?
Temporal lobe
What is the Eustachian tube?
Usually closed; descends from the ossicles in the middle ear; continuous with nasal cavity
Why is it a “problem” that the cochlea is fluid-filled?
Fluid is harder to move by sound waves; needs the ossicles to amplify things
What is the attenuation reflex of the ear?
At the onset of loud sound, muscles attached to ossicles make their movement more rigid, diminishing sound conduction and preventing damage
What frequencies is sound attenuation greatest at?
Low frequencies; not high frequencies
Why is attenuation good for speech recognition?
“picks out” the high-frequencies speech out of low-frequency noise
What happens when auditory spiral ganglion enter the brainstem?
Enter the brainstem via auditory-vestibular nerve; enter the medulla’s dorsal and ventral cochlear nuclei, ipsilateral to where they originated (branches to reach both), then goes through multiple parallel pathways
What happens when cells in the ventral cochlear nucleus project their axons to the superior olive of the brainstem?
Axons of superior olivary neurons project to the lateral lemniscus, then to the inferior colliculus in the midbrain
How does the dorsal cochlear nucleus’s projections to the inferior colliculus differ from the ventral cochlear nucleus’s?
Bypasses the superior olive
Where do all ascending auditory pathways converge?
Inferior colliculus
Where do auditory neurons in the inferior colliculus project their axons to?
The MGN in the temporal lobe, and then to auditory cortex
What is different about cochlear nucleus input from the rest of the other auditory nuclei in the brain stem?
Cochlear nuclei receive input from ipsilateral ear only; other brain stem auditory nuclei receive input from both ears
What is the only way brain stem damage can produce deafness in one ear only?
Damage to the cochlear nuclei
Why do most spiral ganglion cells respond to sound in a limited frequency range?
Because they receive input from only one inner hair cell and so fire APs only when that specific portion of the basilar membrane vibrates
What is characteristic frequency?
The one frequency at which an auditory neuron is most sensitive
What are the two ways sound intensity is encoded?
Firing rates of neurons, number of active neurons
What happens to the basilar membrane vibrations as sound intensity increases?
Greater amplitude of vibrations: causes membrane potential to be more de/hyper polarized, greater rate of AP firing; Greater distance of vibrations: activation of more hair cells, so broader frequency range for responses
What is tonotopy?
Systematic organization of characteristic frequency within an auditory structure
What part of the basilar membrane is more sensitive to high frequency vs low?
High at base, low at apex
What is phase locking?
The consistent firing of a cell at the same phase of a sound wave; fires APs at either peaks/troughs/etc. of the wave
What is the volley principle?
When intermediate sound frequencies are represented by the pooled activity of a number of neurons, each of which fires in phase-locked manner
Why are frequencies above 4Hz represented to tonotopy alone?
Too fast for APs to accurately represent timing; phase locking can’t occur; APs must fire at random phases
What are the two mechanisms used to localize horizontal sound?
Interaural time delay and interaural intensity difference
What causes interaural intensity difference?
Sound shadowing by your head
Which stages of the auditory pathway are monaural vs binaural?
Only the neurons in the cochlear nuclei are monaural; everything else is binaural
Where is the first structure that has binaural hearing?
Superior olive
What is the main contributor for vertical sound localization?
The reflections of the pinna; angles back in due to the little nub in the ear :)
How do neurons in the MGN reach the cortex?
By extending axons through the auditory radiation
What is the laminal cell layout of the auditory cortex?
From Layer 1 to 6: few cells bodies – small pyramidal – small pyramidal – granule – large pyramidal– large pyramidal
What columns are present in auditory cortex?
Frequency-dominant columns
What are isofrequency bands?
Run across A1, with similar characteristic frequencies
What are the two main organizational principles of the auditory cortex?
Tonotopic organization; columns with similar binaural interaction
Mammalian organs of balance/hearing evolved from what structure?
Lateral line organs
How are all hair structures connected in mammals?
By the vestibular labyrinth!
What are the two main structures of the non-auditory vestibular labyrinth?
Otolith organs and semicircular canals
What are otolith organs?
Detect force of gravity, head tilt
What are semicircular canals?
Sensitive to head rotation
What are the otolith organs?
Saccule and utricle
What is Scarpa’s ganglion?
Location of cell bodies from vestibular nerve axons
What is the otolith organ’s macula?
Sensory epithelium, vertically oriented in the saccule, horizontally oriented in the utricle
What are otoliths?
Little calcium carbonate crystals on top of the gelatinous cap of the orolith organs; exert pressure with movement, creating a force on the gelatinous cap and moving the hair bundles
What a kinocilium?
In otolith organs: one especially tall cilia that cause depolarizing excitatory receptor potential when bent
What is the ampulla?
Bulge in the semicircular canal; location of clusters of hair cells within a sheet of crista
What is the cupula?
Gelatinous area in the middle of the ampulla
What is the vestibulo-ocular reflex?
Keeps your eyes pointed in a particular direction throughout other motion; automatically instructs extraocular muscles to keep the eye in place
What is the predominant ionic composition of perilymph?
Low K+, high Na+
What is the predominant ionic composition of endolymph?
High K+, low Na+
What is the endocochlear potential?
80 mV
What causes the endocochlear potential?
Stria vascularis pumps in K+ and pumps out Na+
What do the rods of Corti do?
Span the distance between basilar membrane and reticular lamina: provide structural support
Where do hair cells synapse in the ear?
In neurons whose cell bodies are in the spiral ganglion in the modiolus
What kind of neurons are spiral ganglion cells?
Bipolar
How do TRPA1 channels make stereocilia very sensitive to soft sounds?
Connected to elastic filament tip links to adjacent cilium
How does a tip link work?
When the cell is bent, the tip link is stretched/compressed, increasing or decreasing K+ influx and causing depolarization/hyperpolarization by activating Ca++ channels
Why does K+ influx depolarize hair cells, unlike in most other cells?
Unusually high K+ concentration in endolymph, causing Ek to be 0, also high voltage gradient favoring it
Which type of hair cell can share a single spiral ganglion cell?
Outer hair cells
How many spiral ganglion cells does one inner hair cell feed?
Many!
Why are outer hair cells cochlear amplifiers?
Motor proteins called proteins change the outer hair cell length; so outer hair cells respond with change in length and change in receptor potential, allowing sound signal amplification
