W6: Audition Flashcards
define
Sounds
audible variations in air pressure
almost anything that can move air molecules can generate sound
state
Audible Frequency Range of Humans
20Hz - 20,000Hz
Frequency of Sound
number of compressed or rarefied patches of air that pass by our ears each second
One Cycle of Sound
the distance between successive compressed patches
Hertz (Hz)
units expressing sound frequency, number of cycles per second
kind of a trick question – so like not a number
At what speed to sound waves propagate?
All sound waves propagate at the same speed!
Pitch
Whether a sound is perceived to have high / low tone, as determined by frequency
higher frequency = higher pitch
lower frequency = lower pitch
Intensity / Amplitude
Property of a sound wave - the difference in pressure between compressed and rarefied patches of air
Sound intensity determines the loudness we perceive (louder sounds having higher intensity)
List
First Stages of the Basic Auditory Pathway
Pinna -> Ear Canal -> Tymp. Memb. -> Ossicles -> Oval Window -> Cochlea
- Sound waves move the tumpanic membrane (bubinek)
- Ossicles (maleus, incus, stapes) move the membrane (footplate) at the oval window
- Motion at the oval window moves the fluid in the cochlea
- Movement of the fluid in the cochlea causes a response in sensory neurons (hair cells -> spiral ganglion)
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Pinna
helps collect sounds from a wide area, its convolutions playing a role in localising sounds
define
Ear / Auditory Canal
entrance to the internal ear, extending ca. 2.5cm inside the skull before ending at the tympanic membrane
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Ossicles
Maleus, Incus, Stapes
Series of bones connected to the medial surface of the tympanic membrane.
Smallest bones of the body
latin for “little bones”
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Oval Window
Hole in the bone of the skull (covered by the footplate of the stapes)
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Cochlea
Spiral cavity of the inner ear (fluid-filled) behind the oval window, containing the apparatus for transforming the physical motion of the oval window into a neural response (TRANSDUCTION)
MGN
Medial Geniculate Nucleus
list
Components of the Middle Ear (3)
- ONE tympanic membrane
- TWO tiny muscles attaching to ossicles
- THREE ossicles
Eustachian Tube
Ossicle 1
Maleus (hammer)
attached to tympanic membrane
Ossicle 2
Incus (anvil)
Ossicle 3
Stapes (stirrup)
its movements transmit sound vibrations to cochlea fluids
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Role of Ossicles
sound force amplification
the fluid in the inner ear resists being moved much more than air does do more pressure is needed to vibrate the fluid than air can provide. The ossicles provide this necessary amplification in pressure
because
Pressure = force exerted / SA
state
Two Muscles Attached to the Ossicles
- Tensory Tympanu Muscle
- Stapedius Muscle
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Sound Attenuation
Response to the onset of a loud sound that triggers a neural response that causes these muscles to contract. When these muscles contract, the chain of ossicles becomes more rigid, and sound conduction to the inner ear is greatly diminished. Sound attenuations is much greater at low frequencies than at high frequencies.
role of cohclea - state
Cochlea
Plays role in transforming sound into a neural signal
list
3 Fluid-Filled Chambers of the Cochle
(TOP) Scala Vestibuli
(MIDDLE) Scala Media
(BOTTOM) Scala Tympani
define
Reissner’s Membrane
Separates scala vestibuli from scal media
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Basilar Membrane
Separates scala vestibuli from scala media
@ APEX, scala media is closed off, scala tympani becomes continuous with scala vestibuli at hole in membrane HELICOTREMA
@ BASE, scala vestibuli meets the oval window, scala tymoani meets the round window
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Organ of Corti
Sits upon basilar membrane and contains auditory receptor neurons
Three components:
- hair cells
- rods of corti
- supporting cells
deine
Tectorial Membrane
hangs over organ of corti
Define
Perilymph
Fluid in scala vestibuli and scala tympani with an ionic content similar to that of CSF (low K+, high Na+)
define
Endolymph
Fluid in scala media, extracellular fluid with an ionic content similar to that of intracellular fluid (high K+, low Na+)
define
Stria Vascularis
Endothelium lining one wall of scala media and contacting the endoly,ph, responsible for difference in ionic content which is generated by active transport processes at this site
Endocochlear Potential
2 (Related) Properties of Basilar Membrane
i. Wider at apex than base by factor of ca. 5
ii. Stiffness of membrane decreases from base to apex
Where does higher frequency get ‘collected’ on the basilar membrane?
Base
Where does lower frequency get ‘collected’ on the basilar membrane?
Apex
Place Code
where the vibration happens tells the brain what frequency the sound is
Tonotopy
Frequency-based organisation (like frequency map in the auditory system) – analogous to retinotopy in visual system
Auditory Receptor Cells
located in organ of Corti
converting mechanical energy into changes in membrane polarisation
Stereocilia
At tops of hair cells, extend above reticualr lamina into the endolymph, their tips ending either in the gelatinous substance of tectorial membrane (outer HCs) or just below tectorial membrane (inner HCs)
Organ of Corti Sandwich
TOP: tectorial membrane
MIDDLE: HCs
BOTTOM: Basilar membrane
with reticular lamina holding everything n place (goo)
with rod of corti spanning the layers, providing structural support
function
Receptor Potential
facilitates signal transduction
Steps
How transduction channels are believed to function
- Entry of K+ into hair cells via stereocilia, causing depolarisation
- Depolarisation activates voltage-gated Ca2+ channels
- Entry of Ca2+ triggers release of neurotransmitter glutamate (excitatory)
- Release of glutamate activates spiral ganglion fibers lying postsynaptic to the hair cell
Molecular ID of Ion Channels @ Tip of Stereocilia
Unknown! :D
Projections of Spiral Ganglion Neurons to HC Types (distribution)
- 95% of neurons connect to inner HCs (one HC to many neurons)
- 5% of neurons connect to outer HCs (many HCs to one neuron)
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Role of Outer HCs
Cochlear Amplification
Action of outer HCs on the basilar membrane whereby they seem to act like tiny motors that amplify the movement of the basilar membrane during low-intensity sound stimuli. This amplification system relies on 2 molecular mechnanisms.
2 Molecular Mechanisms on which Cochlear Amplification Depends
- Motor Proetins in Outer HC (prestin)
Prestin does not use ATP (cellular energy) but driven by receptor potential, makit it super speedy :))
- Mysosin in Tip Links
Contractile protein attached to tip links of stereocilia, may somehow rapidly enhance movement of hairs in response to weak sounds
Nerve via Which Afferent Fibers Travel From the Spiral Ganglion
Auditory-Vestibular Nerve (Cranial Nerve VIII)
@ Level of Medulla, Axons Synapse…
- Dorsal Cochlear Nucleus
- Ventral Cochlear Nucleus
each axon branches to innervate both cochlear nuclei ipsilateral to the cochlea where the axons originated
Primary Route from Cochlear Nuclei to Auditory Cortex
Ventral Pathway
Characteristic Frequency (CF)
Frequency at which a neuron is most responsive
Frequency Tuning
Neurons show increased firing rates when stimulated at their CF, and diminished firing at neighbouring frequencies.
LOCATION-BASED SENSITIVITY
2 Interrelated Ways in Which Sound Intensity is Encoded
- Firing Rates of Neurons
- Number of Active Neurons
the loudness we perceive is correlated with the number of active neurons in the auditory nerve (and throughout the auditory pathways) and with the firing rate
Frequency Sensitivity
Largely a consequence of the mechanisms of the basilar membrane, because different portions of the membrane are maximally deformed by the sound of different frequencies
Nature of the Tonotopic Map in the Auditory Nerve
Corresponds with HCs
- Auditory nerve fibers connected to HCs near apex have low CFs
- Auditory nerve fibers connected to HCs near base have high CFs
define
Phase-Locking
Consistent firing of a cell at the same phase of a sound wave, shown in recording made from neurons in the auditory nerve
frequency of sound = frequency of neuron’s APs
happens with up to 5 kHz, then not phase-locked and just tonotopy
define
Volley Principle
It is likely that intermediate sound frequencies are represented by pooled activity of number of neurons, each of which fires in a phase-locked manner
Duplex Theory of Sound
The 2 processes:
- ITD
- IID
Interaural Time Delay (ITD)
For sounds in range 20 Hz - 2000 Hz
Difference in time it takes for sound to reach each ear. When a stimulus moves, sound will reach one ear before the other.
Interaural Intensity Difference
For sounds in range 2,000 Hz - 20,000 Hz
Exists between the 2 ears, because the head effectively casts a sound shadow with a direct relationship between the direction from which the sound comes and the extent to which your head shadows the sound to one ear
Monaural Neurons
Only respond to sound presented to one ear
Binaural Neurons
Responses are influenced by sound at both ears, present at all later stages of processing in the auditory system & important for sound localisation in the horizontal plane
First Structure in Pathway at Which Binaural Neurons are Present
Superior Olive
Delay Lines
Arrival of a spoke from one side is delayed just enough that it coincides with the arrival of a spike from the other side. By arriving at precisely the same time, the APs summate producing a strong EPSP.
Key Player in Vertical Sound Localisation
The pinna!!!
its convolutions :))
Primary Auditory Cortex (names + location)
- A1
- Brodmann’s Area 41
- Temporal Lobe
define
Acoustic Radiation
Pattern of Tonotopic Representation in the A1
- Low frequencies => rostral & lateral regions
- High frequencies => caudal & medial regions
Isofrequency Bands
Run mediolaterally across the A1, containing neurons with fairly similar CFs
Is there a simple classification for the auditory cortical neurons?
NO!!!
They have diverse response properties and mostly intermixed!!
Pattern of Stimuli Processing, Lower to Higher Levels of Auditory Pathway
Lower Levels = deal with simpler stimuli
Higher Levels = deal with more complex stimuli
e.g. Wernicke’s area, not so much processing sound, but ability to interpret spoken language
2 Components of the Vestibular System + Function
- Otolith Organs
- Semicircular Canals
purpose of both = transmit mechanical energy derived from head movement to its hair cells
Otolith Organs
Detect force of gravity & tilts of head
- Sacule
- Utricle
Linear Acceleration
Semicircular Canals
Sensitive to head rotation
- three arcing structures of the labyrinth
Angular Acceleration
Vestibular Macula
NOTHING TO DO WITH RETICULAR MACULA
Push-Pull Arrangement
Primary Vestibular Axons (from cranial nerve VIII) Connect to…
- Medial & Lateral Vestibular Nuclei (same side of brainstem)
- Cerebellum
Vestibular Ocular Reflex (VOR)
Keeps eyes fixated on target, senses rotations of head and immediately commands compensatory movement of eyes in opposite direction - not triggered by visual inputs so good in dark!!
