Auditory system Flashcards
What are the differences between inner and outer hair cells?
Inner - Stereocilia are loosely attached to the tectorial membrane. Approx 3500. The code the frequency and intensity of acoustic stimuli.
Outer - Stereocilia are embedded within the tectorial membrane. Approx 12000 organised in 3 rows. Allow cochlear amplification - change in length in response to membrane vibration in order to amplify sound and increase frequency discrimination.
What is the difference between type 1 and 2 afferents?
Type 1
- Many synapse with a single IHC
- The different axons will have differing sensitivities - range fractionation. - The true auditory afferents
- Axons are well myelinated
- Larger cell bodies
- Have AMPA receptors
Type 2
- Contact up to 100 OHCs
- Only fire APs in response to intense stimulation
- Central axon is weakly myelinated
- Makes local circuits which resynapse with OHCs - little is known about this role.
What are the efferent inputs to the spiral organ?
What is their function?
MOC efferents
- Are frequency tuned.
- Provide inhibitory input directly to OHCs to reduce their electromotility. They increase the signal:noise ratio and help to unmask sounds. They also help in protecting against hearing loss.
- Maison et al (2013) - lesioned MOCs in mice then exposed them to a 8-16kHz sound at 84dB for 1 week - destruction of OHCs at base (high frequency) and consequently a decreased sensitivity to high frequency sounds vs controls.
LOC efferents
- Synapse onto type 1 afferents, thus indirectly affecting IHCs.
- Protecting form noise induced hearing loss. - Liberman et al 2014 - LOC efferents protect the basal 1/2 of the cochlea, MOC efferents the apical 1/2.
What are the causes of stereo cilia /hair cell depolarisation?
As the tectorial and basilar membranes are hinged and different points, membrane movement from changes pressure causes a shearing force on the hair cells.
If the stereo cilia, whose tips are in contact with the tectorial membrane, are deflected towards the kinocillium, tip links are stretched and more MT (mechanotransducer) channels are opened. –> cations (mainly K+) flow into the cell causing depolarisation –> when the cell reaches threshold it will open VGCCs –> release of NT.
How do hair cells adapt?
Slow adaptations
- Involves adjusting the height of the tiplink insertion points, through movement of the Upper Tiplink Density (UTLD) complex; core components of which are myosin VIIa and harmonin b
- To adapt to depolarisation the myosin motor moves down, reducing tippling tension.
Fast adaptations
- When Ca2+ is high it binds directly to, or via an accessory protein, to the MT channels, altering her sensitivity. This makes them more difficult to keep open.
What cells produce endolymph?
Where are they found?
What is endolymph’s composition?
a) Cells in the stria vascularis
b) Scala media
c) High in K+, low in Na+
What are the 3 parts of the cochlear nucleus and their function?
Dorsal cochlear nucleus
- Complex frequency analysis - have complex coding characteristics.
- Receive a variety of ascending and descending inputs.
Anteroventral cochlear nucleus
- Spherical cells from here project to the ipsilateral LSO
- Globular cells project to the medial nucleus of the trapezoid body (MNTB) - intramural intensity difference circuit.
Posteroventral cochlear nucleus
- brief analysis of speech sounds - contains octopus cells
Ventral cochlear nucleus
- Multipolar cells
Define the What and Where pathway for the auditory system.
What evidence is there for this?
Ventral pathway - what/identification - concerned with recognition of familiar sounds to speech (latter a function of the dominant hemisphere) - forms a part of Wernicke’s area.
Dorsal pathway - where - involved in spatial aspects of audition- superior and middle temporal gyrus–> supra marginal gyrus in the parietal lobe + via the arcuate fasiculus Broca’s area.
fMRI in humans
electrode recording in monkeys along the belt areas (Tian et al 2001) - neurons in ant belt for call identification and caudate belt for spatial selectivity
How do the functions of the primary auditory complex (A1), belt areas and parabelt areas differ?
A1
- Wang et al found an area that was selective for marmoset twitter calls but not synthetic vocabulary - e.g time expressed or time reversed. Suggests may contain neurons for natural species specific sounds
Belt areas
- Neurons here are especially selective for species specific sounds.
- major lesions can –> verbal auditory agnosia
Parabelt areas
- Involved in semantics and phonological structure of a learned language. Also involved in integration of hearing with other areas such as wernicke’s.
A1 is tonotopically mapped. It contains neurons with the same preferred frequency (iso frequency bands) with this responding to lower frequencies more rostral, and higher frequencies more caudal. Also has perpendicular striped arrangement of binaural properties.
What’s the difference between harmonics, octaves and fundamental frequency?
How does fundamental frequency convey pitch?
Fundamental frequency f0
- The lowest harmonic in a periodic waveform
Harmonic
- The fourier components of a complex tone, which are integer multiples of f0. 1st harmonic is f0.
Octave
- The doubling of a frequency, and so if 1st frequency was 300Hz, next octave would be 600hz.
The higher the fundamental frequency the higher the pitch.
Which variable domains of sound make up a spectrogram?
A frequency domain plot utilises only 2 domains. Which 2?
Time, Frequency and Amplitude.
Frequency and amplitude
What does the term phase locking mean?
This is only possible up to a what frequency? Why?
AN axons fire in synchrony with the peaks in the waveform for a complex periodic sound.
Approximately 3kHz - AN nerve firing can no longer keep up.
Which parameter of the acoustic signal is the sensation of pitch most related to?
What is it?
The periodicity of the waveform.
The period of repetition of a waveform, which can be distinct from it’s frequency.
When phase-locking is no longer possible, what kind of code is the acoustic signal turned in to?
Takes a DC pattern.
Describe the auditory pathway in 5 steps.
Cochlea –> cochlea nuclei in the medulla –> second order neurons ascend through the inferior colliculus –> synapse onto the MGN of the thalamus –> 3rd order neurons up to the superior temporal gyrus (area A1 - primary auditory cortex)
