Sensory Four - Hearing part two Flashcards
What does central auditory processing lead to?
Central processing of the auditory nerve activity leads to the perception of:
- Intensity (dB)
- Pitch (Hz)
- Localisation of sound.
Whats the final point in the auditory pathway?
The auditory cortex, where hearing, speech and language centers overlap.
What does the central auditory pathway being with?
The spiral ganglion
The spiral ganglion neurons run from the cochlear to:
Through the 8th cranial nerve (vestibulocochlear) to first relay nuclie - which is the Dorsal+Ventral cochlear nucleus.
Where do fibers go from the dorsal+ventral cochlear nucleus?
Ventral Cochlear nucleus -> Sup. Olive complex Left hemishere and on the right hemisphere
Dorsal Cochlear nucleus -> inferior colliculus (bypass via the lateral laminiscus pathway)
Where do fibers go from the sup olive complex on the left and right side?
Left side side goes to the left inferior colliculus
Right side goes to the right inferior colliculus
From the inferior colliculus where do the fibers go?
MGN Medial Gentate Nucleus then on to the auditory cortex in the temporal lobe
What other fibres project into the auditory pathway?
Fibers from the:
Superior Colliculus
Feedback from the auditory cortex - to control and regulate OHC response.
What point in the auditory pathway do the nuclei receive input from both ears?
Sup. olivary complex (first relay nuclei to do this)
Why is the point where the auditory pathway starts to receive input from both ears important clinically?
AS from this point onwards damage to the pathway results in hearing loss in both ears
How does the auditory nerve encode frequencies?
Each auditory nerve responds to a specific frequency with maximum firing rate (go away from this Hz and firing decreases) This property is known as characteristic Hz
How do auditory nerves encode for intensity?
At a constant Hz higher intensity produces higher rate of firing over low intensity sound
How else is intensity encoded?
Firing rate of neurons increases and number of neurons firing increases also when the intensity of sound increases.
Why does a higher intensity sound result in more neurons being recruited?
A higher intensity sound will result in greater movement of the basilar membrane therefore activation of more OHC+IHC leading to the activation of more neurons.
What is the basilar membrane also known as?
The mechanical analyzer of frequencies.
How is the basilar membrane organized?
Tonotopically
The Base is thin and high Hz are detected here while the apex is wide and low Hz sounds are detected here
I.e the neurons innervating each part will have their highest firing rate for the Hz that results in maximum amplitude of that part of the basilar membrane.
How is the cochlear also tonotopically organized?
High Hz detected more medially than lower Hz which is more laterally.
Apart from the tonotopic organisation of the cochlear / basilar membrane what is another important aspect of frequency analysis?
The timing of neuronal firing.
How does freqency of sound and timing of neuronal firing differ?
Low Hz = Phase locking. One Hz = One neuron discharge
Med Hz (1-4KHz) = Phase locking but not every wave i.e every 4th wave = neuron discharge this also relies on tonotopy for Hz discrimination (volley principle)
High Hz (4+KHz) = Random neuron firing, relies solely on tonotopy for Hz discrimination
What is the volley principle?
Up to 4KHz a pool of phase locked activity representative of a particular Hz.
What are the two types of sound localisation?
Horizontal and vertical localisation
Whats the difference between horizontal and vertical localisation?
Horizontal localisation relies on both ears time difference in hearing a sound while vertical relies on one ear.
What is the difference in hearing a sound between ears called?
Interaural time delay
Above what frequency can sound no longer be differentiated in the horizontal plane and why?
2kHz as the interaural delay is not observed, relies instead on interaural intensity difference
Why does a interneural intensity difference exist?
Sound physics means that one ear will always be in a shadow (directional difference) and therefore have lower intensity
What are the two processes of locating sound in the horizontal plane?
Interaural time delay (20-2000Hz)
Interaural intensity difference (2-20kHz)
Where is the interaural time delay detected?
The sup. olive complex (first relay station to receive input from both ears)
How does the sup olive complex detect time delays?
Each neuron in the complex corresponds to a difference in time delay i.e the neuron at which both AP from both ears reaches it at the same time (and summate) results in the detection of which ear/ direction the noise is coming from.
How does the interaural intensity difference work?
Occurs in the Sup olivary complex.
Two types of neurons that do this:
- Excitatory excitatory (EE)
- Excitatory Inhibitory (EI)
It is the combination of these neurons firing that determine intensity of sound
How doe he EE and EI neurons differ in their firing rate?
EE are stimulated by sound in both ears. Less so with sound in the contralateral ear and the least by sound in the ipsilateral ear (left and right olivary complexes)
EI are stimulated the most by contralateral sound, then both ears and the least by the ipsilateral ear.
How is sound localized in the vertical plane?
Interaural time and intensity difference do not exist in the verticle plane.
The ear uses the time difference between deflected sound from the pinna and direct sound to determine localisation in the vertical plane.
What is the primary auditory cortex also called?
Broadmanns area
What is the role of broadmanns area?
To process complex sound along with the adjacent secondary cortex which interprets sound.
How is broadmanns area organized?
Tonotopically
What does tonotopically mean with regards to broadmanns area?
The neurons are frequency and intensity tuned
How is sound processed in broadmanns area?
Related to the ice cube model.
Three permaters
x = Type of binural interaction
EE , EI, EE, EI
y = Characteristic frequency
(rostral to caudal (high Hz))
Z= Cortical layer (1-6) (less important)
What is audiometry?
The measuring of hearing thresholds assessed by objective and subjective methods.
What are the objective methods of audiology?
They are based on electrical recordings along various parts of the auditory pathway, from cochlea to cortex
What are the subjective methods of audiology?
Individual undertaking a perception test (listening one)
What is an example of a objective method?
Electrocochleagraphy
In humans we record the compound cochlear nerve action potential at the round window.
What is an example of a subjective method?
Pure tone audiograms. Measures hearing loss in dB threshold = 0dB
What are the causes of hearing loss?
Noise induced
Age related
Drug induced (ototoxicity)
Inner and middle ear problems
What hearing losses are caused by pathology to IHC and OHC?
OHC damage = incomplete deafness
IHC = Complete deafness, organ of corti collapse. Can be fixed by cochlear implants
What is an example of drug induced deafness?
Aminoglycoside ototoxicity:
Caused by
- Steptomycin
- Antitumor drugs
These drugs cause progressive hearing loss i.e
Partial OHC, Full OHC, Loss of IHC and OHC
What are the types of noise trauma hearing loss?
Acute or chronic
Acute = Temporary hearing loss Chronic = Irreversible, damage to OHC, IHC or nerves
Whats the physiogy behind acute and chronic hearing loss due to trauma?
excess glutamate release (glutamate excitotoxicity) causes temporary hearing loss synaptic plasticity allows recovery except chronic trauma = permanent
What is the most likely theory for hearing loss?
Oxidative stress / free radicles produced by drugs or metabolism. These can cause DNA damage, lipid and protein breakdown.. huge permanent cochlea damage
What are the rates of old age hearing loss ?
Onset 45-54
44% by 69
66% by 79
>90% over age 80
What is age related hearing loss called?
Presbyacusis
loss of high Hz first
What do olfactory receptors axons interact with within the glomeruli?
Mitral cells
periolfactory cells (between glomeruli)
Tufted cells
