Anatomy and Physiology Associated with Cochlear Implantation Flashcards
(22 cards)
Is nerve deafness a common term used for SNHL?
Yes
In most cases of SNHL, the primary site of lesion resides within the cochlea
Cochlear hearing loss results in insufficient transduction of acoustico-mechanical energy into neural impulses at the auditory nerve
Cochlear implants are used to bypass the damaged hair cells and provide direct stimulation to the nerve
What is acoustic hearing?
Sound is influenced by both HRTF and the external auditory meatus
The middle ear amplifies and transmits sound vibrations to the inner ear, overcoming the impedance mismatch between air and the fluid-filled cochlea
What is electric hearing?
Microphone placement alters the natural HRTF and external auditory meatus, minimizing the localization cues normally provided by the auricle - their ability to localize sounds is poorer than those with acoustic hearing
Active middle ear effusion may delay cochlear implant surgery and has been anecdotally associated with reduced hearing during episodes of otitis media or negative middle ear pressure - CI users can also have reduced hearing (pressure of ME might impact the pressure of the inner ear - but exact mechanism is unknown)
*ME status still matters for electric hearing aid CIs
What is the modiolus?
The bony core with which the cochleas turns are wrapped around
What is the typical length of the cochlea?
35 mm
Wraps around approx 2.5 turns
*electrodes never reach the end of the cochlea - the longest electrode is 31 mm (med-el)
What is frequency information shifted upward due to the length of the electrodes?
Lower frequency information is being shifted upward to higher frequency regions due to the limitations of the electrodes
This might contribute to the robotic sounds that patients perceive
Is placement in the cochlea really important?
Yes
The closer to the modiolus, the better
Associated with lower charge levels
Is the cochlea a filter bank?
Yes
Groups of neurons are connected to hair cells which responds to different frequencies/areas of the SM
Cochlea function as a frequency analyzer, , with each place along its length acting like a band-pass filter tuned to a specific range of frequencies
Together, they behave as a bank of overlapping band-pass filters
How many afferent nerve fibers are in the auditory nerve?
Composed of ~ 30,000 afferent nerve fibers
Type I fibers make up 95% of the afferent cochlear nerve fibers
Type II fibers make up the remaining 5%
The cell bodies from these neurons, spiral ganglia, are located in Rosenthal’s canal in the modiolus
How do CIs work in a deafened ear?
In a deafened ear, hair–cell failure severs the connection between the peripheral and central auditory systems
CIs restore the link, bypassing hair cells to stimulate directly the cell bodies in the spiral ganglion
Where are the spiral ganglion cells?
In rosenthals canal of the cochlea
Their peripheral processes innervate the hair cell receptors, and their central processes conduct auditory information to the brain
Could the duration of deafness affect neurons?
Yes
Neuron survival depends on the organ of Corti’s health; damage to hair cells and hearing loss can cause neuron degeneration
Less uniform survival of neurons which would affect the electric stimulation
CIs target ganglion neurons for stimulation
CIs are based on the assumption that there are sufficient number of intact SGNs that are widely distributed along the tonotopic axis
*enough neural survival is needed for electrodes to be distributed
What are the causes of variation in neural survival?
Loss of hair cells
Loss of spontaneous activity
Dieback of peripheral processes
Loss of SGN cell body
Demyelination
CNS changes
What section of the cochlea are CIs inserted into?
Scala tympani
Due to its larger diameter to accommodate the electrode array
Allows insertion below the fragile cochlear duct and in close proximity to the SG cell bodies
Closer proximity to the round window during surgical insertion
Less intracochlear trauma and better preservation of residual hearing
Leads to better implantation outcomes and reduced postoperative vertigo
What happens when the electrode contact is placed in the ionic cochlear fluid and an electric current is applied?
It causes an ionic current to flow toward or away from the contact and causes voltage changes within the immediate fluid environment of the electrode
These voltage changes stimulate the neural membrane, causing sites along the auditory nerve to depolarize and generate action potentials
Does safe stimulation require careful control to prevent tissue damage?
Yes
The stimulus parameters selected may cause neural damage through the mechanisms that lead to corrosion and release of toxic products or the biochemical effect of over stimulation
Do CIs stimulation residual SGNs with controlled electric pulses?
Yes
Each pulse is made up of equal-sized negative and positive phases, designed to deliver no net charge through the electrode at the end of the pulse
Localized electrochemical reactions are reversed during the second phase of the biphasic pulse, ensuring that no net electrochemical products are formed
This charge balancing avoids the accumulation of charge that could produce toxic tissue reactions
*each pulse is equally charged so there is not a build-up
How is loudness coded?
By the number of activated nerve fibers and their rates of firing, which increase with the amplitude of the stimulus current
Once the nerve fibers are stimulated, they fire and propagate neural impulses to the brain
The brain interprets them as sounds
How is frequency coded?
By the place in the cochlea that is stimulated
Low frequencies stimulate electrodes near the apex
High frequencies stimulate electrodes near the base
What can CIs not replicate?
Travelling wave
Spontaneous firing rate (about 60% of auditory nerve fibers fire rapidly at rest (>80 discharges per second) and they are sensitive to low intensity signals, 25% have moderate spontaneous rates and 15% have slow rates (which code high level signals); spontaneous activity helps maintain sensitivity and supports coding across a wide dynamic range)
Phase locking (cochlear nerve fibers fire at the same point in each sound wave cycle, but due to their refractory period, individual fibers cannot respond to every cycle, so groups of fibers collectively encode sound frequency and pitch up to 4 kHz)
Stochasticity (random by phase-related firing of individual cochlear nerve fibers, this randomness allows neighboring groups to collectively encode the temporal structure, frequency, intensity, and duration of sound, enhancing the auditory system’s ability to represent complex acoustic signals)
Is the success of CIs surprising?
Yes because even multichannel CIs cannot replicate the spectral analysis that occurs in the cochlea
CIs bypass the frequency selectivity of the basilar membrane and replace it by a more coarse division of the audible spectrum
What are the three main reasons why CIs are successful in providing speech intelligibility?
Much of natural speech is redundant
Much of the processing capabilities of the ear and the auditory nervous system are redundant
The CNS have an enormous ability to re-wire to changing demand through expression of neural plasticity
