Lecture 15: Hearing 2 Flashcards
List the structures in the inner ear
3 semicircular canals + vestibule of vestibular system, spiral cochlea containing the organ of Corti
How can meningitis be related to the inner and middle ear
There is only a thin layer of bone separating the internal auditory canal (Auditory nerve continuous with the CNS) from the middle ear space. Therefore if bacteria can penetrate it can cause meningitis
Also meningitis bacteria can also travel into the cochlea leading to deafness
Describe the structure of the cochlea, 3 compartments including the types of fluid in there
Fluid filled tube spiralling around a central bony core (modiolus) containing the auditory nerve and vascular supply.
This tube contains perilymph.
It is split into the Scala Vestibuli above and Scala tympani below by the cochlea duct filled with endolymph called Scala media. It contains the auditory sense organ: Organ of Corti
What are the boundaries of the cochlea duct/scala media
- Has high resistance to K+ leakage
Floor: basilar membrane projecting from a bony shelf in the modiolus to the fibrous structure on lat wall.
Medially and superiorly bound by Reissner’s membane.
Laterally bound by stria vascularis: contains rich blood supply for active pumping
What cells produce perilymph and endolymph, what is the ion balance of the fluid in mM and why is maintaining these fluids in compartments important
Perilymph: produced by cells lining the perilymphatic cavity. It has high Na+ [150] but low K+ [5] -
Endolymph: produced by stria vascularis by selective pumping of ions. It has low Na+ [5] but High K+ [150]
It is important because it provides essential nutrients.
- The ion composition difference causes an electric potential of across the scala media (80+mv) and hair cell (-40–70mv) mv).
- High K+ difference means that after it goes in hair cell for depolarisation there is no need for active pumping of K+ out of hair cells due to passive diffusion: makes hair cells sensitive. Doesn’t require a lot of energy for hair cells so no loud blood supply
Meniere’s disease- hearing and balance disorders are caused by abnormal fluid homeostasis (swollen)
Describe the cellular structure of the Organ of Corti: numbers, how arranged, cell types
Organ made of Inner hair cells in a single row (25%) on the medial side and Outer hair cells in 3-5 rows (75%) on lateral side.
These have apical projections into endolymph called Stereocilia embedded into the Tectorial membrane.
20,000 Hair cells surrounded by matrix of structural support cells and sit on top of afferent and efferent axons.
Compare the innervation of Inner hair cells and Outer hair cells
Afferent and efferent including destinations and type of fibre
Both cells are innervated by efferent (motor) and afferent (sensory) but IHC has 90% of sensory innervation, while OHC has most of the efferent.
Afferents: Both send fibres to the cochlea nu. in the brainstem
IHC: Type 1 myelinated nerves
OHC: Type 2 small unmyelinated
Efferents fibres come from the superior olivary complex from same and opposite side
What is the function of OHC, why is it needed
When stimulated by sound, voltage sensitive motor protein Prestin in the cell membrane causes to contract in time with the frequency changes in sound to increase the sensitivity and frequency selectivity of the ear by enhancing the small motion of the cochlear partition
This is an active process to overcome the loss of sound energy due to frictional and inertial forces of the inner ear fluids and organ of corti
Describe the process of sound transduction to the stimulation of the auditory nerve (5)
- Displacement of stapes in the oval window causes corresponding displacement of the round window, initiating a travelling wave of displacement along the basilar membrane and Organ of corti
- For a particular frequency, the wave reaches a peak amplitude of displacement at a specific region of the cochlea where it maximally vibrates and then dissipates rapidly.
- Vertical motion at organ of corti is converted into radial motion of stereocilia
which opens and closes mechanically gated ion channels at the tip of hair cells. - Large movement of K+ (and ca2+) across apical hair cell membrane causes standing current with resultant depolarisation, (and hyperpolarisation) after closing of the ion channel.
- Depolarisation causes opening of Ca2+ channels and release of Ca2+ from internal stores, which leads to release of glutamate at the base of cell, exciting afferent auditory nerve.
How is the cochlea tonotopically organised - spatial representation of sound along length -
(Place coding)
From Base to apex, there is increasing mass and decreasing stiffness.
This means that basal part corresponds to high frequency, apex to lower frequency.
100 um of cochlea is activated for a particular frequency. A single nerve from a single place has a small range of frequencies it can respond to
What is the structure of the mechanically gated ion channels at the tip of stereocilia
How is it able to follow high frequency sounds
Operated by fine actin filaments (cahedrin molecule) going from ion channel on the shaft of stereocilium to the tip of the adjacent stereocillium with myosin and actin filaments on it to actively regulate the tension on the linkages so that it is very sensitive to opening , with 100 channels per hair cell.
Able to follow high frequency sounds due to time constant of operation 20us
What is the temporal coding of frequency of sound (pitch) - not the tonotopic
How is it limited at different frequency
For low frequencies below 1000 Hz, the discharge rates of nerve fibres is also synchronised to the frequency of the stimulus: Phase locking.
The refractory period of the nerve fibre means that sounds higher than 4000Hz can’t do phase locking.
The frequency can be extended by population response: summation of individual nerve fibres firing while others are refractory on the same cell
How is intensity (loudness- amplitude of wave) of sound coded for
- Number of nerve fibres activated by stimulus
- Rate of firing of individual fibres in proportion to intensity
Despite individual nerve fibres having small ranges of discharge rate, there are 3 types of low, middle and high threshold fibres with high ones unsaturate, activated in that order.
What is the common term for Inner ear disease -
injury/death to sensory cells, stereocillia/neurons, loss of fine frequency tuning and intensity detection of hair cells
causes and presentation
Sensorineural deafness:
Caused by: excessive stimulation (noise exposure), congential abnormalities, infections, ototoxic drugs, disturbances in cochlea fluid, trauma, aging
Presentation: Loss of hearing sensitivities at frequencies corresponding to location of damage in the cochlea, loss of frequency discrimination, poor loudness and balance tolerance, poor speech recognition
What is the safe sound level of db to avoid inner ear disease
80 db and below. Every 3 db above this, the time spent in the environment has to be halved to keep safe
