Lecture 4 - Mechanisms of auditory transmission & Ion channelopathies Flashcards
How is sound transmitted?
Through variations in air pressure
What makes up the outer ear?
Pinna - collect sounds
Auditory canal - extends 2.5cm into skull
What makes up the middle ear?
Tympanic membrane - eardrum
Ossicles (malleus, inclus, stapes) - transfer movement of tympanic membrane into movement of oval window
- Stapes - flat bottom or floorplate portion - moves in and out like piston at oval window to transmit sound vibrations to fluids of cochlea
Eustachian tube
What makes up the inner ear?
Cochlea: “snail”; part of auditory system
- hollow tube –32 mm long and 2 mm in diameter; walls made of bone
- rolled up – size of a pea
- base of cochlea are two membrane-covered holes: oval window and round hole
What is the attenuation reflex?
Muscles contract, ossicles more rigid, sound conduction diminished - happens in response to loud sounds
What is in the cross-section of the cochlea?
3 fluid-filled chambers - scala vestibuli, scala media, scala tympani
Apex: S. media is closed, s. tympani and s. vestibuli becomes continuous at helicotrema
Base: S. vestibuli meets oval window; S. tympani meets round window
Cochlea narrows from base to apex but basilar membrane widens from base to apex
Perilymph - low K+(7mM) and high Na+ (140mM): S. tympani and S. vestibuli
Endolymph: high K+ (150mM) and low Na+ (1mM): S. media
Organ of Corti - contains auditory receptor neurons
Why do we assume cochlea is completely rigid?
Cochlea is filled with incompressible fluid, wall is body
Increase in pressure at oval window would result in round window bulging out
What are the 2 structural properties of the basilar membrane?
Wider at apex than base by factor of 5
Stiffness decreases from base to apex - base 100x stiffer
How does neural coding of pitch occur?
High frequency: stiffer base of membrane will vibrate more
Low frequency: waves travel up to apex before dissipation
Where are the cilia of outer hair cells and inner hair cells?
Cilia of outer hair cells in tectorial membrane
Cilia of inner hair cells below tectorial membrane
Inner hair cells: one row, ~3,500
Outer hair cells: three rows, 15,000 to 20,000
What results from the upward motion of basilar membrane?
Entire rods of corti, reticular lamina and hair cells move as a unit
Tectorial membrane holds cilia tips
Lateral movement of reticular membrane bends stereocilia
Cilia are rigid and interconnected by actin filaments, therefore bend as one unit
Depolarisation happens in upward phase (hyperpolarisation happens in downward phase)
How does the bending of stereocilia convert into neural signals?
- Cilia bend
- MET* channels open
- Receptor potential changes
- Influx of K+ depolarizes cell
- Depolarization open Ca2+ channels
- Influx of Ca2+ trigger glutamate release
What is the mechanism for K+ channel opening in the hair cells?
Tip links between stereocilia respond to compressive and tensile forces to open mechanosensitive non-ion selective channel in stereocilia
Why is direction of K+ flow inward in hair cells?
K+ ions mediate depolarization and repolarization of hair cells
Hair cells operate as 2 distinct compartments
(1) Apical end protrudes into scala media containing endolymph
(2) Basal end is bathed in perilymph of scala tympani
Endocochlear potential is 80 mV
-difference between peri-and endolymph potentials
Hair cell is -125 mV more negative than endolymph
How are hair cells innervated by neurons from spiral ganglion?
Number of neurons in spiral ganglion = 35,000 –50,000
>95% of neurons synapse with IHCs (IHC:OHC = 1:3)
1 spiral ganglion fibre receives input from one IHC
1 IHC feeds 10 spiral ganglion neurites
1 ganglion fibre synapses with numerous OHCs
Spiral ganglion is bipolar, neurites to base and side of hair cell, axon project to medulla
Majority of information from cochlea comes from inner hair cells. What is the role of outer hair cells?
They act as cochlear amplifiers
Peak movement of basilar membrane is 100x smaller without it, excessive use of antibiotic kanamycin exclusively damages it
Tiny motors to amplify basilar membrane movement during low intensity sound stimuli
Motor proteins present on hair cell membrane
OHCs respond to sound by change in
•receptor potential and,
•length
Hair cell’s motor:
*ATPs not required
*Very fast
*prestin
What is a tonotopic map?
Characteristic frequency mapped to a position in cochlear nucleus therefore there is a map of the basilar membrane within the cochlear nucleus
What are some forms of hearing loss?
Most common form of hearing loss: involve peripheral auditory system
- Structures that transmit or transduce sounds into neural signals
*Conductive hearing loss - damage to outer or middle ear
*Sensorineural hearing loss - damage to inner ear: e.g., auditory hair cells, auditory nerves
What are some treatments for conductive hearing loss and sensorineural hearing loss?
Hearing aid: treatment for CHL
-microphone, speaker and amplifier
-Help boost sounds to compensate for reduced efficiency
Cochlear implant: treatment for SHL where auditory nerve is intact
-partially restore hearing
-Microphone, digital signal processor, stimulating electrode arrays
-electrical stimulation of nerve
What are the characteristic features of channelopathies?
(1) Intermittent symptoms
* mins, hours or days
* paroxysmal, episodic, periodic
(2) Positive & Negative symptoms
* positive: migraine, myotonia, epilepsy etc
* negative: paralysis and myasthenia
What are some common triggers of channelopathies?
(1) initiation of movement - kinesogenic
* episodic ataxia 1 (EA-1)
(2) Rest after exercise
* hypokalemic periodic paralysis (hypoKPP)
* hyperKPP
(3) Dietary intake
* hypoKPP (CHO load)
* hyperKPP (K+ load)
(4) Stress & Fatigue
* migraine, episodic ataxia, paroxysmal dyskinesia
How is muscle contraction generated from neuron signals?
(1) Action Potential: generated by voltage-gated KV and NaV channels
(2) Depolarization of the muscle membrane activates voltage-gated CaV channels
(3) Sensing of depolarization by CaV channel triggers interaction with
ryanodine receptor (RyR)
(4) RyRs open and Ca2+ release into cytoplasm initiates muscle contraction.
In one example of HYPP or hyperkalemic periodic paralysis, point mutations at the III-IV loop region which forms the “hinge & lid” or inactivation gate. How is inactivation affected?
Inward current: positive charge carriers entering cell
-Na+ ions entering via NaV channels
–amount of current depends on the amount of inactivation
-eg G1306E mutant has much slower inactivation and more Na+ enters and membrane will be more depolarized
Severity depend on length of side chains (G1306-E,A,V)
aa substitution:
E: permanent myotonia
V: moderate exercise-induced myotonia
A: benign, subclinical
G-G: hinge for fast inactivation gate
What is hyperkalemic periodic paralysis?
*dominantly inherited sodium channel disease
* gene affected is skeletal muscle voltage-gated sodium channel SCN4A
(NaV 1.4)
* hyperPP is characterised by:
-episodes of flaccid muscle weakness associated with hyperkalemia with signs of myotonia in the interval between attacks
- 21 missense mutations found by 1999 (gate: linker, hinge or S4)
- triggered by rest after a heavy workload ( eg vigorous exercise)
- key symptoms of stiffness & weakness caused by long-lasting depolarisation of muscle fibre membranes
-note: two populations of SCN4A (NaV 1.4) channels: WT & mutant
What is the mechanism of hyperkalemic periodic paralysis?
•membrane depolarisation to 5-10mV
-WT can recover from inactivation
-lead to repetitive firing: basis for involuntary muscle activity (stiffness)
•membrane depolarisation to 20-30mV
-majority of SCN4A (NaV 1.4) channels inactivated
-muscle become inexcitable (weakness)
NaV 1.7/SCN9A mutations are associated with pain disorders. How does an SCN9A channelopathy cause congenital inability to experience pain?
Painless channels
Nonsense mutations/frameshift mutations leading to premature stop codon
What are the disorders caused by ‘painful channels’?
Inherited erythromelalgia
Paroxysmal extreme pain disorder
Why do patients with IEM or PEPD mutations manifest different pain topology despite ubiquitous expression in sensory neurons?
The familial IEM mutations in SCN9A
that have been characterized to date all shift the voltage-dependence of NaV1.7 activation in a hyperpolarized direction.
Mutant NaV1.7 channels open at more negative membrane potential –> mutant channels open more easily upon membrane depolarization
PEPD-linked SCN9A mutations shift the voltage-dependence of steady-state fast inactivation towards a depolarizing direction.
Mutant NaV1.7 channels close at more depolarized membrane potential –> mutant channels stay open longer
What is the function of voltage-gated calcium channels?
What are some calcium channel disorders?
- couple cell electrical excitability with intracellular signalling
- gene mutations or ablations: to understand roles in
-cardiovascular function, pain, epilepsy, migraine and deafness
human CaV 1.2 L-type Calcium Channel Disorder
-Timothy Syndrome
-possible linkage to schizophrenia and bipolar disorder
human: CaV 2.1 P/Q-type Calcium Channel disorders:
-Familial Hemiplegic Migraine (FHM),
-Spino-cerebellar Ataxia-6 (SCA-6),
-episodic ataxia-2 (EA-2)
-dominant form of inheritance
- FHM: various missense mutations
- SCA-6: expansion of CAG repeats in the C-terminus
- EA-2: truncation of large part of C-terminus
What are the traits of ion channel mutations?
-Mainly affect gating
-Less affect on permeation or selectivity
* possibly such mutations affect conductance or loss of selectivity is embryonal lethal
Many channelopathies –dominant inheritance
–> gain-of-function
defects
eg greater open probability –NaV channel defect in periodic paralysis and myotonia or epilepsy
-due impaired inactivation
-due to enhanced activation
–> loss-of-function defects
–recessive or dominant inheritance
eg due to nonsense mutation or splicing defect
-asymptomatic carrier in heterozygotes
