Module D-09 Flashcards
Which cells are the receptor cells of the vestibular and
cochlear divisions of the inner ear?
Hair cell
describe hair cell
- The apex of a hair cell is the signal transduction site which contains cilia (mostly stereocilia).
- This area is surrounded by endolymph
- The interior of the cilia is continuous to the cytoplasm of the hair cells.
- At the base of a hair cell we find a synaptic terminal,
with vesicles containing excitatory transmitter. - stimulates the afferent fibers of the vestibular and cochlear portions of the vestibulo-cochlear nerve (CN VIII).
What differs composition of Endolymph from Perilymph?
Endolymph - high in K+, very low Na+, low Ca2+, slightly
higher Cl-
Perilymph - Very low K+, high Na+
Hair cells have ______ potentials not _______
graded; AP
Opening of which channels causes depolarization of Hair cells
mechanically gated cation channels (TRPA1 channels)
How do the TRPA1 channels open?
Mechanical force produced by a “tip link” between neighboring stereocilia directly opens the cation channels during deflection of the cilia towards the tallest cilium.
In what state are the TRPA1 channels when the cilia are at rest ?
the channel is slightly open so there is some influx of K+, so some graded potentials will be seen in the hair cell at rest
When will no graded potentials be seen in the hair cell?
when the stereocilia are pointed away from the tallest cilium and the TRPA1 channel completely closes causing hyperpolarization
Influx of which ion during depolarization causes transmitter release from the hair cell?
Voltage-gated calcium channels embedded in the hair cell membrane are opened during depolarization, resulting in an inward current of calcium ions in turn, the release of an
excitatory transmitter
Which are the 2 types of sensodry organs in the vestibular system?
1) Otolith organs
2) Semicircular canal
Which are the 2 otolith organs?
Saccule and utricle
describe the otolith organs
- endolymph filled pockets that contain in their walls a patch of sensory hair cells.
- Their cilia support, at their tips, calcium carbonate crystals glued together with a jelly like material (otolithic
membrane) . - the cilia of the hair cells have different orientations on each side of the center or striola
- In utricle the kinocilium is oriented towards the striola.
- In the saccule, the situation is just the other way round, with the kinocilium oriented away from the striola.
Describe the semicircular canals
endolymph filled pipes which contain a patch of sensory hair cells (crista), with their cilia inserted into a jelly like structure (cupula)
Describe how semicircular canals function
When the endolymph moves relative to the walls of
the canals, induced by angular acceleration (rotation) of the head around one of the axes of the semicircular canals, the cupula with the inserted cilia bends,
which leads to depolarization or hyperpolarization of the hair cells.
What type of acceleration do otolith organs detect?
Linear acceleration
What type of acceleration semicircular canals do detect?
angular acceleration
what are the 3 types of semicircular canals?
horizontal, anterior vertical and posterior vertical
Describe how otolith function
- When the cilia of the sensory hair cells are tilted, either by gravity forces, or by linear acceleration, the cilia bend, which leads to depolarization or hyperpolarization of the hair cells
- It is dependent on the orientation of the hair cells
- Due to the inertia of the calcium carbonate crystals, the cilia of the hair cells in the one half of the striola are tilted away from the kinocilium (inhibition), while the hair cells in the other half are tilted towards the kinocilium(excitation)
What causes the delay in the detection of angular acceleration
The inertia of the endolymph inside the semicircular canal delays its participation in the rotation
In the Horizontal semicircular canal the longest cilia are towards __________ and shortest are towards ________
Occipital ; nasal
The primary vestibular system works at the __________ level with very little reaching consciousness
Subcortical
Is there a cortical primary vestibular area?
No
What is the use of vestibular sensory information?
forms the afferent limb of vestibular reflexes (vestibulo-ocular and vestibulo-spinal reflexes).
What is the use of vestibular reflexes?
stabilize the eyes and the body when the head moves
Describe movement of the endolymph and ampulla in the horizontal semicircular canals when the head is rotated to the left
1) Head rotation to the left induces endolymph flows to the right inside both horizontal semicircular canals
2) The endolymph movement pushes the cupula in the left horizontal semicircular canal towards the back of the head, and the cupula in the right horizontal semicircular canal towards the front (anterior part) of the head.
{{{{{{The ampullae of both horizontal semicircular canals are oriented towards the midline; the canals themselves expand more laterally. Within the ampullae, the
hair cells are localized in the medial walls, with the kinocilium oriented towards the back of the head.
As a consequence, the cilia of the hair cells in the left horizontal semicircular canal are bent towards the back of the head, towards the kinocilium, and they are
getting excited. Cilia of hair cells in the right horizontal semicircular canal are Head rotation to the left
bent towards the anterior of the head, away from the kinocilium, so they are inhibited. }}}}}}}
Describe the state of the horizontal semi-circular canal at rest.
Both horizontal semicircular canal receptor organs are in a neutral state, halfway between maximum excitation and
maximum inhibition
Movement of the head to the left excites the _____ labyrinth and activates the vestibulo-ocular reflex and the eyes turn to the ______
left; right
describe the pathway of the vestibulo-ocular reflex when head is moved to the left
1) The semicircular canals relay the information of head rotation to the vestibular nuclei in the brainstem.
2) Fibers from the excited labyrinth (or the more active semicircular canal receptor organ) which form the afferent limb of the VOR, enter the brainstem at the ponto-medullary junction
3) synapse in the vestibular nucleus.
4) Axons of neurons located in the Vestibular nucleus cross
the body’s midline and synapse in the contralateral Abducens nucleus.
5) the abducens nucleus activates the Lateral
rectus muscle of the right eye and, via fibers crossing the body’s midline end ascending in the medial longitudinal fasciculus (MLF) the oculomotor nucleus.
6) The oculomotor nerve (CN III) then activates the medial rectus muscle of the left eye.
7) The lateral rectus muscle abducts the right eye and the medial rectus muscle adducts the left eye, so both eyes turn towards the right.
Does the Vestibulo-ocular reflex work in the dark?
Yes, it doesn’t need visual input
Speed of Vestibulo-ocular induced eye movements
up to 300° per second
What is Nystagmus?
rhythmically alternating movements of the eyes
What are the 2 phases of nystagmus?
1) a slow phase into one direction
2) a fast phase in the opposite direction
What mechanisms drive the slow phase of nystagmus?
driven by a reflex circuitry
What mechanisms drive the fast phase of nystagmus?
a simple reset mechanism, driven by the saccadic circuitry
The direction of a nystagmus is named after the _____ phase
fast
Physiological nystagmus
vestibulo-ocular nystagmus
Describe the fast and slow phase of vestibulo-ocular nystagmus
The slow phase of the nystagmus is in the opposite direction of the head movement, and is driven by the vestibulo-ocular reflex.
The fast phase (reset) of the nystagmus is in the direction of the head movement, and is driven by a saccade
What is the Oculocephalic Maneuver (Dolls eyes maneuver) ?
The examiner turns the head of the patient in the
horizontal (or vertical) plane and notes whether the ocular excursions in the opposite directions occur.
What does the Oculocephalic Maneuver test?
Used to determine, whether the vestibulo-ocular reflex pathway from the medulla to the midbrain is intact
What is Caloric Testing of the Vestibulo-Ocular Reflex?
procedure the outer ear canal of a patient is irrigated with cold (or warm) water and the examiner observes for conjugate deviation of the eyes, driven by the vestibulo-ocular reflex
What happens when cold and warm water are used to irrigate the outer ear?
Cold water => inhibit that Labyrinth and eye moves towards that side
Warm water => excites that labyrinth and eye moves to the opposite side
Motion sickness aka
Kinetosis
What causes Kinetosis?
individuals experience a discrepancy between vestibular and visual inputs
How does alcohol intoxication effect vestibular system?
by the interactions of blood alcohol with the endolymph, which can cause convection endolymph flows within the semicircular canals causing Vertigo
___________ is an antibiotic that is toxic to vestibular hair cells and should not be used chronically because it accumulates in the ___________
Streptomycin; endolymph
Vestibular damage can cause pathological nystagmus where the ____ component gets directed to the damages labyrinth and the ____ component gets directed to the functional side
slow; fast (because it is sensed as inhibition of the damaged side and relative excitation of the normal side)
