19 Posture + balance Flashcards
What is the structure of the vestibular apparatus?
Bony labyrinth with inner membranous labyrinth.
Semicircular canals + vestibule (utricle + saccule).
How is the vestibule innervated?
Vestibulocochlear nerve.
Vestibular branch: utricular, saccular and ampullary nerve.
What does the vestibular apparatus provide information about?
Head orientation, movement and location in space.
What are the six degrees of freedom?
Translational: x, y, z.
Rotational: roll, pitch, yaw.
What are the structural features of a maculae?
Matrix of supporting cells.
Hair cells possess cilia, penetrate into otolithic membrane (gel containing calcium carbonate).
What are the structural features of hair cells?
Functional features?
One kinocillium.
Many cilia, functionally connected by ‘tip links’.
Bending towards kinocilium increases nerve firing.
Bending away decreases firing.
How do hair cells produce an action potential?
Distortion of sterocilia opens K+ channel. K+ moves in from endolymph. Activates voltage gated Ca channels, triggering glutamate release.
What do macula provide information about?
Static equilibrium (head relative to horizon, 0.5o sensitivity). Dynamic equilibrium (linear acceleration).
How are the maculae orientated?
Horizontal in utricle.
Vertical in saccule.
What do the semicircular canals provide information about?
Rotational movements.
Angular acceleration.
Where are the sensory organs in the semicircular canals located?
Ampulla.
How are the ampulla structured?
Sensory hair cells embedded in cupula (gelatinous mass).
Cupula forms barrier against endolymph movement.
How does head rotation activate the ampullary nerve?
Cupula + cilia distorted by head rotation due to inertia of the endolymph. Cupula bends in opposite direction to the endolymph.
How are the semicircular canals arranged in pairs?
Why?
Ant + post. Post + ant. Two lateral.
Allows comparison - firing will increase on one side and decrease on the other.
Describe the vestibular pathway associated with the cupula:
Afferent to medial vestibular nucleus.
Efferents to neck/body/trunk (vestibulo-ocular reflex).
Describe the vestibular pathway associated with the otoliths:
Afferents to lateral vestibular nucleus.
Efferents to cerebellum + limb motor neurons (maintain posture).
What is the function of the vestibulo-ocular reflex?
Maintain forward vision during head movements.
What is Meniere’s syndrome? Symptoms?
Increased volume of endolymph rupture membranous labyrinth.
Pressure, earache, tinnitus, dizziness.
What is Benign Paroxysmal Positional Vertigo? Cause?
Calcium carbonate crystals become dislodged from otoliths.
Illusion of movement, disorientation, dizziness.
Which drugs can cause ototoxicity? (8)
Antibiotics. Anticancers. Diuretics. Aspirin. Quinines. Metals. Xylene. Toluene.
What does a medulloblastoma cause?
How?
Postural instability.
Cerebellar tumour leads to inability to utilise vestibular information from utriles + saccules.
Describe the vestibulo-ocular reflex.
Rotation to right.
Right horizontal ear canal depolarisation, vestibular afferent increases activity.
Neurons to LVI + RIII fire more.
Neurons to LIII + RVI fire less.
L lateral rectus + R medial rectus contract.
L med rectus + R lat rectus relax.
Eyes rotate leftward.
What is nystagmus?
Slow tracking movement in lone direction, then fast in the other.
How does nystagmus change over time with right rotation?
Start: rightwards nystagmus.
30s endolymph catches up + nystagmus decreases.
End: fluid momentum causes leftwards nystagmus.
What is the basis of electrooculography?
Cornea +vely charged compared to retina.
Eye movement changes voltmeter potential difference.
What is caloric testing?
Lie patient down, irrigate ear with water.
Warm causes endolymph to rise - increasing firing.
Cold causes endolymph to fall - decreasing firing.
What is caloric testing used for?
Checking brainstem function in unconscious patients.
