1.53 The vestibular system Flashcards
What is the structure of the vestibular system?
Bony otic capsule - buried in lateral part of the petrous bone
Vestibule - semi circular canals
Membranous labyrinth containing endolymph surrounded by bony labyrinth containing perilymph
What makes up the vestibule?
Utricle and succule - connected by utriculo-saccular duct
What are the semi-circular canals?
Lateral - horizontal Superior Posterior - vertical Perpendicular from one another Originate from utricle Bulge at one end - ampulla
What do the utricle and saccule contain?
They are otolith organs and contain calcium carbonate
What do the utricle and saccule detect?
Detect changes in head angle and linear acceleration
What is the sensory epithelium called?
Macula
Horizontal - utricle
Verticle - saccule
What do the vestibular hair cells have?
Kinocillium and stereocillia
Bending towards longest depolarises, towards shortest hyperpolarises
What does the macula respond to?
Gravity or linear acceleration
What do the semi-circular canals detect?
Rotational acceleration
Where are the hair cells embedded in the semi-circular canals?
gelatinous cupula in ampulla
What happens when the head rotates in the appropriate plane?
When head rotates in appropriate plane, canal rotates but initially fluid doesn’t owing to its inertia – bends cupola, and stereocilia of hair cells
Detects change in velocity (acceleration)
What happens at constant velocity?
At constant velocity fluid eventually catches up with canal and rotates at same velocity
Cupola dragged back to upright position, so bending of hair cell stereocilia eliminated eliminated
Can only detect a change in velocity not constant velocity
What happens when rotation stops?
cupola bends in the opposite direction
Vestibular axons spontaneously active and so can signal both increase and decrease in firing - even when no stimulation
Why is the central vestibular pathway important?
Important in controlling head, eye and body position
Where does the otolith organ project to and what does that allow?
Otolith organ output projects to lateral vestibular nucleus and via vestibulospinal tract - controls legs to maintain posture
Where does each vestibular nucleus project into?
Lateral - cerebellum & limb motor neurons
Medial - medial longitudinal fasciculus - II, IV , VI and neck motor neurons and thalamus and cortex
Where do semicircular canals project?
Medial vestibular nucleus
What is the function of the vestibular ocular reflex?
Stabilises eyes – keeps them pointing in same direction despite head movements
Compensates for head movement with counter rotation of the eyes
Which part of the vestibular system is important in the vestibular ocular reflex?
Semi - circular canals
Is the reflex affected by the darkness?
No it also works in the dark - doesn’t need vision!!
Does the left vestibular nucleus connect unilaterally or bilaterally with the nuclei of the eye muscles?
Bilaterally, inhibits and excites appropriate ones
What is nystagmus?
Slow movement in opposite direction to rotation followed by a rapid flip back, also seen when rotation stops
defined by the direction of the fast beating
What is acquired nystagmus and what is it caused by?
Eyes move without rotation
Vestibular and brainstem lesion
What is menieres disease?
a disease of unknown cause affecting the membranous labyrinth of the ear, causing progressive deafness and attacks of tinnitus and vertigo.
