6.9 Vestibular System Flashcards
What is the vestibular system
The vestibular system is a sensory system located in the inner ear that maintains balance. It detects movement, head position and spatial orientation. The vestibular nerve relays sensory information about balance and spatial orientation to the brainstem and cortex. The brainstem reflex pathways help to maintain stability and re-establish an equilibrium during movement and the cortex integrates information on perception of gravity and movement
What are the 3 semicircular canals in the inner ear
The anterior, posterior and lateral canals. The canals are arranged in 3 opposite pairs. The canals are all perpendicular to each other and are on 3 different planes of motion
What is the function of semi-circular canals
They respond to rotation movements of the head
Describe the mechanism by which semi-circular canals transduce a signal.
The canals are filled with endolymph fluid and at the base of each canal there is a membranous ampulla. The ampulla has the cupula which is distorted by the movements of the endolymph in the canals, this causes bending of the cilia on hair cells in the ampulla. In each vestibular organ, the hair bundles have a specific orientation (all pointed in the same direction). Hair cells that are pointing in the direction which the head is turning will depolarise and those pointing in the opposite direction will hyperpolarise, one of the canal pairs will depolarise and one will hyperpolarise. The movement of the stereocilia either opens or closes mechanically gated K+ transduction channels, causing an increase or reduction of neurotransmitter release, either increasing or decreasing firing of action potentials in the vestibular nerve - this mechanism is fast and adaptable.
Describe the 2 otolith organs
The 2 otolith organs are the utricule and saccule. They are precisely orientated near the bases of the ampullae of the tubules, at rest the utricles are positioned horizontally and the saccules are positioned vertically. They function to sense head position and linear acceleration/deceleration
What is the function of the saccule
The saccule senses linear vertical acceleration and static vertical head tilt (flexion downwards)
What is the function of the utricle
The utricle senses linear horizontal acceleration and static horizontal head tilt
Describe the structure of utricles and saccules
They contain the macula which has hair cells in a gelatinous matrix which synapse onto the vestibular nerve. The macula is covered by the otoconia which is crystals of calcium carbonate (ear rocks) that add weight to the membrane and move which is essential to sense linear acceleration and gravity for the purpose of maintaining balance.
Describe how ortolith organs send signals
Following a change in head position or linear acceleration/ deceleration, depending on the direction of the hair movement, action potential firing will either increase or decrease compared to the resting level.
Describe the vestibular neuronal pathway
The hair cells of the maccular saccule, maccular utricle and the semicircular canals all synapse onto the vestibular nerve (which is part of the vestibulocochlear nerve CN VIII). The vestibular nerve projects centrally to a complex of four vestibular nuclei in the rostral medulla and caudal pons, left and right vestibular nuclei receive input from both ears. These vestibular nuclei have projections to the spinal cord (controlling head and body position), to the cerebellum (coordinating postural adjustments), to 3 extraocular motor nuclei controlling eye movements and to the thalamus (eventually reaching the cortex allowing for conscious perception of movement and gravity).
What are the 3 major classes of reflexes that control and maintain equilibrium and gaze during movement, posture and muscle tone.
-vestibulo-ocular reflex
-vestibulocollic reflex
-vestibulo-spinal reflex
Describe the vestibulo-occular reflex (VOR)
The vestibulo-occular reflex (VOR) helps keep your eyes pointed in a particular direction, it triggers eye movement that counters the head movement, allowing a gaze to be fixed on a particular point. If the head rotates left, both eyes move to the right due to the contraction of eye muscles triggered by the semicircular canals.
Describe the vestibulocollic reflex
The vestibulocollic reflex (VCR) regulates head position and control of neck muscles, it acts on the neck musculature to stabilise the head, a reflex head movement counters the movement sensed by the vestibular sensory organs.
Describe the vestibulospinal reflex
The vestibulospinal reflex (VSR) regulates the balance and maintenance of upright posture. When the head is tilted to one side, both the canals and otoliths are stimulated which activates the vestibular nerve and nuclei, impulses are then transmitted via the lateral and medial vestibulospinal tracts to the spinal cord. This induces extensor activity on the side to which the head is leaned and flexor activity on the other side.
Describe some diseases of the vestibular system
Merniere’s disease:
-a progressive vestibular condition caused by the build up of endolymph
symptoms include regular dizzy spells, hearing loss, ringing in the ear and feeling of fullness in the ear
can be diagnosed by 2+ vertigo attacks lasting 20+ minutes, hearing loss proved by a test or tinnitus
-medication can treat symptoms but there is no cure
Pyschogenic vertigo:
caused by an environmental or social stimuli giving stress
-gives false or distorted sensation of movement, unsteadiness as a feeling of rocking or swaying when upright, and dizziness as a nonmotion sensation of disordered spatial orientation
-they are vestibular-like symptoms, distinct from those caused by structural abnormality of the inner ear
What are some tests that study how well the inner ear is working
-Electronystagmogram or videonystagmography (ENG or VNG
-Rotary chair testing
-Vestibular evoked myogenic potentials (VEMP) testing
-Computerised dynamic posturography (CDP)
-Video head impulse test (vHIT)
-Electrocochleography (ECoG)
Describe the Video head impulse test (vHIT)
The vHIT is a quantitative investigation of the function of the horizontal vestibulo-ocular reflex. While you focus on a point, your head is turned quickly and unpredictably. The angular velocities of eye and head movements are measured simultaneously using video, and their quotient (the gain) is calculated. If your eyes move off the target when your head is turned, you have a reflex issue. A normal gain is 1 (where the angular velocities are nearly the same), right unilateral vestibulopathy reduces the gain to 0.3 and the bilateral vestibulopathy reduces gain to 0.05.
