vestibular function Flashcards
what is thr vestibular system?
A sensory system essential in the control of posture and balance. Found in the inner ear, it is a series of fluid-filled membranous tubes, (labyrinths), which are embedded in the temporal bone
what does the vestibular apparatus consist of?
of 3 semi-circular canals, the utricle to which the semi-circular canals all connect and the saccule.
what are the otolith organs?
the utricle and saccule
function of the utricle
back/front tilt and horizontal acceleration
function of the saccule
vertical acceleration
function of the semi-circular canals
rotational acceleration.
where are the sensory cells of the semi-circular canals?
embedded in swellings at the base of the bony canals called ampulla.
what are the function of the hair cells inside the swellings
sensory receptors
where are the hair cells embedded in the swellings?
embedded in a flexible gelatinous structure called the cupula that stretches across the entire width of the ampulla and which becomes distorted by movement of the endolymph fluid within the canals.
how does the semi circular canals detect rotational acceleration?
If the skull is rotated left or right from rest, the endolymph at first does not move because of its inertia. However the ampulla moves instantly because it is embedded in the skull.
The inertia of the endolymph produces drag which bends the cupula, and consequently the cilia embedded in it, in the opposite direction to movement. If rotate at constant velocity, the endolymph catches up and rotates at the same speed, removing the shearing forces, but this takes several seconds.
Sudden stop will cause endolymph to continue to move due to momentum creating a continuing sense of movement and dizziness.
what are the two types of hair cells?
there is a single very large kinocilium and a set of progressively smaller stereocilia
what does Distortion of the cilia in the direction of the kinocilium cause?
depolarisation and increased discharge of APs in the vestibular nerve.
what does Distortion of the cilia away from the kinocilium lead to
hyperpolarization and decreased discharge of APs in the vestibular nerve.
whats the purpose of the orientation of the cupua being slightly different?
so the brain can build a 3D image of body position using the pattern of firing and inhibition received.
what is the maculae?
The sensory apparatus of the utricle and saccule
what are otoliths
Embedded in the otolith membrane are CaCO3 crystals
what is the tilt of the head detected by?
the macula in the utricle (horizontal plane);
why are otoliths affected mroe by gravity?
otoliths have a density greater than endolymph and thus are affected more by gravity. Tilting the head moves the otoliths and the otolith membrane in which they are embedded. This distorts the jelly, and moves the cilia.
what happens when the head is tilted backwards?
Backward tilt moves the otolith in the direction of the kinocilium causing depolarisation and increased discharge of APs. Opposite for forward tilt.
what keeps the brain informed of the body’s position in space and any direction of movement.
The distinct pattern of information being sent from the three ampulla of the semi-circular canals, and the two maculae of the otolith organs, to the brain via the vestibular nerve
where do the vestibular nuclei on one side project?
ipsilaterally, bilaterally and contra-laterally to descending motor pathways (and also the extraocular nuclei (extraocular muscles)
where do vestibular nuclei receieve input from?
proprioceptors signalling limb and body position, also from neck and eye muscles
where does the vestibular nuclei project vie?
via the thalamus to the cerebral cortex —> perception of movement and body position = kinaesthesia
what do the vestibular system reflexes involve?
the vestibulocoritcal and vestibulospinal tracts
what are the 3 types of vestibular system reflexes?
Tonic labyrinthine reflexes
Dynamic righting reflexes
Vestibulo-ocular reflexes
what does the Dynamic righting reflexes do
rapid postural adjustments that are made to stop you falling when you trip. Long reflexes, involving extension of all limbs. Most profound in cats.
what does the Vestibulo-ocular reflexes do?
strong association is evident between the vestibular apparatus, the visual apparatus and postural control.
what are the types of Vestibulo-ocular reflexes?
Static reflex
Dynamic Vestibular Nystagmus
what is the Dynamic Vestibular Nystagmus
A series of saccadic eye movements that rotate the eye against the direction of passive rotation of the head and body so that the original direction of gaze is preserved despite head rotating. Think about being on a slowly rotating roundabout
how is Nystagmus is used to test vestibular function
Post-rotatory nystagmus,
Caloric stimulation
what is Post-rotatory nystagmus,
Subjects are rotated in a Barany chair. If rotate to the left then during acceleration get a left nystagmus. At the end of rotation, for about 20 seconds, during deceleration get a right nystagmus.
Due to endolymph catching up and now pushing the cupula in the opposite direction.
what is Caloric stimulation
Simple test for the horizontal SCC. When the outer ear is washed with either cold or warm fluid, the temperature difference from core 37C, gets through the thin bone and sets up convection currents which affect the endolymph. Warm fluid (44C) causes nystagmus towards affected side, cold (30C) causes nystagmus away from affected side (COWS – Cold Opposite, Warm Same).
what gives rise to motion sickness (kinetosis)
Powerful maintained stimulation of the vestibular system can give rise to kinetosis
when is motion sickness most likely to occur?
occur if visual and vestibular system inputs to the cerebellum are in conflict e.g. if the vestibular system indicates rotation but the visual system does not.
what is Labyrinthitis
Acute interference with normal vestibular function as a result of infection = all ANS symptoms + vertigo
what is vertigo?
is the perception (hallucination) of movement in the absence of movement. There may also be nystagmus. Gross impairment of posture and balance, very disabling.
what is Ménière’s disease
Vertigo, nausea, nystagmus and tinnitus = subjective noise. Associated with over production of endolymph causing pressure. The cause is unknown.
