Vestibular System Flashcards
What is the vestibular system?
a sensory system essential in the control of posture and balance- found in the inner ear and it is a series of fluid-filled membranous tubes (labyrinths) which are embedded in the temporal bone
How does this sensory information get back to the brain?
the vestibular nerve which is part of the vestibulocochlear nerve
What is the vestibular apparatus?
3 semi-circular canals, the urticle which the semi-circular canals connect and the saccule
Describe the semi-circular canals in more detail.
they are at right angles to each other (3D) and their swellings at their bases are called ampulla which contain sensory hair cells
What are the otolith organs?
urticle and saccule
What does the saccule do?
vertical acceleration
what does the utricle do?
back/front head tilt and horizontal acceleration
What do semi-circular canals detect?
rotational acceleration
where are and how do the sensory receptors in semi-circular work?
embedded in swellings at the base of the bony canals called ampulla-inside are hair cells which act as sensory receptors and are 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 - distortion triggers action potentials which are transmitted along sensory neurone of the vestibular nerve as it synapses directly with the gelatinous cupula
What happens when the skull is rotated from left to right from rest?
the endolymph at 1st does not move because of its inertia but the ampulla moves instantly as 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
What then happens if you rotate at constant velocity?
the endolymph catches up and rotates at the same speed, removing the shearing forces but this takes several seconds
What happens when you stop suddenly?
the endolymph to continue to move due to momentum creating a continuing sense of movement and dizziness
What are the 2 types of cilia of hair cells?
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 away from the kinocilium cause?
hyperpolarisation and decreased discharge of APs in the vestibular nerve
What does stimulation of the vestibulocochlear nerve cause?
this allows the brain to determine movement in time and space- most integration takes place in the cerebellum
what does the orientation of the cupula allow?
so the brain can build a 3D image of body position using the pattern of firing and inhibition received
what are the sensory receptors of the otolith organs?
the sensory apparatus of utricle and saccule are called the maculae - macula in utricle are on the horizontal plane orientation and in saccule are on the vertical plane orientation - maculae have cilia (one kinocilium and series of stereocilium) just like the ampulla with protrude into gelatinous mass called the otolith membrane - embedded in the otolith membrane are CaCO3 crystals called otoliths
What does tilting the head cause?
the tilt is detected by the macula in the utricle (horizontal plane) and otoliths have a greater density than endolymph and thus are affected more by gravity- tilting the head moves the otoliths and the otolith membrane in which they are embedded which distorts the jelly and moved the cilia
what does backwards head tilt cause?
moves the otolith in the direction of the kinocilium causing depolarisation and increased discharge of APs
what does forward tilt of the head cause?
moves the otolith away from the kinocilium causing hyperpolarisation and decreased discharge of APs
what do vertically orientated macula in the saccule do?
respond to vertical forces eg elevation in a lift and provide orientation of the head when lying down
what does the information from the vestibular nerve do?
it keeps the brain informed of body’s position in space and direction of movement
Where else does the vestibular nerve connect with?
the cerebellar centres which co-ordinate the postural muscles required to maintain balance without synapsing in the medulla
what is vestibular nystagmus?
predictable eye movement in response to stimulation from the vestibular system = 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 the head rotating
Describe right nystagmus.
the direction of nystagmus is the direction of the rapid flick back - right rotation causes right nystagmus- the extent of eye movement is restricted by the bony orbit so when the eyeball comes to the end of its range of movement it rapidly flicks back forward
How is vestibular function tested?
Nystagmus via caloric stimulation where the outer ear is washed out with either warm or cold fluid and the temperature difference from the core 37*c sets up convection currents through thin bone which affects the endolymph - warm fluid causes nystagmus towards the affected side and cold away (COWS- cold opposite and warm same)
What is kinetosis?
motion sickness which is most likely to occur if visual and vestibular system inputs to the cerebellum are in conflict eg if the vestibular system indicates rotation but the visual system does not - the cerebellum generates a sickness signal to the hypothalamus to bring about to bring about the ANS changes= nausea, vomiting, decreased BP, dizziness, sweating and pallor
What is labyrinthitis?
acute interference with normal vestibular function due to infection= all ANS symptoms and vertigo
What is Ménière’s disease?
vertigo, nausea, nystagmus and tinnitus (subjective noise) - associated with overproduction of endolymph causing increased pressure- cause is unknown
What is vertigo?
perception of movement in absence of movement, may also be nystagmus and it is gross impairment of posture and balance which is very disabling
does the visual system compensate for vestibular impairment?
chronic vestibular impairment can be compensated quite well by the visual but problems occur if subjects cannot use visual information
What do lesions of the brain stem cause?
nystagmus at rest
