Neuro 13: Vestibular Systems Flashcards
Which structures in the body are responsible for angular motion of the head?
Semi-circular canals
Which structures in the body sense the acceleration of the head and the strength of gravity?
Otolith organs
What are the two otolith organs?
Saccule
Utricle
What are the main functions of the vestibular system?
Subserve perception of motion in space and tilt
Provide balance reactions to sudden instability of gait/posture
Stabilise the eyes on fixed targets during head movements
Assist in control of heart rate and blood pressure
Provokes motion sickness
Define vertigo.
False perception of movement in space
Define vestibular ataxia.
Instability of gait or posture
What happens to the ability fo the brain to stabilise the eyes in unilateral vestibular lesions?
Vestibular Nystagmus
eyes move in the direction of the lesion
What happens to the ability of the brain to stabilise the eyes in bilateral vestibular lesions?
Oscillopsia
everything appears to be shaking
ability to stabilise the eyes is lost
What are some other consequences of vestibular loss?
Slight impairment of orthostatic control
nausea and vomiting
loss of coordination on directional orientation
What type of cell is involved in the detection of movement in the vestibular system?
Hair cells
Describe the cilia of these hair cells.
One kinocilium and rest are stereocilia
What does the hair cell fibre synapse with and where does it project?
synapses with a primary neurone dendrite
project into the vestibular nuclei in the brainstem
What stimulates hair cells?
in otoliths - deflection of forces by inertial resistance to acceleration
in canals - endolymphatic fluid rotation
Describe how the hair cell receptor potential can be changed?
depolarisation = movement towards the kinocilium
hyperpolarisation = movement away from the kinocilium
Describe how ganglion cell discharge can be changed.
Towards the kinocilium = increased firing frequency
Away from the kinocilium = decreased firing frequency
Describe the orientation and sensitivities of the saccule?
Saccule is oriented vertically with hair cells projecting normal to the plane
sensitive to all combinations of vertical and antero-posterior directions
Describe the orientation and sensitivities of the utricle?
Oriented horizontally with hair cells projecting vertically
sensitive in lateral and antero-posterior directions
How do the otolith organs give a signal of linear acceleration in all 3 dimensions?
Vector sum of utricular and saccular stimulation
Describe the structure and functions of the semi circular canals.
Hair cells project from the ampulla in the wall of the canal and are
uni-directionally oriented so that acceleration to a particular side
stimulates the canals on that side (e.g. rotation of the head to the right
stimulates the right canal, rotation in the other direction inhibits the right
canal activity)
When head rotation decelerates to a stop, the canal on the other side
(left side) is stimulated
Describe the firing of the canals when the head is still.
Each canal has a tonic firing rate so that they equal out when the head is still
What are the effects of loss of canal function on one side?
Unopposed signal coming from the intact cide meaning partial impairment of sensitivity to rotation
Why would a unilateral canal lesion cause vertigo?
Unopposed tonus of the intact canal gives a signal as if the head is rotating to the intact side
Why would acute unilateral vestibular disorder cause vestibular nystagmus?
unopposed tonus of intact canal causes the eyes to be driven to the lesioned side
Where do superior and medial vestibular neurones project?
motor nuclei supplying extraocular muscles
Describe the path of medial vestibular neurones?
The axons of medial vestibular neurones cross the midline and project
to the contralateral abducens (VI) nucleus to abduct the eye on the
opposite side (in the opposite direction to head rotation)
Axons from the abducens nucleus ascend in the MLF to the
contralateral oculomotor nucleus (III) to adduct the other eye (in the
opposite direction to head rotation)
Describe the path of superior vestibular neurones?
project ipsilaterally to the oculomotor and trochlear nuclei to generate vertical vestibulo-occular reflexes
What is oscillopsia?
everything appears to be oscillating
How would you test if a patient has oscillopsia?
Tell the subject to look at a fixed target and then rapidly move their head
if they have bilateral loss of vestibular function then their eyes will be taken off the target by the head swing
What are the effects of bilateral vestibular disorder on the gait?
mild gait ataxia
What are the effects of unilateral vestibular disorder on the gait?
tendency for the body and head to lean or fall to the lesioned side
Describe the path and function of the lateral vestibulo-spinal tract.
Descends ipsilaterally in the ventral funiculus of the spinal cord
axons terminate in the lateral part of the ventral horn
influence motor neurones to limb muscles
Describe the path and function of the medial vestibulo-spinal tract?
descend bilaterally to the MLF
State the common causes of vestibular vertigo.
a. Seconds
Benign Paroxysmal Positional Vertigo (BPPV) b. Minutes
Vertebrobasilar insufficiency c. Hours
Meniere’s Syndrome d. Days
Vestibular neuritis e. Fluctuating/continuous
Uncompensated vestibular lesion f. Silent
Acoustic neuroma
What is BPPV?
Benign paroxysmal positional vertigo
caused by otoconial debris in the canals and is provoked by head movement
debris floating stimulates the ampulla and generated false signals of head rotation
cured by vigorously turning the head in the opposite direction
