Lecture 26 Vestibular system Flashcards
What is the “sense” of balance - 3 parts and the overall route
- The Integration of sensory input:
- visual, vestibular, proprioceptive, touch, and sound localisation - which leads to motor system activation either to make corrective movements or static posture.
- The receive sensory information about spatial orientation as feedback (info about head and eyes separate)
What are the 2 forms of balance controlled by reflexive and conscious actions?
- Static postural equilibrium: (controlling the centre of mass over a base of support),
2.Dynamic balance.
It prevents falling when standing or moving by moving muscles.
What are the 3 roles of the vestibular system
- Senses dynamic or static head position
- Detects the linear and angular acceleration of head
- Important for conscious awareness of head position and reflex control of eye movement.
What is the condition when the vestibular -ocular reflex: (maintenance of eye gaze with motion) doesn’t work
Oscillopsia: sensation that surrounding environment is constant in motion.
Symptom of conditions that affect eye movement or ability to stabilise images as usually the eyes will move in the opposite direction of the head turning to keep the gaze
What is Vertigo, what symptoms follow it
Sensation of world moving when you are not moving due to acute loss of vestibular function.
Can lead to cause collapse, disorientation and nausea.
It is accompanied by autonomic symptoms: pallor, sweating, nausea and vomiting
What causes Motion sickness and an associated condition
Conflict between vestibular, visual and proprioceptive inputs compared with an expected internal model - eg. conflict between stable horizon but bobbing up and down
Long term sense of motion= Mal de debarquement
How does CNS compensate for chronic vestibular problems?
It corrects the abnormal inputs from ears at central nuclei brainstem for those with chronic and gradual loss of vestibular function, so less symptoms (provided other sensory inputs available)
What is the path of signals from the vestibular system including main nerve
- Angular and linear information from the Sensory vestibular apparatus in Inner Ear
- Travels via Vestibular portion of vestibulocochlear nerve - cell bodies in scarpa’s ganglion in int auditory meatus
a) Sup division innervates: Utricle, lat + ant SCC and ant part of saccule
b) Inf division innervates: post part of saccule, Post SCC
- Vestibular nuclei (lat, med, sup, inf) in the medulla under the 4th ventricle.
- Fibres from the nuclei project to the cerebellum, occulomotor nuclei and spinal cord.
What are the 5 sensory organs of the Vestibular system, what fluid do they sit in, and what info do they sense
Within the bony labyrinth filled with perilymph, there is a membranous labyrinth filled with endolymph.
There is 1 sensory organ per semicircle canal- sup, lat, and post (3) which are ampullae. They detect angular acceleration.
There are 2 otolithic organs in the meeting point of the semicircular canals called Saccule and Utricle. They detect static head position and linear acceleration.
How are the Semicircle canals paired between the two ears considering they are all at 90 degrees from each other - and why
When the head moves along a specific plane, the SCC on the same plane are paired for stimulation- they operate bilaterally.
The posterior canal of one ear and anterior/superior canal of the other is in the same plane.
Both Lateral (horizontal) ones are in the same plane
What is the structure of the sensory organ in the SCC
At the end of the SCC is the Ampulla.
Inside the ampulla is the Crista Ampullaris which is a specialised epithelial ridge containing vest. hair cells.
The stereocillium project into the Cupula: gelatinous structure that stretches to the roof of the ampulla
Describe the main structural features of Vestibular sensory cells
(hair cells precursor to cochlear hair cells)
Both types of cells have a large stereocilia called Kinocilium on one edge of the cell, after which there is stepwise size reduction in stereocilia size which gives the cell polarity.
There are type 1 and 2 hair cells in the vest. organs too
How are vestibular hair cells depolarised/ hyperpolarised.
Compare the polarisation of vestibular hair cells in the different organs
Displacement of stereocilia toward the kinocilium causes depolarisation: increased rate of stimulation. Away from kinocilium leads to hyperpolarisation.
Within the ampulla hair cells are aligned in the same direction- toward the front of head
In the Saccule (vertical) (orientated on A-P plane) depolarisation is in the lateral direction from the midline of organ on all sides
In the Utricle (horizontal) (orientated on the M-L plane) depolarisation is in the medial direction towards the midline of the organ
What is the structure of the otolithic organs: Utricle and saccule
Hair cells sit in a matrix of supporting cells arranged in button like sheets called Macula
Stereocilia project into Otolithic membrane (gelatinous).
Sitting on top of this is Otoconia: crystals of calcium carbonate
The macula utriculus is horizontally orientated and macula sacculus is vertical to this.
How does the SCC system detect angular acceleration of the head (rotation)
- As the head turns to one side, the endolymph stays stationery for a moment because of Inertia.
- Thus there is apparent flow of endolymph relative to the bony canal in the opposite direction of head turning.
- This causes deflection of hair cell stereocillia - the hair cells are polarised in opposite directions in paired canals.
- This means that there will be increased activity (from constant baseline firing) on the canal on the same side which was being turned to and decreased activity on the other side’s canal.
- The brain relies on the comparison of activity to determine direction of movement
- In the VOR, vestibular nuclei-> occulomotor nuclei to cause movement of eyes opposite to turning direction
