Vestibular anatomy + physiology Flashcards
What does the inner ear consist of?
Bony labyrinth, a system of passages making up 2 main functional parts
What are the 2 functional parts of bony labyrinth?
- Cochlea - dedicated to hearing
2. Vestibular system - dedicated to Balance
What can bony labyrinth be subdivided into?
- Vestibule
- 3 semicircular canal
- Cochlea
What are the pair of membranous sacs of vestibule?
- Saccule (Sacculus)
2. Utricle (Utriculus)
What do receptors in the vestibule provide for?
sensations of gravity and linear accelerations
What do semicircular canal enclose and what are receptors stimulated by?
Enclose slender semicircular duct
Receptors are stimulated by rotation of the head
What is The vestibular system comprised of ?
two types of sensors: the two otolith organs (the saccule and utricle), which sense linear acceleration (i.e., gravity and translational movements), and the three semicircular canals, which sense angular acceleration in three plane
What is sense of hearing provided by?
Receptors within cochlear duct
What are 2 sets of end organs in inner ear, or labyrinth?
- Semi circular canal
2. Utricle and saccule within vestibule
What is the function of semi circular canal?
Respond to rotational movement (angular head acceleration
What is the function of utricle and saccule within vestibule?
Respond to changes in the position of head with respect to gravity
What are inner ear function and symptoms?
Auditory - hearing loss, tinnitus, hyperacusis
Vestibular - dizziness, vertigo, unsteadiness, imbalance, falls, oscillopsia
What is oscillopsia?
Visual disturbance in which objects in the visual field appear to oscillate
What is hyperacusis?
Heightened sensitivity to particular sounds
What is the vestibular part of the inner ear?
- Saccule
- Utricle
- 3 semicircular canal [anterior, posterior, horizontal] with their ampullary tissue
What does Otolith sense?
sense linear head acceleration
How are the superior and posterior semicircular canal orientated?
Vertically at right angles to each other
Lateral semicircular canal
about 30-degree angle from the horizontal plane
What does orientation of the canal cause?
A different canal to be stimulated by movement of head in different planes
What does horizontal canal detect?
Angular acceleration of head when the head the head is turned
What do superior and posterior canal detect?
Vertical head movement when the head is moved up or down
What do each of 3 semicircular canal have at its base?
A bulbous expansion called ampulla which houses sensory epithelium or crista that contains hair cells
What is semicircular canal?
three fluid-filled bony channels in the inner ear. They are situated at right angles to each other and provide information about orientation to the brain to help maintain balance.
Where is capula located?
Within ampullae of each of the 3 semicircular canal
What is capula?
Gelatinous component of crista ampullaris that extend from crista to the roof of ampullae
What is the function of capula?
When the head rotates, the endolymph filling the semicircular duct initially lags behind due to inertia
The capula is deflected opposite direction od head movement
As endolymph pushes capula, the stereocilia is bent as well, stimulating hair cells within crista ampullaris
Where does ampulla connect with?
utricle
What happens when endolymph moves?
Pushes the gelatinous capula that covers crista ampullaris causes embedded hair cells to bend and send nerve impulses to the brain
Lateral semicircular duct
Axial rotation
Fluid does not move in sync with labyrinth as a result of inertia
Flexion and extension of the head
Anterior duct is most affected
What are Type I + II vestibular hair cells defied by?
afferent innervation patterns
What are the properties of type I hair cells?
- chalice cells
- one afferent nerve calyx
- '’true’’ receptors
[90% afferents]
What are the properties of Type II hair cells
- Cyclindrical cells
- Several afferent + efferent nerve endings
[10% afferents] - '’modulators’’
What are 2 types of vestibular hairs?
- Kinocilium
2. Stereocilia
What are the properties of kinocilium?
One, thicker and longer than stereocilia
Secured to cell by basal body
What are the properties of stereocilia?
50-110 thin stereocilia/hair cells
Height increases across cell to kinocilium
Shorter in macula, in cristae up to 36 microns
Deflection of hair bundle in the + direction?
Toward the longest stereocilia
Excitatory
The cell’s membrane is depolarised with respect to resting potential of about -60mv
Displacement of hair bundle in - direction?
Inhibitory
Deflection in orthogonal direction produces no electrical response
What provides maximal stimulus of vestibular stimulation?
A force parallel to the surface of sensory epithelium which bisects the bundle of stereocilia and passes directly through kinocilium
What comes from utricle and saccule within vestibule?
The information for static equilibrium and linear acceleration
What does saccule and utricle each contain?
A sense organ called macula, where stereocila + their supporting cells are formed
What are 4 subnuclei of vestibular nucleus complex?
- Medial vestibular nucleus
- Lateral vestibular nucleus
- Inferior vestibular nucleus
- Superior vestibular nucleus
What is vestibulo-ocular reflex
Responsible for maintaining eye fixation during head movement
Involves a 3 neuron arc
Obtain objects on fovea
VOR responds to head movement with eye movement that is equal in magnitude but opposite in direction
What is vestibulo-ocular arc?
Neuron from semicircular canal
Neuron in the vestibular nuclei transmit information to eye muscle nuclei
eye motor neuron
What is Head thrust test?
Tests the VOR
This reflex is helpful to maintain corrective eye position during any change in head position + to correct eye movement rapidly so that vision remains on target
What is vestibulo-spinal reflex?
Coordinate head + neck movement with trunk + body, with the goal of maintaining head in upright position
What is lateral vestibulospinal?
Postural changes to compensate for tilts and movements of body
What is medial vestibulospinal?
Stabilizes head position during walking
What is vestibulo-collic reflex?
Acts on neck musculature to stabilize the head with body movements
What are the mechanisms of balance?
- Vision
- Cortex, cerebellum, reticular formation, extra-pyramidal system
- Cortical awareness of head/body/motion
- Cintrol of oculo-motor activity
- Control of posture
- Control od motor skills
- Labyrinthine activity
- Proprioception superificial sensation
What is proprioception?
'’sense to self’’
Ability to sense movement within joints and joint angle, muscle length/tension
The vestibulo cerebellum
Functions as an adaptive processor and re-adjust vestibular input through inhibition
Higher cortical level = most likely in/near parietal or insular cortex
Ascending connection from vestibular nuclei
Anterior horn cells of spinal cord, via vestibulo-spinal tract
Cerebellum
Contralateral vestibular nuclei
Cranial nuclei III, IV, + VI
Pontine reticular formation
Temporal cortex
Cerebral modification
Cerebral function responsible for modification + ability to supress vestibular reflexes
Involvement of parietal cortex + ocular gyrus to visually suppress VOR
