Vestibular System Flashcards
What is the vestibular system, and what are its components?
- Part of the inner ear
- Fluid filled space in the petrous portion of the temporal bone
1. Bony Labyrinth
2. Membranous Labyrinth
What is the bony labyrinth, and what are its components?
- Perilymph filled space, lateral and posterior to the cochlea
1. Vestibule
2. Semicircular Canals
What is the Vestibule?
Small oval central cavity
- continuous with cochlea anteriorly and semicircular canals posteriorly
- Oval and round windows located here
- Contains the otolith organs
What are the semicircular canals?
- 3 canals projecting posterosuperior from the vestibule at right angles from one another
- At one end of each canal is a swelling, called the bony ampulla
- Contain the semicircular ducts
What is the membranous labyrinth and what are its components?
- Endolymph filled spaces suspended within the bony labyrinth
1. Otolith Organs
2. Semicircular Ducts
What are the otolith organs?
- 2 epithelial lined sacs located within the vestibule
1. utricle
2. saccue - Contains the maculae
What are maculae?
- Location of the receptors for translational head movements and orientation of the head relative to gravity
- Where transduction occurs
Describe the macula of the utricle.
- Horizontally placed
- Concerned with motion in the horizontal plane
Describe the macula of the saccule
- Vertically placed
- Concerned with motion in the vertical plane
What are the semicircular ducts?
- Epithelial lined ducts
- Project from the utricle
- located within the semicircular canals
- Contains the membranous ampullae
What are the membranous ampullae and where are they located?
- Location of the receptors for rotational head movements
- Located within the bony ampullae at one end of each semicircular duct.
What is meniere syndrome?
- Disruption of normal endolymph volume
- Abnormal distention of the membranous labyrinth
- Vertigo, tinnitus, hearing loss, and nausea occurs.
What are the maculae and membranous ampullae analagous to in the auditory and visual systems?
- Organ of corti
- Neural Retina
What are the components of the maculae and membranous ampullae?
- Hair Cells
2. Gelatinous Layers
Describe the structure of the hair cells in the vestibular system.
- Hair buncle with stereocilia and a single longer kinocilium
- Stereocilia are arranged in rows, graded by height, tallest next to the kinocilium.
- Tip links: extend from tip of each stereocilium to the next tallest neighbor
What are the types of hair cells in the vestibular system?
- Type I- Pear shaped and innervated by terminals of afferent vestibular cells
- Type II- Columnar shaped and innervated by simple terminals of afferent vestibular cells
What are the two types of gelatinous layers in the maculae and membranous ampullae?
- Otolithic Membrane
2. Cupula
What is the structure and function of the otolithic membrane?
S: Covered by calcium carbonate crystals (otoconia), which are 3 times as dense as endolymph, and not displaced by normal endolymph movements
F: Envelopes the stereocilia and kinocilium of the maculae
What is the structure and function of the Cupula?
S: Attached to the floor, roof, and walls of the membranous ampullae. Same specific density as that of the endolymph.
F: Envelopes the stereocilia and kinocilium of the membranous ampullae
What is the striola?
A curved depression on the otolithic membrane that bisects the maculae.
How are hair cells on the utricular macula organized?
Polarized so that the kinocilium is always on the side toward the striola.
How are hair cells on the saccular macula organized?
Polarized so that the kinocilium is always on the side away from the striola.
What are crista?
- A ridge that the hair cells are embedded in
- Perpendicular to the long axis of the duct
- Located in the membranous ampullae
How are the hair cells in the horizontal semicircular duct organized?
- All arranged with their kinocilium on the side closer to the utricle
How are the hair cells in the anterior and posterior semicircular duct organized?
- All arranged with their kinocilium on the side further from the utricle.
Why are the hair cells in both the maculae and membranous ampullae organized in the way that they are?
So that they can get information from all different directions
Describe the steps to mechanoelectrical transduction of head movements and position.
- Displacement of the gelatinous layers occur, resulting in a bending of the sterocilia of the hair cells.
- Hair bundles are deflected toward the kinocilium, opening ion channels near the tips of the stereocilia, allowing K+ to flow into the cell down its electrochemical gradient.
- Depolarization opens Ca++ channels, increase in Ca++ causes synaptic vesicles to fuse to the membrane and release neurotransmitter.
What type of movements are the maculae sensitive to?
- Transient displacements caused by translational movements of the head (linear acceleration)
- Static displacements caused by tilting the head
Describe the movement that occurs in the otolith organs when the head moves.
- When the head is moved, the otolithic membrane moves with respect to the surrounding endolymph, deflecting the hair cells.
- Hair cells on one side of the striola are depolarized, and on the hair cells are hyperpolarized.
What type of movements are the membranous ampullae sensitive to?
Transient displacement caused by rotational movements of the head (rotational acceleration)
Describe the movement that occurs in the semicircular ducts when the head turns.
- When the head turns in the plane of the semicircular ducts, the duct moves with it, endolymph lags, and moves in the opposite direction.
- Lagging endolymph moves the cupula, deflecting the hair cells.
- The left and right semicircular canals form functional pairs that always respond oppositely to any head movement that affects them.
What are the afferent cells that process vestibular information?
Vestibular (Scarpa’s) Ganglion
What is the location of the cell bodies of the Vestibular Ganglion? What is the distribution?
L: Internal Acoustic Meatus
D: Terminate on all hair cells
What is the location of the cell bodies of the efferent cells? What is their distribution?
L: Brainstem, rostral to the vestibular nuclei
D: Majority terminate directly on Type II hair cells
What is the function of the efferent cells in the vestibular system?
May control the sensitivity of hair cells
How do vestibular ganglion cells transmit information?
-By increasing or decreasing their firing rate
What does the change in firing rate of the vestibular ganglion cells mean?
- The firing can be either transient or sustained
- Signaling either absolute head position or acceleration
What is benign Paroxysmal Positional Vertigo
A condition characterized by brief episodes of vertigo that coincide with particular changes in body position, possibly due to posterior canal abnormalities
What is the central projection of the vestibular ganglion cells?
Vestibular Nerve- formed from axons of the vestibular ganglion cells
What is the structure and function of the vestibular nerve?
S: extends from inner ear through internal auditory meatus into brainstem
F: fibers give off ascending and descending branches to the ipsilateral vestibular nuclear complex or project directly to the cerebellum
What would happen if there was a lesion of the vestibular apparatus or vestibular nerve?
Complaints in dizziness, nausea, and spacial disorientation.
Describe the structure for the vestibular nuclei.
- 4 collections of neuron cell bodies that lie in the rostral medulla and caudal pons
- Large amount of maculae and membranous ampullae convergence over a wide range of neurons in the vestibular nuclei
- Receives input from vestibular nuclei of the opposite side, cerebellum, visual and somatosensory systems.
What is the function of the vestibular nuclei?
Neurons show directional sensitivity for particular head movements and can encode for both angular and linear components of head movements.
-Axons of the vestibular nuclei project to the contralateral nuclei, cerebellum, oculomotor nuclei, spinal cord, and thalamus.
What do the reciprocal connections between the cerebellum and vestibular nuclei constitute?
Important regulatory mechanisms for the control of eye movements, head movements and posture.
What are the components of the Vestibulo-ocular Network
- Vestibular Projects–> Vestibulo-ocular Reflexes
2. Nystagmus
When do VORs occur?
In response to rotational and linear movements
Describe what occurs during a Horizontal VOR and the head turns left.
- Want eye to move to the right.
1. Left horizontal semicircular canal excites left vestibular nuclei
2. Left vestibular nuclei excites Right Abducens nuclei
3. Right abducens nuclei contracts Right Lateral Rectus Muscle
4. Right abducens nuclei excite left oculomotor nuclei, which produce contraction of the Left Medial Rectus Muscle.
5. Results in rightward eye movement to keep object on the fovea
When does Nystagmus occur?
In response to large head rotations, such as 360 degree body turn.
Describe the phases of the Nystagmus.
- Slow Phase- VOR directs the eyes slowly in the direction opposite to the head motion.
- Fast Phase- When the eye reaches the limit of how far it can turn, it moves back rapidly to a central position, moving in the same direction as that of the head.
How are Nystagmus movements named?
For the direction of the fast phase
What is the function of Nystagmus?
Maintain gaze fixation during head movements.
In the Vestibulospinal Network there are Vestibular Projections. What are the two tracts in the projections and where are they?
- Lateral Vestibulospinal Tract- Ventral Funiculus of the Spinal Cord
- Medial Vestibulospinal Tract- Medial Longitudinal Fasciculus of the Cervical Spinal Cord
Where does the vestibulospinal reflex (tilt to the left) occur, and what happens?
- lateral Vestibulospinal Tract
- Left LVST neurons excite alpha motor neurons of left axial and limb extensor muscles.
Where does the vestibulocolic Reflex (Tripping) occur, and what happens?
- Medial Vestibulospinal tract
- Bilateral MVST neurons excite alpha motor neurons of the neck extensors muscles and inhibit neck flexor muscles.
What is the function of the Vestibulospinal Network?
Influences muscle tone and produces reflexive postural adjustments of the body and head.
What is the function of the Vestibulo-Thalamo-Cortical Network?
Generate cognitive perceptions of motion and spatial orientation.
Where do the vestibular projections in the Vestibulo-Thalamo- Cortical Network go?
From Ventral Posterior Lateral Nucleus of the Thalamus–> Primary Somatosensory Cortex–> Secondary Somatosensory Cortex, Posterior Parietal Cortex, and Prefrontal Cortex
OR
From Ventral Posterior Lateral Nucleus of the Thalamus–> Secondary Somatosensory Cortex, Posterior Parietal Cortex, and Prefrontal Cortex
What is Decerebrate Rigidity?
- Rigid extension of the limbs due to transection of the brainstem above the level of the vestibular nuclei.
- Relieved when vestibular nuclei are lesioned.
