Vestibular System Flashcards
What is the general purpose of the vestibular system?
To detect precise changes in orientation of the head and in acceleration. We use vestibular information to “know” our position in space.
What is the vestibular system’s role in the vestibulo-ocular reflex?
The vestibular system talks with the motor nuclei of the eye to keep images stable on the retina despite chaotic head movement and to maintain upright body posure while moving
Where does the output of the vestibular nucleus go?
Spinal cord
Cerebellum
Thalamus
Nuclei of EOM
Medial Longitudinal Fasiculus
- What nuclei does this connect?
- What is the relationship between the vestibular nuclei and the MLF?
- What is this pathway mostly concerned with?
- Where are the cells of origin for the vestibular system / MLF?
- Motor nuclei of 3, 4, 6
- All fibers of the vestibular nuclei contribute fibers to the MLF
- Maintaining orientation in space
- Scarpa’s ganglion
Using the image of the horizontal conjugate gaze circuit below, explain what would happen in a normal person who is wanting to look to the left.
The pontine reticular formation is the point of origin and it sends a signal to the left abducens nucleus in the pons carrying the information necessary to communicate “look left”. The axon carrying this signal bifurcates to synapse on (A) one axon that innervates the left lateral rectus muscle and tells it to contract, and (B) a second axon that crosses the midline, enters the MLF, enters the right oculomotor nucleus in the midbrain, and synapses on a neuron that innervates the right medial rectus muscle and tells it to contract. This allows both the left and the right eye to move to the left simultaneously.
Internuclear Ophthalmoplegia
- Results from damage to what structure?
- Problems with what activity?
- Symptoms / presentation (ex: left side affected)?
- What would cause this to occur bilaterally?
- Results from disruption to the MLF
- Problems with horizontal conjugate gaze
- Symptoms
- Affected (left) eye loses the ability to control the medial rectus –> left eye cannot look right
- Contralateral (right) eye is still able to abduct (look right) but shows nystagmus when doing so
- Horizontal diplopia due to divergence of eyes
- Multiple sclerosis
We don’t need to know the difference between the 4 vestibular nuclei. Just know that there are 4 of them.
However, they are visible in different parts of the brainstem, which is worth knowing.
What 2 are visible in the pons vs. the medulla?
Pons - superior and lateral
Medulla - medial and inferior
What do the following inner ear structures detect:
- Saccule
- Utricle
- Semicircular canals
- Linear vertical acceleration
- Linear horizontal acceleration
- Angular acceleration / rotation
What is perlymph? How is it different from endolymph?
Perilymph is a fluid that is similar to CSF that is in the bony labyrinth
Endolymph is a fluid that is similar to intracellular fluid with a high concentration of K+ and low concentration of Na+. It is in the membranous labyrinth
Describe how the utricle and saccule work.
Vestibular nerve fibers are attached to a basement membrane called the macula, which consists of supporting cells and hair cells that are imbedded in this layer of cells. On top of the hair cells is a calcium carbonate layer of crystals called otoliths in a gelatinous membrane. The hair bundles of the hair cells project into this crystalized layer. When the head moves, the mass of the otoliths in this gel membrane lags behind the movement and results in the mechanical deflection of the hair cells. In this manner, the hair cells are able to detect different accelerations due to the different inertial movement of the otolithic membrane.
Much like the hair cells in the cochlea, the hair cells in the utricle and saccule are […] gated […] channels that, when opened, allow a large influx of […].
Mechanically
K+
K+
Explain how the semicircular canals are able to detect angular acceleration and rotation.
Rotation of head results in inertial flow of endolymph fluid through semicircular canals. Simiarly to otolitic organs, the canals detect acceleration based on the inertia of endolymph filling the canals. The flow of endolymph in the canal can be bidirectional and can persist after movement of the head stops.
What is the ampulla? What is happening in this location to allow detection of angular acceleration in the semicircular canals?
Deflection of cilia in one direction results in the opening of K+ channels and K+ influx leading to […], while deflection in the opposite direction results in closing of the K+ channels and leads to […]
Depolarization
Hyperpolarization
Once vestibular information has been sent to cell bodies in scarpa’s ganglia, where does it go to next? (4 options for 4 nuclei)
- Cerebellum via vestibulocerebellar tract which enters cerebellum through inferior cerebellar peduncle –> coordinates balance and eye movements
- EOM nuclei via ascending MLF –> coordinates eye movements with head movements
- Limb motor neurons via vestibulospinal tract –> excites ipsilateral limb/trunk to oppose body tilt
- Neck motor neurons via descending MLF –> excites ipsilateral neck muscles to oppose head tilt
What is the role of the cortex in the vestibular system?
Descending control appears to increase excitation of the hair cells and may modulate the dynamic range to match expected or anticipated accelerations
What is the vestibulo-ocular reflex?
Correction of eye position to compensate for head movement in order to maintain foveal centering of an image.
What is vestibular nystagmus?
Alternating slow and rapid eye movements indicative of vestibular pathology
What is vertigo?
A sense of rotation. Caloric stimulation test is used to detect vertigo b/c induces sense of movement.
What is Meniere’s syndrome?
Recurring periods of vertigo, potentially due to elevated levels of endolymph
How can antibiotics be damaging to the vestibular system?
Sustained treatment can be toxic to K+ channels leading to the loss of hair cells and thus vestibular and hearing deficits
What causes motion sickness?
When data provided by the vestibular system is not consistent with the data provided by the visual or somatosensory systems
Why does the room spin when you lie down after a rough evening of drinking?
What is the area postrema?
What is its role in alcohol toxicity?
Located beneath obex of 4th ventricle, acts as chemodetector for toxins
Too much EtOH triggers area postrema to initiate events mediated by vagus nerve (peristalsis, flexions of diaphragm…vomiting)
Why do the vestibular nuclei project to the cervical spinal cord?
Vestibulocervical reflex arc excites neck muscles via the descending MLF to oppose head tilt
Why do the vestibular nuclei project to the spinal cord?
Vestibulospinal reflex arc excites same side limbs and trunk extensors to oppose body tilt
Caloric stimulation test
- Most commonly used to test for […]
- Cold water causes the […] phase of nystagmus to the […] side of the filled ear
- Warm water causes the […] phase of nystagmus to the […] side of the filled ear
- Brainstem death
- Fast; opposite
- Fast; same
What are some common symptoms of Meniere’s disease?
Unilateral hearing loss
Tinnitis
Periods of vertigo
What are the afferent pathways used for maintaining stable control of balance and posture?
Vision
Vestibular
Conscious proprioception
Unconscious proprioception
Which efferent motor pathways are used for controlling proximal muscles necessary for postural tone and truncal stability?
Anterior corticospinal
Vestibulospinal
Tectospinal
