Lecture 21: Vestibular System

What are the 3 main functions of the vestibular system?

- detects head position and movement in space

- controls posture and equilibrium

- coordinates eye movements relative to head movements

What symptoms can occur with vestibular system damage?

- severe nausea and vomiting

- profound dizziness

- vertigo (sensation of movement)

What is the function of the inner ear in the vestibular system?

transduces head motion and position

What is the function of the vestibular nuclei?

receive, integrate, and distribute vestibular information

What are the major output pathways of the vestibular nuclei and their functions?

- vestibuloocular: eye movements

- vestibulospinal: posture and balance

- vestibulothalamocortical: conscious perception of movement

- vestibulocerebellar: adaptive processing

What are the 3 components of the bony labyrinth?

cochlea, vestibule, and 3 semicircular canals

What fluid is found in the bony labyrinth surrounding the membranous labyrinth?

perilymph

What fluid fills the membranous labyrinth?

endolymph (high K+ concentration)

What type of movement does the vestibule detect?

linear acceleration

What movements does the utricle detect?

side-to-side movements of the head (lateral plane)

What movements does the saccule detect?

up/down and forward/backward movements of the head (anterior/posterior plane)

What structures are embedded in the otolithic membrane?

crystals (otoliths) and hair cells

What is the sequence of events during head movement that activates vestibular hair cells?

head moves → crystals move → otolithic membrane moves → hair cells deflect → vestibular nerves fire action potentials

What is the function of mechanotransduction in vestibular hair cells?

converts mechanical stimuli into electrochemical activity

What happens when vestibular hair cells deflect toward the kinocilium?

mechanically gated ion channels open → K+ enters → depolarization → voltage-gated Ca2+ channels open → Ca2+ enters → neurotransmitter release → AP in vestibular neuron → information sent to CNS

What triggers neurotransmitter release from vestibular hair cells?

Ca2+ influx through voltage-gated calcium channels

What type of movement do the semicircular canals detect?

angular acceleration and rotation

How many semicircular canals are present on each side of the head?

3

What is the orientation of the anterior and posterior semicircular canals?

vertical plane

What is the orientation of the horizontal semicircular canal?

tilted 30° posteriorly

Sensory organ of the semicircular canals

crista ampullaris

What is the structure of the crista ampullaris?

sensory epithelium lying on connective tissue with hair cell stereocilia embedded in the cupula and innervated by afferent neurons

What is the longest true cilium of vestibular hair cells?

kinocilium

What is the enlarged region of the semicircular canal containing the crista ampullaris?

ampulla

What happens to endolymph when the head moves?

endolymph moves in the opposite direction

What happens when stereocilia deflect toward the kinocilium?

increased action potential firing in vestibular afferents

What happens when stereocilia deflect away from the kinocilium?

decreased action potential firing in vestibular afferents

How do left and right horizontal semicircular canal firing rates compare when the head is at rest?

equivalent tonic resting firing rates

What happens in the left horizontal semicircular canal during leftward head rotation?

endolymph moves toward the left ampulla → stereocilia deflect toward kinocilium → increased left afferent firing

What happens in the right horizontal semicircular canal during leftward head rotation?

stereocilia deflect away from kinocilium → decreased right afferent firing

How does the vestibular system distinguish leftward versus rightward head rotation?

compares increased firing on one side with decreased firing on the opposite side

Where are vestibular afferent neuron cell bodies located?

Scarpa's (vestibular) ganglion

What type of neurons are vestibular afferents?

bipolar neurons

Where does the peripheral process of a vestibular afferent neuron terminate?

vestibular hair cell

What forms the central process of vestibular afferent neurons?

CN VIII

Where do vestibular afferents project?

ipsilaterally to vestibular nuclei and vestibulocerebellum

What are the 4 nuclei of the vestibular nuclear complex?

- superior vestibular nucleus (SVN)

- lateral vestibular nucleus (LVN)

- medial vestibular nucleus (MVN)

- inferior vestibular nucleus (IVN)

Where are the superior and lateral vestibular nuclei located?

caudal pons

Where are the medial and inferior vestibular nuclei located?

rostral medulla

What is the function of the vestibular nuclear complex?

primary processor that receives, processes, and projects vestibular information

What is the function of commissural fibers between vestibular nuclei?

compare firing rates between the left and right sides

Which vestibular pathway controls eye movements?

vestibuloocular pathway

Which vestibular pathway controls posture and balance?

vestibulospinal pathway

Which vestibular pathway provides conscious perception of movement?

vestibulothalamocortical pathway

Which vestibular pathway is involved in adaptive processing?

vestibulocerebellar pathway

What muscles are innervated by the LVST?

extensor axial muscles

What is the function of the LVST?

maintains balance and allows postural changes

What muscles are innervated by the MVST?

neck muscles

What is the function of the MVST?

stabilizes head in relation to body movements

What is the function of the superior and inferior vestibular nuclei in cerebellar pathways?

provide cerebellar afferents terminating in the vestibulocerebellum

Where are the centers for gaze located?

brainstem

Brainstem center responsible for horizontal gaze

paramedian pontine reticular formation (PPRF)

Where is the PPRF located?

pons

What structure is identified as the center for vertical gaze and also contributes to horizontal movements?

medial longitudinal fasciculus (MLF)

Where is the MLF located according to the lecture?

midbrain

How are oblique eye movements coordinated?

horizontal and vertical gaze centers work together

Reflex that produces compensatory eye movements opposite the direction of head movement

vestibuloocular reflex (VOR)

What is the purpose of the VOR?

coordinates eye movements relative to head movements

What type of eye movements occur during the VOR?

slow, gradual, smooth movements rather than saccades

Can the VOR be consciously suppressed?

yes

Does the VOR require visual input?

no

What happens to the eyes when the head turns left during the VOR?

both eyes move smoothly to the right

What is the vestibuloocular pathway during leftward head rotation?

left Scarpa's ganglion → left vestibular nuclei → right PPRF → right abducens nucleus (CN VI) → right lateral rectus → right eye abducts

How does the left eye adduct during the VOR with leftward head rotation?

right PPRF → MLF → left oculomotor nucleus (CN III) → left medial rectus → left eye adducts

What structure connects the horizontal gaze pathway to the contralateral oculomotor nucleus?

MLF

Illusion of movement of the body or environment associated with nystagmus, nausea, and vomiting

vertigo

Where can lesions causing vertigo occur?

anywhere along the vestibular pathway

What percentage of vertigo cases are caused by peripheral lesions?

approximately 75%

(associated with inner ear)

What percentage of vertigo cases are caused by central lesions?

approximately 25%

(associated with brainstem or cerebellum)

Involuntary eye movement also called "dancing eyes"

nystagmus

What are the 2 components of nystagmus?

slow phase and fast phase

(named after direction of the fast component)

What causes physiological nystagmus?

vestibular activation after sustained rotation

What causes pathological nystagmus?

- congenital disorders

- CNS disorders

- toxicity

- alcohol

What is the most common finding in vestibular system disorders?

nystagmus

(also associated with vertigo)

What false sensation occurs with a right CN VIII lesion?

false sense of rotation to the left

What eye movements occur with a right CN VIII lesion?

slow VOR to the right followed by fast saccade to the left

Why does a unilateral vestibular lesion produce a false sense of rotation?

unequal tonic firing rates between the two vestibular systems

What deficit results from an abducens nerve lesion?

impaired lateral gaze on the side of the lesion

What type of strabismus results from an abducens nerve lesion?

medial strabismus

What muscle is affected by a left abducens nerve lesion?

left lateral rectus

Lesion of MLF causing impaired adduction of the ipsilateral eye

internuclear ophthalmoplegia (INO)

What happens to the coordination of CN VI and CN III with an MLF lesion?

actions of abducens and oculomotor nuclei become uncoupled

What eye movement deficit occurs with a right MLF lesion?

right eye fails to adduct during leftward gaze

What are the usual causes of internuclear ophthalmoplegia (INO)?

vascular occlusive disease or multiple sclerosis

What is the key distinction between abducens nerve and MLF lesions?

- abducens lesion causes impaired ipsilateral eye abduction and medial strabismus

- MLF lesion causes impaired ipsilateral eye adduction (INO)