Vestibular A&P

What are the three primary functions of the vestibular system?

Gaze stability, postural stability, and orientation to gravity.

Which reflex primarily maintains gaze stability during head movement?

The vestibulo-ocular reflex (VOR).

Which pathways primarily contribute to vestibular postural stability?

Vestibulospinal pathways, working with ankle, hip, and stepping strategies.

Which vestibular structures primarily provide orientation to gravity?

The otolith organs: the utricle and saccule.

Which three sensory systems contribute most directly to balance?

Vestibular, visual, and somatosensory/proprioceptive systems.

What structures form the bony labyrinth?

The semicircular canals, vestibule, and cochlea.

What structures form the membranous vestibular labyrinth?

The semicircular ducts, utricle, and saccule.

What fluid fills the bony labyrinth around the membranous labyrinth?

Perilymph.

What fluid fills the membranous labyrinth?

Endolymph.

What is the vestibule of the inner ear?

The central portion of the bony labyrinth that contains the utricle and saccule.

What are the three semicircular canals?

Anterior (superior), posterior, and horizontal (lateral) canals.

What type of head motion do the semicircular canals detect?

Angular or rotational acceleration.

What is the ampulla?

The widened portion of each semicircular duct that contains the sensory receptor for angular acceleration.

What is the crista ampullaris?

The sensory epithelium within the ampulla that contains vestibular hair cells.

What is the cupula?

A gelatinous structure over the crista that bends with endolymph movement and deflects the hair cells.

What is the role of vestibular hair cells?

They convert mechanical deflection caused by head movement into changes in vestibular nerve firing.

What are stereocilia?

Graded hair-like projections on vestibular hair cells that bend during mechanical stimulation.

What is the kinocilium?

The tallest polarized projection on a vestibular hair cell that determines whether deflection excites or inhibits the cell.

What happens when vestibular hair bundles deflect toward the kinocilium?

The hair cell depolarizes and vestibular afferent firing increases.

What happens when vestibular hair bundles deflect away from the kinocilium?

The hair cell hyperpolarizes and vestibular afferent firing decreases.

Why is resting or tonic vestibular nerve firing important?

It allows head movement to be encoded through either an increase or decrease in firing rate.

How are the semicircular canals oriented?

They lie in approximately perpendicular planes so rotations in three dimensions can be detected.

How is the horizontal semicircular canal oriented relative to true horizontal?

It is tilted upward approximately 30 degrees when the head is upright.

Which semicircular canals form functional coplanar pairs?

Right anterior with left posterior; left anterior with right posterior; and the two horizontal canals with each other.

What is the vestibular push-pull system?

Head rotation excites a canal on one side while inhibiting its coplanar partner on the opposite side.

What normally occurs in the horizontal canals during a rightward head turn?

The right horizontal canal is excited and the left horizontal canal is inhibited.

What sensory structure detects linear acceleration and gravity in an otolith organ?

The macula.

What is the utricle?

An otolith organ primarily sensitive to horizontal linear acceleration and head tilt relative to gravity.

What is the saccule?

An otolith organ primarily sensitive to vertical linear acceleration and gravity.

What motion is most associated with utricular stimulation?

Horizontal translation, such as acceleration in a vehicle, and lateral head tilt.

What motion is most associated with saccular stimulation?

Vertical translation, such as moving in an elevator.

What is the otolithic membrane?

A gelatinous layer over the macula that contains embedded otoconia and shifts with gravity or linear acceleration.

What are otoconia?

Calcium-carbonate crystals that add mass and inertia to the otolithic membrane.

How do the otolith organs detect linear acceleration?

Inertia causes the otolithic membrane to shift relative to the macula, bending the underlying hair bundles.

What is the usual orientation of the utricular macula when the head is upright?

Approximately horizontal.

What is the usual orientation of the saccular macula when the head is upright?

Approximately vertical.

What is the vestibular portion of cranial nerve VIII?

The nerve division that carries information from vestibular hair cells to the brainstem and cerebellum.

What structures are innervated mainly by the superior vestibular nerve?

The anterior canal, horizontal canal, and utricle.

What structures are innervated mainly by the inferior vestibular nerve?

The posterior canal and saccule.

What is Scarpa's ganglion?

The vestibular ganglion containing the cell bodies of primary vestibular afferent neurons.

Where are the vestibular nuclei located?

Primarily in the pons and medulla of the brainstem.

What are the major central connections of the vestibular nuclei?

The ocular motor nuclei, cerebellum, spinal cord, thalamus/cortex, and autonomic centers.

What is the medial longitudinal fasciculus (MLF)?

A brainstem pathway that links vestibular nuclei with ocular motor nuclei to coordinate conjugate eye movement.

What is the vestibulo-ocular reflex (VOR)?

A reflex that moves the eyes equal and opposite to head movement to keep a visual target stable on the retina.

During a normal rightward head turn, which direction do the eyes move through the VOR?

The eyes move left to maintain fixation on the target.

What is the basic three-neuron arc of the VOR?

Vestibular afferent to vestibular nucleus, vestibular nucleus to ocular motor nucleus, and ocular motor neuron to the extraocular muscle.

What is the vestibulospinal reflex?

Vestibular activation of spinal motor pathways that helps maintain posture, equilibrium, and antigravity muscle activity.

What is the vestibulocollic reflex?

A reflex that activates neck muscles to stabilize the head relative to gravity and body movement.

What is the cervico-ocular reflex?

A reflex using cervical proprioceptive input to help stabilize gaze during trunk or neck movement.

What is the function of the cochlea?

To convert sound vibration into neural signals for hearing.

What artery directly supplies the inner ear?

The labyrinthine artery, which most commonly arises from the anterior inferior cerebellar artery (AICA).

Why can acute vertigo with new unilateral hearing loss indicate an AICA-territory event?

AICA commonly supplies both central vestibular structures and the labyrinthine artery serving the inner ear.

What is sensory reweighting?

The central nervous system's adjustment of how much it relies on vestibular, visual, and somatosensory information for balance.

When does visual input become especially important for balance?

When vestibular or somatosensory information is impaired and the environment provides a stable visual reference.

When does vestibular input become especially important for balance?

In darkness, on unstable surfaces, or during rapid head movement when vision or somatosensation is unreliable.

Why is a patient with vestibular loss and peripheral neuropathy especially unstable in darkness?

Darkness removes visual compensation while vestibular and somatosensory inputs are already impaired.

How does BPPV relate to normal vestibular anatomy?

Otoconia, usually displaced from the utricle, enter a semicircular canal or attach to a cupula and make the canal abnormally gravity-sensitive.