Vestibular System and Labyrinth

Vestibular System Overview

• Animals use the vestibular system to track their head position relative to the world.
• Though vision helps, it cannot differentiate between the animal’s movement and the movement of the external world
• The vestibular labyrinth detects both linear and angular acceleration in any direction through receptors.

Vestibular Labyrinth

• The bony labyrinth's vestibule contains two membranous labyrinth expansions: the saccule (connects to the cochlear duct) and the utricle (connects to the saccule).
• Each semicircular canal has a semicircular duct connected to the utricle at both ends.
• The five aforementioned structures comprise the vestibular section of the labyrinth.
• Otolithic organs, consisting of the utricle and saccule, detect linear acceleration.
• Angular acceleration is detected by the semicircular ducts.
• The vestibular labyrinth contains hair cells, like the cochlea, with stereocilia in a gelatinous mass
• The physical arrangement of the gelatinous masses determines the mechanical sensitivity of the different sections of the labyrinth.

Otolithic Organs and Linear Acceleration

• The macula, a curved patch within the utricle and saccule, houses the otolithic organs' hair cells.
• Stereocilia are inserted into the otolithic membrane, a gelatinous structure with calcium carbonate crystals (otoliths).
• Otolithic membranes are denser than endolymph, making them gravity-responsive.
• Maculae with vertically oriented stereocilia do not respond much to upward force.
• Tilts from a stereocilia-vertical position cause the otolithic membrane to shift and deflect the stereocilia.
• The utricular macula is most sensitive to tilts from a head-upright position; the saccular macula is most sensitive to tilts from a head-sideways position.
• Utricle and saccule can signal tilts in any direction because each macula contains hair cells with varied orientations.
• The density of otolithic membranes makes them sensitive to linear acceleration.
• The utricular macula is most sensitive to linear acceleration in horizontal planes, while the saccular macula is most sensitive to acceleration in sagittal planes.
• In a normal head-upright position, the stereocilia of saccular hair cells are tonically deflected.

Semicircular Canals and Angular Acceleration

• Semicircular ducts have a different mechanical sensitivity strategy.
• Each duct has an ampulla (dilatation) with a partition (crista), which contains hair cells and a gelatinous flap (cupula)
• The cupula's density matches the endolymph, making it gravity-insensitive.
• Initial rotation in a semicircular duct's plane causes the endolymph to lag, pushing on the cupula and deflecting hair cells.
• The endolymph catches up after 10 seconds of constant angular velocity.
• Endolymph continues at the end of rotation, deflecting the cupula in the opposite direction.
• Semicircular ducts are sensitive to angular velocity changes (angular acceleration).
• Angular acceleration in any direction moves endolymph in at least one duct because the three semicircular ducts on each side are in orthogonal planes.

Vestibular Pathways

• Orientation change-related vestibular signals contribute to conscious awareness and postural adjustments and eye movements.
• CNS vestibular pathways reflect these functions.

Vestibular Nuclei and Their Connections

• The vestibular branch of CN VIII primarily terminates in the vestibular nuclei in the CNS.
• These nuclei are largest near the pontomedullary junction, where the eighth nerve enters the brainstem, and are laterally located.
• The vestibular nuclei project to the thalamus, spinal cord, and cranial nerve nuclei III, IV, and VI.
• The cerebellum is involved in coordinating trunk and eye movements and has interconnections with the vestibular nuclei.

Vestibular Projections to Cerebral Cortex

• Conscious awareness of vestibular stimuli is mediated by a pathway from the vestibular nuclei to the thalamus near the ventral posterolateral and posteromedial nuclei (VPL/VPM).
• The thalamus projects vestibular information to cortical areas, including a posterior part of the insula and parietal areas near the somatosensory cortex's head representation.

Vestibulospinal Tracts

• The vestibular nuclei project to the spinal cord via the lateral and medial vestibulospinal tracts.
• The lateral vestibulospinal tract, an uncrossed pathway to extensor muscle motor neurons, reaches all spinal cord levels and primarily mediates postural adjustments.
• The medial vestibulospinal tract is a bilateral projection to neck muscle motor neurons that helps stabilize the head.
• It runs in the medial longitudinal fasciculus (MLF), coordinating head and eye movements.

Vestibulo-ocular Reflex

• It’s critical to prevent images from moving rapidly across the retina.
• Locomotion causes head bobbing, which would move images on the retina unless the eyes compensate.
• The vestibulo-ocular reflex (VOR) compensates with fast, simple, three-neuron reflex arcs
• The afferent limb is vestibular primary afferents; interneurons in the vestibular nuclei project through the MLF and reticular formation to cranial nerves VI, IV, III motor neurons.

Nystagmus

• The VOR compensates for small head movements but not large ones
• Sustained rotation is interrupted by quick eye movements (nystagmus) that stabilize images on the retina for most of the rotation
• If the rotation continues in the dark, endolymph catches up and nystagmus stops. Lights sustain nystagmus via optokinetic nystagmus, caused by visual stimuli. Endolymph keeps going at rotation's end, and nystagmus resumes in the opposite direction.
• Studying rotation-induced nystagmus can yield diagnostic information.
• Irrigation with warm or cool water fools the vestibular system into thinking the head is rotating as it sets up convection currents (caloric nystagmus).
• Nystagmus is a normal response, but vestibular system damage can cause pathologic nystagmus.

Combining Multiple Sources of Position Information

• Hair cells adapt to positions, and semicircular canals stop during constant rotation, thus, the vestibular system is inadequate alone.
• Visual and vestibular information must be combined to compensate.
• Neurons in the vestibular nuclei get visual inputs, and vestibular cortical areas get visual and somatosensory inputs.

Clinical Significance - Romberg's Sign

• Romberg's sign (swaying or falling worsening with eyes closed) indicates defective proprioception or vestibular function because closing eyes removes one of three balance systems.