Unlocking the Mysteries of the Vestibular System A brief discussion of the anatomy and physiology of the vestibular system

Ear Anatomy

Inner Ear Structures: Cochlea Vibration of the tympanic membrane stimulates the cochlea, thus producing sound.

Inner Ear Structures: Semicircular Canals (3) 3 Semicircular Canals: Anterior (superior), Posterior and Horizontal Each canal containing endolymph Sensitive to angular rotation Contains cupula

Inner Ear Structures: Cupula Hair cells deflect with movement of endolymph, thus stimulating (same side) or inhibiting (contralateral side) the inner ear.

Otoconia

Vestibular Nuclei

Summary of Inner Ear Anatomy Cochlea- sound 3 Semicircular Canals- sensitive to angular rotation Cupula- hair cells sensitive to displacement of endolymph Otolith- Saccule and Utricle sensitive to gravity and linear acceleration Vestibular Nuclei- area(s) of brain where impulses travel from vestibularcochlear nerve (CN VIII)

Vestibular System Function Detects linear, angular and rotational movements of the head Gaze stability (VOR) Postural stability/Balance Head and body orientation

Physiology of Vestibular System: How does it work? Sensing Angular Rotation: Push Pull Mechanism (SCC) Sensing Linear Acceleration and Gravity (Otoliths) Motor Response: Vestibulo Ocular Reflex (VOR)

Sensing Angular Rotation: Push-Pull Mechanism 3 SCC structurally arranged to sense head rotation in all angles. Each canal is paired with another on the opposite side of the head. Rotation causes excitation to same side and inhibition to the opposite side. The disparity of firing rates of the two sides allows us to sense rotation and initiates a motor response (VOR). The larger the disparity, the more intense the response.

Push-Pull Mechanism

Vestibulo Ocular Reflex (VOR) Stabilizes vision with head movement. Responds to head rotation by stimulating extraocular musculature. Thus, eyes move in an equal and opposite direction of the head movement. Can be accurate up to 2hz Relationship between head rotation and eye movement is called gain. Normally a 1:1 ratio.

Nystagmus Normal with head movement, typically as a VOR response when eyes are not fixated on a target or when in the absence of light. NOT normal in the absence of head movement. Typically seen as a slow movement phase of the eyes followed by a quick phase in the opposite direction.

Nystagmus Direction of nystagmus is designated by the direction of the quick phase relative to the patient. Nystagmus can be suppressed when concentrating on a visual reference. Pure vertical nystagmus may be more consistent with a central problem.

Nystagmus Video What is the direction of the fast phase? Is this horizontal or vertical nystagmus?

Nystagmus video #2 What is the direction of the fast phase? Is this horizontal or vertical nystagmus? Is this normal? This was a video taken from our IR goggles. This patient was not moving her head at all.

Nystagmus Video #3 What is the direction of the fast phase? Is this purely horizontal or vertical nystagmus? Is this normal?

Nystagmus Video #4 What is the direction of the fast phase? Is this horizontal or vertical nystagmus? Is this normal?