Sensory Systems: The Vestibular System Dr. Jonathan Spindel CSD and ISAT James Madison University

Why? Dizziness and balance problems... 7 million patients annually #1 complaint of patients over age 70 #3 complaint of all patients (after headache and lower back pain) One in three people will have dizziness or balance dysfunction during their life. (85% of these are due to inner ear involvement)

What is “dizziness?” vertigo disequilibrium lightheadedness (also … fuzzy-headed, blurred vision, off-kilter, stumbly, drunk, not right …)

“the four legged stool” vestibular visual proprioceptive diffuse CNS Balance & Motion Management

vestibular visual proprioceptive auditory other Common Central Processing Individual Central Processing Eye Movements Posture Adaptive Controller

The Vestibular System Head Motion: Angular Acceleration Head Motion: Linear Acceleration Head Position: Gravity Semicircular Canals Saccule & Utricle Central Nervous System Other Inputs Spinal Cord & Cerebellum: Posture Forebrain: Orientation Oculomotor System: Eye Movements

The Vestibular System Head Motion: Angular Acceleration Head Motion: Linear Acceleration Head Position: Gravity Semicircular Canals Saccule & Utricle Central Nervous System Other Inputs Spinal Cord & Cerebellum: Posture Forebrain: Orientation Oculomotor System: Eye Movements

The Vestibular Periphery 3 semi-circular canals (sense angular acceleration) 2 otolith organs (maculae); saccule and utricle (sense linear acceleration)

The Sensory Element: Vestibular Hair Cells Transducers converting mechanical energy into neuro-chemical signals interpretated by the brain as motion of the head or head position KEY POINT: Continuous activity with directional sensitivity. Excited by bundle deflection towards KC. Inhibited by bundle deflection away from KC.

Hair Cells in Action

Otolith Organs: Saccule & Utricle Function: Static tilt (gravity) Linear acceleration

Otolith Organs: Static Tilt and Linear Acceleration

Semicircular Canals Function: Angular acceleration VOR

Semicircular Canals: Structure of the “Crista”

SCC System: Organization Canals function in paired sets: right anterior & left posterior right posterior & left anterior right horizontal & left horizontal Motion inducing excitation of one canal induces inhibition of the other. NOTE: Canals are not orthogonal (at right angles to) normal directions of body/head movement, therefore most regular motions stimulate multiple canal sets.

The Vestibular System Head Motion: Angular Acceleration Head Motion: Linear Acceleration Head Position: Gravity Semicircular Canals Saccule & Utricle Central Nervous System Other Inputs Spinal Cord & Cerebellum: Posture Forebrain: Orientation Oculomotor System: Eye Movements

The Vestibulo-Ocular Reflex (Excitatory) “Three-Neuron Arc”

Vestibulo-Ocular Reflex (VOR)

Nystagmus (In general, the objective of most vestibular testing is to look for nystagmus to be there when it should be, and to not be there when it shouldn’t be.) Normal vestibular nystagmus is a series of eye motions which reflexively keep center vision focused on the fovea of the eye during head motion.

The Vestibular System Head Motion: Angular Acceleration Head Motion: Linear Acceleration Head Position: Gravity Semicircular Canals Saccule & Utricle Central Nervous System Other Inputs Spinal Cord & Cerebellum: Posture Forebrain: Orientation Oculomotor System: Eye Movements

Brainstem: Vestibular Nucleus Posterior “Phantom” Aspect

Vestibular Nuclear Complex Lateral VN (LVN) -input from utricle and semicircular canals (maintenance of posture) Medial VN (MVN) & Superior VN (SVN) -input primarily from semicircular canals (primary participant in VOR) Descending (Inferior) VN (DVN) -input from semicircular canals, utricle and saccule, and vermis of the cerebellum (integrate vestibular reflexes with other sensory/cerebellar input) (First site of central sensory interaction integration. Misnomer since only a small number of VNC neurons receive input from vestibular periphery, most receive input from other sensory systems (visual, proprioceptive, auditory, etc.).

The Vestibular System Head Motion: Angular Acceleration Head Motion: Linear Acceleration Head Position: Gravity Semicircular Canals Saccule & Utricle Central Nervous System Other Inputs Spinal Cord & Cerebellum: Posture Forebrain: Orientation Oculomotor System: Eye Movements

Lateral Vestibulo-Spinal Tract Control of head and neck, upper limb, trunk and lower limb movements in response to motion.

Medial Vestibulo-Spinal Tract MVST fibers do not extend beyond the thoracic spinal segments. Primary participants in movements that control head and upper body response to motion.