SHB 2019 Vestibular System Robert J. Frysztak, Ph.D.

CONCEPTS The vestibular system is essential for maintaining the position of the body in space, which, in turn, is important for coordination of motor responses, eye movements, and posture. The vestibular system regulates the movement and position of the head in space in response to signals associated with angular and linear acceleration of the head, as well as the gravitational pull exerted on it.  LEARNING OBJECTIVES FOR THE VESTIBULAR SYSTEM List, describe, and diagram the anatomy and physiology of different components of the vestibular system, including the semicircular canals, saccule, utricle, orientation of hair cells, cupola and otolithic granules. Describe and diagram central vestibular pathways, including their connections to the cerebellum, ascending and descending motor systems, and the visual system. List and describe mechanisms by which maintenance of balance and posture are mediated. Describe clinical disorders associated with the vestibular system. Describe nystagmus, vestibuloocular reflex and the rationale for why we experience dizziness.

The Vestibular System (and Balance) Balance is not normally a “conscious” process, but is essential for the proper functioning of the motor activities, eye movements and normal posture. Proper balance requires functional vestibular, proprioceptive and visual systems to be present, but only 2 of the 3 systems are essential for balance.

The Vestibular Apparatus Located in the petrous portion of the temporal bone just lateral to the internal acoustic meatus along with the cochlea.

* Semicircular canals - anterior, posterior, horizontal. The bony and membranous labyrinths are continuous with those of the auditory system, as are the fluids contained in each (perilymph and endolymph). There are 3 main divisions: Semicircular canals - anterior, posterior, horizontal. Utricle - connects 3 canals - otolithic membrane (gravity & acceleration sensor*). Saccule - continuous with cochlea - otolithic membrane. Remember that the endolymph has a unique chemical make-up which is essential for the creation of the receptor potentials (K+ >> Na+). As in the auditory system, hair cells are the primary receptors. *

The hair cells in the semicircular canals are located in a swelling (ampulla) in each canal, and consists of the ampullary crest (nerve) and the cupola, which is a gelatinous substance which covers the hair cells. Cupula *

Rotation of the head to the LEFT causes an inertial displacement of the fluid and the cupola, which results in bending of the hair cells to the RIGHT. The hair cells are sensitive to movements as small as 0.1/s2 (acceleration force - NOT velocity). When head turns to LEFT Fluid pushes cupula RIGHT Inertial displacement The inertial displacement of the fluid pushes the cupola in the opposite direction due to the weight of the fluid (endolymph) in the canal.

* + Inertial Displacement This also results in differential displacement of the hair cells on the left and right horizontal canals. The cortex perceives these corresponding increases and decreases in output as directional movement (left or right) with a specific “velocity.” The greater the difference, the faster the perceived velocity. LEFT RIGHT * +

Functionally, the hair cells themselves are oriented in specific planes: the horizontal canals are oriented towards the utricle. the anterior & posterior canals are oriented away from the utricle. Semicircular canals Anterior Horizontal Posterior Horizontal Anterior Anterior Posterior Posterior The anterior canal on the right is paired with the posterior canal on the left; The posterior canal on the right is paired with the anterior canal on the left; The horizontal canals are paired. *

Voltage-gated calcium channel The hair cells of the SSC are polarized both structurally and functionally: There is ONE large kinocilium and 40-70 stereocilia per hair cell. The stereocilia are mechanically linked to the kinocilium (tip links similar to auditory system), and thus give the hair cell an orientation axis. Voltage-gated calcium channel Tonically Active The vestibular nerve has a tonic output, and the hair cells serve to modify that output. Bending of the stereocilia TOWARDS the kinocilium causes depolarization of the receptor, an increased release of the NT, and an increase in the number of AP’s in the vestibular nerve. Bending of the stereocilia AWAY from the kinocilium results in hyperpolarization and a decrease in AP’s from the vestibular nerve. There are NO AP’s in the hair cells! Bending of the stereocilia perpendicular to the axis has NO EFFECT. Increased impulse frequency EXCITATION Decreased impulse frequency INHIBITION Tonically Active *

Saccule & Utricle The cupola in the saccule and utricle are also gelatinous, but contain otolithic granules (calcium carbonate crystals), which have weight, and therefore respond to gravity. *

There is no specific plane of orientation of the hair cells in the otolithic organs (although many believe that the kinocilia are oriented towards the striola, an “S” shaped line running through the otolithic membrane of the utricle). The relative output changes with head position, such as laying down, or standing on your head.

Utricle: detects upright position and head tilt; considered the primary gravity sensor; also detects horizontal linear acceleration (not velocity). Saccule: detects general orientation and vertical linear acceleration. Superior vestibular ganglion Inferior vestibular ganglion Utricle Saccule Otolithic organs *

Input to the CNS Hair cell  bipolar cell (AP’s)  vestibular ganglion  CN VIII  Vestibular nuclei. 2 divisions of the vestibular ganglion: Superior: utricle, anterior portion of saccule, horizontal & anterior SSC Inferior: posterior portion of saccule and posterior SSC

* Castro, Merchut, Neafsey, Wurster; Neuroscience: An Outline Approach, 2002

Vestibular nuclei: There are four divisions of the vestibular nuclei, which have distinct functional and anatomical components: Lateral: utricle and saccule - concerned with posture (extensor muscle groups) and vestibulo-ocular reflexes (via MLF). Medial & Superior: SSC and some fibers from utricle (very few fibers from saccule) - concerned with vestibulo-ocular reflexes and coordination of eye & neck muscles. Inferior (Descending): all areas - projects to cerebellum, and coordinates balance with voluntary movements.

MLF: Medial Longitudinal Fasciculus Ascending fibers (MLF) to control eye movements. MLF: Medial Longitudinal Fasciculus MLF ascending fibers Inputs *

MLF: Medial Longitudinal Fasciculus Descending fibers (MLF) to control head & neck movements. MLF: Medial Longitudinal Fasciculus MLF descending fibers *

Cerebellar Influences VESTIBULOCEREBELLUM: associated with the flocculonodular lobe and vestibular nuclei. Receives direct vestibular input from semicircular canals and otolithic organs, as well as indirect input from the vestibular nuclei, lateral geniculate, superior colliculus & pontine nuclei. Output is projected back to vestibular nuclei, to control axial and proximal limb muscles associated with balance and posture; also controls eye movements and coordination of head & eye movements.

Vestibulospinal tracts Lateral Vestibulospinal Tract From the Lateral Vestibular nucleus Uncrossed projection Entire length of the cord - ventral funiculus Proximal limb muscles Maintains balance by acting on the limbs Medial Vestibulospinal Tract (or “descending MLF”) From the Medial Vestibular nucleus Bilateral projection (?) Cervical spinal cord only – medial ventral funiculus Neck muscles Maintains head erect Medial Vestibulospinal Lateral Vestibulospinal Medial vestibulospinal tract (MLF) Lateral vestibulospinal tract

* Summary: Vestibular Pathways Primary Afferents Vestibular nuclei Flocculonodular lobe Secondary Vestibular connections Cerebellum Flocculonodular lobe, Cerebellar vermis and Fastigial nucleus Spinal cord Lateral and medial vestibulospinal tract Brainstem Ascending MLF, Reticular formation, Vestibular commisural connections Thalamic and cortical areas. *

Vestibular system and the eyes Stabilize our gaze during a head movement or when we shift gaze to a new target The effector system is the extraocular muscles. The vestibular system, cerebral cortex, cerebellum and brainstem are all involved. These types of eye movements include: Vestibulo-ocular reflex Optokinetic response (& nystagmus) Smooth pursuit Saccadic eye movements Vergence eye movements

Vestibulo-ocular reflex (VOR) Stabilizes the image on the retina during a rotation of the head and faster than visual tracking As the head rotates the VOR rotates the eyes with the same speed, but in the opposite direction Without this reflex, the image would appear “smeared” upon the retina Active almost all of the time. If the vestibular system is damaged, then you can't focus on objects when the head/body is in motion! You must first stop all movement. Once the head stops moving the eyes remain in that same direction of gaze “stabilization” occurs through the nucleus prepositus hypoglossi tonic activation maintains the activation/activity of the involved cranial nerve nuclei (the 3rd and 6th) Attenuates with tonic stimulation a t t e n u a t i o n *

Vestibulo-ocular reflex Head is rotating to the right The right horizontal canal is activated Right vestibular nucleus is “activated’ The left 6th nucleus (via PPRF) is activated and the left lateral rectus muscle contracts The left PPRF “activates neurons in the right 3rd nucleus and the right medial rectus contracts …both eyes begin to move to the left Flocculus integrates adjustments to the gain of the V-O system: Purkinje cells can inhibit vestibular interneurons. Adaptive feed-forward "open-loop" control. *

Optokinetic Reflex The optokinetic reflex is a combination of a saccade and smooth pursuit eye movements. It is seen when an individual follows a moving object with their eyes, which then moves out of the field of vision at which point their eye moves back to the position it was in when it first saw the object. As long as the stimulus is present, the reflex continues (does not attenuate). The reflex develops at about 6 months of age. When factitious blindness or malingering is suspected, check for optokinetic nystagmus to determine whether there is an intact visual pathway. Click the image to watch: In factitious disorder, an individual feigns, exaggerates, or actually self-induces physical or psychiatric illness to achieve ends such as mobilizing care and concern, ventilating aggression, diminishing guilty feelings, or gratifying dependency wishes. Factitious disorder is the term used to describe a pattern of behavior centered on the exaggeration or outright falsifications of one’s own health problems or the health problems of others. Some people with this disorder fake or exaggerate physical problems; others fake or exaggerate psychological problems or a combination of physical and psychological problems. Factitious disorder differs from a pattern of falsified or exaggerated behavior called malingering. While malingerers make their claims out of a motivation for personal gain, people with factitious disorder have no such motivation.

Saccades, Smooth Pursuit & Vergence Saccadic system: Rapid movements to "catch up" to a target which has moved, using highly stereotyped responses. Velocity of response is determined by the distance of the object from the fovea. Speeds can be up to 900°/sec, leaving very little time for feedback modifications. Adaptable to changes in muscle function/weakness. Horizontal saccades generated by the pontine reticular formation, Vertical saccades generated by the mesencephalic reticular formation (rostral mlf). Saccades are typically under the control of the superior colliculus & cerebral cortex (frontal eye fields). Modulation by the cerebellum is via the flocculus and the dorsal vermis (fastigial n.) Smooth pursuit: Voluntary movement requiring a moving stimulus/target. Velocity is determined by the speed of the target: Maximum velocity of 100°/sec Requires the cerebral cortex to “choose” target. Vergence: Eyes converge on near objects, diverge on far objects. Retinal disparity gauges (perceives) distance. Linked to the accommodation of the lens. Organized in the midbrain, near the oculomotor nuclei. Smooth pursuit Saccade

* What is nystagmus ? Rhythmic back and forth movement of the eyes Usually the movement is slow in one direction (“smooth”) and fast (“saccadic”) in the other When you induce it by spinning yourself around…. The VOR is generating the slow phase in the opposite direction to the spin which helps to keep an eye on a target Once the eye approaches the maximum that it can turn, a saccade will then occur moving the eyes in the same direction as the spin and onto a new target (Optokinetic nystagmus or OKN) Tested clinically by the caloric test (“COWS”) or a spinning drum. Nystagmus occurs during sustained rotation, and has 2 distinct phases: SLOW - opposite the direction of rotation. FAST - returns gaze to starting position (towards rotation). Nystagmus habituates in the dark, due to adaptation of the semicircular canals *. Nystagmus is coordinated in the brainstem by a 3 neuron arc: Semicircular canal - Vestibular nuclei - Oculomotor neurons (III, IV, VI) Therefore eye muscles are coordinated with semicircular canals V-O changes the "head velocity signal" to an "eye velocity signal". Optokinetic system: ·  Responsible for rotary nystagmus, which takes over for vestibular nystagmus in light, and continues to function as long as the visual system is intact. ·  long latency and slow decay; vestibular system still habituates. ·  Adaptive under feed-forward "open-loop" system also. ·  Reflex CAN be suppressed if head AND target are both moving. ·  Visual information from cortex and pretectum integrated to facilitate reflex. *

How do we get dizzy? Vestibular input without vision While spinning in a chair with your eyes closed (the constant motion eventually results in the cupula membrane returning to its baseline) and you suddenly open your eyes Sense of motion via the visual system, but without vestibular “confirmation” (“a disconnect”) Looking out a car window when an adjacent car moves away (false sense of motion) Sense of motion via the vestibular system, but without visual “confirmation” (“a disconnect”) In the cabin of a boat during a storm (motion sickness) Looking at the horizon (stable point) helps to minimize this effect

And Finally . . .

I wonder if it’s edible? Sure smells good.

And Finally . . . Did you notice who is rowing ?