Neurophysiology PH5208: The Vestibular System PDF

Neurophysiology PH5208 The Vestibular System Dr. Moira Jenkins Functions of the Vestibular System • Reports orientation and rotation of the head • Control and coordinate movements • Help maintain muscle tone and posture • Stabilize images on the retina/fovea when the head moves • Equilibrium an...

Neurophysiology PH5208 The Vestibular System Dr. Moira Jenkins Functions of the Vestibular System • Reports orientation and rotation of the head • Control and coordinate movements • Help maintain muscle tone and posture • Stabilize images on the retina/fovea when the head moves • Equilibrium and balance ** Balance is unconscious and has input from 3 body systems vestibular system of inner ear vision proprioception • Vestibular is most important for balance but disruption of any could lead to imbalance, nausea, dizziness, vertigo • Both the vestibular system and the proprioceptor tracts project directly and indirectly to the cerebellum General Anatomy of the Vestibular System • Components are carved out of the petrous part of the temporal bone • The inner ear organ (other portion is the cochlea) • The vestibular apparatus/organ consists of 3 parts 1. semicircular canals (3) Vestibule 2. utricle 3. saccule • There is a membranous portion within the bony portion • Both contain fluid, perilymph outer, endolymph inner • Cranial Nerve -VIII Vestibulocochlear Nerve • Special Senses – equillibrium and hearing, SSA • Otolithic organs: • semicircular canals: − utricle − horizontal (lateral) − saccule − anterior (superior) − posterior The vestibular system is continuous with the cochlea: • these membranous organs are filled with endolymph • hair cells serve as the sensory receptors that transduce forces generated by motions into a neurochemical signal the vestibular sensory organs are responsive to: • gravitational forces induced by changes in orientation (tilt) • acceleration and deceleration forces generated by rotation of the head and translational movements of the entire body Kandel & Schwartz Fig. 40.1 Kinocilium Special Sensory Receptors of the Inner Ear – Hair Cells • Modified epithelial cells that act as mechanoreceptors • Apical end has “hairs” that are 30-50 microvilli (stereocilia) and 1 kinocilium (true cilia) • Head movement is converted to bending forces on the hairs • If stereocilia bend towards kinocilium there is depolarization and neurotransmitter release • If the stereocilia move away from the kinocilium there is inhibition or hyperpolarization of the cell • Synapse is at the dendritic ending of the vestibular afferents • The sensory receptor hair cell is not part of the afferent neuron! • Hair cells also have some efferent input to control sensitivity • Hair cells in semicircular canal are in the Cristae • Hair cells in the utricle and saccule are in the Maculae Hair Cells Stereocilia towards kinocilium → depolarize/excite Stereocilia away from kinocilium → hyperpolarize/inhibit “neutral position” → resting discharge Kandel & Schwartz Fig. 40-2 • deflection of the stereocilia towards the tall edge causes the hair cell to depolarize → increased neurotransmitter release → increased rate of firing of the afferent nerve fiber • deflection away from the tall edge causes the hair cell to hyperpolarize → decreased neurotransmitter release → decreased rate of afferent firing Vestibular perceptions are based upon varying patterns of differential signaling from two populations of hair cells two populations: → the hair cells are segregated into two groups, which have “opposite polarity” differential signaling: → when both populations are displaced in the same direction, the frequency of signaling increases from one, and decreases from the other → This provides increased sensitivity for detection and direction of motion neutral position same frequency (no difference) displacement increased decreased (differential signal) Vestibule – Utricle and Saccule • Detect static head position relative to gravity and linear acceleration • ka: static labyrinth or Otolithic Organs • Macula is the sensory organ of the saccule and utricle • Utricle –macula is in horizontal plane, detects horizontal movement • Saccule -macula is in vertical plane, detects vertical movement • Gel, embedded within it are otoliths/otoconia/statoconia – gel and otolith known as otolithic membrane Otolithic Organs The hair cells are separated into two populations with opposing polarity by a central line, the striola saccule: the orientation is vertical (sagittal plane): • responsive to forward and backward tilt (“pitch” – gravitational force); • also responds to upward vs. downward translational movement (vertical linear acceleration and deceleration – inertial force) utricle: the orientation is horizontal (transverse plane): • responsive to sideways tilt (“roll” - gravitational); • also responds to lateral translational movement (horizontal linear acceleration and deceleration inertial) Berne & Levy Physiology Fig. 8-28 Utricle and Saccule – Otolithic Organs • Hair cells are similar to crista but some are excited even at rest due to the otolith density • Provides static information • Linear or Vertical acceleration stimulates the otolithic membrane and hair cells translational movement Neuroscience Fig. 14.5 reporting (nervous signaling) from the otolithic organs is therefore “ambiguous” → both tilt (gravitational) and translational movement (acceleration/deceleration) will stimulate these receptors in a similar fashion → other senses (e.g. vision) will “clarify” which of these two possibilities caused the change in signaling from the otolithic organs Semicircular Canals • Detect angular acceleration or rotation of the head, ka: kinetic labyrinth • Special roll in coordinating eye and head movement in order to maintain focus (foveation) • Ampullae - opening of the canal into the utricle, area is widened • Sensory organ, Crista are located in this area (crista ampullaris). Hair sensory cells are embedded in a gel, cupula • Cupula and hair cells move when head rotates The cupula is displaced by angular acceleration/head rotation The endolymph and cupula move in the OPPOSITE direction of head rotation Rotate head to left, endolymph moves to right left vestibular nerve stimulated, right inhibited Semicircular Canals - Paired Semicircular canals are paired with canals on either side of head R. Anterior & L. Posterior R& L Horizontal L. Anterior & R. Posterior The 6 canals function as 3 coplanar pairs When hair cells in one of the canals are depolarized, the other Canal in the pair is hyperpolarized The semi-circular canals are organized into 3 co-planar pairs: • horizontal left and right • left anterior + right posterior • right anterior + left posterior Neuroscience Fig. 14.8 each pair is “tuned” to report rotation of the head in their respective plane: • the hair cells in the R and L canals of each pair have an opposite orientation • the hair cells in the canal towards which the head is turning will depolarize; versus those in the opposite canal which will hyperpolarize → this arrangement generates a “differential signal” between the R and L canals, with respect to rotation in the plane of one of these three pairs • the side from which increased signaling arises indicates the direction of rotation • the magnitude of the difference in signaling (increased minus decreased) will be proportional to the velocity of rotation Comparison of Semicircular Canals and Vestibule Canals • Rotary/angular acceleration and deceleration • Crista ampullaris- sensory organ • Cupula • Hair cells Utricle & Saccule • Linear/vertical/horizontal • Maculae- sensory organ • Otoliths (otolithic membrane) • Hair cells Vertigo • Otoliths/canaloliths can become dislodged and end up in a semicircular canal, simulate rotation • Benign paroxysmal positional vertigo BPPV • Most common cause of dizziness, vertigo • Maneuver to dislodge Vestibular Nuclei • At the pontine-medullary junction, posterior-lateral • 4 nuclei on each side of the brainstem • Special somatic afferent SSA The vestibular nuclei, located in the dorsal pons and medulla, receive and then distribute afferent signaling from the vestibular receptors Organized in nuclei that fibers report to medial & superior nuclei: (there is also direct vestibular input to the cerebellum) • semicircular canals (primarily) → vestibulo-ocular reflexes → coordination of eye & neck muscles Kandel & Schwartz Fig. 40.10 lateral nuclei: inferior (descending) nuclei: • otolith organs • otolith organs (primarily) → posture (extensor muscle groups) and vestibulo-ocular reflexes → coordination of balance and voluntary movements Vestibular Nuclei Projections 1. Cerebellum – control posture and coordinate movements, especially eye 2. Spinal cord – 2 pathways to cord from vestibular nuclei VESTIBULOSPINAL Neck Muscles & All Muscles for posture Descending Medial longitudinal Fasciculus aka: Medial Vestibulospinal Tract this descends to the level of the neck to control head and neck movements Vestibulospinal Tract aka: Lateral Vestibulospinal Tract this descends entire level of cord to control the extensors for posture ipsilateral 3. Motor nuclei of extraocular eye muscles in brainstem MLF Muscles that move the eyeballs Cranial Nerves Oculomotor, Trochlear, Abducens Path is the Ascending Medial Longitudinal Fasciculus Vestibulo-ocular Reflex, VOR – coordinate eye muscles so that image stays focused on retina/fovea even when head is rotating (fixation/foveation) **Medial Longitudinal Fasciculus – ascends and descends, main function to coordinate head and neck with vision Sensory input from the semicircular canals contributes to determining the “motor drive” to extraocular muscles The Medial Longitudinal Fasciculus (MLF) ascending tract carries fibers from the vestibular nuclei to the lower motor neurons/ Cranial nerve nuclei 3, 4, 6 The three planes in which the semicircular canals are oriented align to the cardinal positions of gaze Vestibulo-ocular Reflex, VOR – coordinate eye muscles so that image stays focused on retina/fovea even when head is moving (fixation/foveation) Tract – MLF is both ipsilateral and contralateral → Ex: The head turns to the left, the vestibulo-ocular reflex (“VOR”) will cause the eyes to rotate to the right so as to maintain the fixed gaze The VOR provides an example of reflex control over motor neuron activity via inputs arising from a particular set of sensory receptors, to cause a stereotypic motor response to a given stimulus Excitation to one group of muscles, inhibition to the antagonistic group of muscles along the same axis Fun examples of excellent vestibular reflexes Jordan Westerkamp Behind-the-Back Catch – YouTube Miracle on 10th St: Westerkamp "Hail Mary" Catch vs Northwestern - YouTube