Vestibular Sensation & System PDF

Vestibular sensation Sense of self-motion and orientation Balance Reflex that stabilize our vision when our head is moving ◦ Vestibular ocular reflex: eyes automatically move in the opposite direction that our head is moving LINEAR TRANSLATION Axes defined relative to our head (not g...

Vestibular sensation Sense of self-motion and orientation Balance Reflex that stabilize our vision when our head is moving ◦ Vestibular ocular reflex: eyes automatically move in the opposite direction that our head is moving LINEAR TRANSLATION Axes defined relative to our head (not gravity) ◦ X axis: forward movement (through the nose) ◦ Y axis: side to side (through the ears) ◦ Z axis: up and down axis ROTATION: Roll rotation: around x axis (side to side) ◦ Ear towards shoulder Pitch rotation: forwards and backwards rotation around y axis ◦ Ex: yes/nod Yaw rotation: turning on the spot movement (around z axis) ◦ Ex: shaking “no" The inner ear Vestibular organs ◦ 3 semicircular canals: anterior canal, horizontal canal, posterior canal Sense rotational movement (roll, pitch, yaw) Filled with endolymph Otolith organs: utricle and saccule ◦ Sense linear translation movement (x, y, z axis) ◦ Sense gravitational force: knowing which way is upright Hair Cells Receptors: hair cells with stereocilia embedded in a gelatinous medium ◦ Located in cristae of the semicircular canals ◦ Maculae of the 2 otolith organs 2 TYPES 1. Type 1 Bulbous shape Irregular firing pattern at rest (when person is still) 2. Type 2 More elongated Regular firing pattern at rest (when person is still) Stereocilia bending cause change in firing rate of hair cell ◦ Toward kinoclillium (tallest) = increase in firing rate ◦ Away from kinocillium= decrease in firing rate Vestibular transduction Semicircular canals Each canal has an ampulla ◦ Within which is the crista: contains the vestibular hair cells ◦ Hair cells are aligned in the same direction 3 canals on each side arranged orthogonally to each other (right anterior = left posterior) Push-pull arrangement ◦ For each one on the left, there is one on the right with the same preferred axis of rotation ◦ 2 canals with same preferred orientation = push & pull ◦ Same rotation causes an increased firing rate from the hair cells on 1 side = decreased firing rate on the other side Directional sensitivity Each canal is most sensitive to rotation about the axis that passes through the centre of the canal Rotations about axes orthogonal to this preferred axis do not cause movement of the endolymph within the canal ◦ Not cause deflection of the cupula or activation of the hair cells Rotation about an axis close to the preferred axis will be detected ◦ Sum of rotations including preferred axis ◦ Any arbitrary rotations can be expressed as the sum of 3 orthogonal rotations Dynamic Response Changes in firing rate of canal hair cells is proportional to the angular velocity Sudden acceleration = cupula to deflect and firing rate to decrease suddenly Over a prolonged constant rotation, the deflection of the cupula will subside and the firing of the hair cells will return to baseline after ~15s When rotation stops, the reverse happens: with the semi-circular canals signalling a rotation in the opposite direction Otolith organs 2 utricle and saccule on each side ◦ Each contain a macula containing vestibular hair cells ◦ Cilia of the hair cells are embedded within a gelatinous layer topped with crystals of calcium carbonate (otoconia) Linear acceleration and gravity They are indistinguishable Equivalence principle: both linear acceleration and gravity cause displacement of the otoconia, bending the stereocilia of the hair cells and altering their firing rate Otolith dynamics Will fire proportionally to a constant tilt (or acceleration) Movement in the preferred direction cause the stereocilia to bend toward Kinocillium = increase firing rate Movement in the opposite direction cause the stereocilia to bend away from Kinocillium = decrease firing rate