Kinesthetic OrgansOverview• What is the function of the vestibular system?• How is motion detected in the vestibular system?• How does the vestibulo-ocular reflex work?The vestibular systemVestibular System in 2 minutes: https://youtu.be/P3aYqxGesqs3 semicircular canals (anterior, posterior, horizontal)2 otoliths (utricle and saccule)Vestibular apparatus• Semicircular canals Angular accelerations• Otolith organs (utricle and saccule) Linear accelerations Static head positionBox 14A A Primer on Vestibular NavigationOverview• What is the function of the vestibular system?• How is motion detected in the vestibular system?• How does the vestibulo-ocular reflex work?Sensory receptors of the vestibular systemSensory receptors: hair cells - located in the ampullae, utricle and saccule(Part 1)Cross-section of hair cells with kinocilia of a group of hair cells located on the same side of the hair cell.Top view of hair cellsStereociliamove toward kinocilium↓depolarization ↓neurotransmitter release ↓excitation of vestibular nerveMorphological polarization of hair cells in the utricular and saccular maculaeResponse from a otolith afferent fiber in a monkey seated in a tilting chair•Prior tilt: high firing rate (above zero)• firing rate increases or decreases depending on the direction of tilt•Firing rate remains constant as long as tilting force remains constant encoding static forces applied to the headForces acting on the head displace the otolithic membrane of the utricular maculaSchematic representation of the action of otolith at rest (left), when the head is tilted (centre) and during linear acceleration (right). The action of otolith is similar both during tilting of the head and during linear acceleration.Orientation of the utricular and saccular maculae in the headArrows = orientation of kinociliaRange of orientations signal linear forces in every direction the head might moveUtricle and saccule Utricle:  Oriented horizontally  responds to lateraldisplacement of the head. Saccule:  Oriented vertically responds to up/downand forward/backwarddisplacements of the head.The ampulla of the posterior semicircular canalSemicircular canalsSensory receptors: hair cells in the ampullaSense head rotationsFunctional organization of the semicircular canals Canals and ampullae full of endolymphHair bundles involved in into the cupula (gelatinous mass) is a barrier for endolymphHead turns in the plane of one semicircular canal  inertia of endolymph force across the cupula  cupula (and hair cells) displacement in opposite direction of head movement  depolarization or hyperpolarizationResponse of a vestibular nerve axon from semicircular canal to angular acceleration•Prior acceleration: high firing rate (above zero; baseline)• Initial acceleration: firing rate increases  cupuladeflection  despolarization•Constant velocity: return to baseline; cupula returns to non-deflected state•Deceleration: decreased fing rate; cupula defletion in opposite direction• Benign paroxysmal positional vertigo: Unpleasant sensation of head spinning when head moves – due to otoconia debris creating currents in endolymph fluid. • Many causes  concussionsOverview• What is the function of the vestibular system?• How is motion detected in the vestibular system?• How does the vestibulo-ocular reflex work?Functional organization of the semicircular canals2 sets of semicircular canalsVestibular nystagmus. Rotating chair: Head motion results in smooth counter-rotation of the eyes in opposite direction to head motion (slow phase), interrupted at more or less regular intervals by a quick flick in the other direction (quick phase), giving rise to a sawtooth-like eye movement. Initiated by semicircular canalsDynamic contributionsConnections underlying the vestibulo-ocular reflex 1. Head turns left2. Eyes turn right3. Activate: left medial rectus and right lateral rectusGalvanic Vestibular Stimulation (GVS)Application of weak electrical stimulation to mastoid bones that carries through the head and interferes with vestibular system. Leads to postural sway responses. When seated leads to illusion of moving!!GVS elicits sway in the direction opposite to the negative electrode (the side the person perceives they are tilting to). This response is automatic and