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Lecture 13

Lecture 13 - Vestibular, Taste, and Olfaction

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Michelle French

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PSL300H1F L13, Wed Oct. 12, 2011 Net pressure bending stereocilia Vestibular, Taste, and Olfaction  receptor potential Vestibular apparatus • Mechanoreceptors in 2 inner ear structures: • semicircular canals; detect angular acceleration • otolith organs; detect linear forces or accelerations on head(remember F = ma) Semicircular canals • Really are complete circles, not semicircles • Set of 3 on each side of head • oriented orthogonally (right angles) to one another so cover all 3 dimensions in space • not same as hair follicle receptors in skin • hair cell receptors located within enlargement called Hair cell “ampulla” • Stereocilia polarize the hair cell by a gradient in • hair cell stereocilia embedded in the cupula length and ‘kinocilium’ (longest) – long on one side, (gelatinous) which forms a flexible barrier across the short on other ampulla • stereocilia are joined by tip links • bending cilia towards longest one stretches tip links yank on prots in membrane  open up ion (Na, K) channels  depolarization, EPSPs and other receptor potentials thru mechanoreceptors • bending cilia towards shortest one relaxes tip links  closes ion channels hyperpolarization ○ since in normal upright stage – no motion; there is tension so there is some receptor potential (slightly depolarizing) due to background activity, so can signal both directions • horizontal not rly horizontal, at angle • anterior is largest • fluid goes thru each circle • at ampulla, have block of gelatinous substance, flap • rly the cupula that will bend the stereocilia • hair cell stereocilia embedded in gelatinous cupula Stimulus • Rotation of head causes walls of semicircular canals to rotate with head • endolymph inside canals stays put (inertia)  exerts pressure on cupula in direction opposite to the head rotation – cupula fixed in walls of canal  bulge in cupula of stereocilia • only canals oriented within or close to plane of head rotation are stimulated Bilateral effects • For a given canal, hair cell response can be either similar or opposite on the 2 sides of the head • rotating head to right side  depolarizes horizontal canal hair cells on right side (max spiking), hyperpolarizes these afferents on left side of head (no discharge) ○ balanced out • some planes w symmetrical response • usually opposite of pairs have opposite response • gelatinous matrix replaced by calcium carbonate • anterior on one side paired w posterior on other side • pick up linear forces  bend stereocilia • horizontals pair up on both sides ○ could theoretically lose one horizontal canal but need both for centering Adaptation • Hair cells are slowly adapting • BUT: constant rotational velocity gradually elicits endolymph movement thru friction  net movement of fluid • net force on cupula decays over time no hair cell deflection after 30 s  no more signal of rotational velocity so signals acceleration • get opposite effect • pick up direction/influence of gravity since pushing on cupula in other • gave other orientations present within the saccule direction  hyperpolarization • different sets of hair cells • mechanical effect • saccule picks up from fluid vertical orientations/force s • utricle picks up horizontal/sidewa Otolith organs ys • Receptors are also hair cells orientations/force • cilia embedded in matrix containing calcium s carbonate crystals • fracture line ○ effect of net motion of crystals called striola – get • cilia deflected by linear accelerations (forces) flipping of • adapt to constant linear velocities orientations, respond to same plane but opposite directions (like for directional cells in V1) – all info proprioceptive in nature of what head is doing at one time – info goes to Area 3a of S1 – vestibular info, stretching muscle spindles  head mapping of proprioceptive system Olfactory receptors • 2 Chemoreceptor systems we talk about • otolith organs near utricle and saccule • Unique – primordial sensory system developed in ○ difference is orientation mammals • over 1,000 dif receptor genes; ~350 rec prots expressed in humans ○ compared to 3 in eyes for colour ○ clustered together • each olfactory cell contains one receptor type • olfactory cells have their own axon which projects to olfactory bulb in the brain ○ so if lose a cell (ex. Cold), regenerate all the time, so regenerate the axons all the time so recreate the whole pathway Sensory transduction • Resting potential of potential of -45 mV • odorant response is slow depolarization • receptor is G-protein coupledactivates adenylyl • lateral inhibition by neighbouring neurons cyclase  [cAMP] increases cAMP-gated cation channels opened depolarization  action potentials ○ strongest odour dominates ○ project to several parts of brain (some at initial segment of axon autonomic, all limbic – edge of nervous system – emotion experience + motivation) • project straight to cortex without going thru thalamus whereas all other sensory systems go thru thalamus Olfactory Sensory Neurons • rmbr when going thru g-prot couple receptors – need
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