21 Vestibular Apparatus & Olfactory System

21 Vestibular Apparatus & Olfactory System -...

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Unformatted text preview: PSL302Y: Lecture 21, by Prof. MacKay! Wed., Nov. 3, 2010 Vestibular Apparatus & Olfactory System Mechanoreceptors in 2 inner ear structures: Vestibular, Taste & Olfaction 1) semicircular canals: detect angular acceleration 2) Otolith organs: detect linear forces/accelerations (remember F = ma) November 3, 2 1. Semicircular canals - Set of 3 on each side of head - Each is oriented in a plane orthogonal to one another - Hair cell receptors located w/i ampulla - Hair cell stereocilia embedded in the cupula (flexible, gelatinous barrier across ampulla) - Structure: each canal forms circle filled w/ fluid - Enlargement at base of each one (ampullae) - Each canal defines 1 plane of 3D space Vestibular, Taste & Olfaction November 3, - Horizontal canal not exactly horizontal (angled) - Plane on one side not exactly same as on other side of head Vestibular, Taste & Olfaction November 3, 20 stimulus - True for all other canals Rotation of head causes walls of - Ampulla semicircular canals to rotate with head - Hair cells on pedestal at base endolymph inside canals stays put; exerts pressure on cupula in direction opposite to - Cilia stick out into cupula the head rotation - Canal filled w/ endolymph only canals oriented within or close to plane Hairofcell are stimulated head rotation - Cupula bisects canal into two parts, so fluid can't flow from Stereocilia polarize the hair cell by a one end to other gradient in length and `kinocilium' stereocilia are joined by tip links Stimulus bending cilia towards longest one stretches - Rotation of head -> walls of semicircular canals to rotate w/ head s depolarization tip links depo p o - Endolymph inside canals stay put (inertia) bending cilia towards shortest one relaxes - Exerts pressure on cupula in net direction opposite to head rotation = tip links hyperpolarization deflect cilia of hair cells -> produce receptor potential - Only canals oriented w/i plane or close to plane of head rotation are stimulated Hair cells: in polarized arrangement - Stereocilia polarize the hair cell by a gradient in length and kinocilium - Stereocilia are joined by tip links - Bending cilia twds longest one stretches tip links = depolarization (open ion channels) - Bending cilia twds shortest one relaxes tip links ! = hyperpolarization 1 of 6 PSL302Y: Lecture 21, by Prof. MacKay! Wed., Nov. 3, 2010 - Stretch tip links -> open ion channels = depolarization - Relax tip links -> close ion channels = hyperpolarization - Normally: cilia sit vertically -> modest tension in tip links - Triggers some depolarization of hair cell = release vesicles -> produce EPSPs - Slow rate of a.p. production in afferents w/ NO stimulus = background activity - This might seem wasteful, like in visual system (ON & OFF channels) - Background activity in receptor cell: means it&can signal increase & decrease Vestibular, Taste Olfaction - If it didn't, it could only signal depolarization - We can bend in one direction -> increase or decrease a.p. firing rate AND bend in other direction -> hyperpolarization = silence! Bilateral effects - For a given canal, hair cell response can be either similar/opposite on the 2 sides of head - I.e. For ahead to R side: opposite responses on can be Rotating given canal, hair cell response canal either side of head either similar or opposite on the 2 sides of - Depolarizes horiz canal hair cells on R side - Hyperpolarizes cells on L side of head the head - L posterior canal is roughly in same plane as R rotating depolarizes anterior canal head to right side - But if you had rotation in this plane, one side depolarized, other side hyperpolarized November 3, 2010 Bilateral effects horizontal canal hair cells on right side, hyperpolarizes them on left side of head Vestibular, Taste & Olfaction November 3, 2010 Adaptation - Hair cells are slowly adapting receptors: maintain response as long as stimulus present Adaptation - BUT constant rotational velocity gradually elicits endolymph mvt thru friction Hair cells are slowly adapting - Net force on cupula decays over time BUT: constant rotational velocity gradually = No hair cell deflection after 30s elicits endolymph movement through friction net force on cupula decays over time e o ce o cupu dec ys ove e hair cell deflection after 30 s no o 5 Otolith organs Receptors are also hair cells cilia embedded in matrix containing2 calcium carbonate crystals cilia deflected by linear accelerations ( (forces) ) adapt to constant linear velocities of 6 6 Vestibular, Taste & Olfaction November 3, 2 PSL302Y: Lecture 21, by Prof. MacKay! Wed., Nov. 3, 2010 2. Otolith organs - Complementary to semicircular canals Vestibular, Taste ear - Completely separate chambers in inner& Olfaction - Receptors are also hair cells - Cilia embedded in matrix containing calcium carbonate crystals - Cilia deflected by linear accelerations (forces) - Adapt to constant linear velocities - Structure: at bottom, under semicircular canals' ampullae - Two parts: utricleVestibular, Taste & Olfaction + saccule - Inside: hair cells synapses to afferent axons - Stereocilia embedded in gelatinous matrix filled w/ calcium carbonate crystals -> cause cilia deflection Novem November 3, 2010 - Tonically: gravity pulls on stones = no effect - Tilt head backward: gravity = cilia deflection - Various orientations of hair cells to detect various 8 directions of linear forces - Utricle: arrangements in horizontal plane - Arrows: polarization of stereocilia (direction of linear acceleration it detects) - Diff preferred directions of acceleration along diff positions in utricle - Striola: across fracture line, arrangement switches to opposite direction - Saccule: arrangements in vertical plane - Two otolith organs cover pretty much everything b/c two sides don't match up perfectly - Vestibular system: info about position of head in space - Proprioceptive info -> projects straight to proprioceptive map in somatosensory cortex (area 3a of central sulcus) 9 3 of 6 PSL302Y: Lecture 21, by Prof. MacKay! Wed., Nov. 3, 2010 Olfactory receptors: primordial sensory system (1st to evolve) -> some features are stupid! - Chemoreceptors: most sensory systems have finite # receptors that code all kinds of stimuli - BUT +1,000 diff receptor genes! (30,000 coding genes in human genome) - About 350 receptor proteins expressed in humans - Each olfactory cell contains 1 receptor type - Olfactory cells have their own axon which projects to olfactory bulb in brain - Lose olfactory cells all the time: so replaced readily...but must regrow whole axon! Sensory transduction - Resting potential of -45mV (initially slightly depolarized) Vestibular, Taste & Olfaction - Odorant response is slow depolarization Vestibular, Taste & Olfaction November 3, 2010 - Receptor: G-protein coupled (metabotropic receptor) - Activates adenylyl cyclase -> [cAMP] -> cAMP-gated cation channels open - Depolarization -> a.p.'s at initial segment of axon - Odorant binds to G-protein receptor - Adenylyl cyclase -> [cAMP] - cAMP-gated Na+/Ca++ channels open = [Ca++] = depolarization - Conducted into sensory dendrites -> initial segment of axon (high density of voltagegated Na+ channels) = summation of receptor potentials = a.p. generated -> brain - Amplification of signaling: olfaction very sensitive! November 3, 20 Vestibular, Taste & Olfaction November 3, 2010 Olfactory bulb: target of olfactory axons - Axons project thru bottom of skull = cribiform bone - Location: sits over nose, on bottom of frontal lobe, either side of midline - All axons of same receptor type (in bunches, don't collect in 1 nerve) terminate in a common glomerulus of mitral cell dendrites - Altogether constitute Ist cranial nerve -Glomerulus: knot bulb Olfactory of dendritic branches of post-synaptic mitral cell + olfactory axon terminals Axons of olfactory receptors project -Inhibitory interneurons = lateral through bottom of skull (cribiform bone) inhibition btwn adjacent mitral cells -If one is strongly activated, silence both all axons of same receptor type terminate in on either side = one mitral cell a common `glomerulus' of stimulus dominate -Mitral dendrites cells target piniform cortex (sensory cortex for olfaction) + amygdala + hypothalamus -But NOT thalamus: all other sensory systems go here first -Olfaction straight -> piniform cortex - Hypothalamus related to ANS & amygdala related to basal ganglia (-> ANS) 12 4 of 6 12 Vestibular, Taste & Olfaction November 3, 2010 PSL302Y: Lecture 21, by Prof. MacKay! Population code for odor Wed., Nov. 3, 2010 different olfactory receptors Population code for odor set of responsive olfactory cells activates a receptors...use population code, b/c no way we can have all the - With allset of glomeruli (and hence mitral these diff olfactory specific receptors for everything we can smell in envmt! cells) mitral cells project to usually activates # of diff olfactory receptors - Specific odour `limbic' cortex - Set of responsive olfactory cells activates a specific set of glomeruli (and hence mitral cells) - Mitral cells project to limbic cortex - "edge of cerebral cortex" -> assoc w/ emotional experience, learning & memory BLUE: PHENOLS RED: PENTANOIC ACID Specific odor usually activates a number of lar, Taste & Olfaction -Mouse olfactory bulb: huge relative to rest of brain (vs. ours) -Colours = territory of mitral cells activated by specific smell -Blue: activated by phenols, vs. red: activated by pentanoic acid -Population Novemberdifferent spatial regions for each odour codes: 3, 2010 -Some regions overlap: so mitral cells can respond to +1 smell -Diff smells activate diff sets of mitral cells altogether Diagram: upside-down brain (underside of frontal lobe) -Midline = olfactory bulb (another on other hemisphere) -Olfactory bulb: mitral cell axons -> piniform cortex -Location: edge of temporal lobe, part of limbic cortex (-> 13 memory?), above amygdala -At limbic cortex: further processing of olfactory stimuli -Use population code: greater degree of selectivity -Passed onto other regions of brain -Finally projects to thalamus (after processing!): then sent off to other assoc regions Vestibular, Taste & Olfaction -Assoc cortex: in orbitofrontal region of frontal lobe ("at the back of the eye sockets") -Taste region (gustatory) beside olfactory area Vestibular, Taste & Olfaction Novem November 3, 2010 Taste bud - Cluster of taste receptor cells + supporting cells - Embedded in epithelium of tongue + palate - Some receptor cells have synaptic cxn to sensory axons Taste Bud - Some do not: have to influence sensory axons indirectly - Do not have own axons: if receptor cell damaged, don't have to cluster of taste axon as cells and supporting regrow the receptor well, just the cell cells embedded in epithelium of tongue and out taste pore (`psuedo-cilia') - Structure: Tip of taste cell stick palate in mucous-lined epithelium -p Some direct synaptic some receptor cells havesynapses w/ sensory afferents y p - At tip of to sensory axons; some do not connection tongue: on top of papillae = fungiform taste buds - At back of tongue: in folds (protect buds from physical damage) - Side = folate taste buds - Back = circumvallate buds - W/i each taste bud: full complement of all types of taste receptor cells 14 Taste Bud (contains all types of taste receptor cell, TRC) Taste buds located within various papillae. 15 Taste Bud (contains all types of taste receptor cell, TRC) 5 of 6 Taste buds located within various papillae. Primary taste classes Ion channel coupled receptors: PSL302Y: Lecture 21, by Prof. MacKay! 1) sour (PKD2L1, a TRP ion-channel receptor) 2) salty (amiloride-sensitive Na+ channel?) Wed., Nov. 3, 2010 G-protein coupled receptors: (`T1R' and `T2R' families) Primary taste receptor classes 4) bitter (T2R family) Ion-channel coupled receptors: directly open cation channels -> (meat-like flavor; T1R1 + T1R3 dimer) 5) umami private lines 1) SOUR (PKD2L1, a TRP ion-channel receptor) 2) SALTY (amiloride-sensitive Na+ channel) G-protein coupled receptors: T1R and T2R families -> party lines 3) SWEET (T1R2 + T1R3 dimer) Neural coding of taste 4) BITTER (T2R family) 5) UMAMI (meat-like/MSG-like flavor; T1R1 + T1R3 dimer) - Metabotropic: amplify signals ...to taste efficiently, mustwarm receptor cell responds to one of the 5 each up receptors Taste Receptor Cells Taste Bud Salt and sour receptor cells Taste receptor cells synapse onto their own afferents lines') activity in single taste cells however, does (`private - SALT + SOUR receptor cells synapse onto their own well-defined The other 3 notofuniquely identify a given flavor, and types receptor cell, cell axon afferents = private lines sweet, bitter and umami, do not nociceptor directly synapse onto afferents. - Other 3 types (SWEET + BITTER + UMAMI) don't directlythere is much convergence of afferents (hot spicy) They release trnasmitter (probably ATP) into the extracellular space, synapse onto any axon afferents eventually binds either to code required to identify specific population where it afferent axons or to the - Release transmitter (probably ATP) into ECSnonspecific tastes: ratio of activity across a set of ionotropic-type (salt/sour) receptor salty cells. Signal g from - Transmitter eventually binds either to non-specifictransmissionreceptors sour sweet, bitterreceptor cells and umami is via a receptor sweet afferent axons or to ionotropic-type (salt-sour)`party line'. A population code is required to discriminate bitter nonspecific cells = party lines these tastes. afferents umami Nociceptors (for hot-spicy - A population code is required to discriminate these tastesare located in sensation) - Nociceptors (for hot-spicy sensation) are located inepithelium outside of taste buds. epithelium outside of taste buds -> but added in with taste signals in brain Vestibular, Taste & Olfaction 3) sweet (T1R2 + T1R3 dimer) Nov classes of tastants Neural coding of taste - Each receptor cell responds to 1 of 5 classes of tastants - Activity in single taste cells however, does not uniquely identify a given flavour - Can't rely on particular afferent to define taste - And there is much convergence of afferents - Population code required to identify specific tastes - Ratio of activity across set of receptor cells - Pathway: conveyed thru cranial nerves VIIth & IXth - Terminate in taste region of medulla: Nucleus of Solitary Tract (receives all kinds of chemoreceptive info - more about it on ANS) - Projects up to parabrachial nucleus - Ends up in thalamus: no interest in spatial order so projects to both sides = primary gustatory cortex (G1), located Vestibular, Taste & Olfaction down in lateral fissure (insula) Taste of water: G1 located in insula - Water might be added to list of primary tastes (w/ our current 5) - It makes a big diffce whether interested in that taste or not - Not thirsty vs. thirsty = diff regions of insula activated maximally - Whether they want stimulus or not determines how big the region of cortex responds Insula November 3, 2010 Taste of Water (fMRI scan) thirsty not thirsty 6 of 6 ...
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This note was uploaded on 03/27/2012 for the course PSL PSL300 taught by Professor Mackayfrench during the Fall '11 term at University of Toronto- Toronto.

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