Spatial Orientation and the Vestibular System ⢠vestibular organs ...
Ch. 12- Spatial Orientation and the Vestibular System •
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vestibular organs/system= set of five specialized organs- three semicircular canals and two otolith organs- located in each inner ear that sense head motion and head orientation with respect to gravity. Make a predominant contribution to our sense of tilt and our sense of selfmotion ◦ sixth sense= contributes to clear vision when we move and helps us maintain balance when we stand ◦ appeared early in evolutionary history and have remained relatively unchanged= fundamental ◦ largely automatic system- vestibular perception is often relegated to the attentional background, and may responses evoked by the system are felexive spatial orientation= a sense consisting of three interacting sensory modalities, the senses of linear motion, angular motion and tilt (our sense of tilt and self motion). Dizzines (nonspecifically indicates any form or perveiced spatial disorientation with or without instability), spatial disorientation (any impairment of spaital orientation. More specifically, any impairment of our sense of linear motion, angular motion or tilt), imabalnce (lack of balance; unsteadiness; falling over), blurred vision, illusory self-motion= promotes awareness of our vestibular sensations vestibular system= reflexively rotates eyeballs in the sockets to compensate for head rotation= helps stabilize images on the retina compensate for head movement more readily than hand movement because of vestibulo-ocular reflex often don't perceive your head rotating because vestibular system performs its job automatically Modalities and Qualities of Spatial Orientation sense of spatial orientation is based on three sensory modalities: ◦ angular motion= rotational motion like the rotation of a spinning top/swinging saloon doors that rotate back and forth ◦ linear motion= translational motion like the predominant movement of a train car/ bobblehead doll ◦ tilt= attaining a sloping position like that of the leaning tower of Pisa ◦ modalities like senses NOT qualities because energy is transduced (coverted from one form to another) ▪ color and brightness are different interpretations of the same light= qualities ▪ seeing and hearing involve different types of stimulation energy= light and pressure waves= modalities ▪ perceiving angular motion, linear motion and tilt= requires three different kinds of stimulation energy to be sense- angular acceleration, linear acceleration, gravity= modalities Sensing Angular Motion, Linear Motion and Tilt sensed b y two types of vestibular sense organs: semicircular canals and otolith organs ◦ semicircular canals= any of three toroidal tubes in the vestibular system that sense ANGULAR MOTION ◦ otolith organs= either of two mechanical structures (utricle and saccule) in the vestibular system that sense both linear acceleration (change in linear velocity) and gravity ▪ predominant contribution to sense of linear motion (sense of translation) and sense of
head tilt • 2 sense organs (semicircular canals and otolith organs) but 3 modalities? ◦ Otolith organs= transduce both gravity and linear acceleration= 2 distinct modalities ▪ stimulation energies for gravity and linear acceleration are different so otolith organs attempt to separate the signals of the two ▪ so overall= 3 interacting sensory modalities= a sense of angular motion, a sense of linear motion, a sense of tilt= parallel the three different stimulation energies- angular acceleration, linear acceleration, gravity • Basic qualities of spatial orientation: AMPLITUDE AND DIRECTION ◦ each of the three spatial orientation modalities includes two quantities: amplitude and direction ◦ amplitude= speed/ strength with which a movement is carried out ◦ direction= 3 directions for angular/linear motion, and 2 directions for tilt (because “yaw” causes head rotations to align with gravity so there is no change in tilt) • Mammalian Vestibular System • vestibular organs= found in the inner ear right next to the cochlea ◦ each inner ear= has one vestibular labyrinth/system= and each system has 5 sense organs= three semicircular canals that sense rotational/angular movement, and 2 otolith organs that sense gravity and linear acceleration ◦ NEITHER otolith organs/semicircular canals respond to CONSTANT velocity= they respond to changes in velocity called acceleration ▪ constant motion, whether angular or linear does not result in vestibular signals that directly indicate motion ▪ otolith organs transduce both linear acceleration and gravity into a single neural signal sent to the brain • Hair Cells= Mechanical transducers ◦ hair cells= any cell that has stereocilia for transducing mechanical movement in the inner ear into neural activity sent to the brain; some also receive input from the brain ◦ act as mechanoreceptors= a sensory receptor that responds to mechanical stimulation (pressure, vibration, movement)= stereocilia deflection causes a change in the hair cell voltage which alters neurotransmitter release, which in turn, evokes action potentials in those vestibular nerve fibers that have 1 or more synapses on the hair cell= afferent neurons carry these action potentials to the brain ◦ absence of stimulation= hair cells have a negative charge/voltage and release neurotransmitter at a constant rake= constant rate of action potentials ▪ changes in the hair cell voltage =(receptor potential= a change in voltage across the membrane of a sensory receptor cell (Ex. Hair cell in vestibular system) in response to stimulation= are proportional to the bending of the hair cell bundles and control the rate at which the hair cells release neurotransmitters when a hair cell bends toward the tallest stereocilia= hair cell voltage becomes less negative= depolarization= increases release of neurotransmitter= increase in action potential rate (excitation) • opposite= bend away from tallest stereocilia, hyperpolarization, inhibition • summary: rate of action potentials transmitted by afferent neurons depends following the hair cell receptor potential **so amplitude of motion is proportional to amount of hair cell deflection= receptor potential= rate of action potentials= encoding of amplitude
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Semicircular Canals each inner ear has three semicircular canals- horizontal/lateral, anterior/superior, posterior= roughly orthogonal to one another ampulla= expansion of each canal duct that includes the canal's cupula, crista, and hair cells where transduction occurs crista= any of the specialized detectors of angular motion located in each semicircular canal in a swelling called the ampulla cilium= any of the hairlike structures that move as part of vestibular transduction= includes kinocilia and stereocilia three semicircular canals are maximally sensitive to rotations around the axis perpendicular to it and insensitive to rotations about axes that fall in the plane of that canal= yields direction coding for head rotation absence of any rotation= most afferent neurons fire with a nearly constant rate of 100 action potentials per second ( high relative to spontaneous rates for nerve fibers for other sensory systems= allows neurons to decrease the firing rate for rotations in one direction while increasing the rate for rotations int he opposite direction) semicircular canasl= functional pairs= push-pull arrangement (opposite increase/decrease in firing rates) ◦ two horizontal canals (right/left) lie in the same plane= 1 pair ◦ right anterior and left posterior canals= 2nd pair= push pull ◦ left anterior and right posterior canals= 3rd pair change in neural activity is proportional to the angular velocity of the head
Otolith Organs 2 otolith organs for sensing gravity and linear acceleration utricle and saccule ◦ each of these organs consists of small oval-shaped, fluid-filled sac ◦ and an area called the macula= any of specialized detectors of linear acceleration and gravity found in each otolith organ ◦ each utricular macula contains about 30,000 hair cells and each saccular macula contains about 16000 hair cells ◦ utricular and sacular maculae are innervated by about 4000 neurons each ◦ macula= planar and sensitive to shear forces (forces parallel to the macular plane) whereas perpendicular forces have little influence on neural response ◦ otoconia= tiny calcium carbonate stones in the ear that provide inertial mass for the otolith organs, enabling them to sense gravity and linear acceleration= found in the gelatinous structures that cilia of otolith hair cells are incased in= millions of these in the utricle and sacculae= denser than surrounding fluid so pulled by both gravitational force and inertial force due to linear acceleration= resulting displacement of otoconia drags the gelatinous layer, moving the hair cell stereocilia, changes in the hair cell receptor potentials, change in the rate of action potentials of afferent neurons **Recall 1. hair cell receptor potentials increase when hair bundle tips move toward the LARGEST stereocilia 2. receptor potentials decrease for movements in the opposite directional 3. direction of rotation is coded by excitation from a semicircular canal on side side and inhibition on the other side
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similar mechanisms for otolith organ macula= populations of hair cells on each macula have their stereocilia oriented in opposite directions tilts/linear accelerations in opposite directions cause opposite changes in firing rate= so a tilt or an acceleration that maximally excites a hair cell and afferent neuron on one side of the striola (central band, of which on opposite sides hair cells are orientated in opposite directions) will maximally inhibit a hair cell and afferent neuron on the opposite side larger accelerations/gravitational shear forces= move otolith organs otoconia more= greater deflection of hair cell bundles, larger changes in receptor potentials b/c receptor potential evoked in a given hair cell is proportional to the component of gravity/linear acceleration that is aligned with sensitive axis of that hair cell= larger changes in the hair cell receptor potential lead to larger changes in the rate of action potentials sent to the brain via afferent neurons DIRECTIONAL coding in otolith organs arises from anatomical orientation ◦ plane of utricular macula= horizontal ◦ plane of saccular macula= vertical ◦ sensitive to gravity/linear acceleration IN THE PLANE of the macula, and insensitive to gravity/linear acceleration PERPENDICULAR to the macula= utricle is sensitive primarily to HORIZONTAL linear accelerations and gravitational forces while saccule is sensitive to primarily VERTICAL linear accelerations and the vertical gravitational forces ◦ other component of direction coding= Variations in the orientation of different hair cells= different hair cells respond maximally to different movement directions (direction of maximal sensitivity varying systematically across the plane of each macula from forwardback sensitivity to side-side sensitivity)
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vestibular organs/system= set of five specialized organs- three semicircular canals and two otolith organs- located in each inner ear that sense head motion and head orientation with respect to gravity. Make a predominant contribution to our sense of tilt and our sense of selfmotion ◦ sixth sense= contributes to clear vision when we move and helps us maintain balance when we stand ◦ appeared early in evolutionary history and have remained relatively unchanged= fundamental ◦ largely automatic system- vestibular perception is often relegated to the attentional background, and may responses evoked by the system are felexive spatial orientation= a sense consisting of three interacting sensory modalities, the senses of linear motion, angular motion and tilt (our sense of tilt and self motion). Dizzines (nonspecifically indicates any form or perveiced spatial disorientation with or without instability), spatial disorientation (any impairment of spaital orientation. More specifically, any impairment of our sense of linear motion, angular motion or tilt), imabalnce (lack of balance; unsteadiness; falling over), blurred vision, illusory self-motion= promotes awareness of our vestibular sensations vestibular system= reflexively rotates eyeballs in the sockets to compensate for head rotation= helps stabilize images on the retina compensate for head movement more readily than hand movement because of vestibulo-ocular reflex often don't perceive your head rotating because vestibular system performs its job automatically Modalities and Qualities of Spatial Orientation sense of spatial orientation is based on three sensory modalities: ◦ angular motion= rotational motion like the rotation of a spinning top/swinging saloon doors that rotate back and forth ◦ linear motion= translational motion like the predominant movement of a train car/ bobblehead doll ◦ tilt= attaining a sloping position like that of the leaning tower of Pisa ◦ modalities like senses NOT qualities because energy is transduced (coverted from one form to another) ▪ color and brightness are different interpretations of the same light= qualities ▪ seeing and hearing involve different types of stimulation energy= light and pressure waves= modalities ▪ perceiving angular motion, linear motion and tilt= requires three different kinds of stimulation energy to be sense- angular acceleration, linear acceleration, gravity= modalities Sensing Angular Motion, Linear Motion and Tilt sensed b y two types of vestibular sense organs: semicircular canals and otolith organs ◦ semicircular canals= any of three toroidal tubes in the vestibular system that sense ANGULAR MOTION ◦ otolith organs= either of two mechanical structures (utricle and saccule) in the vestibular system that sense both linear acceleration (change in linear velocity) and gravity ▪ predominant contribution to sense of linear motion (sense of translation) and sense of
head tilt • 2 sense organs (semicircular canals and otolith organs) but 3 modalities? ◦ Otolith organs= transduce both gravity and linear acceleration= 2 distinct modalities ▪ stimulation energies for gravity and linear acceleration are different so otolith organs attempt to separate the signals of the two ▪ so overall= 3 interacting sensory modalities= a sense of angular motion, a sense of linear motion, a sense of tilt= parallel the three different stimulation energies- angular acceleration, linear acceleration, gravity • Basic qualities of spatial orientation: AMPLITUDE AND DIRECTION ◦ each of the three spatial orientation modalities includes two quantities: amplitude and direction ◦ amplitude= speed/ strength with which a movement is carried out ◦ direction= 3 directions for angular/linear motion, and 2 directions for tilt (because “yaw” causes head rotations to align with gravity so there is no change in tilt) • Mammalian Vestibular System • vestibular organs= found in the inner ear right next to the cochlea ◦ each inner ear= has one vestibular labyrinth/system= and each system has 5 sense organs= three semicircular canals that sense rotational/angular movement, and 2 otolith organs that sense gravity and linear acceleration ◦ NEITHER otolith organs/semicircular canals respond to CONSTANT velocity= they respond to changes in velocity called acceleration ▪ constant motion, whether angular or linear does not result in vestibular signals that directly indicate motion ▪ otolith organs transduce both linear acceleration and gravity into a single neural signal sent to the brain • Hair Cells= Mechanical transducers ◦ hair cells= any cell that has stereocilia for transducing mechanical movement in the inner ear into neural activity sent to the brain; some also receive input from the brain ◦ act as mechanoreceptors= a sensory receptor that responds to mechanical stimulation (pressure, vibration, movement)= stereocilia deflection causes a change in the hair cell voltage which alters neurotransmitter release, which in turn, evokes action potentials in those vestibular nerve fibers that have 1 or more synapses on the hair cell= afferent neurons carry these action potentials to the brain ◦ absence of stimulation= hair cells have a negative charge/voltage and release neurotransmitter at a constant rake= constant rate of action potentials ▪ changes in the hair cell voltage =(receptor potential= a change in voltage across the membrane of a sensory receptor cell (Ex. Hair cell in vestibular system) in response to stimulation= are proportional to the bending of the hair cell bundles and control the rate at which the hair cells release neurotransmitters when a hair cell bends toward the tallest stereocilia= hair cell voltage becomes less negative= depolarization= increases release of neurotransmitter= increase in action potential rate (excitation) • opposite= bend away from tallest stereocilia, hyperpolarization, inhibition • summary: rate of action potentials transmitted by afferent neurons depends following the hair cell receptor potential **so amplitude of motion is proportional to amount of hair cell deflection= receptor potential= rate of action potentials= encoding of amplitude
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Semicircular Canals each inner ear has three semicircular canals- horizontal/lateral, anterior/superior, posterior= roughly orthogonal to one another ampulla= expansion of each canal duct that includes the canal's cupula, crista, and hair cells where transduction occurs crista= any of the specialized detectors of angular motion located in each semicircular canal in a swelling called the ampulla cilium= any of the hairlike structures that move as part of vestibular transduction= includes kinocilia and stereocilia three semicircular canals are maximally sensitive to rotations around the axis perpendicular to it and insensitive to rotations about axes that fall in the plane of that canal= yields direction coding for head rotation absence of any rotation= most afferent neurons fire with a nearly constant rate of 100 action potentials per second ( high relative to spontaneous rates for nerve fibers for other sensory systems= allows neurons to decrease the firing rate for rotations in one direction while increasing the rate for rotations int he opposite direction) semicircular canasl= functional pairs= push-pull arrangement (opposite increase/decrease in firing rates) ◦ two horizontal canals (right/left) lie in the same plane= 1 pair ◦ right anterior and left posterior canals= 2nd pair= push pull ◦ left anterior and right posterior canals= 3rd pair change in neural activity is proportional to the angular velocity of the head
Otolith Organs 2 otolith organs for sensing gravity and linear acceleration utricle and saccule ◦ each of these organs consists of small oval-shaped, fluid-filled sac ◦ and an area called the macula= any of specialized detectors of linear acceleration and gravity found in each otolith organ ◦ each utricular macula contains about 30,000 hair cells and each saccular macula contains about 16000 hair cells ◦ utricular and sacular maculae are innervated by about 4000 neurons each ◦ macula= planar and sensitive to shear forces (forces parallel to the macular plane) whereas perpendicular forces have little influence on neural response ◦ otoconia= tiny calcium carbonate stones in the ear that provide inertial mass for the otolith organs, enabling them to sense gravity and linear acceleration= found in the gelatinous structures that cilia of otolith hair cells are incased in= millions of these in the utricle and sacculae= denser than surrounding fluid so pulled by both gravitational force and inertial force due to linear acceleration= resulting displacement of otoconia drags the gelatinous layer, moving the hair cell stereocilia, changes in the hair cell receptor potentials, change in the rate of action potentials of afferent neurons **Recall 1. hair cell receptor potentials increase when hair bundle tips move toward the LARGEST stereocilia 2. receptor potentials decrease for movements in the opposite directional 3. direction of rotation is coded by excitation from a semicircular canal on side side and inhibition on the other side
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similar mechanisms for otolith organ macula= populations of hair cells on each macula have their stereocilia oriented in opposite directions tilts/linear accelerations in opposite directions cause opposite changes in firing rate= so a tilt or an acceleration that maximally excites a hair cell and afferent neuron on one side of the striola (central band, of which on opposite sides hair cells are orientated in opposite directions) will maximally inhibit a hair cell and afferent neuron on the opposite side larger accelerations/gravitational shear forces= move otolith organs otoconia more= greater deflection of hair cell bundles, larger changes in receptor potentials b/c receptor potential evoked in a given hair cell is proportional to the component of gravity/linear acceleration that is aligned with sensitive axis of that hair cell= larger changes in the hair cell receptor potential lead to larger changes in the rate of action potentials sent to the brain via afferent neurons DIRECTIONAL coding in otolith organs arises from anatomical orientation ◦ plane of utricular macula= horizontal ◦ plane of saccular macula= vertical ◦ sensitive to gravity/linear acceleration IN THE PLANE of the macula, and insensitive to gravity/linear acceleration PERPENDICULAR to the macula= utricle is sensitive primarily to HORIZONTAL linear accelerations and gravitational forces while saccule is sensitive to primarily VERTICAL linear accelerations and the vertical gravitational forces ◦ other component of direction coding= Variations in the orientation of different hair cells= different hair cells respond maximally to different movement directions (direction of maximal sensitivity varying systematically across the plane of each macula from forwardback sensitivity to side-side sensitivity)
