Chapter 15 - PSYB51

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Mathias Niemier

Chapter 15 – Spatial Orientation and Vestibular System  Vestibular system: set of 5 organs – 3 semicircular canals and 2 otolith organs – located in each inner ear that sense head motion and head orientation with respect to gravity  Spatial orientation: sense comprised of 3 interacting sensory modalities – sense of linear motion, angular motion and tilt  Vestibular “sixth sense” provides fundamental contributions that are often overlooked  Fundamental nature of vestibular system is emphasized by fact that vestibular organs appeared very early in evolutionary history and have remained relatively unchanged  Explanation may be that it wasn’t until 19 century that scientists understood that vestibular system is specialized set of organs  Another explanation may be inconspicuous nature of vestibular sense Modalities and Qualities of Spatial Orientation  Sense of spatial orientation is based on 3 sensory modalities – senses of linear motion, angular motion and tilt  Key lies in energy transduced and receptors  Sensing linear motion, angular motion and tilt involves different receptors and/or stimulation energy – modalities  Vestibular system transduces 3 distinct stimulation energies – gravity, angular acceleration and linear acceleration – using 2 different types of sense organs  Otolith organs: mechanical structures in vestibular system that sense both linear acceleration and gravity o Force of gravity, pulling us toward center of Earth, trasduced by otolith organs  Semicircular canals: 3 toroidal tubes in vestibular system that sense angular motion  Sensitivity of vestibular system to acceleration for otolith organs – demonstrates that vestibular system is principally sensitive to changes in motion  Otolith organs sense 2 stimuli with physically distinct causes – gravity and linear acceleration o Makes signals from otolith organs ambiguous because otolith organs trasduce both linear acceleration and relative orientation of gravity into neural signal  Each 3 spatial orientation modalities includes 2 qualities – direction and amplitude o Amplitude: magnitude of displacement (increase or decrease) of head movement 3 Spatial Orientaiton Modalities – Angular Motion, Linear Motion and Tilt  Sense of angular motion: spatial orientation modality that senses motion resulting from rotation  Sense of linear motion: spatial orientation modality that senses translation  Sense of tilt: spatial orientation modality that senses head inclination with respect to gravity  Relatively pure linear motion is more difficult to achieve passively  To experience sense of tilt, close eyes and nod head saying “yes”, then pitch head forward and hold it there for 7 seconds and pitch head backward andhold Basic Qualities of Spatial Orientation – Direction and Amplitude  Direction o Direction is quality for each of spatial orientation modalities o Define simple coordinate system that moves with head o X-axis always points forward, y-axis always points out the left ear and z-axis always points out of top of head o Head can rotate with roll angular velocity, as tilt left or right from upright, or can rotate with pitch angular velocity, as nodding “yes” or head can rotate with yaw angular velocity when shaking “no” o Roll, pitch and yaw angular velocities always defined with respect to head o 3 dimensions of linear motion o As for angular velocity, translations are defined relative coordinate system that moves with head o Translation along a straight line in any direction can be expressed as some amount of each of 3 perpendicular transitions o Translation along x-axis, y-axis and z-axis can occur when subject is lying down o Each orientation has 2 tilt directions  When upright, might experience pitch tilt forward or backward  Might experience roll tilt to left or right  3 rotation direction would be yaw rotation but wouldn’t yield change in tilt of head with respect to gravity  When head rotations align with gravity no change in head tilt o 3 directions for angular velocity and translation, but only 2 directions for tilt  Amplitude o Second quality of spatial orientation is amplitude o For linear motion, can perceive translation having high velocity (speeding) or low velocity (car crowded parking lot) or low velocity o Can perceive rotation with high amplitude (vigorously shaking head) or low amplitude o Tilt amplitudes can be small (nod head) or large (lie down or hang upside down) Peripheral Structure of Mammalian Vestibular System  To sense various motions of head, need sensors found in vestibular system  Vestibular system’s alternate name – vestibular labyrinth – reflects anatomical complexity of vestibular apparatus o Size of large pea or small marble and found in inner ear next to cochlea  Vestibular organs respond primarily to head motion – both linear and angular – head tilt with respect to gravity  Each inner ear has one vestibular labyrinth, and each vestibular labyrinth includes 5 sense organs – 3 semicircular canals that sense rotational motion, and 2 otolith organs that sense gravity and linear acceleration  Hair Cells – Mechanical Transducers o Hair cells: cells that support stereocilia that transduce mechanical movement in vestibular labyrinth into neural activity sent to brain stem o Mechanoreceptors: sensory receptors that are responsive to mechanical stimulation (pressure, vibration, movement) o Pressure waves in cochlea move cochlear membranes that, in turn, bend hair cell stereocilia  Similar for vestibular organs, except that hair cell motion is caused by head motion, which causes hair cell stereocilia to deflect  Stereocilia deflection causes change in hair cell voltage, which alters neurotransmitter release, evokes action potentials in vestibular nerve fibers that have 1 or more dendritic synapses with hair cell o Receptor potential: change in voltage of sensory receptor cells – hair cells for vestibular system – in response to stimulation o When hair cells bends toward tallest stereocilia, hair cell voltage becomes less negative o Voltage change is called depolarization because hair cell becomes less polarized than negative resting potential o Through series of bioelectrochemcial reactions, hair cell depolarization increases release of neurotransmitter, causing increase in action potential rate (excitation_ o If hair cell is bent away from tallest stereocilia, cell potential becomes more negative (hyperpolarizes), causing decrease in reelase of neurotransmitter and decrease in action potential rate (inhibition) o Rate of action potentials transmitted by afferent neurons increases or decreases following hair cell receptor potential o General principle applies for otolith organs – acceleration in one direction yields increase in receptor potentials of some hair cells, while acceleration in opposite direction yields decreases in receptor potentials for those same hair cells  Semicircular Canals o Semicircular canal reflects gross anatomy of structure, which has circular shape of toroid or doughnut o Space called osseous (bony) canal because carved out of mastoid bone – filled with fluid called perilymph o Remaining length of toroid passes through vestibule o Second, smaller toroid is found inside larger toroid and smaller one is formed by membrane filled with fluid called endolymph and in cross section it has diameter of about 0.3 mm, which is little thicker than think human hair o Each swelling is an ampulla o Within endolymph space of each ampulla, angular motion detectors are assembled into sensory epithelium –crista o Crista: specialized detectors of angular motion located in each semicircular canal in swelling called ampula. Each consists of small ridge, which has epithelium made up of about 7000 hair cells and associated supporting cells and nerve fibers o When head rotates, inertia of edolypmh cuases it to lag behind motion of head which leads to tiny deflections of hair cells o Rotations in one direction yield increases in receptor potential of all hair cells that semicircular canal and concomitant increases in action potential rate for all neurons that innervate semicircular canal – rotations in opposite direction yield decreases in hair cell receptor potentials and concomitant decreases in rate of action potentials  Coding of Direction n Semicircular Canals o Each inner ear has 3 semicircular canals – horizontal (or lateral), anterior (or superior) and posterior o 3 canals are maximally sensitive to rotations in different planes, yielding part of direction coding for head rotation o Each canal is maximally sensitive to rotations around axis perpendicular to it and insensitive to rotations about axes that fall in plane of that canal o Semicircular canals are organized as functional pairs in what is called a push-pull arrangement o 2 horizontal canals – one on right of head and one of left – lie in same plane and form one of 3 functional canal pairs  Horizontal canal afferent neurons on right all increase firing rate for yaw head turns to right (shaking “no” and horizontal canal neurons on left decrease firing rate  Anterior and posterior canals are minimally sensitive to yaw rotations o In contrast to horizontal canal arrangement, mirror symmetry of semicircular canals in left and right ears yields functi
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