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Vestibular system Nuno Coelho 6th february Animal Body Function VIII Master Degree - Veterinary Medicine Summary 1. Introduction: the role of vestibular system 2. Vestibular organ: from a functional point of view 3. Transduction of v...

Vestibular system Nuno Coelho 6th february Animal Body Function VIII Master Degree - Veterinary Medicine Summary 1. Introduction: the role of vestibular system 2. Vestibular organ: from a functional point of view 3. Transduction of vestibular stimulus 4. Detection of body and head movement rotary aceleration and deceleration Linear aceleration and deceleration Head tilt 5. Central pathways 6. Vestibular reflexes for teaching purposes only 1) Introduction To coordinate posture and locomotion, animal’s brain needs to know the orientation of the body vestibular system informs the brain about the position and motion of the head and body’s orientation gives a sense of balance/equilibrium Acceleration of the body tilt of the head Vestibular system Providing reflexes that triggers linear acceleration of the head postural adjustements to keep detects: body position rotary acceleration of the head for teaching purposes only 1) Introduction @Rumin8er, 2014 (Youtube channel); https://www.youtube.com/watch?v=nsDv-vNiy_0&t=26s for teaching purposes only  1) Introduction Lesion of the vestibular system are common in veterinary medicine – e.g. of clinical symptons: head tilt, compulsive rotary movements, nystagmus @Rumin8er, 2014 (Youtube channel); https://www.youtube.com/watch?v=t4NNToOa284 for teaching purposes only 1) Introduction To coordinate posture and locomotion, animal’s brain needs to know the orientation of the body The function of the vestibular system with some similarities with the : also with hair cells excitation, but different receptor organs (utricle, saccule and semicircular canals openstax, 2024 for teaching purposes only  2) Vestibular organ Vestivular system – bilateral receptor system inside the inner ear Inner ear with: bony labyrinth: composed by “caverns and tunnels”, houses portions of the vestibular and auditory systems (cochlea) The receptor organs of those systems are part of the membranous labyrinth with specialized epithelium (sensory cells) and supporting The membranous labyrinth is filled with endolymph (rich in K+) and is cells within the bony lab separated from the bony labyrinth by perilymph (rich in Na+) for teaching purposes only  2) Vestibular organ Reece, 2015 otosurgeryatlas.stanford.edu The vestibular portion of the membranous labyrinth has 2 main set of components: three semicircular ducts: at approximately right angles to each other and arranged in 3 orthogonal planes; each with an enlargement at its end – ampulla (a) for teaching purposes only utricle and saccule: a pair of chamber structures (a.k.a. otolith organs)  2) Vestibular organ Each vestibular structure of the membranous labyrinth displays hair cells: hair cells are the basis of the sensory receptor organ each hair cells with numerous stereocilia in its appex, arranged by size; and kinocilium in one margim of the apical cell surface hair cell synapses with a sensory neuron Klein, 2020 for teaching purposes only  2) Vestibular organ 2 types of vestibulary sensory receptors/organs 1) Crista ampullaris 2) Macula Klein, 2020 for teaching purposes only  2) Vestibular organ: crista ampullaris Reece, 2015 1) Crista ampullaris: located in each ampulla, aimed to detect angular acceleration and deceleration sensory hair cells (receptor cells) separated by supporting cells hair cells: kinocilium and several stereocilia apically – orderly fashion: kinocilium in one margin and stereocilia in the remaining surface; stereocilia disposed in ascending height Cupula: gelatinous structure with cilia embedded Tip link connecting stereocilia attached to the roof of the ampulla angular/rotary acceleration agitates endolymph that pushed cupula and bends cilia for teaching purposes only  2) Vestibular organ: orientation of sensory cellls Orientation in crista ampullaris Semicircular duct: lateral: horizontal plane the anterior and posterior semicircular anterior and posterior: ducts have their hair cells arranged with vertical plane the kinocilia on the side away from the utricle In the crista ampullaris of the lateral semicircular duct, the sensory hair cells are all arranged with their kinocilia on the side of the utricle for teaching purposes only Reece, 2015  2) Vestibular organ: macula 2) Macula: looking like a small spot, is placed in utricle and saccule and responds primarily to linear acceleration or deceleration, static head tilt and gravity similar structure sensory elements compared to crista: kinocilum and stereocilia and gelatinous layer ≠s: statoconial (or otolith) membrane this membrane covered by heavy crystals of calcium carbonate (otoliths) – statoconia – function as inertial mass (high density) for teaching purposes only Klein, 2020 Reece, 2015  2) Vestibular organ: orientation of sensory cellls Orientation in macula Reece, 2015 utricle Sensory cells of the macula are arranged in an orderly pattern Kinocilia of the sensory cells of the utricle are always on the side toward the striola Kinocilia of the sensory cells of the saccule are oriented on the side away from the striola saccule striola: curved ridge that runs This particular arrangement makes utricular and through the middle of the macula saccular sensory cells directionally sensitive to a wide variety of linear head movements and positions for teaching purposes only  3) Transduction of vestibular stimulus resting potential of hair cells: upright position Movement of stereocilia towards kinocilium: depolarization and ↑ firing rate Movement of stereocilia away from kinocilium: hyperlarization and ↓ firing rate Reece, 2015 for teaching purposes only  3) Transduction of vestibular stimulus K+-rich endolymph in contact with apical surface of hair cells deflection of the stereocilia toward kinocilum by movement/position changes tip links stretch and open K+ channels (mechanically-gated) → K+ inflow depolarization and opening of voltage-gated Ca2+ channels ↑ release of neurotransmitters, vestibular nerve fibers Reece, 2015 depolarization and increasing in their firing rate for teaching purposes only  3) Transduction of vestibular stimulus K+-rich endolymph in contact with apical surface of hair cells deflection of the stereocilia away from kinocilum opening of K+ channels in the basal part of the hair cells → K+ outflow hyperpolarization decreasing in their firing rate Reece, 2015 for teaching purposes only 4) Detection of head movement 1) static tilt of the head Vestibular system 2) linear acceleration of the head detects: 3) rotary acceleration of the head for teaching purposes only  4) Detection of head movement: head tilt Maculae detect head tilting relative to gravity head tilts to one side, gravity displaces the heavy statoconial membrane toward the tilted side (if Reece, 2015 remains tilted – stationary tilted) shearing force on cilia may force them to bend toward kinocilia, for instance in this case, increases firing rate in vestibular nerve for teaching purposes only adapted from Klein, 2020 4) Detection of head movement: head tilt https://nba.uth.tmc.edu/neuroscience/s2/chapter10.html the macula of the utricle is oriented in the horizontal plane macula of the saccula is in the ventricular plane for teaching purposes only 4) Detection of head movement: linear acceleration When the head is stationary and upright, there is little or no bending of the hair cell cilia, in the macula When the head accelerates in a straight line (linear acceleration), the hair cells accelerate in the same direction, but the heavy otolith layer lags behind (due to its inertia) otolith drag in the opposite direction → hair cell cilia bending, by way of the interposed gelatinous layer, in the adapted from Klein, 2020 direction opposite the acceleration for teaching purposes only  4) Detection of head movement: linear acceleration Due to the specific spatial organization of the receptor components in utricle and saccule, these vestibular structures can transduce linear acceleration of the head: Linear acceleration in a given direction results: hair cell cilia bending in the opposite direction due to otolith drag some Hair cells whose cilia are bent directly toward the kinocilium (green regions) will be more depolarized, greatly increasing the action potential frequency in their associated sensory neurons Hair cells whose cilia are bent directly away from kinocilium (orange regions) will be hyperpolarized, reducing the action potential frequency in their associated neurons Hair cells whose cilia are bent along other axes will be less Klein, 2020 significantly affected for teaching purposes only  4) Detection of head movement: linear acceleration for each direction of acceleration, there is a specific pattern of depolarization vs hyperpolarization vs. no alteration in firing rates This topographical pattern of hair cell bending and changes in action potential firing will be specific for linear acceleration (and head tilt) in any specific direction The CNS can decipher these various patterns of neural activity to determine the onset and direction of linear acceleration and to initiate an appropriate compensatory response Klein, 2020 for teaching purposes only  4) Detection of head movement: rotary acceleration Reece, 2015; Klein, 2020 Head rotation (rotary acceleration/decceleration): If head accelerates: Head is still: endolymph and the cupula remain 1) semicircular ducts and receptor organs (ampulla) rotate still and nerve fibers of the crista ampullaris in 2) endolymph lags behind the turning motion due inertia each ear fire at the same rate 3) This places a shear force on the cupula, displacing it in the opposite direction to that of rotation bending the cilia: toward the kinocilium: firing of the vestibular nerve↑ for teaching purposes only away from the kinocilium : ↓ firing rate  4) Detection of head movement: rotary acceleration https://nba.uth.tmc.edu/neuroscience/m/s2/image s/html5/10-1.html Head rotation (rotary acceleration): So, When the head turns, fluid in one or more semicircular ducts pushes against the cupula and bends the cilia of the hair cells Fluid in the corresponding semicircular duct on the opposite side of the head moves in the opposite direction for teaching purposes only  4) Detection of head movement: rotary acceleration nba.uth.tmc.edu The semicircular ducts work in pairs to detect head movements (angular acceleration) A turn of the head excites the receptors in one ampulla and inhibits receptors in the ampulla on the other side https://nba.uth.tmc.edu/neuroscience/m/s2/chapter10.html for teaching purposes only  4) Detection of head movement: rotary acceleration Klein, 2020 Semicircular ducts located on opposite sides of the head, but in the same plane (co-planar) work as a pair to send information to brain the brain interprets reciprocal changes in sensory action potential frequency as resulting from clockwise or counterclockwise acceleration or deceleration in a given plane rotary acceleration/deceleration in any given plane affects all 3 pairs of semicircular ducts but each pair to different degrees In this way, the bilateral system of six semicircular ducts detects the direction of both rotary acceleration of the head and modulates particular CNS structures to produce the appropriate reflex response for teaching purposes only  5) Central pathways from vestibular nerve vestibular hair cells synapse on sensory neurons whose axons constitute VIII cranial nerve vestibular nuclear complex (in medulla and pons) vestibular nuclear complex second-order neurons may project to: through vestibulospinal tracts to spinal cord inputs from utricle and saccule, to provide excitatory facilitation to motor neurons of muscles of the trunk and linbs in response to linear acceleration or static head tilt Klein, 2020 for teaching purposes only  5) Central pathways from vestibular nerve vestibular hair cells synapse on sensory neurons whose axons constitute VIII cranial nerve vestibular nuclear complex (in medulla and pons) vestibular nuclear complex second-order neurons may project to: through medial longitudinal fasciculus to cranial nerve nuclei inputs from crista ampullaris to cranial nerve nuclei (oculomotor, trochlear, abducens) to produce compensatory eye movements in responde to rotary acceleration to cerebellum, through flocculonodular lobe fine-tuning the coordination of postural and oculomotor reflexes to cerebral cortex in conscious vestibular sensations Klein, 2020 for teaching purposes only 6) Vestibular reflexes Vestibulospinal reflex The vestibular control of skeletal muscles of the trunk and limbs is mediated by the vestibulospinal tracts descend in the ventral funiculus of the spinal cord and modulate motor neurons via spinal interneurons maintain balance by facilitating lower motor neurons for extensor muscles and inhibiting lower motor neurons for flexor muscles Reece, 2015 for teaching purposes only  6) Vestibular reflexes Vestibulocollic reflex This reflex acts to stabilize the head with the neck musculature elicited by rotational acceleration if an animal rolls to one side, the head moves in the opposite direction to the roll more effective in animals that make minimal eye movements (pidgeons, owls, etc.), to stabilize the gaze Reece, 2015 for teaching purposes only  6) Vestibular reflexes Vestibuloocular reflex vestibular reflex that control the eye’s extraocular muscles coordinates eye and head movements to keep eyes fixed on a specific field when head moves (linear or rotation) – compensatory eyes movement maintain a steady gaze when head moves by turning slowly in a direction opposite to the head rotation, until reaches its limit of rotation, and then swiftly move back in the direction of the head rotation – normal nystagmus for teaching purposes only Reece, 2015 6) Vestibular reflexes Vestibuloocular reflex Nystagmus: involuntary eye movements Nystagmus may occur under pathologies of the vestibular system - spontaneous nystagmus A persisting head tilt, falling, and compulsive circling or rolling may be also present oriented in a consistent pattern with respect to the side of a peripheral lesion of the vestibular system abnormal, asymmetrical action potential inputs to the brainstem from the vestibular apparatus on the two sides of the head for teaching purposes only The end! Thank you for your attention!! References Klein, B. G. (2020). Cunningham's textbook of veterinary physiology, 6th edition. Elsevier Health Sciences. Reece, W. O., Erickson, H. H., Goff, J. P., & Uemura, E. E. (Eds.). (2015). Dukes' physiology of domestic animals. 13th edition, John Wiley & Sons. Hall, J. E., & Hall, M. E. (2020). Guyton and Hall textbook of medical physiology. Elsevier Health Sciences. Britannica, Encyclopedia. (2022). Encyclopedia Britannica, in https://www.britannica.com/