NPB 110B Fall 2024 Lecture 26 Vestibular System I PDF

Lecture 26: Vestibular system I 11/13/2024 Sources: Neuroscience (Purves et al.) Principles of Neural Science (Kandel, Schwartz and Jessell) From Neuron to Brain (Nicholls et al.) Quantitative Physiology for Engineers (Feher) Medical Physiol...

Lecture 26: Vestibular system I 11/13/2024 Sources: Neuroscience (Purves et al.) Principles of Neural Science (Kandel, Schwartz and Jessell) From Neuron to Brain (Nicholls et al.) Quantitative Physiology for Engineers (Feher) Medical Physiology (Boron and Boulpaep) Human Physiology (Widmaier, Raff and Strang) Human Physiology (Silverthorn) Topics Vestibular Hair Cells Semicircular Canals Otolith Organs  Overview The vestibular system—composed of the otolith organs (saccule and utricle) and semicircular canals—is responsible for sensing motion, postural stability and image stabilization during head movement. The vestibular receptors are located in the labyrinth of each inner ear and provide the brain with information about motion of the head and about the orientation of the head relative to gravity. http://im-09-tb.blogspot.com/2009_03_01_archive.html  Overview These functions go largely unnoticed except when experiencing unusual conditions (motion sickness: nausea, dizziness) In addition to the maintenance of balance and posture, the vestibular system assists in coordinating body position with other sensory information. http://im-09-tb.blogspot.com/2009_03_01_archive.html Vestibular Hair Cells Depolarization Hyperpolarization Similar to the auditory system, motion is detected by the vestibular sensory organs through the deflections of hairs cells (kinocilium and stereocilia) Hair cells hyperpolarize or depolarize depending on the direction of deflection of the stereocilia Vestibular Hair Cells Depolarization Hyperpolarization Depolarization is caused by the stereocilia movement towards the kinocilium Hyperpolarization is caused by stereocilia movement away from the kinocilium Different polarization affects the firing rate of the primary vestibular afferents to the brainstem  Vestibular Hair Cells At rest there is spontaneous activity in the afferent neurons due to open 𝐶𝑎2+ channels causing a slow, constant neurotransmitter release. Motion in one direction (short hairs tipping in the direction of tall hairs) produces an electrical disturbance in the hair cell that stimulates the firing of cranial 8th nerve fibers. Motion in the other direction (tall hairs tipping in the direction of short hairs) inhibits the firing of cranial 8th nerve fibers. Vestibular Hair Cells Depolarization/hyperpo larization depends, respectively, on the 𝐾 + - rich endolymph and the 𝐾 + -poor perilymph on the basal and lateral portions of hair cells. Deflection of the stereocilia toward the kinocilium causes 𝐾 + channels in the apical portions of the stereocilia and kinocilium to open. 𝐾 + flows into the cell from the endolymph, depolarizing the cell membrane. Depolarization causes voltage-gated 𝐶𝑎2+ channels at the base to open, allowing 𝐶𝑎2+ to enter the cell, causing synaptic vesicles to release their transmitter (aspartate or glutamate). Vestibular Hair Cells Afferent fibers respond by depolarizing and increasing firing rate. When the stimulus subsides, the stereocilia and kinocilium return to their resting position. Most 𝐶𝑎2+ channels close and voltage-gated 𝐾 + channels open. returning the cell membrane to its resting potential. Deflection of the stereocilia away from the kinocilium causes 𝐾 + channels to open, allowing 𝐾 + to flow out from the cell resulting in hyperpolarization Decreases the rate of neurotransmitter release and firing rate of afferent fibers. Semicircular Canals Six total semicircular canals (three on each side). Canals are approximately oriented at right angles (orthogonal) to each other. Each canal responds to angular head acceleration during rotation of the head in their plane Each canal consists of a membranous canal (filled with endolymph: high 𝐾 + , low 𝑁𝑎+ ) within a bony canal The space between the membranous canal and bony canal is filled with perilymph (low 𝐾 + , high 𝑁𝑎 + ) Semicircular Canals Each canal has an ampulla: a region with hair cells (crista ampullaris) and a gelatinous structure that connects the hair cell region to the opposite side (cupula) Angular head motion leads to inertially-driven endolymph motion Endolymph motion causes cupula deflections which causes stereocilia movement towards or away from the kinocilium. Motion leads to differences in transmitter release from hair cells and increased and decreased discharge rates of the afferent nerve proportional to the angular head velocity.  Semicircular Canals are Functionally Paired The right and left horizontal canals lie in the same plane The right anterior and left posterior canals lie in the same plane The left anterior and right posterior canals lie in the same plane Coplanar canals act as push-pull pairs—angular movement that leads to excitation in one pair will lead to inhibition in the other. http://www.network54.com/Forum/52812/thread/1251377687/last-1251835526/Left+to+right+Vs+right+to+left. Similarities Between the Auditory and Vestibular Systems Although the auditory system senses the external environment and the vestibular system senses movements of itself, there are similarities between the two systems: 1. Common Fluid System. The membranous labyrinth is one continuous fluid system serving both hearing and balance. If there’s something wrong with this fluid system, both hearing and balance will be affected (e.g., Meniere’s Disease) 2. Hair Cell Motion Detectors. Hearing and balance both involve the detection of motion. In both cases, the motion detectors are hair cells that operate on nearly identical principles. 3. Innervation. Hearing and balance are innervated by separate branches of the same cranial nerve (8th). Otolith Organs: Utricle and Saccule The utricle lies horizontally, and the saccule is oriented vertically The organs provide information about linear acceleration (back and forth, up and down) and changes in head position relative to the forces of gravity. Fitzpatrick and Day, Journal of Applied Physiology, 2004  Otolith Organs: Utricle and Saccule Each organ has a sheet of hair cells (the macula) whose cilia are embedded in a gelatinous mass with a clump of small calcium carbonate crystals called otoliths (ear stones). Scanning electron micrograph of calcium carbonate crystals in the utricular macula of the cat Each crystal is about 50 µm long. In humans, they are 3 - 30 µm long Lindeman, 1973 Otolith Organs: Utricle and Saccule Linear acceleration, either due to gravity or translational motion, causes ineritally-driven motion of the crystals which applies a shear force to the stereocillia of the hair cells. Similar to the semicircular canals, motion towards the kinocilium results in excitation and away results in inhibition http://biology.nicerweb.com/med/SAVE/otolith.html Otolith Organs: Utricle and Saccule The striola, a central stripe that divides the macula into two regions, is the boundary for the reversal of hair-cell polarity Utricle hair cells are oriented towards the striola Saccule hair cells are oriented away from the striola The striola is curved so otolithic organs are sensitive to linear motion in multiple trajectories Otolith Organs vs Semicircular Canals Similarity: both sensory organs use hair cells to detect motion Difference: semicircular canal hair cells are oriented in the same direction, whereas otolith hair cells are oriented in opposing directions so that there are some hair cells within each otolith organ that are excited by acceleration in opposite directions Summary Within the inner ear are specialized sensory receptors (otolith organs and semicircular canals) responsible for the perception of forces associated with head movement and gravity. Motion is detected by the vestibular sensory organs through the deflections of hairs cells (kinocilium and stereocilia) Motion is sensed by the relative movement of hair cells to the head which causes changes in electrical potential, transmitter release and firing rate. Semicircular canal hair cells are oriented in the same direction, whereas otolith hair cells are oriented in opposing directions The auditory and vestibular systems are similar in terms of a common fluid system, hair cell motion detectors and Innervation.