Vestibular System | Biol2051/52 Lecture 3 PDF

Vestibular System Biol2051/52 – Lecture 3 James Dillon ([email protected])  Lecture structure & Learning Outcomes Mechanotransduction Auditory system (Lecture 2) Cellular and molecular anatomy underlying hearing Vestibular system (Lecture 3) Located in the ear Important for reflexive mov...

Vestibular System Biol2051/52 – Lecture 3 James Dillon ([email protected])  Lecture structure & Learning Outcomes Mechanotransduction Auditory system (Lecture 2) Cellular and molecular anatomy underlying hearing Vestibular system (Lecture 3) Located in the ear Important for reflexive movements Balance Learning outcomes: Be able to describe the specialized sense organs involved in balance at a molecular and cellular level Be able to describe the circuitry that processes this type of sensory information in the CNS  The Vestibular System Proprioception limb position “Dizziness Disorders” (e.g. Meniere’s disease) Vestibular System Head position, self-motion & spatial orientation Auditory System Hearing Vestibular System: Function and Structure Vestibular System: Function and Structure The vestibular system is joined to the cochlea It has the same fluids as the cochlea The fluids have the same ionic profiles  Vestibular Navigation Movement in 3D space can take place in 3 different axis Movement can either be translational, and static head tilts (primarily detected by otolith organs) OR Movement can be rotational (detected by semi-  Vestibular Hair Cells Hair cells are located in the utricle, saccule (otolith organs) and the ampullae of the semi- circular canals Vestibular Hair Cells  Vestibular Hair Cells: Spontaneous Activity A recording from a vestibular axon from the utricle Before and after the head tilt there is electrical activity in the axon In the absence of stimulation some of the ion channels in the stereocilia are open There is a tonic release of neurotransmitter Spontaneous activity in the vestibular nerve Changes in the spontaneous activity of the vestibular nerve cells Structure of the otolith organs: utricle and saccule The utricle and saccule are called otolith organs Sensory epithelium is called the ‘macula’ The otolithic membrane is a gel layer with calcium carbonate crystals Deflection of hair cells by otoconia in utricle and saccule Information about the head relative to gravity: static head tilts and movements in a Static straight line Head Tilts Deflection of hair cells by otoconia in utricle and saccule Information about the head relative to gravity: static head tilts and movements in a straight line Linear Head Accelerations Deflection of hair cells by otoconia in utricle and saccule - + Hair cell stereocilia bundles are orientated towards the striola The striola acts as an axis of symmetry and hair bundles on either side of the striola have opposing morphological polarization This sets up a differential pattern of excitability encoding head position Utricle detects movements in horizontal plane Saccule detects movements in the vertical plane  Orientation of the saccular and utricular macula The saccular macula is orientated vertically and the utricular macula is orientated horizontally Utricle Responds to translational movements of the head in the horizontal plane (left-right) and to side ways head tilts (left right) Saccule Responds to translational movements of the head in the vertical plane (up-down) BOTH utricle and saccule detect front-back translational movement and front back head- Sensing Linear Accelerations of the Head A diagram of the activity in a utricle nerve axon as a head tilt is performed Summary of the otolith organs Semicircular canals: ampulla house sensory epithelium The semicircular canals detect head rotation Semicircular canals: ampulla house sensory epithelium Hair cells are aligned oppositely in each pair of semicircular canals Sensing Angular Accelerations of the Head A diagram of the activity in a utricle semi-circular canal nerve axon as a head tilt rotation is performed  Summary – Semicircular canals There are 3 semi-circular canals in each ear. The specialised sensor is the HAIR CELL located in the ampullae of the semicircular canals Open K+ channels depolarise the hair cell and triggers neurotransmitter (glutamate) release. There is a tonic release of neurotransmitter at rest. (Transduction) Specific movements of the stereocilia and the activation of different populations of hair cells can create different patterns of electrical activity and signalling (Encoding) Hair cells have the opposite orientation in corresponding semicircular canals in the left and right ear. Head movements create different signals from the left and right ear, that are integrated in the brainstem allowing us to sense the direction of rotational head movements. (Integration and  Equilibrium pathways Visual Input Cutaneous Proprioceptive Vestibular Input Input ra n ial Input of c p art II r I t ib ula rve V e Ves N Vestibular Coordinated Cerebellum Nuclei processing (Brainstem) Output to spinal cord and Output to motorneurons Output to Thalamus motorneurons of external eye muscles of the limb and torso Cortex Control of eye movement Maintenance of balance Perception of motion and posture and self awareness What is the Vestibulo-ocular Reflex? A reflex that controls the movement of the eye sockets when the head moves The reflexive eye-movement moves the eye in the opposite direction to the head Stabilizing gaze when the head is moving Because it is a reflex it is not under conscious control, it is an automatic response by the vestibular system and very fast! Oscillopsia: a condition caused by damage to the vestibular system (bouncing vision) Equilibrium pathway to CNS: Vestibulo- ocular Reflex A diagram of the neural circuit that underpins the vestibular ocular reflex and how the activity in the neural circuit changes with head movements in the horizontal plane.  Vestibular-Cervical and Vestibular-Spinal Reflex DESCENDING Pathway Medial co- Lateral and ordinates medial nucleus co- the VCR ordinates the VSR Signals Signals from the from the otoliths travel to semi- the lateral circular vestibular nucleus canal and nerve cells control carry signals via neck the vestibulospinal muscles tract to the spinal and head FAST! Fewer cord position via synaptic this Equilibrium pathway to CNS: Vestibular Pathways to the thalamus and cortex ASCENDING Pathway Respond to proprioceptiv e, visual and vestibular signals Multi- Sensory Multi- Integration Sensory ! Integration! Body orientation in 3D space Summary – vestibular system  Provides information about position, orientation and motion of head and body  Receptor cells located in utricle, saccule and semicircular canals (ampulla)  Movements cause a stimulation of distinct subsets of hair cells in the saccule, utricle and semi-circular canals  Important for co-ordinating reflex movements for maintaining gaze stability and posture