Vestibular System Lecture Notes PDF

PHYL 2041 -- LECTURE 11 Vestibular System Dr. Stefan Krueger Dept. of Physiology & Biophysics [email protected]  Vestibular System: Topics 11.1.Vestibular Labyrinth 11.2. Central Processing of Vestibular Information ...

PHYL 2041 -- LECTURE 11 Vestibular System Dr. Stefan Krueger Dept. of Physiology & Biophysics [email protected]  Vestibular System: Topics 11.1.Vestibular Labyrinth 11.2. Central Processing of Vestibular Information  Vestibular Labyrinth 11.1.1. Stimuli Detected by the Vestibular System 11.1.2. Structure of the Vestibular Labyrinth 11.1.3. Structure of the Semicircular Canals 11.1.4. Transduction of Angular Acceleration in Semicircular Canals 11.1.5. Structure of the Otolith Organs 11.1.6. Transduction of Linear Acceleration in Otolith Organs Back to main index  11.1.1.Vestibular Stimuli Linear acceleration Function of the vestibular system: Control of eye, head and body positions during movements Spatial sensation Vestibular system stimulated by: Acceleration, i.e. change of velocity with time Acceleration ensues when force acts on a mass Vestibular system can detect linear and angular accelerations Back to index Angular acceleration  11.1.2.Vestibular Labyrinth Vestibular labyrinth consists of: Vestibular part of cranial nerve VIII ‣ Semicircular canals Scarpa’s detect angular (=rotational) acceleration ganglion ‣ Otolith organs: Utricle detects horizontal acceleration Saccule detects vertical acceleration Semicircular canals, utricle, saccule, and cochlear duct form membranous labyrinth, contain endolymph Bony labyrinth encloses membranous labyrinth, Utricle Saccule Cochlea contains perilymph Semicircular Otolith Innervation by vestibular part of eighth cranial canals organs nerve. Cell bodies of vestibular neurons located in Scarpa’s ganglion (= vestibular Vestibular labyrinth nerve ganglion) Back to index  11.1.3. Semicircular canals Three semicircular canals orientated orthogonal to each other. Each canal has a corresponding canal on other side in the same plane of rotation. Each canal has ampulla (= bulge). Ampulla Ampulla contains sensory epithelium: Crista Cupula Crista contains hair cells (= sensory receptor Endolymph cells). Innervated by vestibular neurons Stereocilia of hair cells located in cupula (= Hair cells gelatinous cap). Endolymph surrounds cupula Crista A erents of vestibular neurons Bottom image from: Derrickson Human Physiology 2nd Ed. Wiley. Back to index ff  11.1.4. Transduction of Angular Acceleration in Semicircular Canals (1) Rotation of head results in movement of labyrinth walls and cupula; inert From: Derrickson Human Physiology endolymph slower to move Endolymph exerts force on cupula, causing stereocilia to bend Bending towards kinocilium (largest cilium) causes mechanically gated ion channels to Channels Channels open, bending away from kinocilium leads to closed fully open channel closure Channel opening → K+ in ux → depolarization → more glutamate release onto vestibular neuron → res action potentials at higher frequency Channel closure → hyperpolarization → less glutamate release → AP frequency lower Low ring rate High ring rate Next slide fi fi fi fl  11.1.4. Transduction of Angular Acceleration in Semicircular Canals (2) Head rotation causes depolarization of hair cells in a semicircular channel in one ear and hyperpolarization of hair cells in the complementary semicircular channel on the other side Firing frequencies of vestibular neurons innervating semicircular canal in either ear change in opposite ways When head does not move, vestibular neurons re at background rate Back to index fi  11.1.5. Otolith Organs Adapted from: Purves et al. (eds.) Neuroscience. Sinauer. Utricle and saccule contain sensory Otoconia epithelium called macula, which contains Otolithic membrane hair cells and support cells. Stereocilia of hair cells projecting into otolithic membrane, a gelatinous layer. Surface of otolithic membrane contains otoconia, tiny calcium carbonate crystals. Hair cells Back to index  11.1.6. Transduction of Linear Acceleration in Otolith Organs (1) Linear acceleration or head tilt creates force on otoconia. Otolithic membrane moves with otoconia, causing bending of hair cell cilia. Head tilt Bending towards largest stereocilium causes mechanically gated ion channels to open and hair cells to depolarize; bending away from largest stereocilium elicits channel closure and hyperpolarization. Basal ring frequency of vestibular neurons increased or decreased with linear acceleration and head tilt. Gravitational force Bending of cilia perpendicular to preferred direction does not cause response: Direction-selectivity Next slide fi  11.1.6. Transduction of Linear Acceleration in Otolith Organs (2) Direction of depolarization Macula of utricle is in horizontal plane, macula of saccule in vertical plane In addition, hair cells on each macula have different direction-selectivity: Direction Macula of utricle of linear acceleration can be precisely detected in all directions. Saccular and utricular maculae on one side of head are mirror images to those on the other side: Linear acceleration has opposite effects on ring of vestibular neurons in either ear. Macula of saccule Back to index fi  Central Processing of Vestibular Information 11.2.1. Central Vestibular Pathways: Overview 11.2.2.Vestibulo-Ocular Re ex 11.2.3.Vestibulospinal Projections 11.2.4.Vestibulocerebellar Pathway 11.2.5.Vestibular Pathways to the Cortex 11.2.6.Vestibular Dysfunction Back to main index fl  11.2.1. Central Vestibular Pathways Primary vestibular neurons project to: vestibular nuclei, cerebellum Vestibular nuclei also receive proprioceptive and cerebellar input Bear et al. (eds.) Neuroscience Lippincott Dual role of vestibular system: ‣ Provides rapid re exes ‣ Imparts sense of spatial orientation and self-motion ➡ Many neurons in vestibular nuclei function as premotor neurons and give rise to ascending sensory projections Projections from vestibular nuclei to: 1) Extraocular motor neurons (vestibulo-ocular re ex) 2) Spinal motor neurons (postural re exes) 3) Cerebellum (coordination of postural adjustments) 4) Thalamus (→ cortex; sense of spacial orientation, self-motion) Back to index fl fl fl  11.2.2. Vestibulo-Ocular Re ex (1) Activity Produces eye movements that counter High Low Compensatory eye movement head movements for gaze xation Medial rectus muscle Excitation of contralateral lateral rectus Lateral rectus muscle Oculomotor muscle and ipsilateral medial rectus nerve (III) muscle via medial vestibular nucleus, Oculomotor abducens nucleus and oculomotor nucleus nucleus Abducens Head rotation => physiological nystagmus nerve (VI) Abducens nucleus Head rotation Medial vestibular Scarpa’s nucleus Slow eye Ganglion movement Fast saccade Next slide Head rotation fi fl  11.2.2. Vestibulo-Ocular Re ex (2) Irrigated ear Vestibulo-ocular re ex (VOR) used diagnostically to test for lesions at brainstem or midbrain level Medial Caloric testing: Ear irrigation with cold/warm water. longitudinal fasciculus Allows to separately assess circuitry on either side Brainstem and distinguish brainstem and midbrain lesions. Midbrain Left ear Midbrain lesion involving No lesion Lesion of lower brainstem irrigated with medial longitudinal fasciculus Cold water Warm water Unchanged VOR; no saccade Lateral movement of the eye Absent VOR if patient unconscious only on the less active side Back to index fl fl  11.2.3. Vestibulospinal Projections 1) Medial vestibulospinal tract Vestibulocervical re ex (VCR): Postural adjustments of head (e.g., downward pitch of body leads to head being pulled up) Medial vestibular nucleus → medial vestibulospinal tract →motor neurons Corneil & Musallam in Encyclopedia of Neuroscience innervating neck muscles (bilateral) 2) Lateral vestibulospinal tract Vestibulospinal re ex (VSR): Limb extension to prevent falling Lateral vestibular nucleus → lateral vestibulospinal tract → ipsilateral motor neurons of extensor limb muscles Back to index fl fl  11.2.4. Vestibulocerebellar Pathway Reciprocal connections from and to vestibulocerebellum Abducens nucleus (VOR) Function Vestibulo- cerebellum Integrate vestibular signals with infor- mation about voluntary movements, e.g. to distinguish passive from self-generated movements Plasticity of vestibular output and sensorimotor learning (e.g.VOR gain Vestibular nerve changes in response to unilateral Vestibular vestibular damage or mismatched nuclei vestibular and visual inputs) Vestibulospinal tract Back to index  11.2.5. Vestibular Pathways to the Cortex Multiple cortical areas in parietal and temporal cortex receive vestibular input via ventroposterior thalamic nucleus Ventro- posterior These cortical areas are multisensory: Also thalamic nucleus receive somatosensory and visual input Parieto-insular vestibular cortex Example parieto-insular vestibular cortex (PIVC): Direct electrical stimu- lation in humans elicits strong vestibular sensations; fMRI shows activation in response to vestibular stimulation, but also to proprioceptive and visual input. Proprioceptive input Lesions in PIVC: Altered perception of Projections from personal and extrapersonal space. superior and medial vestibular nucleus Adapted from: Purves et al. (eds.) Neuroscience. Sinauer. Back to index  11.2.6. Vestibular Dysfunction Causes of acute vestibular dysfunction: Physiological nystagmus Labyrinthitis (in ammation of the inner ear) Trauma to labyrinth or vestibular nerve, e.g. after surgical removal of acoustic neuroma Benign Paroxysmal Positional Vertigo (BPPV): Pieces of otoliths in semicircular canals Meniere’s Disease (buildup of endolymph) Increased ring Decreased ring Symptoms of vestibular dysfunction: Spontaneous nystagmus Unilateral acute dysfunction: Spontaneous nystagmus, vertigo (dizziness) and nausea, often tinnitus (involvement of cochlea) If dysfunction in both labyrinths (e.g. inherited), only mild symptoms: Proprioceptive and visual inputs can compensate in most situations. Baseline ring No ring Recovery if dysfunction persists: Plasticity Back to index fi fi fi fi fl