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Unformatted text preview: Vestibular Function and Anatomy
UTMB Grand Rounds April 14, 2004
Gordon Shields, MD
Arun Gadre, MD System of balance Membranous and bony labyrinth embedded in petrous bone 5 distinct end organs – 3 semicircular canals: superior, lateral, posterior
– 2 otolith organs: utricle and saccule Semicircular canals sense angular acceleration Otolithic organs (utricle and saccule) sense linear acceleration Embryology
Embryology 3rd week of embryonic development
Otic placode formed from neuroectoderm and ectoderm
Otocyst or otic vesicle 4th week Embryology
Embryology Endolymphatic duct forms Utricular chamber becomes utricle/semicircular canals Saccular chamber becomes saccule/cochlea Separation of saccule and cochleaductus reuniens Embryology
Embryology Week 3 sensory epithelia develop from ectoderm
3 cristae, 2 maculae Vestibulocochlear ganglion starts as one then spits into superior and inferior divisions – Superior division: Superior/lateral canals, utricle
– Inferior division: saccule, posterior canal (via singular nerve) Semicircular canals are orthogonal to each other
Lateral canal inclined to 30 degrees
Superior/postereor canals 45 degrees off of sagittal plane Utricle is in horizontal plane
Saccule is in vertical plane Anatomy
Anatomy There are five openings into area of utricle
Saccule in spherical recess
Utricle in elliptical recess 45% from AICA
24% superior cerebellar artery
Two divisions: anterior vestibular and common cochlear artery Superior vestibular nerve: superior canal, lateral canal, utricle
Inferior vestibular nerve: posterior canal and saccule Membranous labyrinth is surrounded by perilymph Endolymph fills the vestibular end organs along with the cochlea Perilymph – Similar to extracellular fluid
– K+=10mEQ, Na+=140mEq/L
– Unclear whether this is ultrafiltrate of CSF or blood
– Drains via venules and middle ear mucosa Endolymph – Similar to intracellular fluid
– K+=144mEq/L, Na+=5mEq/L
– Produced by marginal cells in stria vascularis from perilymph at the cochlea and from dark cells in the cristae and maculae
– Absorbed in endolymphatic sac which connected by endolymphatic, utricular and saccular ducts Sensory structures
Sensory structures Ampulla of the semicircular canals Dilated end of canal Contains sensory neuroepithelium, cupula, supporting cells Cupula is gelatinous mass extending across at right angle
Extends completely across, not responsive to gravity
Crista ampullaris is made up of sensory hair cells and supporting cells Sensory cells are either Type I or Type II
Type I cells are flask shaped and have chalice shaped calyx ending One chalice may synapse with 24 Type I cells
Type II cells – cylinder shaped, multiple efferent and afferent boutons Hair cells have 50100 stereocilia and a single kinocilium. stereocilia are not true cilia, they are graded in height with tallest nearest the kinocilium. Kinocilium is located on one end of cell giving each cell a polarity Has 9+2 arrangement of microtubule doublets Lacks inner dynein arms, and central portion of microtubules not present near ends – may mean they are immobile or weakly mobile Each afferent neuron has a baseline firing rate Deflection of stereocilia toward kinocilium results in an increase in the firing rate of the afferent neuron Deflection away causes a decrease in the firing rate kinocilia are located closest to utricle in lateral canals and are on canalicular side in other canals Ampullopetal flow (toward the ampulla) excitatory in lateral canals, inhibitory in superior/posterior canals Ampullofugal flow (away from the ampulla) has opposite effect Semicircular canals are paired Horizontal canals
Right superior/left posterior
Left superior/right posterior
Allow redundant reception of movement
– Explains compensation after unilateral vestibular loss
– Otolithic organs
Otolithic organs Utricle and saccule sense linear acceleration Cilia from hair cells are embedded in gelatinous layer Otoliths or otoconia are on upper surface Calcium carbonate or calcite
Specific gravity of otolithic membrane is 2.712.94
Central region of otolithic membrane is called the striola Saccule has hair cells oriented away from the striola
Utricle has hair cells oriented towards the striola
Striola is curved so otolithic organs are sensitive to linear motion in multiple trajectories Central connections
Central connections Scarpa’s ganglion is in the internal auditory canal Contains bipolar ganglion cells of first order neurons Superior and inferior divisions form common bundle which enters brainstem No primary vestibular afferents cross the midline Afferent fibers terminate in the vestibular nuclei in floor of fourth ventricle
– Superior vestibular nucleus
– Lateral vestibular nucleus
– Medial vestibular nucleus
– Descending vestibular nucleus Vestibular nuclei project to – Cerebellum
– Extraocular nuclei
– Spinal cord
– Contralateral vestibular nuclei Senses and controls motion
Information is combined with that from visual system and proprioceptive system
Maintains balance and compensates for effects of head motion Vestibuloocular reflex – Membranous labyrinth moves with head motion
– Endolymph does not causing relative motion
– Cupula on right canal deflected towards utricle causing increase in firing rate, left deflects away causing a decrease in firing rate.
– Reflex causes movement of eyes to the left with saccades to right
– Stabilizes visual image If acceleration stops, and spin to right is at constant velocity, sensation of motion stops after 1420 seconds as does nystagmus Cupula only takes 810 seconds to return to equilibrium position Vestibular integrator is the term for the prolongation and is mediated by the vestibular nuclei and cerebellum Vestibulospinal Reflex
Vestibulospinal Reflex Senses head movement and head relative to gravity Projects to antigravity muscles via 3 major pathways:
– Lateral vestibulospinal tract
– Medial vestibulospinal tract
– Reticulospinal tract How do calorics work?
How do calorics work? Patient is lying down with horizontal canals oriented vertically (ampulla up)
Cold water irrigation causes endolymph in lateral portion to become dense and fall causing deflection of cupula away from utricle with a decrease in the firing rate
This causes nystagmus with fast phase (beat) away from the stimulus With warm water irrigation column of endolymph becomes less dense, rises and causes deflection of cupula toward the utricle
Results in increase firing rate and nystagmus which beats towards the stimulation
COWS (cold opposite, warm same) Sources
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Baloh RW. Dizziness, Hearing Loss, and Tinnitus. Philadephia, F.A. Davis Company, 1998.
Jahn AF, SantosSacchi J. Physiology of the Ear. Second edition. San Diego, Singular, 2001.
Friedman I, Ballantyne J. Ultrastructural Atlas of the Inner Ear. London, Butterworth & Co., 1984.
Janfaza P, Nadol JB. Temporal Bone and Ear. In: Janfaza P ed. Surgical Anatomy of the Head and Neck. Philadelphia, Lippincott Williams & Wilkins, 2001:419479.
Wall C, Vrabec JT. Vestibular Function and Anatomy. In: Head & Neck SurgeryOtolaryngolog. Philadelphia, Lippincott Williams & Wilkins, 2001:16411650. ...
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