<p>TSM55: THE VESTIBULAR SYSTEM 29/10/08</p><p>LEARNING OUTCOMES</p><p>Describe the vestibular system</p><p>THE VESTIBULAR APPARATUS</p><p> The vestibular apparatus is contained within the membranous labyrinth of the inner ear, comprising:</p><p> o Two endolymph-filled sacs in the vestibule</p><p>. Utricle – larger of the two, oval-shaped, at the base of the semicircular canals</p><p>. Saccule – rounded, more central in the vestibule, drains the cochlear canal</p><p> o Three perpendicular semicircular canals, each with a basal expansion – the ampulla</p><p>. Superior – angled at roughly 30˚ to the midline on the transverse plane</p><p>. Posterior – angled at roughly 150˚ to the midline on the transverse plane</p><p>. Lateral – flat at 90˚ to the midline in the transverse plane</p><p> The utricle and saccule are lined with a sensory epithelium called the macula</p><p> o This has a vertical orientation in the saccule and a horizontal orientation in the utricle</p><p> o Contains vestibular hair cells with stereocillia of similar function to auditory hair cells</p><p> Crystals of calcium carbonate (otoliths) are embedded in a gelatinous membrane overlying the macula</p><p> o Crystals respond to movement, linear acceleration and gravity, distorting the stereocillia</p><p> o Bending of the stereocillia causes hyperpolarisation or depolarisation of the hair cells</p><p> The semicircular canals contain hair cells concentrated in a cupula – a gelatinous bulb in the ampulla</p><p> o Endolymph flowing in the ducts bends the cupula during accelerated rotation of the head</p><p> o At constant velocities the cupula equilibrates with fluid flow</p><p>THE VESTIBULAR PATHWAY</p><p> Primary afferent vestibular nerve fibres have their cell bodies in the vestibular ganglion</p><p> o Found in the lateral end of the internal acoustic meatus</p><p> Axons project up to the vestibular nuclei in the rostral medulla / caudal pons  Second order neurones project from the vestibular nuclei to a number of regions:</p><p> o Spinal cord – descending the vestibulospinal tract; alters tone of the muscles of posture</p><p> o Oculomotor nuclei – CNIII, CNIV and CNVI; maintains visual fixation during head movement</p><p> o Vestibulocerebellum – caudal inferior surface of cerebellum; assists in motor control</p><p> o Cortex – via the VPL of the thalamus; provides conscious awareness of head movements</p><p>Explain the role of the vestibular system in maintaining posture and balance</p><p> The vestibulo-ocular reflex compensates for rotational head movements to maintain visual fixation</p><p> o Rotation of the head triggers activity in the semicircular canals and an afferent transmission</p><p> o Primary afferents synapse in vestibular nuclei which relay the signal to the oculomotor nuclei</p><p> o Efferent fibres to the extra-ocular muscles cause appropriate counter-rotation of the eyes</p><p> Motion sickness arises effectively as a result of contradictory visual and vestibular sensory information</p><p> Ménière’s disease results from a severe dysfunction of the semicircular canals</p><p> o Recurrent vertigo, tinnitus, fluctuating deafness, nausea, vomiting</p><p> o Thought to be due to excessive endolymph production / ineffective drainage into the CSF</p><p> o Treated using anticholinergics and antihistamines</p>