Vestibular system 2020 ho.pdf

The Vestibular System Laura Andreae [email protected] Learning outcomes By the end of this lecture, you should be able to: name the sensory inputs that result in a sense of balance describe the anatomy of the vestibular apparatus explain how hair cells respond to movement in semicircula...

The Vestibular System Laura Andreae [email protected] Learning outcomes By the end of this lecture, you should be able to: name the sensory inputs that result in a sense of balance describe the anatomy of the vestibular apparatus explain how hair cells respond to movement in semicircular canals and otolithic organs and predict how directional movement increases or decreases their firing rate explain the different types of movement that the semicircular canals and otolithic organs (saccule & utricle) respond to list the connections of the vestibular nuclei and explain their relevance provide a few examples for disorders that affect the vestibular system How to coordinate balance with movement, ensure posture Vestibular system Vision Balance posture ‘righting reflex’ Proprioception The vestibular apparatus External ear Middle ear Inner ear …located deep in the petrous part of the temporal bone The vestibular apparatus …located deep in the petrous part of the temporal bone The bony labyrinth Vestibule The membranous labyrinth Endolymph High K, low Na Perilymph High Na, low K  Voltage difference Sensory receptors of vestibular system: hair cells stereocilia microvilli tight junction supporting cell hair cell Release glutamate at synapse with afferent fibre primary afferent fibre Sensory receptors of vestibular system: hair cells Kinocilium Axis of polarity 40-70 actin-rich stereocilia Sensory receptors of vestibular system: hair cells Stereocilia are connected by tip links Sensory receptors of vestibular system: hair cells K+ ©2010 by The Physiological Society Furness D N et al. J Physiol 2010;588:765-772 Sensory receptors of vestibular system: hair cells Sensory receptors of vestibular system: hair cells How do hair cells respond to motion and position changes? 1. Semicircular ducts Inside the ampulla is the ampullary crest… Hair cells cupula ampullary crest How do hair cells respond to motion and position changes? 1. Semicircular ducts All hair cells in ampulla are arranged so that the axis of polarity always points in one direction, eg in horizontal semicircular duct they go towards utricle. Therefore, because hair cells only respond along their axis, each ampulla responds with depolarization in one direction, and hyperpolarization in the other. How do hair cells respond to motion and position changes? 1. Semicircular ducts Angular (or rotational) Acceleration DYNAMIC How do hair cells respond to motion and position changes? 2. Utricle Otoconia Otoconia: crystals of calcium carbonate Macula Otolithic membrane How do hair cells respond to motion and position changes? 2. Utricle When upright, macula roughly horizontal, so otoliths rest directly on it. If head is tilted, gravity acts on heavy otolithic mass so it sags in direction of tilt and bends hair cells. So movement is NOT REQUIRED. This means static head position can be detected. Macula can also detect linear acceleration (frequent fluctuations lead to motion sickness). How do hair cells respond to motion and position changes? 2. Utricle Cilia of hair cells in utricle macula don’t all face in the same direction… …they all point towards a curving landmark – the striola This means the utricle can respond to tilt or linear acceleration in many directions. Tilt in any direction will depolarize some cells and hyperpolarize others – leads to complex signal to brain with accurate measure of head position How do hair cells respond to motion and position changes? 2. Saccule Saccule essentially like utricle: also has a macula, but this is oriented vertically when person in upright position. Therefore responds to vertically directed linear force and detects position of head in space striola In the saccule, cilia also oriented in same direction relative to striola, but unlike utricle, they point away from striola. Therefore effectively point in different directions Vestibular nerve and ganglion Central connections trigeminal nerve (cV) trochlear nerve (cIV) facial nerve (cVII) pons cerebellum abducens nerve (cVI) vestibulocochlear nerve (cVIII) cVIII joins the brain stem in the cerebellopontine angle Central connections Cross section through medulla: Vestibular nuclei Inferior cerebellar peduncle olive Central connections Oculomotor n Trochlear n mlf Vestibulo-cerebellar fibres Abducens n Vestibular nuclei eg: lateral – utricle (tilt), med/sup – semicircular ducts mlf Vestibulospinal tract Central connections Ascending and descending tracts from vestibular nuclei: Project to nuclei of extra-oculomotor nerves and to cervical spinal cord to co-ordinate head movements with eye movements -this is the vestibulo-ocular reflex “small rotation of the head is accompanied by movement of the eyes through the same angle but in opposite direction” – ‘doll’s eyes’ Project to cerebellum and lower spinal levels to co-ordinate extensor and flexor muscles to maintain balance and posture Some disorders of vestibular system Ménière’s Disease: too much endolymph and distention of membranous labyrinth => attacks of severe vertigo, nausea, nystagmus, hearing loss and tinnitus, plus eventual permanent progressive hearing loss. Benign Positional Vertigo: otoconia dislodged from utricle and migrate into semicircular ducts (often posterior one). When head moves, gravity-dependent movement of otoconia causes abnormal fluid displacement in the affected semicircular duct and resultant vertigo Acoustic neuroma: benign tumour of the myelin-forming cells of the vestibulocochlear nerve (CN VIII) (also called vestibular schwannoma) – located in the cerebellopontine angle. Suggested reading The Head and Neck: Dean and Pegington Fundamental Neuroscience: Haines Principles of Neural Science: Kandel and Schwartz