TSM55: the Vestibular System 29/10/08

TSM55: THE VESTIBULAR SYSTEM 29/10/08

LEARNING OUTCOMES

Describe the vestibular system

THE VESTIBULAR APPARATUS

 The vestibular apparatus is contained within the membranous labyrinth of the inner ear, comprising:

o Two endolymph-filled sacs in the vestibule

. Utricle – larger of the two, oval-shaped, at the base of the semicircular canals

. Saccule – rounded, more central in the vestibule, drains the cochlear canal

o Three perpendicular semicircular canals, each with a basal expansion – the ampulla

. Superior – angled at roughly 30˚ to the midline on the transverse plane

. Posterior – angled at roughly 150˚ to the midline on the transverse plane

. Lateral – flat at 90˚ to the midline in the transverse plane

 The utricle and saccule are lined with a sensory epithelium called the macula

o This has a vertical orientation in the saccule and a horizontal orientation in the utricle

o Contains vestibular hair cells with stereocillia of similar function to auditory hair cells

 Crystals of calcium carbonate (otoliths) are embedded in a gelatinous membrane overlying the macula

o Crystals respond to movement, linear acceleration and gravity, distorting the stereocillia

o Bending of the stereocillia causes hyperpolarisation or depolarisation of the hair cells

 The semicircular canals contain hair cells concentrated in a cupula – a gelatinous bulb in the ampulla

o Endolymph flowing in the ducts bends the cupula during accelerated rotation of the head

o At constant velocities the cupula equilibrates with fluid flow

THE VESTIBULAR PATHWAY

 Primary afferent vestibular nerve fibres have their cell bodies in the vestibular ganglion

o Found in the lateral end of the internal acoustic meatus

 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:

o Spinal cord – descending the vestibulospinal tract; alters tone of the muscles of posture

o Oculomotor nuclei – CNIII, CNIV and CNVI; maintains visual fixation during head movement

o Vestibulocerebellum – caudal inferior surface of cerebellum; assists in motor control

o Cortex – via the VPL of the thalamus; provides conscious awareness of head movements

Explain the role of the vestibular system in maintaining posture and balance

 The vestibulo-ocular reflex compensates for rotational head movements to maintain visual fixation

o Rotation of the head triggers activity in the semicircular canals and an afferent transmission

o Primary afferents synapse in vestibular nuclei which relay the signal to the oculomotor nuclei

o Efferent fibres to the extra-ocular muscles cause appropriate counter-rotation of the eyes

 Motion sickness arises effectively as a result of contradictory visual and vestibular sensory information

 Ménière’s disease results from a severe dysfunction of the semicircular canals

o Recurrent vertigo, tinnitus, fluctuating deafness, nausea, vomiting

o Thought to be due to excessive endolymph production / ineffective drainage into the CSF

o Treated using anticholinergics and antihistamines