Vestibulocochlear: An overview Ken Wu ken.wu09@imperial.ac.uk Thursday 17 th November 2011

Vestibulocochlear: An overview

Ken Wuken.wu09@imperial.ac.uk

Thursday 17th November 2011

Disclaimer

• This tutorial is a simple and conceptual guide to the vestibulocochlear system

• If there are any conflicts between my slides and the lecturers, THE LECTURER IS ALWAYS RIGHT…

• …maybe not always but they set your exams so if in doubt, refer back to their teaching

The Vestibular System

• Located in the inner ear• Semicircular canals– Anterior vertical– Posterior vertical– Horizontal

• Otolith organs– Utricule– Saccule

Semicircular Canals

• Detect angular acceleration• 3 sets covering rotation in the 3 planes– Anterior vertical = coronal (head – shoulder) – Posterior vertical = sagittal (head nodding)– Horizontal = transverse (head shaking)

Mechanism - Demo

• Cilia of hair cells within endolymph• Inertia movement of endolymph causes hair

cells to deform• Displacement of hair cells causes

depolarisation

Angular acceleration

• Bilateral stimulation• Rotation to one side stimulates the same side

AND inhibits the opposite side• There is a tonic firing rate – normally the left

and right balance out

Otolith organs

• Detects gravity and linear acceleration• Saccule– Arranged in vertical plane– Hair cells are horizontal– Therefore detects vertical movement

• Utricule– Arranged in horizontal plane– Hair cells are vertical– Therefore detects horizontal movement

Mechanism - Demo

• Cilia of hair cells covered by otoliths – a gelatinous matrix containing calcium carbonate crystals

• Inertia of otoliths cause hair cells to deform• Displacement of hair cells cause

depolarisation

Gravity and Linear acceleration

• Once hair cells are displaced, they stay displaced– Therefore tonic firing exists in the saccule due to

the presence of gravity– It also means head tilt and lying down are also

detected even after several hours (e.g. sleep)• Inertia of the otoliths in linear acceleration

temporarily causes displacement of the utricule hair cells

Vestibular pathways

• Vestibular nerve ganglion (Scarpa’s ganglion)• Vestibular nerve• Vestibulocochlear nerve• Vestibular nuclei– In the brainstem, at the floor of the 4th ventricle

Vestibulo-ocular

• Vestibular nucleus stimulates contralateral VI nucleus

• VI nucleus– Abduct eye– Stimulates contralateral III nucleus – adducts

opposite eye• Causes vestibulo-oculor reflex– Head rotate left, eyes moves right to maintain gaze– Function is to maintain gaze

Vestibulo-spinal

• Lateral vestibulo spinal tract– Ipsilateral– Influence limb muscles

• Medial vestibulo spinal tract– Bilateral– Influence neck and back muscles

Vestibular disorders

• Vestibular nystagmus– Unopposed tonus of intact canal– Eyes driven to lesioned side– Fast saccade beat to intact side

• Vestibular ataxia– Unopposed tonus of intact canal– Body/head fall towards lesioned side

Ear

• Sound conduction• Sound transduction• Sound pathways

Sound conduction

• Outer ear– Pinna, external acoustic meatus– Sound collection and conduction

• Middle ear– Air filled chamber in bone• Malleus• Incus• Stapes (smallest bone in the body)

– Sound amplification

Middle ear protection

• Reflex contraction of muscles dampens amplitude– Tensor tympani – malleus– Stapedius – stapes• Stapedius supplied by VII, thus Bell’s palsy causes

hyperacusis

• Eustacian tube allows pressure equalisation

Conductive deafness

• Wax• Otitis media• Otosclerosis of ossicles• Perforated tympanic membrane• Congenital malformations

Cochlear

• Pressure equalisation by oval and round window movements

Sound transduction

• Sound waves causes vibration of vestibular and basilar membranes

Organ of Corti

• Basilar membrane vibration• Tectorial membrance provides shear force– Stereocilia displaced away from modiolus (central

axis of cochlea• K channels open - depolarisation

– Stereocilia displaced towards modiolus• K channels closed - hyperpolarisation

• Endolymph provides the ions

Demo

Pitch

• Higher frequencies towards the base of basilar membrane

• Lower frequencies towards the apex of basilar membrane

Auditory pathways

• Bilateral• Tonotropy– Pattern of pitch is preserved

• Lateral inhibition• Inferior collicus– To Reticular Activating System• Startle reflex

Auditory cortex

• Primary– In temporal lobe near central sulcus– Subdivided areas according to frequencies– Analyses duration, intensity and sound patterns

• Secondary– Complex sound patterns– Higher functions e.g. speech

Weber and Rinne

• Rinne– Pinna vs mastoid process– Rinne +ve = pinna > mastoid

• Normal!!!• Sensorineural deafness

– Rinne –ve = mastoid vs pinna• Conductive deafness

• Weber– Midline of forehead– Equally loud = normal– L > R

• R sensorineural deafness• L conductive deafness

Any questions?

• Email me at ken.wu09@imperial.ac.uk

• Good luck!