Vestibulocochlear: An overview Ken Wu Thursday 17 th 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 4 th 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? me at Good luck!