Vestibular Apparatus and Equilibrium Dr Than Kyaw December 2011

Functional Structure of Ear External ear Middle ear Inner ear Cochlea and Hearing Vestibular appratus and Balance

Structure of Ear External ear Pinna: cartilagenous Movable Localizing and picking up sound Tympanic membrane (ear drum) separate middle ear from external ear

Structure of Ear Middle ear Three Ossicles: malleus, incus, stapes (also k/s hammer, anvil, stirrup) Veatibular (oval) window Cochlear (round) window Auditory tube (eustachian tube) - communicate with the pharynx

Structure of Ear Internal ear 1. Cochlear portion (sensory for hearing) 2. Vestibular portion (sensory for equilibrium - Both organs in the temporal bone (bony cavity: osseous labyrinth) - Supplied by 2 branches of vestibulocochlear nerve

Cochlear portion (sensory for hearing) Cochlear: spiral shaped Its base at the level of oval window

Cochlear duct (Scala media) endolymph Cochlear (Cross section) Scala vestibuli (above) filled with perilymph Cochlear duct (Cross section) endolymph Scala tympani (below) filled with perilymph

Cochlear duct (Scala media) endolymph Organ of Corti - along with the cochlear duct convert sound waves to nerve impulses

Mechanism of hearing Sound (Air pressure waves) Tympanic membrane Captured by Pinna Vibratory Scala vestibuli Vestibular window Auditory ossicles Perilymph The wave is transmitted to the scala media and from there to the scala tympani. Displacement of hair cell cilia against tectorial membrane caused by oscillations of basilar membrane (resulting from dissipation of sound waves) causes the hair cells to depolarize and create a nerve impluse. Organ of Corti The impulse is transmitted to auditory cerebral cortex through vestibulocochlear nerve.

Base

Hearing Frequency limit Human - 20 to 20,000 cps Dog – up to 50,000 cps

Vestibular apparatus/system Control body stability Movement Posture Balance/Equilibrium

Vestibular System Vestibule 3 semi-circular canals - anterior, lateral, posterior - perpendicuar to each other The utricle The saccule Ampulla These organs contain the sensory hair receptors: the maculae (for the utricle & saccule) and cristae (ampullae).

Macuae and cristae hair cells embedded in and otolithic membrane Otolithic membrane – gelatinousc material - contain otoliths: calcium carbonate crystals relatively heavy - utricle receptors – horizontal plain - saccule receptors – vertical plain

Mechanism of equilibrium Linear acceleration (Macular sensors) - Gliding stress to the hair cells This force register position of the head Due to weight of otoliths – sufficient inertia to sense linear acceleration or deceleration of the head - Pull of gravity - Position of the head

1- Supporting cells 4- Membrane otolithique 2- Hair cell 5- nerve fibers 3- Cilia 6- Otolithes

Maculae in Saccule & utricle Linear acceleration (Macular sensors) Saccule Utricle Maculae in the: Saccule : is responsible for vertical acceleration Utricle: Is responsible for horizontal acceleration Maculae in Saccule & utricle

Linear Acceleration Stimuli When the head starts or stops moving in a linear acceleration otolothic membrane slides backward or forward over hair cells the hair cells will bend

Linear Acceleration Stimuli When the hair bends towards the kinocilium the hair cell depolarize faster steam of impulse is sent to the brain When the hair bends in the opposite direction the hair cells hyperpolarize Slower impulse generation NOTE: It is important to understand that the maculae is responsible for the change in acceleration only. Because the hair cell can adapt it quickly Nerve Action Potential

Rotational acceleration (Ampullary crista) - Ampullary crista detect any plane of rotational acceleration or deceleration - Hair cells of cristae are stimulated when the head is moved. - Mechanical action through the endolymph

Ampulla

Rotational acceleration The receptors for Dynamic equilibrium are the ampulla which is found in the semicircular canals. In each ampulla is a small elevation called a crista. Each crista is made up of hair (receptor) cells and supporting cells, and covered by a jelly-like material known as the cupula. Movement of the cupola stimulates the hair cells Ampulla

Rotational acceleration The ampulla is responsible for the change in rotational movement, as continuous rotation does not stimulate the ampulla. when the head starts moving in a rotationally the endolymph in the semicircular ducts move in the direction opposite to the body’s direction deforming the crista in the duct causes depolarization If the body continues to rotate at a constant rate The endolymph moves at the same direction and speed as the body and stop the movement of hair cells

Rotational acceleration When we suddenly stop moving, the endolymph keeps on moving in the opposite direction hyperpolarization of the hair cells that will tell the brain that we have stopped movement.