Vestibular Apparatus and Equilibrium

Dr Than KyawDecember 2011

Functional Structure of EarExternal earMiddle earInner 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 ear1. 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(Cross section)

Scala vestibuli(above) filled with perilymph

Scala tympani(below) filled with perilymph

Cochlear duct(Cross section)endolymph

Cochlear duct(Scala media)endolymph

Organ of Corti - along with the cochlear duct convert sound waves to nerve impulses

Cochlear duct(Scala media)endolymph

Mechanism of hearing

Sound(Air pressure

waves)

Captured by Pinna

Tympanicmembrane

Auditory ossicles

Vibratory

Vestibular window

PerilymphScala vestibuli

Organ of Corti

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.

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 the:– Saccule : is responsible for

vertical acceleration– Utricle: Is responsible for

horizontal acceleration

Saccule

Utricle

Maculae in Saccule & utricle

Linear acceleration(Macular sensors)

• 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

Linear Acceleration Stimuli

• When the hair bends towards the kinocilium the hair cell depolarize faster steam of impulse is sent to the brain

Nerve Action Potential

When the hair bends in the opposite direction the hair cells hyperpolarize Slower impulse generationNOTE: It is important to understand

that the maculae is responsible for the change in acceleration only. Because the hair cell can adapt it quickly

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

• 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.

Rotational acceleration