AUDITION

The Human Ear

Divisions of the Ear

The ear is divided into three main areas: • • • Outer (external) ear Middle ear Inner ear

Outer Ear

• The outer ear consists of: – an outer, funnel-like structure called the auricle(or pinna) – an S-shaped tube, the external auditory meatus.

The shapes of the outer ear (the auricle) and the external auditory canal can help collect sound waves travelling through air, amplify them and direct them to the middle ear.

Auricle or Pinna

• • It is composed of irregular plate of elastic cartilage covered with skin, and an occasional hair.

Many muscles are present, but most modern humans cannot move or wiggle ears

Middle Ear

• The middle ear consists of: – an air-filled space in the temporal bone called the tympanic cavity, – – an eardrum or tympanum (tympanic membrane), and three small bones called auditory ossicles.

• Sound waves entering the external auditory canal eventually hit the tympanic membrane, or eardrum (tympanum = drum), the boundary between the outer and middle ears.

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Tympanic Membrane

It is stretched across the end of the external auditory canal It is cone-shaped, with the apex of the cone directed inward. Its cone shape is attached to one of the auditory ossicles (malleus). air molecules push against the membrane, causing it to vibrate at the same frequency as the sound wave.

Eustachian Tube

It is NOT considered a part of the ear – it is part of the nasopharynx Air enters the middle ear through the pharyngotympanic tube (Eustachian tube)to – ensure that atmospheric pressure is maintained either side of the tympanic membrane – to equalize pressure for correct vibration

The Ossicles

The auditory ossicles are the smallest bones in the body and are named according to their shape: Footplate of stapes 1. malleus (hammer) 2. incus (anvil) 3. stapes (stirrup) tiny ligaments suspend the ossicles, and minisynovial joints link them together into a chain that spans the middle ear cavity.

The middle ear also contains two small skeletal muscles that are attached to the auditory ossicles

Arrangement of the ossicles

• • The handle of the malleus is secured to the eardrum the base of the stapes fits into the oval window.

• • • •

Middle Ear Skeletal muscles

The tensor tympani muscle is attached to the malleus The stapedius muscle attaches the Stapes These muscles are the effectors in the protective tympanic reflex – they contract in response to loud sounds that might damage the hearing receptors.

The ossicles transmit the vibratory motion of the eardrum to the oval window

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The Oval Window

It is the intersection of the middle ear with the inner ear, and is directly contacted by the stapes’ footplate

Inner Ear

• The transmission of the vibration to the oval window by the footplate of the stapes, sets the fluids of the inner ear into motion, eventually exciting the hearing receptors.

• These launch an impulse that is sent to the temporal lobe of the brain • The inner ear is innervated by the eighth cranial nerve in all vertebrates.

Inner Ear

• • Consists of a complex system of The cochlea , dedicating to hearing; converting sound pressure impulses from the outer ear into electrical impulses which are passed on to the brain via the auditory nerve.

The vestibular system , dedicated to balance Together, they are called the inner ear labyrinth.

The Labyrinth

• • • There are two structures within this system – the osseous labyrinth – bony canal in temporal bone – membranous labyrinth (lies inside the osseous labyrinth) Between the osseous and membranous labyrinths is a fluid, called perilymph, that is secreted by cells in the wall of the bony canal.

The membranous labyrinth contains another fluid, called endolymph

Cranial Nerve 8 – Vestibulo chochlear nerves

Vestibular System calcium carbonate crystals exist in the succule and utricle of the vastibular system Hair cells present in the cochlea and semicircular canals

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The Vestibular System

Works with the eyes and visual cortex to keep objects in focus when the head is moving. Joint and muscle receptors also help maintain balance. The brain receives, interprets, and processes the information from these systems to control balance.

Its semicircular canals uses fluids and detection cells called hair cells to detect rotational motion, angle, etc.

Its otoliths (Saccule and utricle) contain calcium carbonate crystals to help detect horizontal motion

The Chochlea

• • • • It is the auditory portion of the inner ear. It is a spiral-shaped cavity in the bony labyrinth It has 2.5 turns around its axis The main part of the cochlea is the Organ of Corti - the sensory organ of hearing – it is distributed along the partition separating fluid chambers in the coiled tapered tube of the cochlea.

How does the cochlea help us hear?

• • • • • • The cochlea is filled with a watery liquid, the perilymph perilymph moves in response to the vibrations coming from the middle ear via the oval window. As the fluid moves, the organ of Corti moves thousands of hair cells sense the motion via their cilia, and convert that motion to electrical signals These signals are communicated via neurotransmitters (chemical signal)to many thousands of primary auditory neurons. These neurons transform the signals into action potentials (electrochemical signals), which travel along the auditory nerve to structures in the brain for further processing.

Cochlear nerve fibers Hair cells

THE END