ENT 318 Artificial Organs Physiology of Ear Lecturer: Ahmad Nasrul Norali
The Ear
The Ear Components of hearing mechanism - Outer Ear - Middle Ear - Inner Ear - Central Auditory Nervous System
Major Divisions of the Ear Peripheral Mechanism Central Mechanism
Structures of the Outer Ear Auricle (Pinna) Collects sound waves Helps in sound localization Most efficient in directing high frequency sounds to the eardrum approx. 5-6 db
External Auditory Canal Approximately 1¼ inch in length S shaped Lined with cerumen glands Outer 1/3rd cartilage; inner 2/3rds mastoid bone Increases sound pressure at the tympanic membrane by as much as 5-6 db (due to acoustic resonance)
Mastoid Process of Temporal Bone Bony ridge behind the auricle Provides support to the external ear and posterior wall of the middle ear cavity Hardest bone in body, protects cochlea and vestibular system Contains air cavities which can be reservoir for infection
Tympanic Membrane Thin membrane Forms boundary between outer and middle ear Vibrates in response to sound Changes acoustical energy into mechanical energy
The Ossicles The Ossicular Chain A: Malleus B: Incus C: Stapes Ossicles are smallest bones in the body Act as a lever system Footplate of stapes enters oval window of the cochlea
Eustachian Tube (AKA: The Equalizer ) Lined with mucous membrane; connects middle ear to back of the throat (nasopharynx) Equalizes air pressure Normally closed except during yawning or swallowing Not a part of the hearing process
Stapedius Muscle Connects the stapes to the middle ear wall Contracts in response to loud sounds; known as the Acoustic Reflex Changes stapes mode of vibration; makes it less efficient and reduce loudness perceived
Structure of The Inner Ear Cochlea - Snail-shaped organ with a series of fluid-filled tunnels; converts mechanical energy into electrical energy
Structures of the Inner Ear (Cont.) Oval Window located at the footplate of the stapes; when the footplate vibrates, the cochlear fluid is set into motion. Round Window functions as the pressure relief port for the fluid set into motion initially by the movement of the stapes in the oval window.
Organ of Corti The end organ of hearing; contains stereocilia and hair cells. Cochlear fluids
Hair Cells Frequency-specific High pitch sounds = base of cochlea Low pitch sounds = apex of cochlea When the basilar membrane moves, a shearing action between the tectorial membrane and the organ of Corti causes hair cells to bend
Vestibular System Consists of three semicircular canals Shares fluid with the cochlea Controls balance No part in hearing process Monitors the position of the head in space Cochlea & Vestibular system comprise the inner ear
Central Auditory System 8th Cranial Nerve or Auditory Nerve carries signals from cochlea to brain Fibers of the auditory nerve are present in the hair cells of the inner ear Auditory Cortex: Temporal lobe of the brain where sound is perceived and analyzed
How Sound Travels Through The Ear... 1. Acoustic energy, in the form of sound waves, is channeled into the ear canal by the pinna 2. Sound waves hit the tympanic membrane and cause it to vibrate, like a drum, changing it into mechanical energy 3. The malleus, which is attached to the tympanic membrane, starts the ossicles into motion 4. The stapes moves in and out of the oval window of the cochlea creating a fluid motion 5. The fluid movement causes membranes in the Organ of Corti to shear against the hair cells 6. This creates an electrical signal which is sent up the Auditory Nerve to the brain The brain interprets it as sound! 20
Transduction of sound into an auditory perception Sound is a propagating pressure wave. Perception of sound involves the electrical activity of neurons in the auditory cortex of the brain. The transduction process is the means by which the pressure waves in air (a mechanical stimulus) is converted into neural activity (action potentials). This process involves a number of stages, some of which involve conduction and impedance matching.
The path of sound ear canal vibrate tympanic membrane vibrate ossicles (3 bones: Malleus, Incus, Stapes) vibrate oval window of cochlea create waves in cochlea fluid create standing waves in basilar membrane movement of hair cells generates electrical activity through mechanically gated ionic channels hair cells stimulate the auditory nerve series of action potentials up to the auditory cortex.
Hearing process
Cochlear Mechanics Basilar membrane : The spectral analyser Basilar membrane (BM) is approx 33mm long in humans Apex of BM is wide and relatively loose Base of BM is thinner and more stiff Variations in length and stiffness provides BM with a continuum of resonant frequencies along its length: low frequencies at apex and high frequencies at base A wave with a particular frequency produces a maximum displacement at a particular portion of the basilar membrane: tonotopic organization BM is heavily damped beyond the resonant frequency Travelling wave velocity is in range 1-20m/sec and is frequency dependent (velocity is reduced apically for low frequencies)
Frequency analysis in inner ear High frequency waves vibrate the basal part of the basilar membrane, dissipate energy and then die out. Lower frequency waves travel further towards apex before dying out.
The Ear
The Ear
Mechanism of Hearing by Organ of Coti Vibration of the basilar membrane produces shear forces that bend the stereocilia (hairs protruding from the hair cells) against the tectorial membrane Movement of the stereocilia either cause the hair cell to depolarise or hyperpolarise, depending upon the direction of movement Changes in the membrane potential of the hair cell generate an AP in the nerve fibre attached to the hair cell.
The Ear Outer Ear: amplification and direction filtering. Middle Ear: impedance matching. Cochlear & Basilar Membrane: spectral analyser. Organ of Corti: Acoustic to neuro-electric transducer. Inner Hair Cells: Synapse type I AN fibres projecting centrally. Outer Hair Cells: Amplification and improved frequency tuning.
Hearing Impairment Hair cells are responsible for translating mechanical information into neural information. Thus, with damaged hair cells, the auditory system has no way of transforming acoustic pressure waves (sound) to neural impulses which in turn leads to hearing impairment. Hair cells damage might cause by diseases (e.g meningitis, Meniere s). If large number of hair cells or auditory neurons throughout the cochlea are damaged, then the person with such a loss is diagnosed as profoundly deaf. There is research which shows that the most common cause of deafness is the loss of hair cells rather than the loss of auditory neurons. So, the remaining neurons could be used by exciting it with electrical stimulation. This make way to the wide use of cochlear implants.
Hearing Impairment.
Cause of hearing loss