Auditory Physiology

Auditory Physiology

1. Human Hearing Ability

• The human ear can hear sounds in the frequency range of 20 – 20,000 Hz.

• The ear is most sensitive to frequencies around 1000 Hz.

• The frequencies of normal conversations fall within the range of 500-2000 Hz or 250-4000 Hz.

• Sound intensity depends on sound pressure.

• The threshold of hearing is 0 dB and the threshold of pain is 120 dB.

2. Structure of the Auditory System

• The auditory system comprises:

• Outer ear: Pinna, external auditory canal.

• Middle ear: Tympanic cavity, Eustachian tube.

• Inner ear: Bony labyrinth, membranous labyrinth, inner ear fluid.

• Brain: Central auditory processing area.

• Boundaries:

• Between the outer and middle ear is the tympanic membrane (malleus attached to it).

• Between the inner and middle ear are the round window and oval window (stapes attached to it).

3. Functions of the Outer Ear

• Main function: Receiving and resonating sound.

• Pinna:

• Collects sound.

• Directs sound.

• Amplifies sound (increases by 5-6 dB).

• External auditory canal:

• Conducts sound.

• Increases sound frequency (length 2.5 cm, S-shaped).

• Warms air before it reaches the eardrum.

• Amplifies sound (increases by ~15 dB) at frequencies of 1500-7000 Hz.

• Hair-bearing skin epithelium: Secretes sebum, keeps the ear warm and softens the skin.

• Cerumen (earwax): Epithelium (hydrophobic, water-repellent) secreted in the outer 1/3 of the external auditory canal, with functions:

• Prevents dust, sand, and bacteria.

• Has a taste => repels insects.

• Moisturizes and lubricates the eardrum surface.

• pH=6.1 (acid) => Resistant to bacteria, viruses, and fungi.

• Cleans the external auditory canal.

• Earwax plug: Cerumen impacted and pushed inward => forming an earwax plug.

4. Functions of the Middle Ear

• The middle ear is an air-filled cavity located in the temporal bone.

• Tympanic membrane: Transforms sound waves into mechanical vibrations.

• Sound transmission from air to cochlear fluid: Loss of ~30 dB due to impedance difference (air has lower impedance than cochlear fluid).

• The ear can recover the ~30 dB lost through:

• Area difference of the membrane: The tympanic membrane area is larger than the oval window area => focusing force from the eardrum to the oval window, recovering 25 dB.

• Lever effect: The action of the ossicular chain following the lever principle, recovering 2-3 dB.

• Total recovery: Close to 30 dB.

• Phase difference: Sound travels to the oval window first, and then travels back to the round window later.

• Mechanism to protect the ear from loud sounds:

• Stapedius muscle: Pulls the stapes out.

• Tensor tympani muscle: Presses the stapes in.

• Both muscles contracting will immobilize the ossicular chain => protecting the ear from loud sounds (high frequencies).

• Tympanic membrane perforation: Loss of phase difference.

• Disarticulation of the ossicular chain: Loss of lever effect.

• Tympanic membrane perforation + disarticulation of the ossicular chain: Loss of 40-45 dB.

• Eustachian tube: Regulates air pressure, protects and drains.

5. Functions of the Inner Ear

• The inner ear is located in the middle 1/3 of the petrous bone.

• The inner ear consists of:

• Bony labyrinth: Contains perilymph fluid.

• Membranous labyrinth: Contains endolymph fluid.

• Organ of Corti:

• Has 16,000 hair cells (30-100 stereocilia).

• Above: Stereocilia contact the tectorial membrane.

• Below: The base of the hair cells contacts the first-order sensory neuron fibers (bodies in the spiral ganglion).

• Consists of outer hair cells and inner hair cells: Separated by the tunnel of Corti.

• Outer hair cells: Have 3 rows (13,500 cells), have cilia that contact the tectorial membrane, amplify small sounds, classify frequencies.

• Inner hair cells: Have 1 row (3500 cells), do not have cilia in direct contact with the tectorial membrane, synapse with the centripetal auditory nerve fibers.

• Where the neuro-electrical signal is received: Where it synapses with the centripetal auditory nerve fibers.

• Tonotopy theory: Differences in sound frequency are related to the width and stiffness of the basilar membrane, “standing wave” ripple.

• High-frequency sounds: Short wavelengths, vibrate near the base of the cochlea, stiff and narrow basilar membrane.

• Low-frequency sounds: Long wavelengths, vibrate near the apex of the cochlea, flexible and wide basilar membrane.

• Receptor potential: Generated by hair cells in the basilar membrane vibrating, converting mechanical stimulation into electrical signals.

• Pressure changes in the cochlea:

• Movement of hair cells (leaning against the tectorial membrane).

• Stereocilia bend => generating receptor potential.

• Generating auditory nerve current => connecting to the auditory nerve fibers.

6. Hearing Impairment

• Cochlear implant: Indicated in cases of hearing impairment due to hair cell destruction.

• Mechanism of action: The device converts mechanical stimulation into electrical signals => transmitted to the VIIIth cranial nerve => to the brain.

• Types of hearing loss:

• Sensory neural deafness: Damage to the cochlea or auditory nerve.

• Conductive deafness: Damage to the outer ear or middle ear.

• Mixed deafness: Both.

• Neurological hearing loss: Damage to the VIIIth cranial nerve (often hair cells).

• Causes: Noise, poisoning (antibiotics, anticancer drugs).

• Conductive hearing loss: Vibrations do not reach the hair cells (disruption of the conduction mechanism).

• Middle ear pathology: Acute otitis media, middle ear sclerosis.

• Presbycusis: Gradual degeneration of hair cells in the Organ of Corti.

7. Tinnitus

• Tinnitus is common in the elderly.