Auditory periphery : The outer ear, middle ear, and inner ear, ending at the nerve fibers exiting the inner ear.
Auditory central nervous system : The ascending and descending auditory pathways, which centers in the brainstem and cortex.
Tonotopic organization : The systematic mapping of sound frequency to the place of maximum stimulation within the auditory system that begins in the cochlea and is preserved through the auditory cortex.
MAIN COMPONENTS OF THE HEARING MECHANISM:
Divided into 4 parts (by function):
Central Auditory Nervous System
STRUCTURES OF THE OUTER EAR
Gathers sound waves
Aids in localization
Amplifies sound approx. 5-6 dB
The outer ear serves a variety of functions:
1. It protects the more delicate middle and inner ears from foreign bodies.
2. It boosts or amplifies high-frequency sounds.
3. The outer ear provides the primary cue for the determination of the elevation of a sound’s source.
4. The outer ear assists in distinguishing sounds that arise from in front of the listener from those that arise from behind the listener.
EXTERNAL AUDITORY CANAL:
Approx. 1 inch long
“ S” shaped
Outer 1/3 surrounded by cartilage; inner 2/3 by mastoid bone
Allows air to warm before reaching TM
Isolates TM from physical damage
Cerumen glands moisten/soften skin
Forms boundary between outer and middle ear
Vibrates in response to sound waves
Changes acoustical (sound) energy into mechanical energy
The middle ear consists of a small air-filled cavity lined with a mucous membrane that forms the link between the air-filled outer ear and the fluid-filled inner ear.
This link is accomplished mechanically via three tiny bones, the ossicles:
The purpose of the elaborate link between the air-filled outer ear and the fluid-filled inner ear is to compensate for the loss of energy that would occur if sound waves struck the fluid-filled inner ear directly.
The middle ear compensates for this loss of sound energy through two primary mechanisms:
1. The areal ratio of the tympanic membrane to the footplate of the stapes
2. The lever action of the ossicles
EUSTACHIAN TUBE ( “THE EQUALIZER”)
Mucous-lined (protection), connects middle ear cavity to nasopharynx
“ Equalizes” air pressure in middle ear
Normally closed, opens under certain conditions
May allow a pathway for infection
Children “grow out of” most middle ear problems as this tube lengthens and becomes more vertical
Attaches to stapes
Contracts in response to loud sounds; (the Acoustic Reflex)
Changes stapes mode of vibration; makes it less efficient and reduce loudness perceived
Absent acoustic reflex could signal conductive loss or marked sensorineural loss
STRUCTURES OF THE INNER EAR: THE COCHLEA
Snail shaped cavity within mastoid bone
2 ½ turns, 3 fluid-filled chambers
Scala Media contains Organ of Corti Converts mechanical energy to electrical energy
TRANSMISSION OF SOUND TO THE INNER EAR
The route of sound to the inner ear follows this pathway:
Outer ear – pinna, auditory canal, eardrum
Middle ear – malleus, incus, and stapes to the oval window
Inner ear – scalas vestibuli and tympani to the cochlear duct
Dynamic equilibrium – angular and rotary head movements
Figure 8.16a, b
Static Equilibrium - Rest Slide 8.31
Maculae – receptors in the vestibule
Report on the position of the head
Send information via the vestibular nerve
Anatomy of the maculae
Hair cells are embedded in the otolithic membrane
Otoliths (tiny stones) float in a gel around the hair cells
Movements cause otoliths to bend the hair cells
Function of Maculae Slide 8.32 Figure 8.15
Dynamic Equilibrium - Movement Slide 8.33a
Crista ampullaris – receptors in the semicircular canals
Tuft of hair cells
Cupula (gelatinous cap) covers the hair cells
Action of angular head movements
The cupula stimulates the hair cells
An impulse is sent via the vestibular nerve to the cerebellum
EFFECT OF GRAVITY ON UTRICULAR RECEPTOR CELLS Figure 15.36
The vestibular apparatus DOES NOT automatically compensate for forces acting on the body… it sends warning signals to the CNS which initiates the appropriate “righting” compensations to keep your body balanced, weight distributed, & eyes focused on location .
AUDITORY PATHWAY TO THE BRAIN
Organ of Corti spiral ganglion (in cochlear nerve) cochlear nuclei of medulla superior olivary nucleus (pons/ medullary junction) along the lateral lemniscal tract to inferior colliculus (midbrain) medial geniculate body of thalamus auditory cortex in temporal lobe