Ear

OB J E C T I V ES

Describe the anatomy of the structures in the three main regions of the ear.
• List the major events in the physiology of hearing.
• Identify the receptor organs for hearing, and describe how they function.
• Describe the auditory pathways.
describe deafness and types of deafness
Hearing

• Hearing

• Organ of hearing: organ of Corti
• Sensory receptor :cochlea duct
• Nerve: cochlear nerve
• Common characteristics – receptor for hearing & equilibrium
• Hair cells bathed in endolymph
• Hairs (cilia) of the cells are embedded in a dense mass
• Movement of the mass of hair cells relative to one another stretches/bends the cilia
• Stretching/ bending of cilia in 1 direction increases impulse generation


Hearing
• Outline of hearing (3 major steps)
• 1.Airborne sound consists of vibrations: alternate phases of condensation & rarefaction; the auditory apparatus convert these vibrations in air to vibrations in the inner ear fluid
• 2.Vibrations lead to bending of cilia of hair cells (of the Organ of Corti); Generate nerve impulse: transmit along auditory nerve to higher centers of hearing.
• Sound

pressure wave produced by air by a vibrating body

results from the back & forth vibration of the particles of the medium (air)
resulting in alternate phases of condensations (compressions) and rarefactions
• Properties of sound:
Greater amplitude (intensity of sound wave),louder the sound expressed as decibel (dB) 1 dB = 0.1 bel
1 bel: the logarithm of the ratio of the intensity of the sound and a standard sound
0 dB: not absence of sound,sound level of an intensity equal to that of the standard threshold pressure
140 dB is potentially damaging to auditory receptor the organ of Corti.
• Greater frequency, higher the pitch.
• Soundwaves that have repeating patterns perceived as musical sounds
Aperiodic nonrepeating vibrations is noisiness

Threshold varies with the pitch of sound:
Greatest sensitivity range 1000-4000 Hz
Audible frequency range: 20 – 20,000 Hz

• Threshold for human hearing

• Hearing mechanism

• Collection & concentration of sound waves by ear lobes (pinna)
Vibration of tympanic membrane in harmony with the frequency of sound source
• Movements of 3 ossicles
• In & out movement of the footplate of stapes at the oval window of cochlea pressing on the fluid in the cochlea
•

Impedance matching – function of the middle ear

• Auditory receptors of the inner ear operate in a fluid environment
• underwater sound receiver
• Effective transfer of sound energy from air (lower acoustic resistance/impedance) to fluid (higher acoustic resistance/impedance)
• is due to amplification of the pressure by:
• large ratio btwn the areas of tympanic membrane & stapes footplate-oval window (17:1)
• pressure = force/area
• mechanical advantage of the ossicular lever system
• 1:3
• amplified 17 times

• 3 chambers of cochlea & cochlea nerve

Round window allows for fluid displacement in the cochlea

because fluid of the inner ear is not compressible,inward movement of the stapes footplate is allowed because of the yielding of the thin membrane which covers the round window
this is essential to the transmission process since it provides elastic relief for the fluid of the inner ear thus permitting movement of the stapes & the structure of the inner ear

• Mechano-electrical event

Hair cells & their stereocilia are ‘wedged’ between tectorial membrane & reticular lamina
Vibrations transmitted by stapes
produce displacement of basilar membrane
up & down movement
Shearing movement between the tectorial membrane & reticular lamina
bends the hairs (stereocilia) of the hair cells of organ of Corti
Opening of mechanically-gated K+ channes in the stereocilia
K+ influx from endolymph into the stereocilia
depolarization of the hair cell membrane
Opening of voltage-gated Ca2+ channels
Ca2+ influx
Release excitatory neurotransmitter glutamate
• When basilar membrane bends towards scala vestibuli (medially)
• hair cells depolarized
• Opposite direction will lead to hyperpolarization
• generating alternating hair cell receptor potential

Auditory Pathways

Receptors
cochlear division of 8th CN
Cochlear nuclei in the medulla oblongata
Inferior colliculi
Medial geniculate bodies of thalamus
Thalamic radiation
Cortical auditory centres in temporal lobe

Sounds of different frequencies travel different distances down the basilar membrane

low pitched – apex
high pitched – base
Why?
the width & stiffness of the basilar membrane vary from the apex to the base
Pitch discrimination

Intensity discrimination

Greater the degree of displacement of the hair cells
more hair cells displaced
hence more nerve fibres are stimulated

Deafness

Types of deafness:
1.Conductive deafness
pathology in external/middle ear characterized by impaired sound conduction
common causes:
a.plugging of the external auditory canals with wax (cerumen) or foreign bodies, otitis externa, inflammation of the outer ear,swimmer’s ear.

b.otitis media: inflammation of the middle ear causing fluid accumulation , hyperemic swelling & increased mucus production associated with an upper respiratory infection leads to temporary closing of the Eustachian tube,so -ve pressure develops within the middle ear ,distention of the tympanic membrane which leads to perforation of the eardrum.


c.Osteosclerosis:bone is resorbed and replaced with sclerotic bone which grows over oval window leads to fixation of the stapes in the oval window

2.Sensorineural deafness/nerve deafness

pathology in cochlear/auditory neural pathways
common causes
• Presbycusis : hearing loss with aging
• loss of hair cells & neurons
• Ototoxicity: aminoglycoside antibiotics (streptomycin, gentamycin), obstruct the mechanosensitive channels in the stereocilia of the hair cells cause cells to degrade
• damage to hair cells by prolonged exposure to noise
• tumors in; 8th cranial nerve , cerebellopontine angle
• vascular damage in the medulla
3.Mixed deafness

Audiometry

Audiometer – measure auditory acuity
pure tones of various frequencies through earphones
At each frequency
threshold intensity is determined & plotted on a graph as a % of normal hearing
Provides objective measurement of the degree of deafness
and the tonal range that is most affected