Special Senses:
Equilibrium & Hearing
Honors Anatomy & Physiology
Ear Anatomy
External Ear
• Pinna (Auricle)
▫ elastic cartilage covered
w/thin skin
▫ Directs sound waves into
auditory canal
• External Acoustic
Meatus (auditory canal)
▫ Short curved tube carved
into temporal bone
▫ Lined w/Ceruminous
glands – secrete
cerumen (ear wax) to
repel insects and trap
pathogens
• Tympanic Membrane
(Ear Drum)
▫ Connective tissue
membrnae
▫ Sound waves cause it to
vibrate
Ear Structures
• Middle Ear
▫ Air-filled mucosa-lined cavity
▫ Phargnotympanic (Eustachian)
tube
 links middle ear to nasopharynx
 Flattened until needed to equalize
w/external air pressure
 Important for free vibrations of ear
drum
▫ Auditory Ossicles (smallest bones in body)
 Malleus (Hammer)
 Incus (Anvil)
 Stapes (Stirrup)
 Transmit vibrations to oval
window (small opening) of inner
ear
Internal Ear (Labyrinth)
• Osseous Labyrinth
• Filled w/perilymph (similar to CSF)
▫ Vestibule
 Houses receptors that respond to gravity &
changes of head position
▫ Cochlea
 Snail coil the size of a split pea
 Spiral organ of Corti – receptor organ for hearing
▫ Semi-Circular Canals
 Oriented in X,Y,Z planes
 Receptors respond to rotational movements of the
head
• Membranous Labyrinth
▫ Sacs & ducts contained w/in the bony labryinth
▫ Filled w/endolymph – K+ rich intracellular
fluid
Properties of Sound
• Sound – alternating areas of high
and low pressure causing molecules to
vibrate…travels slower than light
• Illustrated as a sine wave defined by:
• Frequency – number of waves that
pass a given point in a certain time
▫ Human hearing 20-20,000 waves per
second (Hz)
• Wavelength – distance between 2
consecutive crests
• Pitch – perceived sound frequencies
(higher freq = higher pitch)
• Amplitude – height of sine wave –
sound intensity…interpreted as
loudness measured in decibels (dB)
Physiology of Hearing
• Sound waves vibrate tympanic membrane transferred to
auditory ossicles and then the oval window
• Bulging membrane of round window creating pressure
waves within perilymph of vestibular duct
• Pressure waves distort basilar membrane, vibrating hair
cells against tectoral membrane
• Information relayed via cochlear nerve
• Hair cells – receptors of inner ear
▫ Stereocilia:80-100 microvilli on free surface moved by
external forces causing displacement in one direction
increasing neurotransmitter release, displacement in
opposite direction decreases neurotransmitter release
• Pitch determined by which part of cochlear duct is
stimulated
• Volume determined by how many cells are stimulated
Vestibular Apparatus:
Equilibrium & Orientation
• Dynamic Equilibrium
▫ Maintains balance during sudden movements
▫ Semicircular Ducts (anterior, posterior & lateral)
▫ Rotational Motion
 Ampulla – swollen region w/sensory “hair” cells
 Movement of endolymph stimulates hair cells
• Static Equilibrium
▫ Maintains posture and stability when motionless
▫ Vestibule
▫ Gravity & Linear Acceleration
 Osteolith (CaCO3) sits on hair cells and can shift
• Vestibular branch of vestibular cochlear nerve integrate sensory info from
both sides, and relay info to cerebellum, cerebral cortex, & brain stem
Hearing Deficits
• Conduction Deafness
▫ Blocks transfer of vibration from tympanic
membrane to oval window
▫ Ear wax buildup, trapped water, scarring or
perforation of tympanic membrane, overgrowth of
ossicles
• Sensorinueral Deafness
▫ Within the cochlea or auditory pathway
▫ Broken sterocilia (loud sounds), drugs destroy
hair cells, bacterial infection kill hair cells
▫ Cochlear implants convert sound into electrical
signals can be inserted into temporal bone
American Sign
Language
Can you spell your
name?