Transcript 2013 Chapter 9: Hearing and Equilibrium

Chapter 9: Hearing and
Equilibrium
Equilibrium and Hearing
• Both of these senses are provided by the
internal ear which is located in the temporal
bone.
• Equilibrium informs us of our body’s position
in space
• Hearing enables us to detect and interpret
sound waves
• Both use hair cells which are mechano
receptors
Anatomy of the Ear
• Divided into three anatomical regions
– External ear
• Collects and direct sound waves toward middle ear
– Middle ear
• Amplify sound waves and transmit them to inner ear
– Internal ear
• Contains the sensory organs for hearing and
equilibrium
External Ear
• Includes the auricle or pinna which surround
the entrance to the external acoustic meatus
• Ends at the tympanic membrane.
Middle Ear
• Connected to the nasopharynx by the auditory
tube (eustachian tube).
• Encloses and protects the auditory ossicles
which connect the tympanic membrane to the
internal ear.
– Malleus: attached to tympanic membrane
– Incus: middle bone
– Stapes: attached to the oval window of the inner
ear.
Internal Ear
• Senses of equilibrium and hearing are
provided by the receptors within the internal
ear.
• These receptors are protected by the bony
labyrinth which is fused with the temporal
bone
• The bony labyrinth surrounds the
membranous labyrinth which is a collection of
tubes and chambers.
• The membranous labyrinth is filled with
endolymph and between the bony and
membranous labyrinths is another fluid called
perilymph.
• Bony labyrinth has three parts
– Vestibule: receptors for gravity and acceleration
– Semicircular canals: rotation of the head.
– Cochlea: hearing.
Equilibriium
• Dynamic equilibrium : aids us maintaining our
balance when the head and body move
suddenly
• Static equilibrium: maintains our posture and
stability when the body is motionless.
• Semicircular canals monitor rotational
movement of the head which is part of
dynamic equilibrium
• Structures in the maculae respond to gravity
and linear acceleration.
Hearing
• The receptors for hearing are hair cells similar
to those of equilibrium.
• Their placement in the cochlea shields them
from stimuli other than sound
• The auditory ossicles convert the pressure
waves of air to pressure pulses in the
perilymph at the oval window.
• The pressure pulses stimulate hair cells along
the cochlear spiral.
• The frequency (pitch) of the perceived sound
is determined by which part of the cochlear
duct is stimulated. (units hertz)
• The intensity (volume) of the perceived sound
is determined by how many hair cells at that
location are stimulated. (units decibels)
6 steps
• 1. Sound waves arrive at the tympanic
membrane.
• 2. Movement of the tympanic membrane
causes displacement of the auditory ossicles.
• 3. The movement of the stapes at the oval
window establishes pressure waves in the
perilymphs of the inner ear.
• 4. The pressure waves distort the basilar
membrane on their way to the round window
of the tympanic duct.
• 5. Vibration of the basilar membrane causes
vibration of hair cells against the tectorial
membrane.
• 6. Information about the region and intensity
of stimulation is relayed to the CNS over the
cochlear branch of cranial nerve VIII.
Aging and the Senses
• Smell: olfactory receptor cells are regularly
replaced by cell division but this decreases
with age. The receptors also become less
sensitive.
• Taste: reduction in number and sensitivity of
taste buds. Begin life with around 10,000
taste buds but number declines quickly after
age 50.
Vision and age
• With age the lens loses its elasticity and
stiffens. Seeing objects close up becomes a
problem—called presbyopia.
• Cataracts: loss of transparency in the lens.
• Gradual loss of rods with age: need more light
to read.
• Macular degeneration: growth and
proliferation of blood vessels in the retina.
Hearing
• The tympanic membrane loses some elasticity.
• It becomes difficult to hear high pitched
sounds.
• Progressive hearing loss that occurs with aging
is presbycusis.
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