Transcript Special Senses: The Ear

Special Senses: The Ear
By the end of this class you should
understand:
• The properties of sound waves as they relate
to hearing
• The three regions of the ear and their
functions
• How the inner ear creates the senses of
hearing and balance
Sensory Neurons
• Recall there are five
types of neurons,
classified by the type of
stimulus they respond to
– Mechanoreceptor
– Thermoreceptor
– Nociceptor (Pain
receptor)
– Chemoreceptor
– Photoreceptor
Mechanoreceptors
• Mechanoreceptors depend on
mechanically gated ion
channels
– When the cell is deformed,
they open and allow sodium
into the cell
• There are many types of
mechanoreceptors
• The type of interest today is
called the hair cell
Hair Cell
• Hair cells are unique in that
they do not have an axon
• The “hair” of the hair cell
(also known as stereocilia)
bend when there are
vibrations
• The bending allows positively
charged ions into the cell
• This depolarizes the cell
membrane causing
neurotransmitters to be
released
Vibrations
• The ear is responsible for
detecting two different
types of movements
using hair cells
– Sound: vibrations of the air
– Vestibular sense:
acceleration of the head
• Both of these occur in the
inner ear
Parts of the Ear
Vestibular Sense
• Sense of balance or
equilibrium
• Can sense both linear
and angular
acceleration
– The vestibule detects
linear acceleration
– The semicircular canals
detect angular
acceleration
Acceleration Types
• Linear acceleration is in a
straight line
– Detectable in a car that is
braking or gunning the engine
– Also tells you which way is up
• Angular acceleration is
spinning or rolling
– Detectable doing somersaults
or spinning in a circle
• In both cases the hair cells are
stimulated by the linear or
angular movements of heavy
crystals called otoliths
Acceleration and Nausea
• The brain must integrate signals
from the different senses
– A combination of forebrain and
midbrain work
• When these signals do not
match the brain struggles to
integrate them
– Nausea (“carsickness” and
“seasickness”) may result from
feeling acceleration in the
vestibular sense but not visually
detecting any acceleration
Angular Acceleration
• Spinning rapidly for an
extended period of time can
cause the semicircular canals
to build up a lot of angular
momentum
• When you stop spinning you
experience dizziness because
the semicircular canals are
still spinning
– Causes the world to “spin”
because your eyes are trying
to follow the movement of the
canals
Nature of Sound
• The hair cells for the
vestibular sense respond to
the movement of heavy
crystals
• The hair cells for the sense of
sound respond instead to the
vibrations caused by sound
• To understand this, it is
important to understand the
nature of sound
Pressure Waves
• Sound is caused by pressure
waves moving through a
medium (air, water, etc)
– A single pressure wave will
only sound like a crack or
boom, not a tone
– Back-and-forth differences
in air pressure at a certain
frequency produce a
constant tone
• The faster the waves of air
pressure arrive at the ear,
the higher the perceived
pitch
Pitch and Volume
• The bigger the waves (the more
energy is carried), the higher the
volume
– Higher amplitude, measured in
decibels
– Its all relative! The closer, the louder
the sound
• The faster the waves arrive, the
higher the pitch of the sound
– Higher frequency, measured in hertz
– Same unit used for light frequency but
light waves have millions of times
higher frequencies
Resonance
• A complication is that any
object capable of producing or
containing sound waves has a
resonance frequency
• The best example of resonance
is making waves in a bathtub
slowly bigger and bigger by
moving your own body in time
with the waves
– Sound waves can have this
happen as well!
– Note the different lengths on
harp/piano strings
Resonance in the Cochlea
• There is a spiral-shaped
section of the temporal bone
called the cochlea
– The cochlea serves as a
resonating chamber
– Vibrations from the ear are
transmitted to pressure waves in
the cochlea
• Much like piano strings, there
is a high-frequency to lowfrequency resonance of the
hair cells in the cochlea
Parts of the Ear
Pressure Waves
• Pressure waves are
transmitted through the
outer ear
– Pinna (the part of the ear you
can touch)
– Auditory canal (the part of the
ear you can put a Q-tip in)
– The Tympanic
membrane/eardrum forms the
border between the outer and
middle ear
Middle Ear
• The middle ear is a cavity with
three tiny bones called ossicles
that attach to the tympanic
membrane at one end and the
inner ear at the other
– The ossicles are unique to
mammals
• When air vibrations cause the
eardrum to vibrate, the three
bones transmit these
vibrations to the inner ear
Middle Ear Ossicles
• The three ossicles are
malleus, incus and stapes
– Hammer, anvil, and stirrup,
named for their shapes
– Malleus is attached to the
eardrum, stapes pounds on
the oval window
• The auditory tube connects
this space to the nasal
passage
– If blocked due to illness, can
result in headaches/earaches
and reduced hearing
Oval Window
• Just as the tympanic membrane
forms the border between the
outer and middle ear, the oval
window forms the border between
the middle and inner ear
• The oval window is where pressure
waves enter the cochlea
– The faster the vibrations transmitted
onto the eardrum, the faster the
vibrations transmitted into the oval
window
The Cochlea
• The cochlea is filled with a
liquid called perilymph that
transmits vibrations from the
oval window
• The hair cells plus the
membrane they attach to and
the sensory neurons form the
Spiral Organ, or Organ of Corti
• The axons from these sensory
neurons bundle together to
become the cochlear nerve
The Nerves!
• The cochlear nerve leaves the
cochlea heading for the brain
• The vestibular organs produce
another nerve called the
vestibular nerve
• These two nerves actually join
to become the
vestibulocochlear nerve which is
processed by the thalamus and
midbrain before transmitting to
the temporal lobe
That’s our show!
• Next Monday is review day! Bring your
questions!
• One week from today is Exam #2!