Transcript The Human Eye - Burroughs Middle School

Your Senses
• A loud “boom!” is heard across the street. You
turn quickly and duck slightly. Your senses are
in action! The unexpected noise produces
impulses in sensory receptors in your ears.
These impulses travel to your brain, where
they create a sense of awareness called a
sensation. The sensory receptors in your eyes
send impulses to your brain, which recognizes
the sound as being a car crash.
The Four Senses
•
•
•
•
Sight
Touch
Hearing
Taste
The Human Eye
Your Sense of Sight!
Cornea
• The cornea is a thin, clear membrane that
covers the front of the eye
• It protects the eye while allowing light to enter
Pupil
• Light from the front of the eye enters through
an opening called a pupil
Iris
• The pupil is surrounded by the iris, a ring of
muscle
• The iris controls the amount of light that enters
the eye and gives the eye its color
• In bright light, the iris contracts, which makes
the pupil smaller in diameter
• In dim light, the iris relaxes, which dilates the
pupil to let in more light
Lens
• The oval-shaped piece of clear, curved
material behind the iris.
• The lens refracts, or bends, the light rays and
focuses the image, before sending it off to the
retina in the back of the eye
• Muscles in the eye change the shape of the
lens in order to focus light onto the retina
• When you look at objects that are close to the
eye, the lens becomes more curved. It becomes
flatter when you look at distant objects
Nearsightedness
• If the lens focuses light in front of the retina,
this results in nearsightedness
• Cause: the cornea’s curvature is too steep for
the shape of the eye, causing distant objects to
appear blurry
Farsightedness
• Here the person can see distant objects better
than close up objects
• It results from the lens focusing light behind
the retina
• Children with mild to moderate cases of
farsightedness can see distances far and near
without correction because the muscles and
lens within the eye can overcome the
farsightedness
To correct near or farsightedness you need
to change the way the light is bent using
contacts, eye glasses or surgery
Focusing the Light
• Light travels in a straight line until it passes
through the cornea and the lens.
Retina
• A layer of light-sensitive cells, called
photoreceptors, found in the back of the eye
• They respond to light energy
• These photoreceptors create electrical impulses
• The brain perceives these impulses as light
Two Types of Photoreceptors
ROD CELLS
• Retinal cells that are
sensitive to dim light
• Important in night vision
• Impulses from rod cells are
interpreted as black and
white images
• Impulse travels along an
axon
CONE CELLS
• Retinal cells that are very
sensitive to bright light
• Impulses from cones allow
you to see detail and color
• Impulse travels along an
axon
FOVEA
• Located in the back of the
eye
• It is responsible for sharp,
central vision necessary in
reading, watching
television, driving, or in
any other activity that
requires seeing detail
Optic
Nerve
The impulse
leaves the back
of each eye
through an optic
nerve. The optic
nerve carries the
impulses to your
brain, where the
impulses are
interpreted
What does 20/20 vision mean?
• 20/20 vision means that if you stand 20 feet
away from an eye chart, you can see what the
human eye is supposed to see at 20 feet away
• 20/40 vision means that you standing 20 feet
away from the chart, you see what a normal
human can see when standing 40 feet away
• 20/ 200 is the cut off for legal blindness
The Sense of Hearing
(auditory processing)
The human ear is the most complex sensory system in the
human body. Vision and smell are extraordinary, too, but they
do not match the complexity of turning waves of sound
pressure from air molecules banging against the ear drum in to
the neural signals that get sent to the brain and interpreted as
sound.
What is a Sound wave?
It is just a wave of sound. Music is just a bunch of sound waves, arranged
in a nice pattern. A sound wave is comprised of the changes in tone of air
pressure through compression and refraction.
The Human Ear
The Outer Ear
Sounds are pressure changes in the air that
travel in waves.
The outer ear collects these waves which
travel down your ear canal to enter the middle
ear
• The ear canal channels sound waves to your
ear drum, a thin, sensitive membrane stretched
tightly over your middle ear
• The waves cause your ear drum to vibrate
Middle Ear
The middle ear lies between the tympanic membrane (ear drum) and the
inner ear
Like the outer ear, the middle ear space is filled with air and contains three
small bones called the ossicles: the hammer (malleus), anvil (incus), and the
stirrup (stapes)
These bones amplify the pressure of waves from the vibrating tympanic
membrane which provides an efficient transfer of sound energy to the fluid
filled inner ear
The Inner
Ear
1.The stirrup touches a
liquid filled sac and
the vibrations are
passed on to the inner
ear’s cochlea
2.The hollow channels
of the inner ear are
filled with fluid
The cochlea, semicircular canals, and the
auditory nerve make
up the inner ear
Cochlea
A snail-shaped organ of the inner ear filled
with fluid
Inside the cochlea there are hundreds of
special hair-like cells attached to nerve
fibers, which can transmit information to
the brain by changing sound waves to
nerve impulses
The brain processes the sounds we hear
and lets us distinguish between different
types of sound
Auditory Nerve
The waves are turned into a nerve
impulse in the cochlea and sent to
the brain for interpretation via the
auditory nerve.
Exposure to too much noise can damage ear cells and lead
to hearing loss. Hair cells in the cochlea do not replace
themselves naturally, so take care of the healthy ones you
have !
Damaged ear cells in the
cochlea
Normal ear cells in the cochlea
The Inner Ear and Your Balance
• The semi-circular canals in the inner ear help
you maintain your balance (vestibular system)
• Also fluid filled with nerve cells to send
messages to the brain
• The vestibular system works closely with the
visual cortex and skeletal muscles to help
maintain balance
The Eustachian Tube
• Tube that connects your middle to your throat
• The tube allows air to pass between the tympanic cavity
and the outside of the ear by way of the throat and
mouth
• It is important to maintain equal pressure on both sides
of the ear drum, which is important for normal hearing
• For example, if a person goes from a high altitude to a
low one pressure on the outside of the ear gets greater
and greater, pushing the eardrum inward, out of its
normal position, and hearing may be inpaired