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The Senses

Chapter 15 Section 3

• Vision, hearing, balance,

smell, taste, and touch
• Each of your major senses
picks up a specific type of
stimulus from your
• The sense organs change
those stimuli into nerve
impulses and send the
impulses to your brain
How Light Enters the Eye
• Eyes respond to the external stimulus
of light
• They convert that stimulus into
impulses that your brain interprets,
and enables you to see
• Light 1st strikes the cornea- clear tissue
that covers the eye
• Light then passes through a fluid filled
chamber to the pupil
• Pupil- the opening through which light
enters the eye
• Iris- circular structure that surrounds
the pupil and regulates the amount
of light entering the eye (eye color)
How Light is Focused
• Light passes through the pupil and strikes the
• Lens- flexible structure that focuses light
• Images produced by the lens are upside down
and reversed
• Light then passes through a transparent, jellylike
fluid and strikes the retina
• Retina- layer of receptor cells that lines the back
 of the eye
• Receptor cells are rods and cones
• Rod cells work best in dim light
• Cone cells work well in bright light and see color
• Impulses begin after light strikes the receptor
cells and travel to the cerebrum through the
optic nerve
• The brain turns the image right-side up and
combines the images from both eyes in the
Correcting Vision Problems
Nearsightedness Farsightedness

• Nearsightedness- CAN • Farsightedness- CAN see

see nearby objects distant objects clearly
clearly, but have
trouble seeing objects • Nearby objects are
far away blurry
• Caused by an eyeball • The lens of the eye
that is too long bends light from
nearby objects and do
• Distant objects do not not focus
 focus sharply on the • Corrected with convex
retina lenses

• Ears are the sense
organs that
respond to the
external stimulus
of sound
• Ears convert sound
to nerve impulses
that the brain
How Sound is
 Sound Vibrations and

Produced Ears

• Sound is produced by • Structure of the ear

vibrations functions to receive
• Vibrations create sound vibrations
waves that move
outward from the
• Waves consist of
moving particles,
such as molecules
of air
• Waves also travel
through liquids and
Structure of the Ear
• 3 parts of the ear- outer ear, middle ear, and inner
• Outer ear shaped as a funnel that enables the outer
ear to gather sound waves
• Eardrum- separates the outer ear from the middle
ear; membrane that vibrates when sound waves
strike it
• Middle ear- 3 smallest bones of the body- hammer,
anvil, and stirrup
• Eardrum makes the hammer vibrate, hammer
 vibrations to the anvil, anvil
 pass them to the stirrup
How You Hear
• Stirrup vibrates against a thin membrane that covers
the opening of the inner ear
• Membrane channels the vibrations into the fluid in
the cochlea
• Cochlea- snail-shaped tube that is lined with
receptors that respond to sound
• Fluid in the cochlea
 vibrates and stimulates
 these receptors
• Sensory neurons then
 send nerve impulses to
 the cerebrum through
 the auditory nerve
Internal Stimuli and Balance
• Ear also controls sense of balance
• Semicircular canals- structures in the ear that are responsible
for your sense of balance above the cochlea
• Canals are full of fluid and are lined with tiny cells that have
hairlike extensions
• Movement causes fluid to move and make hairlike extensions
• Bending creates stimulus that produces nerve impulses in
sensory neurons
• Impulses travel to the cerebellum to determine which way your
head is moving
Smell and Taste
• Smell and taste
depend on
chemical that
responses in
receptors in the
nose and mouth
• 50 different odors
• 4 tastes- bitter,
sweet, salty, and
• Flavor is
determined by
both smell and
• Sense of touch is found all
 over the body
• Skin is the largest sense
 organ
• Receptors responding to light touch are in
the upper part of the dermis; also let you
feel textures
• Receptors deeper in the dermis pick up the
feeling of pressure
• Receptors responding to temperature and
pain are found in the dermis
• What function do the senses perform in the body?

• Describe the process by which your eyes produce an
image of your surroundings. Begin at the point at
which light is focused by the lens.

• How do sound vibrations affect structures in the ear to
produce the sensation of hearing?

• How are the senses of taste and smell similar? How are
they different?