Vision involves light entering the eye, refracting through the cornea and lens, and projecting onto the retina where rods and cones detect light and detect shape and movement. The optic nerve then carries visual impulses to the brain which interprets them into the correct image. Hearing involves sound waves stimulating nerve cells in the ear that send impulses to the brain, where they are interpreted as sound. Smell occurs when odor molecules stimulate olfactory cells in the nasal passages, sending impulses to the brain for identification. Taste is detected by taste buds on the tongue that send impulses to the brain when dissolved foods contact them.

1. Senses
Chapter 15

2. Vision
• As light enters the eye, it passes through the cornea (transparent section
at front of eye) and is refracted.
• Then light passes through a lens and is refracted again.
• The lens directs the light onto the retina (which is tissue at the back of
the eye that is sensitive to light)
• Rods and Cones are found in the retina. (Cells) Cones respond to bright
light and color. Rods respond to dim light.
• They are used to detect shape and movement.

3. • The light stimulates impulses to pass to the optic nerve.
• The optic nerve carries the impulses to the vision area of
the brain. The image is upside down and reversed.
• The brain interprets the image correctly and you see
what you are looking at.
• The brain interprets images received from both eyes and
blends them into one image. This gives you a sense of
distance.

4. Hearing
• Sound waves are necessary for hearing and they can
travel through solids, liquids, and gases.
 Sound waves reach your ear and stimulate nerve cells
deep within your ear.
 Impulses from these cells are sent to the brain.
 When the sound impulse reaches the hearing area of the
brain, it responds and you hear a sound.

5. • The ear has three sections:
 Outer ear
 Intercepts sound waves and funnels them down the ear canal to the middle ear
 Middle ear
 Ear drum vibrates
 Three tiny bones: hammer, anvil and stirrup (vibrate)
 Inner ear
 Cochlea and semicircular canals
 Cochlea is fluid-filled structure that vibrates once the stirrup vibrates.
 The vibrations bend sensory hair cells in the cochlea which cause electrical
impulses to be sent to the brain by a nerve.
 Depending on how the nerve endings are stimulated, you hear a different type of
sound.

6. • Balance
 Structures in your inner ear control your balance.
 Cristae ampullaris
 Maculae

7. Smell
• You smell food because molecules from the food move into
the air.
• If they enter your nasal passages, these molecules
stimulate sensitive nerve cells (olfactory cells).
• Olfactory cells are kept moist by mucus and when
molecules in the air dissolve in the moisture, the cells
become stimulated.

8. • If enough molecules are present, an impulse
starts in these cells, then travels to the brains
where it is interpreted.
• If it is recognized by a previous experience, you
can identify the odor. If you don’t recognize it, it
is remembered and identified next time.

9. Taste
• Taste buds on your tongue are the major sensory
receptors for taste.
• Five taste sensations
 Sweet
 Salty
 Sour
 Bitter
 MSG

10. • In order to taste something, it has to be dissolved in
water so saliva begins the process of tasting.
• When a solution of saliva and food washes over taste
buds, impulses are sent to your brain.
• The brain interprets the impulses, and you identify the
tastes.

11. Smell and Taste (working together)
• Smell is needed to identify some foods.
• Saliva in your mouth mixes with the food, odors
travel up the nasal passage in the back of your
throat.
• Olfactory cells are stimulated and the taste and
smell is sensed.

12. Drugs affect the nervous system
• Depressant
 Alcohol (disrupts neurons cell functions)
 Destroys brain and liver cells
• Stimulants
 Any substance that speeds up the activity of the
central nervous system
 Caffeine