temperature of the water in a lake before jumping in. You might stick one bare foot in the
water. Neurons in the skin on your foot would sense the temperature of the water and send
a message about it to your central nervous system The frontal lobe of the cerebrum would
process the information. It might decide that the water is really cold and send a message to
your muscles to pull your foot out of the water.
In some cases, messages about pain or temperature don’t travel all the way to and from the
brain. Instead, they travel only as far as the spinal cord, and the spinal cord responds to
the messages by giving orders to the muscles. When messages bypass the brain in this way,
it forms areflex arc, like the ones shown inFigures20.29,20.30and20.31.
First image:
Second image:
Third Image
Taste and Smell
Your sense of taste is controlled by sensory neurons on your tongue that detect chemicals
in food. The neurons are grouped in bundles withintaste buds(Figure20.32). There are
five different types of taste neurons on the tongue. Each type detects a different taste. The
tastes are sweet, salty, sour, bitter, and umami, which is a meaty taste. When taste neurons
detect chemicals, they send messages to the brain about them. The brain, in turn, decides
what tastes you are sensing.
Your sense of smell also involves sensory neurons that detect chemicals. The neurons are
found in the nose, and they detect chemicals in the air. Unlike taste neurons, which can
detect only five different tastes, the sensory neurons in the nose can detect thousands of
different odors.
Have you ever noticed that you lose your sense of taste when your nose is stuffed up? That’s
because your sense of smell contributes greatly to your ability to taste of food. As you eat,
airborne molecules of food chemicals enter your nose. You experience the taste and smell
at the same time. Being able to smell as well as taste food greatly increases the number of
different tastes you are able to sense. For example, you can use your sense of taste alone to
learn that a food is sweet, but you have to use your sense of smell as well to learn that the
food tastes like strawberry cheesecake.
Why These Senses Matter
The senses of hearing, balance, touch, taste, and smell enrich our lives each day. The sense
of hearing lets us listen to our favorite music. The sense of balance helps us play the sports
we like. The sense of touch allows us to use a keyboard to text our friends. The senses of