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Unit 19: Biology Part 2 Page 76 of 137
Touch your brain. This is called a reflex
arc.
If you’ve ever accidently touched a
hot stove, prickly cactus plant, or a
block of ice, you know how strong
the sense of touch can be. Touch
is the sense of pain, pressure, or
temperature. All of our skin
contains sensory neurons. When
you touch something, messages go
from the sensory neurons to the Figure 62 – A reflex arc causes
brain, which interprets what you’ve the spinal cord to send a
touched.
message pulling your hand
Although all of the skin, as well as away from a hot stove.
many internal organs, have You often hear people say they did
sensory neurons allowing them to something, “without thinking.” The
experience touch, certain areas of truth, however, is that our brain is
the body, including the palms of
the hands, soles of the feet, lips, usually very much involved in the
and tongue, have more sensory things we do. A reflex arc is one of
the rare times when the brain is
neurons than other parts of the truly not involved. Other
body. This makes them more
sensitive to touch. responses may come later. You
may scream in pain, say “ouch” or
As we’ve learned, messages travel cry after touching the hot stove.
from sense organs, like the skin, to All these involve your brain and
the brain very quickly. There are therefore happen after the reflex
times, however, when something arc action of removing your hands.
we touch is so dangerous that our It is only the initial movement, to
body does not even want to take prevent damage to the tissue of
the time to send the message to the skin, that is brain-free.
the brain. For example, if you
touch a hot stove, the message Taste and Smell
that you are touching something Neurons found in your nose detect
hot will only go to your spinal cord, chemicals in the air. These
instead of making it all the way to neurons send messages to your
brain, and the brain determines
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