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Chapter 4 Section 4
Other Senses
Obj: Identify the chemical, skin,
and body senses.
Smell
People do not have as strong a sense of smell as
many animals. Dogs use seven times as much
of the cerebral cortex for smell as people do.
Some dogs are used to sniff out drugs or
explosives in suitcases or to track lost children
or objects.
But smell is important to people too. Without smell,
you would not be able to taste as much as you
do. For example, if your sense of smell were not
working, an onion and an apple would taste very
much alike to you.
Odors of substances are detected by receptor
neurons high in each nostril. Receptor neurons
react when molecules of the substance in the
form of a gas come into contact with them. The
receptors send information about the odors to the
brain via the olfactory nerve.
Our sense of smell adapts quickly. We adapt rapidly
even to annoying odors. This may be fortunate if
we are in a locker room. It may not be fortunate if
harmful fumes, such as from cars, are present-we
may lose awareness of the smoke or fumes even
though danger remains. One odor can also be
masked by another, which is how air fresheners
work.
Taste
Why would your dog gobble up a piece of a
candy bar, but your cat turn up its nose at
it? Dogs can taste sweetness, but cats
cannot.
The four basic taste qualities are sweetness,
sourness, saltiness, and bitterness. Do you
think that you taste more than these four
qualities? What you are experiencing is the
flavor of food, which involves odor, texture,
and temperature as well as taste.
If you have a cold, for
example, food tastes
flat because you cannot
smell it. The reason
older people sometimes
complain that their food
has little “taste” is
usually that they have
experienced a loss of
the sense of smell, not
taste. Thus, they
perceive less of the
flavor of their food.
Older people often
spice their food heavily
to enhance its flavor.
Taste is sensed through receptor neurons
located on taste buds on the tongue. Some
people have low sensitivity for sweetness
and may have to use twice as much sugar
to sweeten their food as others who are
more sensitive to sweetness. People who
claim to enjoy very bitter foods may actually
be “taste blind” to them. Sensitivities to
different tastes can be inherited.
By eating hot foods and
scraping your tongue,
you regularly kill off
many taste cells. But
you need not be
alarmed. Taste cells
reproduce rapidly
enough to completely
renew themselves in a
week. The taste system
is thus one of the most
resilient of all the body’s
sensory systems. It is
very rare for anyone to
suffer a complete
permanent taste loss.
The Skin Senses
What we normally call touch is better called the skin
senses because touch is a combination of
pressure, temperature, and pain. Humans have
distinct sensory receptors for pressure,
temperature, and pain, but some nerve endings
may receive more than one type of sensory input.
Our skin senses are vitally important to us.
Studies have shown that premature infants grow
more quickly and stay healthier if they are
touched. And older people seem to do better if
they have a dog or cat to stroke and cuddle.
• Pressure – Your body is covered with
hairs, some of them very tiny. Sensory
receptors located around the roots of hair
cells fire where the skin is touched. Other
structures beneath the skin are also
sensitive to pressure. Different parts of the
body are more sensitive to pressure than
others. The fingertips, lips, nose, and
cheeks are more sensitive than the
shoulders, thighs, and calves.
• Temperature –
Sensations of
temperature are
relative. When your
body temperature is at
a normal 98.6 F, you
might perceive
another person’s skin
as being warm. When
you are feverish,
though, the other
person’s skin might
seem cool.
The receptors for temperature are neurons just
beneath the skin. When skin temperature
increases because you touch something warm,
receptors for warmth fire. Decreases in skin
temperature, such as those that occur when you
put a cool, moist cloth on your forehead, cause
receptors for cold to fire.
We adapt to differences in temperature. Have you
ever walked out of an air-conditioned building into
the hot sunshine? At first, the heat really hit you,
but soon the sensation faded as you adapted to
the warmth. In the same way, when you first jump
into a swimming pool, the water may seem cold.
Yet, after a few moments, the water feels warmer
as your body adjusts to it.
• Pain – Pain tells you something is wrong.
It is also adaptive-it motivates us to do
something to stop it.
Headaches, backaches, toothaches-these
are only a few of the types of pain most of
us experience from time to time. More
serious health problems-such as arthritis,
cancer, or wounds-also cause pain. Not all
areas of the body are equally sensitive to
pain. The more pain receptors located in a
particular area of our skin, the more
sensitive that area is.
Once a person gets hurt, everything
happens very quickly. Pain originates at
the point of contact. The pain message is
sent from the point of contact to the spinal
cord to the thalamus in the brain. Then it is
projected to the cerebral cortex, where the
person registers the location and severity
of the pain. Chemicals called
prostaglandins help the body transmit pain
messages to the brain. Aspirin and
ibuprofen are common pain-fighting drugs
that work by curbing production of
prostaglandins.
Simple remedies like rubbing and scratching
an injured area sometimes help relieve
pain. Gate theory suggests that only a
certain amount of information can be
processed by the nervous system at a time.
Rubbing or scratching the area transmits
sensations to the brain that compete with
the pain messages for attention. Thus,
many neurons cannot get their pain
messages to the brain. It is as if too many
calls are flooding a switchboard. The
flooding prevents many or all of the calls
from getting through.
One of the more fascinating facts in
psychology is that many people
experience pain in limbs that are no longer
there. More than half of combat veterans
with amputated limbs report feeling pain in
the missing, or “phantom,” limbs. In such
cases, there is no current tissue damage,
but the pain is real. The pain appears to
involve activation of nerves in the stump of
the missing limb, along with activation of
neural circuits that have memories
connected with the limb.
Body Senses
Body senses are the senses
that people are least
aware of. But do not let
that fool you. Without
them, you would have to
pay attention just to stay
upright, to lift your legs to
go downstairs, or even to
put food in your mouth.
• The Vestibular Sense – Stand up. Now close
your eyes. Do you have to look in a mirror to be
certain that you are still upright? No of course
not. Your vestibular sense tells you whether
you are physically upright without your having to
use your eyes. Sensory organs located in the
ears monitor your body’s motion and position in
relation to gravity. Your vestibular sense enables
you to keep your balance. It tells you whether
you are upside down or not and lets you know
when you are falling. It also informs you of
whether your body is changing speeds, such as
in accelerating automobiles.
• Kinesthesis – Ask some friends to close their
eyes. Then ask them to touch their noses with
their index fingers. How close to their noses did
they come? Many of them were probably right
on the mark, while others could only come close.
How did they locate their noses? Their eyes were
closed, so they could not see their hands. They
touched their noses through kinesthesis.
Kinesthesis is the sense that informs people
about the position and motion of their bodies. In
kinesthesis, sensory information is fed to the
brain from sensory organs in the joints, tendons,
and muscles.