Your brain has distorted maps within its sensory and motor areas called ho-
munculi that show the amount of cells dedicated to any given area of the body.
Maps of the body within the brain • Figure 7.11
Frontal plane through
postcentral gyrus
a.
Frontal section of
primary
somatosensory
area
in
right
cerebral
hemisphere
eas such as the hands and mouth have more cells devoted
to them than do the legs or arms. This makes sense be-
cause we have fine motor skills and need detailed control
of muscular movements in our fingers and hands and to
make sounds with our mouths. Motor units in these areas
are quite small, so numerous neurons would be required
to control the muscle cells. The large motor skills associ-
ated with moving our legs or arms do not require such
fine control. The motor units in these areas of the body
are quite large, requiring fewer neurons to regulate the
muscle movements.
The hemispheres of the brain are symmetrical but
vary slightly. Both have sensory and motor functions. Al-
though information is generally processed by both hemi-
spheres, the focus tends to vary a bit, depending on which
hemisphere is dominant. The left hemisphere is inclined
to address information from a more logical, linear reason-
ing perspective; the right hemisphere tends to introduce
more of an artistic, imaginative perspective. For many
b. Frontal section of primary motor area in right
cerebral hemisphere
individuals, the primary speech centers associated with
vocabulary, grammar, and speech production are found in
the left hemisphere, but the right hemisphere helps to add
intonation and contextual qualities to our speech. Most in-
dividuals use one hemisphere slightly more than the other
to process information. This is referred to as
brain la tera l-
iza tio n
or, more commonly,
brain dom inance.
Despite the fact that we have the ability to control mus-
cle functions on both sides of the body, we usually elect to
choose a particular side to lead the activity. About three-
quarters of the population are right-handed, with most of
the remaining individuals being left-handed. A very small
percentage
of the
population
is
ambidextrous
(equal
dominance)—a trait that is often “learned” rather than in-
born. For many of us, attempting to use the non-dominant
hand to perform a task easily done by the dominant hand
can require great concentration and often frustration.
Now let’s turn to the spinal cord to see how the brain
communicates with the outside world.
The Central Nervous System Coordinates All Nervous Activity
205
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