402
UNIT THREE
over from one side of the brain to the other within the brain-
stem and descend as lateral corticospinal tracts. Other F
bers,
in the anterior corticospinal tracts, cross over at various lev-
els of the spinal cord (see F
g. 11.13).
In the spinal cord, the corticospinal F bers synapse with
motor neurons in the gray matter of the anterior horns.
Axons of the motor neurons lead outward through peripheral
nerves to voluntary muscles. Impulses transmitted on these
pathways in special patterns and frequencies are responsible
for F
ne movements in skeletal muscles. More speciF
cally, as
f gure 11.18
shows, cells in the upper portions of the motor
areas send impulses to muscles in the thighs and legs; those
in the middle portions control muscles in the arms and fore-
arms; and those in lower portions activate muscles of the
head, face, and tongue.
The
reticulospinal
and
rubrospinal tracts
coordinate and
control motor functions that maintain balance and posture.
Many of these F
bers pass into the basal nuclei on the way
to the spinal cord. Some of the impulses conducted on these
pathways normally inhibit muscular actions.
In addition to the primary motor areas, certain other
regions of the frontal lobe control motor functions. ±or
example, a region called the
motor speech area,
also known
as
Broca’s area,
is in the frontal lobe, usually in the left hemi-
sphere, just anterior to the primary motor cortex and superior
to the lateral sulcus. The motor speech area is important in
generating the complex muscular actions of the mouth, tongue,
and larynx, which make speech possible (see fig. 11.17).
Bundles of axons directly and indirectly connect the motor
speech area to the sensory speech area. A person with an
injury to this area may be able to understand spoken words
but may be unable to speak.
Above the motor speech area is a region called the
fron-
tal eye F
eld.
The motor cortex in this area controls volun-
tary movements of the eyes and eyelids. Nearby is the cortex
responsible for movements of the head that direct the eyes.
Another region just in front of the primary motor area con-
trols the muscular movements of the hands and F ngers that
make such skills as writing possible (see F g. 11.17).
Table
11.5
summarizes the functions of the cerebral lobes.
An injury to the motor system may impair the ability to produce pur-
poseful muscular movements. Such a condition that aF
ects use of the
upper and lower limbs, head, or eyes is called
apraxia
. When apraxia
affects the speech muscles, disrupting speaking ability, it is called
aphasia
.
PRACTICE
15
How does the brain form during early development?
16
Describe the cerebrum.
17
List the general functions of the cerebrum.
18
Where in the brain are the sensory areas located?
19
Explain the functions of association areas.
20
Where in the brain are the motor areas located?
Association Areas
Association areas are neither primarily sensory nor motor.
They connect with each other and other brain structures.
These areas occupy the anterior portions of the frontal lobes
and are widespread in the lateral portions of the parietal,
temporal, and occipital lobes. They analyze and interpret
sensory experiences and help provide memory, reasoning,
verbalizing, judgment, and emotions (F
g. 11.17).
The association areas of the frontal lobes provide higher
intellectual processes, such as concentrating, planning, and
complex problem solving. The anterior and inferior portions of
these lobes (prefrontal areas) control emotional behavior and
produce awareness of the possible consequences of behavior.
The parietal lobes have association areas that help inter-
pret sensory information and aid in understanding speech
and choosing words to express thoughts and feelings.
Awareness of the form of objects, including one’s own body
parts, stems from the posterior regions of these lobes.
The association areas of the temporal lobes and the
regions at the posterior ends of the lateral sulci interpret
complex sensory experiences, such as those needed to under-
stand speech and to read. These regions also store memories
of visual scenes, music, and other complex sensory patterns.
The occipital lobes have association areas adjacent to
the visual centers. These are important in analyzing visual
patterns and combining visual images with other sensory
experiences—as when one recognizes another person.
A person with
dyslexia
sees letters separately and must learn to read
diF
erently than people whose nervous systems can group letters into
words. Three to 10% of people have dyslexia. The condition probably
has several causes, with inborn visual and perceptual skills interacting
with the way the child learns to read. Dyslexia has nothing to do with
intelligence—many brilliant thinkers were “slow” in school because
educators did not know how to help them.
Wernicke's area corresonds closely to a brain region that
has been referred to as a
"general interpretive area,"
near
where the occipital, parietal, and temporal lobes meet. The
general interpretive area processes sensory information from
all three of these association areas. It plays a role in integrat-
ing visual, auditory, and other sensory information and then
interpreting a situation. ±or example, you hear a familiar
voice, look up from your notes, see a friend from class, and
realize that it is time for your study group.
Motor Areas
The
primary motor areas
of the cerebral cortex lie in the pre-
central gyri of the frontal lobes just in front of the central
sulcus and in the anterior wall of this sulcus (F
g. 11.17). The
nervous tissue in these regions contains many large
pyrami-
dal cells,
named for their pyramid-shaped cell bodies.
Impulses from the pyramidal cells move downward
through the brainstem and into the spinal cord on the
corti-
cospinal tracts.
Most of the nerve F
bers in these tracts cross
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