Chapter 12
The Central Nervous System
433
12
about these areas, do not confuse the sensory and motor ar-
eas of the cortex with sensory and motor neurons. All neu-
rons in the cortex are interneurons.
Each hemisphere is chiefly concerned with the sensory and
motor functions of the
contralateral
(literally, “opposite”)
side of the body.
Although largely symmetrical in structure, the two hemi-
spheres are not entirely equal in function. Instead, there is
lateralization (specialization) of cortical functions.
And finally, keep in mind that our approach is a gross over-
simplification. No functional area of the cortex acts alone,
and conscious behavior involves the entire cortex in one way
or another.
Motor Areas
Te following
motor areas
of the cortex, which
control voluntary movement, lie in the posterior part of the frontal
lobes: primary motor cortex, premotor cortex, Broca’s area, and
the frontal eye field (
Figure 12.6a
, dark and light red areas).
1.
Primary motor cortex.
Te
primary (somatic) motor cor-
tex
is located in the precentral gyrus of the frontal lobe of
each hemisphere (Figure 12.6, dark red area). Large neurons,
called
pyramidal cells
, in these gyri allow us to consciously
control the precise or skilled voluntary movements of our
skeletal muscles. Teir long axons, which project to the spi-
nal cord, form the massive voluntary motor tracts called
py-
ramidal tracts
or
corticospinal tracts
(kor
0
tĭ-ko-spi
9
nal). All
other descending motor tracts issue from brain stem nuclei
and consist of chains of two or more neurons.
Te entire body is represented spatially in the primary
motor cortex of each hemisphere. For example, the py-
ramidal cells that control foot movements are in one place
and those that control hand movements are in another.
Such mapping of the body in CNS structures is called
so-
matotopy
(so
0
mah-to-to
9
pe).
As illustrated in
Figure 12.7
(p. 435), in somatotopy the
body is represented upside down—with the head at the in-
ferolateral part of the precentral gyrus, and the toes at the
superomedial end. Most of the neurons in these gyri con-
trol muscles in body areas having the most precise motor
control—that is, the face, tongue, and hands. Consequently,
these regions are disproportionately large in the caricature-
like
motor homunculus
(ho-mung
9
ku-lus; “little man”)
drawn in Figure 12.7. Te motor innervation of the body is
contralateral: In other words, the le± primary motor gyrus
controls muscles on the right side of the body, and vice versa.
Te motor homunculus view of the primary motor cor-
tex, shown at the le± in Figure 12.7, implies a one-to-one
correspondence between cortical neurons and the muscles
they control, but this is somewhat misleading. In fact, a
given muscle is controlled by multiple spots on the cortex,
and individual cortical neurons actually send impulses to
more than one muscle. In other words, individual pyrami-
dal motor neurons control muscles that work together in a
synergistic way to perform a given movement.
For example, reaching forward with one arm involves
some muscles acting at the shoulder and some acting at
the elbow. Instead of the discrete map offered by the motor
Central sulcus
Longitudinal
fissure
Left frontal
lobe
Left temporal
lobe
Areas active
in speech and
hearing (fMRI)
Figure 12.5
Functional neuroimaging (fMRI) of the cerebral
cortex.
Speaking and hearing increase activity (blood flow, shown
by yellow and orange areas) in the posterior frontal and superior
temporal lobes, respectively.
homunculus, the primary motor cortex map is an orderly
but fuzzy map with neurons arranged in useful ways to
control and coordinate sets of muscles. Neurons control-
ling the arm, for instance, intermingle and overlap with
those controlling the hand and shoulder. However, neu-
rons controlling unrelated movements, such as those con-
trolling the arm and those controlling body trunk muscles,
do not cooperate in motor activity.
Tus, the motor homunculus is useful to show that
broad areas of the primary cortex are devoted to the leg,
arm, torso, and head, but neuron organization within those
broad areas is much more diffuse than initially imagined.
2.
Premotor cortex.
Just anterior to the precentral gyrus in the
frontal lobe is the
premotor cortex
(see Figure 12.6, light
red area). Te premotor cortex helps plan movements. Tis
region selects and sequences basic motor movements into
more complex tasks, such as playing a musical instrument or
typing. Using highly processed sensory information received
from other cortical areas, it can control voluntary actions
that depend on sensory feedback, such as moving an arm
through a maze to grasp a hidden object.
Te premotor cortex coordinates the movement of sev-
eral muscle groups either simultaneously or sequentially,
mainly by sending activating impulses to the primary mo-
tor cortex. However, the premotor cortex also influences
motor activity more directly by supplying about 15% of
pyramidal tract fibers. Tink of this region as the staging
area for skilled motor activities.
3.
Broca’s area.
Broca’s area
(bro
9
kahz) lies anterior to the
inferior region of the premotor area. It has long been
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