Chapter 6 The Nervous System 209
to as convolutions. No two brains look exactly
alike in their pattern of convolutions. However,
the major sulci are arranged in the same pattern
in all human brains.
The sulci divide the brain into four regions
called lobes. The four lobes of the brain are the
frontal, parietal, occipital, and temporal.
Like the sulci, fi ssures are uniformly
positioned, deep grooves in the brain. The
longitudinal fi ssure runs the length of the brain
and divides it into left and right hemispheres. As
a result, the lobes are paired on the left and right
sides of the body. Neural communications to and
from the right side of the body are controlled by
the left brain, and communications with the left
side of the body are controlled by the right brain.
The frontal lobes, located behind the
forehead in the most anterior portion of the
brain, are sectioned off from the rest of the brain
by the central sulcus (Figure 6.9B). Just anterior
to the central sulcus is the primary motor cortex,
which sends neural impulses to the skeletal
muscles to initiate and control the development of
muscle tension and movement of our body parts.
As Figure 6.10 shows, scientists have
mapped the primary motor cortex so that we
know which body parts are controlled in each
region of the cortex. Notice that relatively
small regions of the cortex control major body
segments, such as the trunk, pelvis, thigh, and
arm. Much larger regions of the cortex are
allocated for control of smaller body segments,
such as the hands, lips, and tongue.
Why is this the case? If you think about it,
the body parts associated with larger areas of the
motor cortex are the ones capable of the more
fi ne-tuned movements. Such movements require
the activation of more nerves.
Tongue
Jaw
Motor
Knee
Sensory
Lips
Face
Eye
Brow
Neck
Thumb
Fingers
Hand
Wrist
Elbow
Arm
Shoulder
Trunk
Hip
Swallowing
Leg
Hip Trunk
Neck
Head
Arm
ElbowForearmHandFingers
Thumb
Eye
Nose
Face
Lips
Teeth
Gums
Jaw
Tongue
Pharynx
Intra-
abdominal
Toes
Genitals
Primary motor cortex
(in front of central sulcus)
Primary somatic sensory cortex
(behind central sulcus)
Figure 6.10 The primary motor and somatic sensory cortexes, with mapped regions of motor output and
sensory input depicted. Why do smaller areas of the body, such as the fingers or lips, require more nerves than
larger areas, such as the shoulder or trunk?