Chapter 12
The Central Nervous System
435
12
not impair muscle strength and the ability to perform the dis-
crete individual movements. For example, if the premotor area
controlling the flight of your fingers over a computer keyboard
were damaged, you couldn’t type with your usual speed, but you
could still make the same movements with your fingers. Repro-
gramming the skill into another set of premotor neurons would
require practice, just as the initial learning process did.
Sensory Areas
Areas concerned with conscious awareness of
sensation, the sensory areas of the cortex, occur in the parietal,
insular, temporal, and occipital lobes (see Figure 12.6, dark and
light blue areas).
1.
Primary somatosensory cortex.
Te
primary somatosen-
sory cortex
resides in the postcentral gyrus of the parietal
lobe, just posterior to the primary motor cortex. Neurons
in this gyrus receive information from the general (so-
matic) sensory receptors in the skin and from propriocep-
tors (position sense receptors) in skeletal muscles, joints,
and tendons. Te neurons then identify the body region
being stimulated, an ability called
spatial discrimination
.
considered to be (1) present in one hemisphere only (usu-
ally the le±) and (2) a special
motor speech area
that directs
the muscles involved in speech production. However, im-
aging studies indicate that Broca’s area also becomes active
as we prepare to speak and even as we think about (plan)
many voluntary motor activities other than speech.
4.
Frontal eye field.
Te
frontal eye field
is located partially
in and anterior to the premotor cortex and superior to
Broca’s area. Tis cortical region controls voluntary move-
ment of the eyes.
Homeostatic Imbalance
12.1
Damage to localized areas of the
primary motor cortex
(as from
a stroke) paralyzes the body muscles controlled by those areas.
If the lesion is in the right hemisphere, the le± side of the body
will be paralyzed. Only
voluntary
control is lost, however, as the
muscles can still contract reflexively.
Destruction of the
premotor cortex
, or part of it, results in
loss of the motor skill(s) programmed by that region, but does
Genitals
Foot
Toes
Leg
Hip
Trunk
Neck
Head
Arm
Elbow
Forearm
Hand
Fingers
Thumb
Eye
Nose
Face
Lips
Teeth
Gums
Jaw
Tongue
Pharynx
Intra-
abdominal
Swallowing
Tongue
Jaw
Lips
Face
Eye
Brow
Neck
Thumb
Fingers
Hand
Wrist
Elbow
Arm
Shoulder
Trunk
Hip
Knee
Knee
Primary motor
cortex
(precentral gyrus)
Primary somato-
sensory cortex
(postcentral gyrus)
Motor
Motor map in
precentral gyrus
Sensory
Sensory map in
postcentral gyrus
Posterior
Anterior
Figure 12.7
Body maps in the primary motor cortex and somatosensory cortex of
the cerebrum.
The relative amount and location of cortical tissue devoted to each function is
proportional to the distorted body diagrams (homunculi).
previous page 469 Human Anatomy and Physiology (9th ed ) 2012 read online next page 471 Human Anatomy and Physiology (9th ed ) 2012 read online Home Toggle text on/off