CHAPTER 16
Control of Posture & Movement 245peripheral area in which they produce movement, providing
the basis for feedback control of movement. Some of this
input may be direct and some is relayed from other parts of
the cortex. The current view is that M1 neurons represent
movements of groups of muscles for different tasks.
SUPPLEMENTARY MOTOR AREA
For the most part, the supplementary motor area projects to
the motor cortex. This region also contains a map of the body,
but it is less precise than in M1. It appears to be involved pri-
marily in organizing or planning motor sequences, while M1
executes the movements. Lesions of this area in monkeys pro-
duce awkwardness in performing complex activities and diffi-
culty with bimanual coordination.
When human subjects count to themselves without speak-
ing, the motor cortex is quiescent, but when they speak the
numbers aloud as they count, blood flow increases in M1 and
the supplementary motor area. Thus, the supplementary
motor area as well as M1 is involved in voluntary movement
when the movements being performed are complex and
involve planning. Blood flow increases whether or not a
planned movement is carried out. The increase occurs
whether the movement is performed by the contralateral or
the ipsilateral hand.
PREMOTOR CORTEX
The premotor cortex, which also contains a somatotopic map,
receives input from sensory regions of the parietal cortex and
projects to M1, the spinal cord, and the brain stem reticular for-
mation. Its function is still incompletely understood, but it may
be concerned with setting posture at the start of a planned move-
ment and with getting the individual prepared to move. It is
most involved in control of proximal limb muscles needed to
orient the body for movement.POSTERIOR PARIETAL CORTEX
In addition to providing fibers that run in the corticospinal
and corticobulbar tracts, the somatic sensory area and related
portions of the posterior parietal lobe project to the premotor
area. Lesions of the somatic sensory area cause defects in mo-
tor performance that are characterized by inability to execute
learned sequences of movements such as eating with a knife
and fork. Some of the neurons in area 5 (Figure 16–3) are con-
cerned with aiming the hands toward an object and manipu-
lating it, whereas some of the neurons in area 7 are concerned
with hand–eye coordination.ROLE IN MOVEMENT
The corticospinal and corticobulbar system is the primary
pathway for the initiation of skilled voluntary movement. This
does not mean that movement—even skilled movement—is
impossible without it. Nonmammalian vertebrates have es-
sentially no corticospinal and corticobulbar system, but they
move with great agility. Cats and dogs stand, walk, and run af-
ter complete destruction of this system. Only in primates are
relatively marked deficits produced.
Careful section of the pyramids producing highly selective
destruction of the lateral corticospinal tract in laboratory pri-
mates produces prompt and sustained loss of the ability to
grasp small objects between two fingers and to make isolated
movements of the wrists. However, the animal can still use the
hand in a gross fashion and can stand and walk. These deficits
are consistent with loss of control of the distal musculature of
the limbs, which is concerned with fine-skilled movements.
On the other hand, lesions of the ventral corticospinal tract
produce axial muscle deficits that cause difficulty with bal-
ance, walking, and climbing.PLASTICITY
A striking discovery made possible by PET and fMRI is that the
motor cortex shows the same kind of plasticity as the sensory
cortex (Chapter 11). For example, the finger areas of the contra-
lateral motor cortex enlarge as a pattern of rapid finger move-
ment is learned with the fingers of one hand; this change is
detectable at 1 week and maximal at 4 weeks. Cortical areas of
output to other muscles also increase in size when motor learning
involves these muscles. When a small focal ischemic lesion is pro-
duced in the hand area of the motor cortex of monkeys, the hand
area may reappear, with return of motor function, in an adjacentFIGURE 16–5
Motor homunculus.
The figure represents, on a
coronal section of the precentral gyrus, the location of the cortical rep-
resentation of the various parts. The size of the various parts is propor-
tionate to the cortical area devoted to them. Compare with Figure 11–4.
(Reproduced with permission from Penfield W, Rasmussen G:
The Cere-
bral Cortex of Man.
Macmillan, 1950.)
ToesAnkleKneeHipTrunk
ShoulderElbowWristHand
LittleRing
MiddleIndex
ThumbNeck
BrowEyelid and eyeball
Face
LipsVOCALIZATIO
NSA
LIV
AT
IONMAS
TICATIO
NTongueJaw
Swallowing