the axons cross to the opposite side of the CNS (spinal cord). The axons then descend
as the lateral corticospinal tract, within the lateral funiculus (Figure 12-6). Thus, the left
cerebral hemisphere commands the right side of the body, and the right cerebral
hemisphere controls the left side of the body.
12-30. EXTRAPYRAMIDAL MOTOR PATHWAYS
The extrapyramidal motor pathways are concerned with automatic
(nonvolitional) control of body parts. This particularly includes patterned, sequential
movements or actions. Thus, the major command system of the human nervous
system uses these pathways. There are several extrapyramidal motor pathways.
Having multisynaptic circuits throughout the CNS, they use many intermediate relays
before reaching the effector organs. The cerebellum of the brain plays a major role in
extrapyramidal pathways; the cerebellum is the major center for coordinating the
patterned sequential actions of the body, such as walking.
Section IX. LEVELS OF CONTROL IN THE HUMAN NERVOUS SYSTEM
a. General Concept. The human nervous system can be thought of as a
series of steps or levels (Figure 12-11). Each level is more complex than the level just
below. No level is completely overpowered by upper levels, but each level is controlled
or guided by the next upper level as it functions.
b. Changes With Development or Injury.
(1) Babinski's reflex involves dorsiflexion of the big toe when the sole of the
foot is stimulated. It can be normally observed in infants up to 18 months of age. As
the pyramidal motor pathways develops completely, this reflex disappears. However, if
the pyramidal motor system is injured, the Babinski reflex tends to return.
(2) Thus, it is possible to evaluate the extent of development of an individual
by identifying the highest level of control. In the case of injury, the highest active level
of control helps determine the site of the injury.