The Central Nervous System 235
regions of the cerebral cortex, basal nuclei, and cerebellum
that participate in this motor control have numerous synaptic
interconnections, and they can influence movement only indi-
rectly by means of stimulation or inhibition of the nuclei that
give rise to the extrapyramidal tracts. Notice that this motor
control differs from that exerted by the neurons of the precen-
tral gyrus, which send fibers directly down to the spinal cord
in the pyramidal tracts.
The reticulospinal tracts are the major descending path-
ways of the extrapyramidal system. These tracts originate in
the reticular formation of the brain stem, which receives either
stimulatory or inhibitory input from the cerebrum and the cer-
ebellum. There are no descending tracts from the cerebellum;
the cerebellum can influence motor activity only indirectly by
its effect on the vestibular nuclei, red nucleus, and basal nuclei
(which send axons to the reticular formation). These nuclei, in
turn, send axons down the spinal cord via the vestibulospinal
Figure 8.25 Descending corticospinal (pyramidal)
motor tracts. These tracts contain axons that pass from the
precentral gyrus of the cerebral cortex down the spinal cord
to make synapses with spinal interneurons and lower motor
neurons.
Primary motor
area of cerebral
cortex
Internal
capsule
Thalamus
Skeletal
muscle
Anterior
corticospinal
tract
Lateral
corticospinal
tract
Cervical
spinal
cord
Lumbar
spinal
cord
Medulla
oblongata
Pyramid
tracts, rubrospinal tracts, and reticulospinal tracts, respec-
tively ( fig. 8.26 ). Neural control of skeletal muscle is explained
in more detail in chapter 12.
CLINICAL APPLICATION
Babinski’s reflex (or the Babinski sign ) occurs normally
in infants and children under two years old. This reflex
involves an upward movement of the big toe and a fan-
ning of the other toes when the sole of the foot is stimu-
lated, due to a reflex contraction of the extensor muscles
of the toes. Once the central nervous system has devel-
oped more fully, the descending corticospinal (pyramidal)
tracts inhibit this reflex. In adults, the same stimulation
usually evokes a downward flexion, or curling, of the toes.
A Babinski reflex in an adult can indicate CNS damage due
to such conditions as spinal cord injury, amyotrophic lat-
eral sclerosis (ALS), multiple sclerosis (MS), brain tumors,
and others.
Clinical Investigation CLUES
Kevin’s big toe extended upward when the sole of his
right foot was stroked, but his left big toe instead curled
in response to the same procedure.
- What caused these differing responses?
Figure 8.26 The higher motor neuron control of
skeletal muscles. The pyramidal (corticospinal) tracts are
shown in pink and the descending motor pathways from the
brain stem that are controlled by the extrapyramidal system are
shown in black.
Cerebral cortex
Thalamus Cerebellum
Red nucleus
Vestibular nucleus
Lower motor neurons
Brain stem
reticular
formation
Basal nuclei
Pyramidal (corticospinal) tracts
Reticulospinal
tract
Rubrospinal
tract
Vestibulospinal
tract