Human Physiology, 14th edition (2016)

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