390 Chapter 12
The stretch reflex, with its reciprocal innervations,
involves the muscles of one limb only and is controlled by
only one segment of the spinal cord. More complex reflexes
involve muscles controlled by numerous spinal cord segments
and affect muscles on the contralateral side of the cord. Such
reflexes involve double reciprocal innervation of muscles.
Double reciprocal innervation is illustrated by the crossed-
extensor reflex. If you step on a tack with your right foot, for
example, this foot is withdrawn by contraction of the flexors
and relaxation of the extensors of your right leg. The contra-
lateral left leg, by contrast, extends to help support your body
during this withdrawal reflex. The extensors of your left leg
contract while its flexors relax. These events are illustrated in
figure 12.31.Upper Motor Neuron Control
of Skeletal Muscles
As previously described, upper motor neurons are neurons in
the brain that influence the control of skeletal muscle by lower
motor neurons (alpha and gamma motoneurons). Neurons in
the precentral gyrus of the cerebral cortex contribute axons that
cross to the contralateral sides in the pyramids of the medulla
oblongata; these tracts are thus called pyramidal tracts (chap-
ter 8, see figs. 8.25 and 8.26). The pyramidal tracts include the
lateral and ventral corticospinal tracts. Neurons in other areas
of the brain produce the extrapyramidal tracts. The major
extrapyramidal tract is the reticulospinal tract, which origi-
nates in the reticular formation of the medulla oblongata and
pons. Brain areas that influence the activity of extrapyrami-
dal tracts are believed to produce the inhibition of lower motor
neurons described in the preceding section.When a limb is flexed, for example, the antagonistic
extensor muscles are passively stretched. Extension of a limb
similarly stretches the antagonistic flexor muscles. If the mono-
synaptic stretch reflexes were not inhibited, reflex contraction
of the antagonistic muscles would always interfere with the
intended movement. Fortunately, whenever the agonist mus-
cles are stimulated to contract, the alpha and gamma motoneu-
rons that stimulate the antagonist muscles are inhibited.
Figure 12.31 The crossed-extensor reflex. This complex reflex demonstrates double reciprocal innervation.
Extensor Flexor Extensor Flexor
++- Flexor contracts ––
and extensor relaxes
to withdraw foot
2. Extensor contracts and
flexor relaxes in
contralateral leg
to support weight
CLINICAL APPLICATION
Damage to spinal nerves, or to the cell bodies of lower motor
neurons (by poliovirus, for example), produces a flaccid
paralysis, characterized by reduced muscle tone, depressed
stretch reflexes, and atrophy. Damage to upper motor neu-
rons or descending motor tracts at first produces spinal shock
in which there is a flaccid paralysis. This is followed in a few
weeks by spastic paralysis, characterized by increased
muscle tone, exaggerated stretch reflexes, and other signs of
hyperactive lower motor neurons. These result because upper
motor neurons normally exert an inhibitory effect on lower
alpha and gamma motor neurons. When this inhibition is
removed—for example by spinal cord damage—the gamma
motoneurons become hyperactive and the spindles become
overly sensitive to stretch. This is responsible for spasticity and
for clonus, in which agonist and then antagonist muscles con-
tract because of stretch reflexes to produce alternating move-
ments. Clonus can be demonstrated by forcefully dorsiflecting
the patient’s foot (pushing it up) and then releasing it. Exten-
sion stretches the antagonistic flexor muscles, which contract
to produce the opposite plantar flexion, thereby stretching the
extensor muscles to elicit their reflex contraction.