Human Physiology, 14th edition (2016)

Muscle 387

of gamma neurons thereby enhances the stretch reflex. These

neurons are also important in the voluntary control of skeletal

muscles, as described next.

Coactivation of Alpha

and Gamma Motoneurons

Most of the fibers in the descending motor tracts synapse

with interneurons in the spinal cord; only about 10% of the

descending fibers synapse directly with the lower motor neu-

rons. It is likely that very rapid movements are produced

by direct synapses with the lower motor neurons, whereas

most other movements are produced indirectly via synapses

with spinal interneurons, which in turn stimulate the motor

neurons.

Upper motor neurons —interneurons in the brain that con-

tribute fibers to descending motor tracts—usually stimulate

alpha and gamma motoneurons simultaneously. Such stimula-

tion is known as coactivation. Stimulation of alpha motoneu-

rons results in muscle contraction and shortening; activation

of gamma motoneurons stimulates contraction of the intra-

fusal fibers, and thus “takes out the slack” that would other-

wise be present in the spindles as the muscles shorten. In this

way, the spindles remain under tension and provide informa-

tion about the length of the muscle even while the muscle is

shortening.

Under normal conditions, the activity of gamma moto-

neurons is maintained at the level needed to keep the muscle

spindles under proper tension while the muscles are relaxed.

Undue relaxation of the muscles is prevented by stretch and

activation of the spindles, which in turn elicits a reflex con-

traction (described in the next section). This mechanism pro-

duces a normal resting muscle length and state of tension, or

muscle tone.

Skeletal Muscle Reflexes

Although skeletal muscles are often called voluntary muscles

because they are controlled by descending motor pathways that

are under conscious control, they often contract in an uncon-

scious, reflex fashion in response to particular stimuli. In the

simplest type of reflex, a skeletal muscle contracts in response

to the stimulus of muscle stretch. More complex reflexes

involve inhibition of antagonistic muscles and regulation of a

number of muscles on both sides of the body.

The Monosynaptic Stretch Reflex

Reflex contraction of skeletal muscles occurs in response

to sensory input and does not depend on the activation of

upper motor neurons. The reflex arc, which describes the

nerve impulse pathway from sensory to motor endings in

such reflexes, involves only a few synapses within the CNS.

The simplest of all reflexes—the muscle stretch reflex —

consists of only one synapse within the CNS. The sensory

Alpha and Gamma Motoneurons

In the spinal cord, two types of lower motor neurons innervate
skeletal muscles. The motor neurons that innervate the extra-
fusal muscle fibers are called alpha motoneurons; those that
innervate the intrafusal fibers are called gamma motoneurons
( fig. 12.27 ). The alpha motoneurons are faster conducting (60
to 90 meters per second) than the thinner gamma motoneurons
(10 to 40 meters per second). Because only the extrafusal mus-
cle fibers are sufficiently strong and numerous to cause a mus-
cle to shorten, only stimulation by the alpha motoneurons can
cause muscle contraction that results in skeletal movements.
The intrafusal fibers of the muscle spindle are stimulated
to contract by gamma motoneurons, which represent one-third
of all efferent fibers in spinal nerves. However, because the
intrafusal fibers are too few in number and their contraction is
too weak to cause a muscle to shorten, stimulation by gamma
motoneurons results only in isometric contraction of the spin-
dles. Because myofibrils are present in the poles but absent
in the central regions of intrafusal fibers, the more distensi-
ble central region of the intrafusal fiber is pulled toward the
ends in response to stimulation by gamma motoneurons. As a
result, the spindle is tightened. This effect of gamma motoneu-
rons, which is sometimes termed active stretch of the spindles,
serves to increase the sensitivity of the spindles when the entire
muscle is passively stretched by external forces. The activation

Clinical Investigation CLUES

The physician told Mia to stretch slowly and not to

bounce while she stretched her leg muscles.

• What happens when muscles are stretched that
  might cause muscle spasm?


• How would a slow stretch of the muscles be less
  likely to cause painful spasm than a more rapid
  stretch?

FITNESS APPLICATION

Rapid stretching of skeletal muscles produces very forceful

muscle contractions as a result of the activation of primary

and secondary endings in the muscle spindles that elicit

the monosynaptic stretch reflex. This can result in painful

muscle spasms, as may occur, for example, when muscles

are forcefully pulled in the process of setting broken bones.

Painful muscle spasms may be avoided in physical exercise

by performing slow stretches (not “bouncing”) that are not

too excessive, thereby stimulating mainly the secondary

endings in the muscle spindles, with little stimulation of the

primary endings.