Human Physiology, 14th edition (2016)

(Tina Sui) #1
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.
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