Page 493 - Fundamentals of anatomy physiology

480 Unit 3 Control and Regulation
13 Both sensory and motor neurons innervate each intrafusal
fiber. The dendrites of the sensory neuron spiral around the intra- Stretch reflexes are only one type of postural reflex. There are
fusal fiber in a central sensory region. Axons from spinal motor
neurons form neuromuscular junctions on either end of this fiber.
Motor neurons innervating intrafusal fibers are called gamma
motor neurons. Their axons are called gamma efferents.
An intrafusal fiber has one set of myofibrils at each end.
These myofibrils run from the end of the intrafusal fiber only to
the sarcolemma in the central region that is closely monitored
by the sensory neuron. The gamma efferents enable the CNS to
adjust the sensitivity of the muscle spindle. Before seeing how
this is accomplished, let’s see how the muscle spindle normally
functions as a sensory receptor and look at its effects on the
surrounding extrafusal fibers.
The sensory neuron is always active, conducting impulses
to the CNS. The axon enters the CNS in a dorsal root and syn-
apses on motor neurons in the anterior gray horn of the spinal
cord. Axon collaterals distribute the information to the brain,
providing information about the muscle spindle. Stretching
the central portion of the intrafusal fiber distorts the dendrites
and stimulates the sensory neuron, increasing the frequency of
action potential generation. Compressing the central portion
inhibits the sensory neuron, decreasing the frequency of action
potential generation.
The axon of the sensory neuron synapses on CNS motor neu-
rons that control the extrafusal muscle fibers of the same muscle.
An increase in sensory neuron stimulation, caused by stretching
of the intrafusal fiber, increases stimulation to the motor neuron
controlling the surrounding extrafusal fibers, so muscle tone in-
creases. This increase provides automatic resistance that reduces
the chance of muscle damage due to overstretching. The patellar
reflex and similar reflexes serve this function.
A decrease in the stimulation of the sensory neuron, due
to compression of the intrafusal fiber, leads to a decrease in
the stimulation of the motor neuron controlling the surround-
ing extrafusal fibers, so muscle tone decreases. This decrease
reduces resistance to the movement under way. For example,
if your elbow is flexed and you let gravity extend it, the triceps
brachii muscle, which is compressed by this movement, relaxes.
Many stretch reflexes are postural reflexes—reflexes that
help us maintain a normal upright posture. Standing, for exam-
ple, involves a cooperative effort by many muscle groups. Some
of these muscles work in opposition to one another, exerting
forces that keep the body’s weight balanced over the feet. If your
body leans forward, stretch receptors in your calf muscles are
stimulated. Those muscles then respond by contracting, return-
ing your body to an upright position. If the muscles overcom-
pensate and your body begins to lean back, your calf muscles
relax. But then stretch receptors in muscles of your shins and
thighs are stimulated, and the problem is corrected immediately.
Postural muscles generally maintain a firm muscle tone
and have extremely sensitive stretch receptors. As a result, very
fine adjustments are continually being made, and you are not
aware of the cycles of contraction and relaxation that occur.
many complex polysynaptic postural reflexes.
Now let’s return to the role of the gamma efferents. They let
the CNS adjust the sensitivity of muscle spindles. Gamma effer-
ents play a vital role whenever voluntary contractions change the
length of a muscle. Impulses arriving over gamma efferents cause
the contraction of myofibrils in the intrafusal fibers as the biceps
brachii muscle shortens. The myofibrils pull on the sarcolemma
in the central portion of the intrafusal fiber—the region moni-
tored by the sensory neuron—until that membrane is stretched to
its normal resting length. As a result, the muscle spindles remain
sensitive to any externally imposed changes in muscle length. For
this reason, if someone drops a ball into your palm when your
elbow is partially flexed, the muscle spindles automatically adjust
the muscle tone to compensate for the increased load.
Polysynaptic Reflexes
Polysynaptic reflexes can produce far more complicated re-
sponses than can monosynaptic reflexes. One reason is that
the interneurons involved can control several muscle groups.
Moreover, these interneurons may produce either excitatory
or inhibitory postsynaptic potentials (EPSPs or IPSPs) at CNS
motor nuclei. As a result, the response can involve the stimula-
tion of some muscles and the inhibition of others.
The Tendon Reflex
The stretch reflex regulates the length of a skeletal muscle. The
tendon reflex monitors the external tension produced during
a muscular contraction and prevents tearing or breaking of the
tendons. The sensory receptors for this reflex have not been
identified, but they are distinct from both muscle spindles and
proprioceptors in tendons. The receptors are stimulated when
the collagen fibers are stretched to a dangerous degree. These
receptors activate sensory neurons that stimulate inhibitory
interneurons in the spinal cord. These interneurons in turn
innervate the motor neurons controlling the skeletal muscle.
The greater the tension in the tendon, the greater is the inhibi-
tory effect on the motor neurons. As a result, a skeletal muscle
generally cannot develop enough tension to break its tendons.
Withdrawal Reflexes
Withdrawal reflexes move affected parts of the body away
from a stimulus. Painful stimuli trigger the strongest with-
drawal reflexes, but these reflexes are sometimes initiated by the
stimulation of touch receptors or pressure receptors.
The flexor reflex, a representative withdrawal reflex, af-
fects the muscles of a limb (Spotlight Figure 13–14). Recall
from Chapters 9 and 11 that flexion reduces the angle between
two articulating bones, and that the contractions of flexor mus-
cles perform this movement. pp. 295, 363 If you grab an