514
UNIT 3
Regulation and Integration of the Body
13
a closer look at the functional anatomy of these proprioceptors
and their roles in certain spinal reflexes.
Functional Anatomy of Muscle Spindles
Each muscle spindle consists of three to ten modified skeletal
muscle fibers called
intrafusal muscle fibers
(
intra
5
within;
fusal
5
the spindle) enclosed in a connective tissue capsule
(Figure 13.16)
. Tese fibers are less than one-quarter the size
of the effector fibers of the muscle, called
extrafusal muscle
fibers
.
Te central regions of the intrafusal fibers lack myofilaments
and are noncontractile. Tese regions are the receptive surfaces
of the spindle. ±wo types of afferent endings send sensory in-
puts to the CNS:
Anulospiral endings
(also called
primary sensory endings
)
are the endings of large axons that wrap around the spindle
center. Tey are stimulated by both the rate and degree of
stretch.
Flower spray endings
(also called
secondary sensory end-
ings
) are formed by smaller axons that supply the spindle
ends. Tey are stimulated only by degree of stretch.
Te intrafusal muscle fibers have contractile regions at their
ends, which are the only areas containing actin and myosin
myofilaments. Tese regions are innervated by
gamma (γ)
efferent fibers
that arise from small motor neurons in the ven-
tral horn of the spinal cord. Tese γ motor fibers, which main-
tain spindle sensitivity (as described shortly), are distinct from
the
alpha (
a
) efferent fibers
of the large
alpha (
a
) motor neu-
rons
that stimulate the extrafusal muscle fibers to contract.
Te muscle spindle is stretched (and excited) in one of two
ways:
By applying an external force that lengthens the entire mus-
cle, such as when we carry a heavy weight or when antago-
nistic muscles contract (external stretch)
By activating the γ motor neurons that stimulate the distal
ends of the intrafusal fibers to contract, thereby stretching
the middle of the spindle (internal stretch).
Whenever the muscle spindle is stretched, its associated sen-
sory neurons transmit impulses at higher frequency to the spi-
nal cord
(Figure 13.17a)
.
During voluntary skeletal muscle contraction, the muscle
shortens. If the intrafusal muscle fibers didn’t contract along
with the extrafusal fibers, the muscle spindle would go slack and
cease generating action potentials (Figure 13.17b). At this point
it would be unable to signal further changes in muscle length,
so it would be useless.
Fortunately,
a
-γ coactivation
prevents this from happening.
Descending fibers of motor pathways synapse with both
a
and γ
motor neurons, and motor impulses are simultaneously sent to
the large extrafusal fibers and to muscle spindle intrafusal fibers.
Stimulating the intrafusal fibers maintains the spindle’s tension
(and sensitivity) during muscle contraction, so that the brain
continues to be notified of changes in the muscle length (Fig-
ure 13.17b). Without such a system, information on changes in
muscle length would cease to flow from contracting muscles.
However, the brain is “advised” of most spinal reflex activity
and can facilitate, inhibit, or adapt it, depending on the circum-
stances (as we described in the example of the hot water–filled
pot). Moreover, continuous facilitating signals from the brain are
required for normal spinal reflex activity. As we saw in Chapter
12,
spinal shock
occurs when the spinal cord is transected, imme-
diately depressing all functions controlled by the cord.
±ests of somatic reflexes are important clinically to assess
the condition of the nervous system. Exaggerated, distorted, or
absent reflexes indicate degeneration or pathology of specific
nervous system regions, o²en before other signs appear.
Stretch and Tendon Reflexes
What information does your nervous system need to smoothly
coordinate the activity of your skeletal muscles? ±wo types of in-
formation about the current state of a muscle are key. First, the
nervous system needs to know the length of the muscle. Tis infor-
mation comes from the
muscle spindles
, found in skeletal muscles.
Second, it needs to know the amount of tension in the muscle and
its associated tendons.
Tendon organs
provide this information.
Tese two types of proprioceptors play an important role in
spinal reflexes and also provide essential feedback to the cer-
ebral cortex and cerebellum. Just remember, muscle spind
l
es
measure
l
ength and
t
endon organs measure
t
ension. Let’s take
Flower spray endings
(secondary sensory
endings)
Efferent (motor)
fiber to muscle spindle
Anulo-
spiral
endings
(primary
sensory
endings)
Capsule
(connective
tissue)
Muscle
spindle
Tendon
Sensory
fiber
Tendon organ
γ
Efferent
(motor) fiber
to extrafusal
muscle fibers
Extrafusal
muscle
fiber
Intrafusal
muscle
fibers
α
Figure 13.16
Anatomy of the muscle spindle and tendon
organ.
Notice the afferent fibers from and efferent fibers to the
muscle spindle. Myelin has been omitted from all nerve fibers
for clarity.
previous page 548 Human Anatomy and Physiology (9th ed ) 2012 read online next page 550 Human Anatomy and Physiology (9th ed ) 2012 read online Home Toggle text on/off