Regulation and Integration of the Body
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
the spindle) enclosed in a connective tissue capsule
. Tese ﬁbers are less than one-quarter the size
of the eﬀector ﬁbers of the muscle, called
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:
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
Flower spray endings
secondary sensory end-
) 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
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
) eﬀerent ﬁbers
of the large
) motor neu-
that stimulate the extrafusal muscle ﬁbers to contract.
Te muscle spindle is stretched (and excited) in one of two
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-
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.
prevents this from happening.
Descending ﬁbers of motor pathways synapse with both
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
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
, found in skeletal muscles.
Second, it needs to know the amount of tension in the muscle and
its associated tendons.
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
endon organs measure
ension. Let’s take
Flower spray endings
fiber to muscle spindle
Anatomy of the muscle spindle and tendon
Notice the afferent ﬁbers from and efferent ﬁbers to the
muscle spindle. Myelin has been omitted from all nerve ﬁbers