CHAPTER 6
Muscle
107
Figure 6-7A.
Simplif
ed schematic diagram o± the intra±usal
muscle f
bers o± a muscle spindle receptor.
Muscle spindles
(a type of
stretch receptor
) play an important
role in the control of voluntary movement, constantly moni-
toring the length of each muscle and the rate of change of that
length. Each spindle contains 10 to 15 specialized muscle F bers
(
intra±usal f bers
) innervated by sensory and motor nerve F bers
and surrounded by a fl uid-F lled
connective tissue capsule
.
Muscle spindles are generally about 1.5 mm in length and are
anchored at each end to connective tissue attached to ordinary
muscle F bers (
extra±usal f bers
). The spindle is stretched when
the muscle lengthens and is shortened when the muscle itself
becomes shorter. A given muscle will contain from a few dozen
to a few hundred spindles distributed throughout the bulk of the
muscle (
small drawing
). Two general types of muscle F bers are
included in spindles:
nuclear bag f bers
(which have a swelling
in the middle of the F ber where most of the nuclei are concen-
trated) and
nuclear chain f bers
(which are smaller in diameter
and have a single row of nuclei). A typical human muscle spin-
dle contains three to F ve nuclear bag F bers and 8 to 10 nuclear
chain F bers. There are several highly specialized receptors associ-
ated with the sensory nerve endings, which are able to measure
(1) muscle length, (2) change in muscle length, and (3) rate of
change of muscle length. The sensory axons form two types of
endings: (1)
primary
(or
annulospiral
)
endings
(
green
) in which
the axon wraps around the equator of nuclear bag or nuclear
chain F bers and (2)
secondary (fl ower-spray) endings
(
green
),
which are more common on nuclear chain F bers. The two ends
of each intrafusal F ber consist of contractile muscle very similar
to that of the extrafusal F bers (
striated region in drawing
). These
contractile portions of the intrafusal F bers are innervated by
small-diameter myelinated motor axons (
gamma motor neurons
or
±usimotor neurons
[
blue
]). This innervation causes the intra-
fusal F bers to shorten when the muscle as a whole shortens and
to relax when the muscle as a whole lengthens, therefore main-
taining the sensitivity of the length-sensitive stretch receptors in
their optimum range and providing accurate information about
the state of the muscle to the motor centers of the CNS.
J.Lynch
Nuclear chain
fiber
Connective tissue
capsule
Gamma motor
ending on
contractile portion
of fiber
Attached to
extrafusal
fibers
Nuclear bag
fiber
Secondary
(flower-spray)
ending
Contractile portion
of intrafusal
muscle fiber
Primary
(annulospiral)
endings
Muscle spindles
Muscle
A
Perimysium
Nucleus
Muscle spindles
Capsule
Capsule
(fibroblast)
(fibroblast)
Intrafusal fiber
Intrafusal fiber
Intrafusal fiber
Capsule
(fibroblast)
B
Figure 6-7B.
Skeletal muscle—muscle spindle, cross
section.
H&E,
3
272; inset
3
680
±ascicles of skeletal muscle separated by
perimysium
are
illustrated (see ±ig. 6-2). Several
muscle spindles
can be seen
in tangential section in the central fascicle. The fl
attened
f
bro-
blasts
making up the capsule can be seen in the
inset
, as well as
F ve or six
intra±usal f
bers
. In general, muscles that are used in
delicate, highly controlled movements contain the largest num-
bers of muscle spindles. The intrinsic muscles of the hand, for
example, contain a relatively larger number of spindles than do
larger muscles, such as the quadriceps and gluteus maximus,
which are specialized for producing large amounts of force.
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