Human Physiology, 14th edition (2016)

386 Chapter 12

the nuclear bag fibers, have their nuclei arranged in a loose

aggregate in the central regions of the fibers. The other type

of intrafusal fibers, called nuclear chain fibers, have their

nuclei arranged in rows. Two types of sensory neurons serve

these intrafusal fibers. Primary, or annulospiral, sensory

endings wrap around the central regions of the nuclear bag

and chain fibers ( fig. 12.27 ), and secondary, or flower-

spray, endings are located over the contracting poles of the

nuclear chain fibers.

Because the spindles are arranged in parallel with the

extra-fusal muscle fibers, stretching a muscle causes its spin-

dles to stretch. This stimulates both the primary and secondary

sensory endings. The spindle apparatus thus serves as a length

detector because the frequency of impulses produced in the

primary and secondary endings is proportional to the length of

the muscle. The primary endings, however, are most stimulated

at the onset of stretch, whereas the secondary endings respond

in a more tonic (sustained) fashion as stretch is maintained.

Sudden, rapid stretching of a muscle activates both types of

sensory endings, and is thus a more powerful stimulus for the

muscle spindles than a slower, more gradual stretching that

has less of an effect on the primary sensory endings. Thus, the

force of this reflex contraction is greater in response to rapid

stretch than to gradual stretch.

Muscle Spindle Apparatus

In order for the nervous system to control skeletal movements
properly, it must receive continuous sensory feedback concern-
ing the effects of its actions. This sensory information includes
(1) the tension that the muscle exerts on its tendons, provided
by the Golgi tendon organs, and (2) muscle length, provided
by the muscle spindle apparatus. The spindle apparatus, so
called because it is wider in the center and tapers toward the
ends, functions as a length detector. Muscles that require the
finest degree of control, such as the muscles of the hand, have
the highest density of spindles.
Each spindle apparatus contains several thin muscle cells
called intrafusal fibers ( fusus 5 spindle) packaged within a
connective tissue sheath. Like the stronger and more numerous
“ordinary” muscle fibers outside the spindles—the extra-fusal
fibers —the spindles insert into tendons on each end of the
muscle. Spindles are therefore said to be in parallel with the
extra-fusal fibers.
Unlike the extrafusal fibers, which contain myofibrils
along their entire length, the contractile apparatus is absent
from the central regions of the intrafusal fibers. The cen-
tral, non-contracting part of an intrafusal fiber contains
nuclei. There are two types of intrafusal fibers. One type,

Figure 12.27 The location and structure of a muscle spindle. ( a ) A muscle spindle within a skeletal muscle. ( b ) The
structure and innervation of a muscle spindle.

Tendon

Bone

(a) (b)

Skeletal

muscle

Peripheral

nerve (motor

and sensory

nerve fibers)

Muscle

spindle

Extrafusal fibers

Intrafusal fibers:

Nuclear chain

fibers

Nuclear bag

fiber

Connective

tissue sheath

Afferent nerve fibers

(sensory):

Primary fiber

Annulospiral

endings

Secondary fiber

Flower-spray

endings

Efferent nerve fibers

(motor):

Gamma fiber

Alpha fiber

Motor end plates