Download THE MUSCLE SPINDLE Anatomical Structures of the Spindle

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Chapter Three
Figure 3-2. Organization of muscle from macroscopic to microscopic levels. Reprinted with permission from Oatis CA. Kinesiology: The Mechanics and
Pathomechanics of Human Movement. Philadelphia,
Pa: Lippincott Williams & Wilkins; 2004:46.
Figure 3-1. Successive connective tissue sheaths within muscle. Reprinted with permission from Oatis
CA. Kinesiology: The Mechanics and Pathomechanics
of Human Movement. Philadelphia, Pa: Lippincott
Williams & Wilkins; 2004:47.
which contains only actin (thin) filaments. The darker area
is the A-band, which contains alternating actin and myosin
(thick) filaments. The Z-line consists of a connective tissue
network that bisects the I-band, anchors the thin filaments,
and provides structural integrity to the sarcomere. The Hzone, located in the middle of the A-band, is the region of
thick filaments not overlapped by thin filaments. The Mband bisects the H-zone and represents the middle of the
sarcomere. The M-band consists of protein structures that
support the arrangement of the myosin filaments. During
muscle contraction, the sarcomere I-band and H-zone
decrease in length while the length of the A-band remains
constant.2,3
THE MUSCLE SPINDLE
Figure 3-3. Actin-myosin organization of two successive sarcomeres showing A, H, and I bands as well
as Z-lines. Reprinted with permission from Oatis
CA. Kinesiology: The Mechanics and Pathomechanics
of Human Movement. Philadelphia, Pa: Lippincott
Williams & Wilkins; 2004:46.
The muscle spindle is a long, thin structure located
adjacent and parallel to muscle fibers and is composed of
multiple components that have both afferent and efferent
innervation (Figures 3-4a and 3-4b). The muscle spindle
functions as a stretch receptor and responds to static and
dynamic length changes of skeletal muscle.4-6 This complex
receptor is found in all muscles, primarily in extremity, intercostal, and cervical muscles.
Anatomical Structures of the Spindle
Apparatus
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Intrafusal muscle fibers are generally only 2 to 4 mm in
length. They are fusiform in shape, widest in the center
and tapered at ends. These fibers are composed of two
types of fibers:
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