Page 308 - Atlas of Histology with Functional Correlations

FIGURE 8.8 ■ Skeletal muscle with a muscle spindle (transverse section).

Frozen section stained with modified Van Gieson method (hematoxylin, picric
acid–ponceau stain). Left, medium magnification; right, high magnification.
Courtesy of Dr. Mark DeSantis, Professor Emeritus, WWAMI Medical Program,
University of Idaho, Moscow, ID.

FUNCTIONAL CORRELATIONS 8.2 ■ Muscle

Spindles

Muscle spindles are specialized stretch receptors located parallel to muscle
fibers in nearly all skeletal muscles. Their main function is to detect changes
in the length of the muscle fibers. An increase in the length of muscle fibers

stimulates the muscle spindle and sends impulses via the afferent (sensory)
axons into the spinal cord. These impulses result in a stretch reflex that
immediately causes contraction of the extrafusal muscle fibers, thereby
shortening the stretched muscle and producing movement. A decrease in

skeletal muscle length stops the stimulation of the muscle spindle fibers and
the conduction of its impulses to the spinal cord.

The simple stretch reflex arc illustrates the function of these receptors.
Gently tapping the patellar tendon on the knee with a rubber mallet stretches
the skeletal muscle and stimulates the muscle spindle. This action results in

rapid muscle contraction of the stretched muscle and produces an involuntary
knee-jerk response, or stretch reflex.

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