1.6.1 - Proprioceptors (Stretching)
Description
This article is from the Stretching FAQ, by Brad Appleton Brad_Appleton@ivhs.mot.com with numerous contributions by others.
1.6.1 - Proprioceptors (Stretching)
The nerve endings that relay all the information about the musculoskeletal
system to the central nervous system are called "proprioceptors".
Proprioceptors (also called "mechanoreceptors") are the source of all
"proprioception": the perception of one's own body position and movement.
The proprioceptors detect any changes in physical displacement (movement or
position) and any changes in tension, or force, within the body. They are
found in all nerve endings of the joints, muscles, and tendons. The
proprioceptors related to stretching are located in the tendons and in the
muscle fibers.
There are two kinds of muscle fibers: "intrafusal muscle fibers" and
"extrafusal muscle fibers". Extrafusil fibers are the ones that contain
myofibrils (See "1.2 - Muscle Composition") and are what is usually meant
when we talk about muscle fibers. Intrafusal fibers are also called "muscle
spindles" and lie parallel to the extrafusal fibers. Muscle spindles, or
"stretch receptors", are the primary proprioceptors in the muscle. Another
proprioceptor that comes into play during stretching is located in the
tendon near the end of the muscle fiber and is called the "golgi tendon
organ". A third type of proprioceptor, called a "pacinian corpuscle", is
located close to the golgi tendon organ and is responsible for detecting
changes in movement and pressure within the body.
When the extrafusal fibers of a muscle lengthen, so do the intrafusal
fibers (muscle spindles). The muscle spindle contains two different types
of fibers (or stretch receptors) which are sensitive to the change in
muscle length and the rate of change in muscle length. When muscles
contract it places tension on the tendons where the golgi tendon organ is
located. The golgi tendon organ is sensitive to the change in tension and
the rate of change of the tension.
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