Intro | Golgi Tendon Organ | Motor Neuron | Muscle | Muscle Spindle | Sensory Neurons | Tendon
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The Muscle Spindle is a proprioceptive receptor composed of intrafusal fibers that are located within the extrafusal fibers of skeletal muscle. The intrafusal fibers of the muscle spindle run parallel to the extrafusal fibers of the muscle, attaching to them along short segments. When the extrafusal fibers of skeletal muscle are stretched, the intrafusal fibers of the spindle stretch as well. This stretching of the muscle spindle indicates the length of the muscle. When stretching occurs, the sensory neuron from the muscle spindle signals the motor neurons located within the ventral horn of the spinal cord. This signal causes the motor neurons to fire, resulting in contraction of the muscle. This reflex arc provides negative feedback. The muscular contraction induced by the stretching works against or negates further muscle stretch. This mechanism helps to maintain proper muscle tension or tone. Although the muscle spindle helps to maintain proper muscle tension, unlike the Golgi tendon organ, it is not an indicator of muscle tension, but rather of muscle length.
The fusiform shape of the muscle spindles underlies the labels given to the two types of fibers composing skeletal muscle (Parent, 1996). Intrafusal fibers are so named for their contribution to the spindle structure. Extrafusal fibers are those located outside the structure of the spindles. Muscle spindles are composed of very fine muscle fibers of lengths varying between 4 and 10 millimeters. The intrafusal fibers composing the muscle spindle are so small that they contribute little to the force of contraction. Some specialized muscle spindles are sensitive to the rate of change of muscle length. The density of muscle spindles is greatest in those muscles involved in precise movements such as those controlling the hand.
There are three types of muscle spindles. They are distinguished based on the arrangement of nuclei within the intrafusal fibers and on the function of the fibers. The first type, or nuclear chain fibers, are relatively thin and have nuclei that are arranged in a single row throughout the length of their fibers. The second type, called nuclear bag fibers, have an expanded region midway along their length. Nuclei are clustered with greatest density in these expanded regions. These fibers are thicker than the nuclear chain fibers. Finally, dynamic nuclear bag fibers are sensitive to changes in stretch of the fibers, in contrast to the static nuclear bag fibers which are sensitive to stationary, steady state stretch of the muscles (the new stabilized length of a muscle).
Parent, A. (1996). Carpenter's human neuroanatomy (9th ed.). London: Williams & Wilkins.