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Muscle Spindles definition: Skeletal muscle structures that function as the mechanoreceptors responsible for the stretch or myotactic reflex (REFLEX, STRETCH). They are composed of a bundle of encapsulated SKELETAL MUSCLE FIBERS, i.e., the intrafusal fibers (nuclear bag1 fibers, nuclear bag2 fibers, and nuclear chain fibers) innervated by sensory neurons.
Muscle Spindle definition: The Muscle Spindle is composed of intrafusal fibers that are located within and run parallel to the extrafusal fibers of skeletal muscle. When the extrafusal fibers are stretched, the intrafusal fibers of the spindle stretch as well. 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.
 
Examples
 

Animal experiments suggest that an increase in sympathetic outflow can depress muscle spindle sensitivity.

Nis Hjortskov et al.

BMC Musculoskeletal Disorders 6 (7), 01 Jul 2005

As the muscle spindle may be influenced by sympathetic modulation, we hypothesized that a state of high sympathetic activity as during mental stress would affect the proprioceptive output from the muscle spindles in the back muscles leading to alterations in proprioception and position sense acuity.

Nis Hjortskov et al.

BMC Musculoskeletal Disorders 6 (7), 01 Jul 2005

The muscle spindle afferents play a major role in the sensation of position and movement [ 13 - 15 ] and factors altering the muscle spindle sensitivity may affect the proprioception.

Nis Hjortskov et al.

BMC Musculoskeletal Disorders 6 (7), 01 Jul 2005

Animal studies have shown the presence of sympathetic fibres [ 16 ] and adrenergic receptors inside the muscle spindle [ 17 ], and demonstrated how sympathetic stimulation during a muscle stretch reduced the muscle spindle sensitivity in rabbit jaw muscles [ 18 , 19 ].

Nis Hjortskov et al.

BMC Musculoskeletal Disorders 6 (7), 01 Jul 2005

It has however been suggested that when motor tasks requiring precision and continuous proprioceptive feedback are performed in work situations with strong excitement and stress, the enhanced muscle sympathetic outflow may affect motor performance through the muscle spindle system [ 18 ].

Nis Hjortskov et al.

BMC Musculoskeletal Disorders 6 (7), 01 Jul 2005

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