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Muscle structure is innervated by both sensory and motor neuron axons. The muscle spindle's functions are to send proprioceptive information about the muscle to the central nervous system, and to respond to muscle stretching.
Muscle spindles are found within the fleshy portions of muscles, embedded in so-called extrafusal muscle fibres. They are composed of 3-10 intrafusal muscle fibres, of which there are three types:
Muscle spindles are encapsulated by connective tissue, and are aligned parallel to extrafusal muscle fibers, unlike Golgi tendon organs, which are oriented in series.
The muscle spindle has both sensory and motor components.
These motorneurons are classified as static or dynamic according to their pattern of innervation and their physiological effects.
The function of the gamma motor neuron neuromuscular junction is not to supplement the general muscle contraction provided by extrafusal fibers, but to modify the sensitivity of the muscle spindle to stretch. Upon release of acetylcholine by the gamma neuron, the end portions of the intrafusal muscle fibers contract, thus deliberately elongating the non-contractile central portions of intrafusal muscle fibers. This opens stretch-sensitive ion channels of the centrally-positioned sensory axons, leading to an influx of sodium ions. This raises the resting potential of these axons, thereby increasing the probability of action potential firing, thus increasing the sensitivity of the muscle spindle.
When a muscle is stretched, primary sensory fibers (Group Ia afferent neurons) of the muscle spindle respond to both the velocity and the degree of stretch, and send this information to the spinal cord. Likewise, secondary sensory fibers (Group II afferent neurons) detect and send information about the degree of stretch (but not the velocity thereof) to the CNS. This information is transmitted monosynaptically to an alpha efferent motor fiber, which activates extrafusal fibers of the muscle to contract, thereby reducing stretch, and polysynaptically through an interneuron to another alpha motoneuron, which inhibits contraction in the antagonising muscles.
PNF stretching, or proprioceptive neuromuscular facilitation, is a method of flexibility training that reduces the automatic reflex action in order allow muscles to lengthen.
It is also believed that muscle spindles play a critical role in sensorimotor development.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Muscle_spindle". A list of authors is available in Wikipedia.|