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A muscle spindle is a specialized muscle structure innervated by both sensory and motor neuron axons. Its 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 fibers. They are composed of 3-10 intrafusal muscle fibers, of which there are three types, dynamic nuclear bag fibers (bag1 fibres), static nuclear bag fibers (bag2 fibres) and nuclear chain fibers and the axons of sensory neurons. Axons of motoneurons also terminate in muscle spindles; they make synapses at either or both of the ends of the intrafusal muscle fibers and regulate spindle sensitivity. 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. Primary and secondary sensory fibers spiral around and terminate on the central portions of intrafusal fibers, providing the sensory component of the structure via stretch-sensitive ion-channels of the axons. The motor component is provided by gamma motoneurons and beta motoneurons that innervate the spindle and cause a slight contraction of the end portions of the intrafusal muscle fibers when activated. The gamma, or fusimotor, axons only innervate the intrafusal muscle fibres whereas the beta, or skeletofusimotor, axons innervate both extrafusal and intrafusal muscle fibres. These motornurons are classified as static or dynamic according to their pattern of innervation and their physiological effects. The static axons innervate the chain or bag2 fibres while the dynamic axons innervate the bag1 fibres and increase the velocity sensitivity of the Ia afferents.
The function of the gamma motoneuron 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 opposing 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.
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