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Muscle spindle

A muscle spindle is a specialized musclestructure innervated by both sensoryand motor neuronaxons. Its functions are to send proprioceptiveinformation about the muscle to the central nervous system, and to respond to muscle stretching.

Inhaltsverzeichnis

1 Anatomy
2 Sensitivity Modification
3 Stretch reflex
4 Development
5 See also

Anatomy

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 two types, nuclear bag fibersand nuclear chain fibersand the axonsof sensory neurons. Axonsof motoneuronsalso terminate in muscle spindles; they make synapsesat either or both of the ends of the intrafusal muscle fibersand regulate spindle sensitivity. Muscle spindles are encapsulated by connective tissue, and are aligned parallelto extrafusal muscle fibers, unlike Golgi tendon organs, which are oriented in series.

The muscle spindle has both sensory and motor components. Primaryand secondarysensory fibers spiral around and terminate on the central portions of intrafusal fibers, providing the sensory component of the structure via stretch-sensitive ion-channelsof the axons. The motor component is provided by gamma motoneuronsand beta motoneurons that innervate the spindle and cause a slight contraction of the end portions of the intrafusal muscle fiberswhen 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 abd increase the velocity sensitivity of the Ia afferents.

Sensitivity Modification

The function of the gamma motoneuronneuromuscular junctionis 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 acetylcholineby the gamma neuron, the end portions of the intrafusal muscle fiberscontract, thus deliberately elongating the non-contractile central portions of intrafusal muscle fibers. This opens stretch-sensitive ion channelsof the centrally-positioned sensory axons, leading to an influx of sodiumions. This raises the resting potentialof these axons, thereby increasing the probability of action potentialfiring, thus increasing the sensitivity of the muscle spindle.

Stretch reflex

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 monosynapticallyto an alpha efferent motor fiber, which activates extrafusal fibers of the muscle to contract, thereby reducing stretch, and polysynapticallythrough an interneuronto 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.

Development

It is also believed that muscle spindles play a critical role in sensorimotordevelopment.