Muscle spindle: Encyclopedia II - Muscle spindle - Anatomy

Muscle spindle - 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 fibers 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 abd increase the velocity sensitivity of the Ia afferents.

Adapted from the Wikipedia article "Anatomy", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki