<unk> s are found within the belly of a skeletal muscle.<unk> s are fusiform<unk> -shaped), and the specialized fibers that make up the<unk>  are called intrafusal muscle fibers. The regular muscle fibers outside of the<unk>  are called extrafusal muscle fibers.<unk> s have a capsule of connective tissue, and run parallel to the extrafusal muscle fibers. Composition<unk> s are composed of 5-14 muscle fibers, of which there are three types: dynamic nuclear bag fibers (bag1 fibers), static nuclear bag fibers (bag2 fibers), and nuclear chain fibers. Light microscope photograph of a<unk> . HE stain. Primary type Ia sensory fibers (large diameter) spiral around all intrafusal muscle fibres, ending near the middle of each fibre. Secondary type II sensory fibers (medium diameter) end adjacent to the central regions of the static bag and chain fibres. These fibres send information by stretch-sensitive mechanically-gated ion-channels of the axons. The motor part of the<unk>  is provided by motor neurons: up to a dozen gamma motor neurons also known as fusimotor neurons. These activate the muscle fibres within the<unk> . Gamma motor neurons supply only muscle fibres within the<unk> , whereas beta motor neurons supply muscle fibres both within and outside of the<unk> . Activation of the neurons causes a contraction and stiffening of the end parts of the<unk>  muscle fibers. Fusimotor neurons are classified as static or dynamic according to the type of muscle fibers they innervate and their effects on the responses of the Ia and II sensory neurons innervating the central, non-contractile part of the<unk> . * The static axons innervate the chain or static bag2 fibers. They increase the firing rate of Ia and II afferents at a given muscle length (see schematic of fusimotor action below). * The dynamic axons innervate the bag1 intrafusal muscle fibers. They increase the stretch-sensitivity of the Ia afferents by stiffening the bag1 intrafusal fibers. Efferent nerve fibers of gamma motor neurons also terminate in<unk> s; they make synapses at either or both of the ends of the intrafusal muscle fibers and regulate the sensitivity of the sensory afferents, which are located in the non-contractile central (equatorial) region.
Muscle spindle
Efferent nerve fiber
Postganglionic nerve fibers
Nerve plexus
Nerve fascicle
Muscle tissue
Cerebrospinal fibers
Sensory nerve