Muscle spindle | EPFL Graph Search

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Muscle spindles are stretch receptors within the body of a skeletal muscle that primarily detect changes in the length of the muscle. They convey length information to the central nervous system via afferent nerve fibers. This information can be processed by the brain as proprioception. The responses of muscle spindles to changes in length also play an important role in regulating the contraction of muscles, for example, by activating motor neurons via the stretch reflex to resist muscle stretch. The muscle spindle has both sensory and motor components. Sensory information conveyed by primary type Ia sensory fibers which spiral around muscle fibres within the spindle, and secondary type II sensory fibers Activation of muscle fibres within the spindle by up to a dozen gamma motor neurons and to a lesser extent by one or two beta motor neurons Muscle spindles are found within the belly of a skeletal muscle. Muscle spindles are fusiform (spindle-shaped), and the specialized fibers that make up the muscle spindle are called intrafusal muscle fibers. The regular muscle fibers outside of the spindle are called extrafusal muscle fibers. Muscle spindles have a capsule of connective tissue, and run parallel to the extrafusal muscle fibers. Muscle spindles 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. 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 spindle is provided by motor neurons: up to a dozen gamma motor neurons also known as fusimotor neurons. These activate the muscle fibres within the spindle.

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