Describe the structure and the connections of the muscle spindles and explain how they contribute to the control of movement
Muscle spindles are one of two proprioceptors found in voluntary muscles. They are found in parallel to contractile elements, and as such respond to muscle length and the rate of change of muscle length.
Whilst Golgi tendon organs, the other proprioceptor, respond to muscle tension or force and are found in tendons in series. Although muscle spindles are found in striated muscle, they are outnumbered by striated muscle fibres.
The spindles are found in a fluid-filled capsule (2-4mm long and a few hundred µm in diameter), the ends of which attach to exterior sheaths of neighbouring muscle fibres. Inside the capsule are about half a dozen intrafusal fibres, or modified muscle fibres, that have contractile ends and nuclei in the middle. These intrafusal fibres can either be described as ‘nuclear chain fibres’ (thinner fibres, with their nuclei arranged in a row), or ‘nuclear bag fibres’ (fibres with their nuclei arranged into a bulge).
The central potions of both of these fibre types are innervated by larger, primary fibres of group 1a via annulospiral endings.
Meanwhile, smaller, secondary fibres of group 2 terminate mainly on the ‘nuclear chain fibres’ and on a more peripheral location via annulospiral and flower spray endings. The primary fibres are considered to be dynamic, enabling pronounced adaptation as they fire maximally at sudden stretch (at a rate proportional to the rate of stretch) and only decrease in their firing when the muscle is held at its new length. Whereas, secondary fibres are considered to be static and non-adapting as they signal in proportion to the stretch of the spindle, with their firing rate mirroring the instantaneous value of the muscle length.
In muscle spindles, the adaptation of the sensory receptors is almost all of the energy-filtering kind, whereby static information is disregarded before it reaches.