Extrafusal fibers: large, bulk of muscle, activated by alpha motoneurons

Intrafusal fibers: small, encapsulates in sheath to form the muscle spindle, run in parallel witht he extrafusal fibers, signal both static and dynamic changes in muscle length

Nuclear bag: most sensitive to rapid stretch, provides information about the rate of change in muscle length, group Ia afferent fibers signal dynamic velocity

Nuclear chain: shorter/thinner fibers that respond to static changes in muscle length

Sensory information from spindles are mediated by group II afferent fibers signal static change in length

Spindle is innervated by gamma motor neurons, which reset the length of the spindle to make it sensitive to stretch.  In other words, it makes the spindle contract.

Excitation of Ia fibers stimulates the homonomyous alpha motor neurons so the muscle contacts.

Without information about rate of stretch (from nuclear bag/Ia fibers) clonus would occur.

-In series with extrafusal fibers

-innervated by group Ib fibers and is sensitive to muscle force/contraction

-Detects excessive force to prevent tendon damage

-When Ib fibers is excited it goes through an inhibitory neuron and inhibits the alpha-motorneuron causes muscle relaxation