Thin filaments are composed of globular actin subunits known as G-actin. A protein known as tropomyosin is also located at intervals in the thin filaments. Another protein, troponin, is attached to the tropomyosin. The lengths of the H and I bands decrease, whereas the A bands stay the same length during contraction. The sarcoplasmic reticulum is a modified endoplasmic reticulum that surrounds the myofibrils.
Alpha motoneurons innervate the ordinary, or extrafusal, muscle fibers.
Spindles consist of several intrafusal fibers wrapped together. The alpha motoneuron stimulates the extrafusal muscle fibers to contract, thus relieving the stretch. The activity of gamma motoneurons tightens the spindles, thus making them more sensitive to stretch and better able to monitor the length of the muscle, even during muscle shortening. This is a disynaptic reflex, because the sensory neurons synapse with interneurons, which in turn make inhibitory synapses with motoneurons. The sensory input also crosses the spinal cord to cause stimulation of extensor and inhibition of flexor muscles in the contralateral leg.