Intrafusal muscle fibers are skeletal muscle fibers that serve as specialized sensory organs (proprioceptors) that detect the amount and rate of change in length of a muscle and are innervated by 2 axons, one sensory and one motor, and constitute the muscle spindle. Intrafusal muscle fibers are walled off from the rest of the muscle by a collagen sheath. This sheath has a spindle or "fusiform" shape, hence the name "intrafusal."
There are two types of intrafusal muscle fibers: nuclear bag and nuclear chain fibers. They bear two types of sensory ending, known as annulospiral and flower-spray endings. both the ends of these fibers contract but the central region only stretches and does not contract.
They are innervated by gamma motor neurons. It is by the sensory information from these two intrafusal fiber types that one is able to judge the position of their muscle, and the rate at which it is changing.