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.
They are proprioceptors, that constitute the muscle spindle and are innervated by two axons, one sensory and one motor.
They are walled off from the rest of the muscle by a collagen sheath that has a spindle or fusiform” shape.
Anatomically, there are two types of intrafusal muscle fibers: nuclear bag and nuclear chain fibers.
Intrafusal muscle fibers have two types of sensory ending, known as annulospiral and flower-spray endings, both of which contract.
The central region of intrafusal muscle fibers stretch but does not contract.
Intrafusal muscle fibers are innervated by gamma motor neurons and beta motor neurons.
The sensory information from these two intrafusal muscle fiber types allow for judging the position of the muscle, and the rate at which it is changing.
Extrafusal muscle fibers, which contract, generating skeletal movement and are innervated by alpha motor neurons.