Extrafusal muscle fiber
Extrafusal muscle fibers are the skeletal standard muscle fibers that are innervated by alpha motor neurons and generate tension by contracting, thereby allowing for skeletal movement. "The Public Library of Science says that in most mammals, skeletal muscle comprises about 55% of individual body mass and plays vital roles in locomotion, heat production during periods of cold stress, and overall metabolism." They make up the large mass of skeletal muscle tissue and are attached to bone by fibrous tissue extensions.
Each alpha motor neuron and the extrafusal muscle fibers innervated by it make up a motor unit. The connection between the alpha motor neuron and the extrafusal muscle fiber is a neuromuscular junction, where the neuron's signal, the action potential, is transduced to the muscle fiber by the neurotransmitter acetylcholine.
Extrafusal muscle fibers are not to be confused with intrafusal muscle fibers, which are innervated by sensory nerve endings in central noncontractile parts and by gamma motor neurons in contractile ends and thus serve as a sensory proprioceptor.
Extrafusal muscle fibers can be generated in vitro from pluripotent stem cells through directed differentiation. This allows study of their formation and physiology.
