extrafusal muscle fiber extrafusal muscle fiber extrafusal muscle fibers are the standard skeletal muscle fibers that are innervated by alpha motor neurons and generate tension by contracting, thereby allowing for skeletal movement. they make up the large mass of skeletal striated muscle tissue and are attached to bone by fibrous tissue extensions (tendons). extrafusal muscle fiberdetailspart ofskeletal muscleidentifierslatinmyofibra extrafusalisthh3.03.00.0.00007 anatomical terminology 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 (in a dish) from pluripotent stem cells through directed differentiation. this allows study of their formation and physiology.