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Extrafusal muscle fibers are the 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 muscle tissue and are attached to bone by fibrous tissue extensions, called the tendons.
A motor unit is made up of an alpha motor neuron and the extrafusal muscle fibers innervated by it.
The alpha motor neuron and the extrafusal muscle fiber connect at the neuromuscular junction where the neuron’s signal, the action potential, is transduced to the muscle fiber by the neurotransmitter acetylcholine.
Intrafusal muscle fibers 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.