Extrafusal muscle fibers are the standard skeletal muscle fibers that are innervated by

alpha motor neuron Alpha (α) motor neurons (also called alpha motoneurons), are large, multipolar neuron, multipolar lower motor neurons of the brainstem and spinal cord. They innervate extrafusal muscle fibers of skeletal muscle and are directly responsible for i ...

s and generate tension by contracting, thereby allowing for skeletal movement. They make up the large mass of skeletal

striated muscle tissue Striated muscle tissue is a muscle tissue that features repeating functional units called sarcomeres. Under the microscope, sarcomeres are visible along muscle fibers, giving a striated appearance to the tissue. The two types of striated muscle a ...

and are attached to bone by fibrous tissue extensions (

tendon A tendon or sinew is a tough band of fibrous connective tissue, dense fibrous connective tissue that connects skeletal muscle, muscle to bone. It sends the mechanical forces of muscle contraction to the skeletal system, while withstanding tensi ...

s). Each alpha motor neuron and the extrafusal muscle fibers innervated by it make up a

motor unit In biology, a motor unit is made up of a motor neuron and all of the skeletal muscle fibers innervated by the neuron's axon terminals, including the neuromuscular junctions between the neuron and the fibres. Groups of motor units often work tog ...

. The connection between the alpha motor neuron and the extrafusal muscle fiber is a

neuromuscular junction A neuromuscular junction (or myoneural junction) is a chemical synapse between a motor neuron and a muscle fiber. It allows the motor neuron to transmit a signal to the muscle fiber, causing muscle contraction. Muscles require innervation to ...

, where the neuron's signal, the

action potential An action potential (also known as a nerve impulse or "spike" when in a neuron) is a series of quick changes in voltage across a cell membrane. An action potential occurs when the membrane potential of a specific Cell (biology), cell rapidly ri ...

, is transduced to the muscle fiber by the

neurotransmitter A neurotransmitter is a signaling molecule secreted by a neuron to affect another cell across a Chemical synapse, synapse. The cell receiving the signal, or target cell, may be another neuron, but could also be a gland or muscle cell. Neurotra ...

acetylcholine Acetylcholine (ACh) is an organic compound that functions in the brain and body of many types of animals (including humans) as a neurotransmitter. Its name is derived from its chemical structure: it is an ester of acetic acid and choline. Par ...

. 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.

See also

References

Further reading