Extrafusal muscle fiber is a term given to standard muscle fibers as to distinguish them from intrafusal muscle fiber

Intrafusal muscle fiber

Intrafusal muscle fibers are skeletal muscle fibers that comprise the muscle spindle and are innervated by gamma motor neurons. These fibers are a proprioceptor that detect the amount and rate of change of length in a muscle. These fibers are walled off from the rest of the muscle by a collagen...

s. Extrafusal muscle fibers are innervated by alpha motor neuron

Alpha motor neuron

Alpha motor neurons are large lower motor neurons of the brainstem and spinal cord. They innervate extrafusal muscle fibers of skeletal muscle and are directly responsible for initiating their contraction...

s and generate tension by contracting, thereby allowing for skeletal movement. Extrafusal muscle fibers are not to be confused with intrafusal muscle fiber

Intrafusal muscle fiber

Intrafusal muscle fibers are skeletal muscle fibers that comprise the muscle spindle and are innervated by gamma motor neurons. These fibers are a proprioceptor that detect the amount and rate of change of length in a muscle. These fibers are walled off from the rest of the muscle by a collagen...

s which are innervated by gamma motor neurons and thus serve as a sensory proprioceptor.

The alpha motor neuron and the extrafusal muscle fibers it innervates make up the motor unit

Motor unit

”A motor unit is a single α-motor neuron and all of the corresponding muscle fibers it innervates; all of these fibers will be of the same type . When a motor unit is activated, all of its fibers contract...

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

Neuromuscular junction

A neuromuscular junction is the synapse or junction of the axon terminal of a motor neuron with the motor end plate, the highly-excitable region of muscle fiber plasma membrane responsible for initiation of action potentials across the muscle's surface, ultimately causing the muscle to contract...

, where the neuron's signal, the action potential

Action potential

In physiology, an action potential is a short-lasting event in which the electrical membrane potential of a cell rapidly rises and falls, following a consistent trajectory. Action potentials occur in several types of animal cells, called excitable cells, which include neurons, muscle cells, and...

, is transduced to the muscle fiber by the neurotransmitter

Neurotransmitter

Neurotransmitters are endogenous chemicals that transmit signals from a neuron to a target cell across a synapse. Neurotransmitters are packaged into synaptic vesicles clustered beneath the membrane on the presynaptic side of a synapse, and are released into the synaptic cleft, where they bind to...

 acetylcholine

Acetylcholine

The chemical compound acetylcholine is a neurotransmitter in both the peripheral nervous system and central nervous system in many organisms including humans...

.

See also

• Intrafusal muscle fiber
  Intrafusal muscle fiber
  Intrafusal muscle fibers are skeletal muscle fibers that comprise the muscle spindle and are innervated by gamma motor neurons. These fibers are a proprioceptor that detect the amount and rate of change of length in a muscle. These fibers are walled off from the rest of the muscle by a collagen...
  
  
• Type Ia sensory fiber
  Type Ia sensory fiber
  Type Ia Sensory Fiber also called Primary Afferent Fiber is a type of sensory fiber. It is a component of a muscle fiber's muscle spindle which keeps track of how fast a muscle stretch changes .-Function of muscle spindles:...
  
  
• Type II sensory fiber
  Type II sensory fiber
  Type II sensory fiber is a type of sensory fiber, the second of the two main groups of stretch receptors. They are non-adapting, meaning that they keep responding even when the muscle has stopped changing its length. They are the second most highly myelinated fibers in the body.Their firing rate...
  
  
• Alpha motor neuron
  Alpha motor neuron
  Alpha motor neurons are large lower motor neurons of the brainstem and spinal cord. They innervate extrafusal muscle fibers of skeletal muscle and are directly responsible for initiating their contraction...
  
  
• Gamma motor neuron
• Beta motor neuron
  Beta motor neuron
  A beta motor neuron is a kind of lower motor neuron, along with alpha motor neuron and gamma motor neuron. These motor neurons innervate intrafusal fibers of muscle spindles with collaterals to extrafusal fibers . Axons of beta motor neurons are myelinated...