Extrafusal muscle fibers

Muscle fibers responsible for contraction

Extrafusal muscle fibers are the standard skeletal muscle fibers that are responsible for generating force and causing muscle contraction. These fibers are innervated by alpha motor neurons and are the primary effectors of muscle contraction.

Structure

Extrafusal muscle fibers are organized into fascicles, which are bundles of fibers surrounded by connective tissue. Each fiber is a single, multinucleated cell that contains myofibrils, the contractile elements of the muscle. The myofibrils are composed of repeating units called sarcomeres, which are the basic functional units of muscle contraction.

Function

The primary function of extrafusal muscle fibers is to contract and generate movement. When an alpha motor neuron sends a signal, it releases the neurotransmitter acetylcholine at the neuromuscular junction, causing the muscle fiber to depolarize and contract. This process is known as the sliding filament theory of muscle contraction.

Innervation

Extrafusal muscle fibers are innervated by alpha motor neurons, which originate in the anterior horn of the spinal cord. These neurons transmit signals from the central nervous system to the muscle fibers, initiating contraction. The strength and precision of muscle contraction are controlled by the number of motor units activated and the frequency of their activation.

Comparison with Intrafusal Fibers

Extrafusal muscle fibers should not be confused with intrafusal muscle fibers, which are found within the muscle spindle. Intrafusal fibers are involved in the detection of muscle stretch and are innervated by gamma motor neurons. They play a role in the proprioceptive feedback mechanism that helps maintain muscle tone and posture.

Clinical Significance

Damage to the alpha motor neurons or the extrafusal muscle fibers themselves can lead to muscle weakness or paralysis. Conditions such as amyotrophic lateral sclerosis (ALS) and spinal muscular atrophy (SMA) affect the motor neurons, leading to progressive muscle weakness.

See also

• Muscle spindle
• Motor unit
• Neuromuscular junction
• Sliding filament theory