Which muscles are cited as examples of extrafusal fibers?

Which muscles are cited as examples of extrafusal fibers?

• A. Masseter and biceps
• B. Intrafusal fibers in muscle spindles
• C. Cardiac muscle fibers
• D. Smooth muscle fibers

Answer: (A)

Extrafusal fibers are the main force-generating fibers of skeletal muscles, producing voluntary movement when activated by alpha motor neurons. The masseter and the biceps are classic skeletal muscles composed of these contractile fibers, so they serve as standard examples of extrafusal fibers. Intrafusal fibers, by contrast, reside inside muscle spindles and mainly provide sensory information about stretch, not primary force. Cardiac muscle fibers make up the heart, and smooth muscle fibers line the walls of hollow organs; neither is extrafusal skeletal muscle.

Extrafusal fibers are the main force-generating fibers of skeletal muscles, producing voluntary movement when activated by alpha motor neurons. The masseter and the biceps are classic skeletal muscles composed of these contractile fibers, so they serve as standard examples of extrafusal fibers. Intrafusal fibers, by contrast, reside inside muscle spindles and mainly provide sensory information about stretch, not primary force. Cardiac muscle fibers make up the heart, and smooth muscle fibers line the walls of hollow organs; neither is extrafusal skeletal muscle.