When throwing a baseball, which structure detects and responds to rapid stretch by reflexively increasing muscle activity?

When throwing a baseball, which structure detects and responds to rapid stretch by reflexively increasing muscle activity?

• A. Golgi tendon organ
• B. Muscle spindle
• C. Extrafusal muscle
• D. Pacinian corpuscle

Answer: (B)

Muscle spindles sense how much and how quickly a muscle is lengthening. When a rapid stretch occurs, the spindle’s Ia sensory fibers send signals to the spinal cord and quickly excite the same muscle via alpha motor neurons, producing a reflex contraction that resists the stretch. This reflexive increase in activity helps generate force during a throw and protects the muscle from overstretch. The other options don’t fit: Golgi tendon organs respond to tension and tend to inhibit contraction to protect tendons; Pacinian corpuscles detect vibration and texture changes; extrafusal muscle fibers are the contracting fibers themselves, not receptors.

Muscle spindles sense how much and how quickly a muscle is lengthening. When a rapid stretch occurs, the spindle’s Ia sensory fibers send signals to the spinal cord and quickly excite the same muscle via alpha motor neurons, producing a reflex contraction that resists the stretch. This reflexive increase in activity helps generate force during a throw and protects the muscle from overstretch. The other options don’t fit: Golgi tendon organs respond to tension and tend to inhibit contraction to protect tendons; Pacinian corpuscles detect vibration and texture changes; extrafusal muscle fibers are the contracting fibers themselves, not receptors.