Which of the following structures is found within a muscle spindle?

• A. Tendon organ
• B. Myelin sheath
• C. Motor end plate
• D. Intrafusal muscle fibers

Answer: (D)

Verified step by step guidance

1. Understand the context: Muscle spindles are specialized sensory organs located within skeletal muscles. They play a critical role in detecting changes in muscle length and are involved in proprioception, which is the body's ability to sense its position and movement.
2. Identify the components of a muscle spindle: Muscle spindles contain intrafusal muscle fibers, sensory nerve endings, and motor nerve endings. Intrafusal muscle fibers are the specialized muscle fibers within the spindle that detect stretch and send signals to the central nervous system.
3. Clarify why the other options are incorrect: Tendon organs (Golgi tendon organs) are located in tendons, not within muscle spindles. Myelin sheath is a protective covering around nerve fibers, but it is not a structure within the muscle spindle. The motor end plate is part of the neuromuscular junction, where motor neurons connect to muscle fibers, but it is not found within the muscle spindle.
4. Focus on the correct answer: Intrafusal muscle fibers are the key structural component within muscle spindles. They are surrounded by a capsule and work alongside sensory neurons to detect muscle stretch.
5. Summarize the concept: Muscle spindles are sensory organs containing intrafusal muscle fibers, which are essential for detecting changes in muscle length and contributing to reflexes like the stretch reflex.