**Core Concept**
Nuclear bag fibers are specialized sensory neurons found in the muscle spindle, a proprioceptive sensory organ that detects changes in muscle length and velocity. They play a crucial role in the regulation of muscle tone and reflexes, particularly in the stretch reflex.

**Why the Correct Answer is Right**
Nuclear bag fibers are sensitive to stretch and are responsible for detecting changes in muscle length and velocity. They contain a high concentration of sensory receptors called nuclear bag receptors, which are activated by the stretching of the muscle spindle. This activation triggers a signal that is transmitted to the central nervous system, where it is processed and used to regulate muscle tone and reflexes. The nuclear bag fibers are particularly important in the regulation of reflexes, such as the stretch reflex, which helps to maintain muscle tone and prevent excessive muscle lengthening.

**Why Each Wrong Option is Incorrect**
**Option A:** This is incorrect because nuclear bag fibers are not responsible for detecting changes in muscle tension.
**Option B:** This is incorrect because nuclear bag fibers do not detect changes in muscle force.
**Option C:** This is incorrect because nuclear bag fibers are not responsible for detecting changes in joint position.

**Clinical Pearl / High-Yield Fact**
The muscle spindle is a critical component of proprioception, and nuclear bag fibers play a key role in its function. Understanding the role of nuclear bag fibers is essential for diagnosing and managing disorders of muscle tone and reflexes.

**Correct Answer:** C. Nuclear bag fibers detect changes in muscle length and velocity.