**Core Concept**
Muscle spindles are specialized proprioceptive sensory receptors found within the belly of skeletal muscles. They play a crucial role in detecting changes in muscle length and velocity, thereby contributing to the regulation of muscle tone and movement. The primary function of muscle spindles is to sense stretch and provide information to the central nervous system for reflexive adjustments.

**Why the Correct Answer is Right**
Muscle spindles contain intrafusal fibers, which are divided into nuclear bag and nuclear chain fibers. These fibers are innervated by sensory afferents, known as Ia and II fibers. The Ia fibers are responsible for detecting changes in muscle length and velocity, while the II fibers detect changes in the rate of change of length. The sensory information from muscle spindles is transmitted to the spinal cord, where it can trigger reflexive adjustments in muscle tone. The Ia fibers are also involved in the stretch reflex, which helps to maintain muscle tone and prevent excessive stretching.

**Why Each Wrong Option is Incorrect**
* **Option A:** Muscle spindles do not primarily detect changes in muscle force or tension.
* **Option B:** While muscle spindles do detect changes in muscle length, they are not the primary receptors responsible for detecting changes in joint angle or position.
* **Option D:** Muscle spindles are not responsible for detecting changes in muscle temperature or metabolic byproducts.

**Clinical Pearl / High-Yield Fact**
Muscle spindles play a critical role in the regulation of muscle tone and movement, and their dysfunction can lead to abnormalities in reflexes and movement patterns. For example, patients with peripheral neuropathy may exhibit decreased or absent deep tendon reflexes due to damage to the sensory afferents that innervate muscle spindles.

**Correct Answer: C. Muscle spindle detects changes in muscle length and velocity.**