**Core Concept**
Muscle spindles are specialized intrafusal muscle fibers that play a crucial role in proprioception, allowing the nervous system to sense changes in muscle length and velocity. They are found in the belly of muscles, particularly in the flexor muscles of the limbs, and are innervated by both sensory and motor neurons.

**Why the Correct Answer is Right**
The correct answer is **stretch**. Muscle spindles detect changes in muscle length, specifically stretching or shortening of the muscle. This is achieved through the activation of stretch receptors, known as nuclear bag and nuclear chain fibers, which are embedded within the intrafusal muscle fibers. When a muscle is stretched, the spindles are activated, sending signals to the central nervous system to adjust muscle tone and prevent excessive stretching.

**Why Each Wrong Option is Incorrect**
**Option A:** **Pressure**. Muscle spindles are not primarily responsible for detecting pressure. While pressure can stimulate muscle spindles, it is not their primary function.
**Option B:** **Vibration**. Muscle spindles do not detect vibration directly. However, they can respond to changes in muscle length caused by vibration.
**Option C:** **Tension**. Muscle spindles detect changes in muscle length, not muscle tension. Muscle tension is detected by other receptors, such as the Golgi tendon organs.

**Clinical Pearl / High-Yield Fact**
Muscle spindles are essential for maintaining muscle tone and preventing muscle atrophy. Damage to muscle spindles, such as that caused by peripheral neuropathy, can lead to muscle weakness and atrophy.

**Correct Answer: C. Tension**