**Core Concept**
The Golgi tendon apparatus is a proprioceptive sensory receptor organ that plays a crucial role in detecting and conveying information about muscle tension to the central nervous system (CNS). This apparatus is responsible for regulating muscle tone and preventing excessive muscle tension, which can lead to injury.

**Why the Correct Answer is Right**
The Golgi tendon apparatus is sensitive to changes in muscle tension, specifically the rate of tension increase. When muscle tension exceeds a certain threshold, the Golgi tendon apparatus is activated, sending signals to the CNS to relax the muscle and prevent further tension. This mechanism is mediated by specialized sensory neurons that innervate the muscle spindle and Golgi tendon apparatus, enabling the CNS to monitor and regulate muscle activity. The Golgi tendon apparatus is primarily responsible for detecting changes in muscle tension, making option A the correct answer.

**Why Each Wrong Option is Incorrect**
**Option B:** The length of the muscle is primarily detected by the muscle spindle, another type of proprioceptive sensory receptor organ. While the muscle spindle does play a role in detecting changes in muscle length, it is not responsible for conveying information about muscle tension to the CNS.

**Option C:** The rate of contraction is not directly detected by the Golgi tendon apparatus. While the rate of contraction can affect muscle tension, the Golgi tendon apparatus is primarily sensitive to changes in muscle tension, not the rate of contraction.

**Option D:** Blood supply is not a factor in the functioning of the Golgi tendon apparatus. The Golgi tendon apparatus relies on mechanical stimuli, such as changes in muscle tension, to activate and convey signals to the CNS.

**Clinical Pearl / High-Yield Fact**
The Golgi tendon apparatus is a critical component of the proprioceptive system, enabling the CNS to monitor and regulate muscle activity. Damage to the Golgi tendon apparatus can lead to impaired proprioception and muscle control, making it a potential area of focus for rehabilitation and physical therapy.

✓ Correct Answer: A. Tension in muscle