**Core Concept**
Reciprocal inhibition is a reflex mechanism in which the contraction of one muscle group leads to the relaxation of another muscle group, resulting in a balanced movement. This concept is crucial in understanding the coordination and regulation of muscle activity in the body.

**Why the Correct Answer is Right**
Reciprocal inhibition is primarily achieved through the action of inhibitory interneurons, which release neurotransmitters such as glycine and GABA to suppress the activity of motor neurons. When a muscle contracts, the corresponding motor neuron is activated, while the inhibitory interneuron releases neurotransmitters to inhibit the motor neuron of the opposing muscle group, leading to relaxation. This mechanism ensures smooth and coordinated movement.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because reciprocal inhibition involves the release of inhibitory neurotransmitters, not excitatory neurotransmitters like glutamate, which would actually increase muscle activity.

**Option B:** This option is incorrect because reciprocal inhibition is not primarily achieved through the action of proprioceptors, which detect changes in muscle length and joint position, but rather through the action of inhibitory interneurons.

**Option C:** This option is incorrect because reciprocal inhibition is a reflex mechanism that occurs at the level of the spinal cord, not at the level of the brain.

**Clinical Pearl / High-Yield Fact**
Reciprocal inhibition is a fundamental principle in understanding the coordination and regulation of muscle activity, and is essential for the development of normal movement patterns. It is also an important concept in the rehabilitation of patients with muscle imbalances or neurological disorders.

**Correct Answer: D**