What sensory receptor within the skeletal muscle is activated in response to changes in muscle length?

Muscle spindles are specialized sensory receptors located within the skeletal muscle that play a critical role in detecting changes in muscle length and the rate of that change. When a muscle is stretched, the muscle spindles are activated and send signals to the central nervous system regarding the length and stretch of the muscle. This information is vital for coordinating muscle contractions and maintaining posture, as it helps the body adjust the tension in muscles accordingly.

The other options involve different functions: Golgi tendon organs are responsive to changes in muscle tension rather than length, synapses refer to the junctions between neurons where neurotransmission occurs, and neurotransmitters are chemical messengers that transmit signals across synapses. Thus, muscle spindles are the appropriate choice for sensing muscle length changes specifically.