Final answer:
Muscle spindles are proprioceptive sensors that monitor muscle length and stretch, while Golgi tendon organs detect muscle contraction force. Together, they enable the body to control muscle movement and posture unconsciously, important for actions like the knee j erk reflex.
Step-by-step explanation:
The muscle spindle monitors muscle length or the force of contraction. Muscle spindles are stretch receptors that detect the amount of stretch or lengthening of muscles, functioning to maintain a constant length in response to changes in muscle stretch. In contrast, Golgi tendon organs monitor the force of muscle contraction, detecting tension within the muscle.
These proprioceptive components are essential for the body to adjust and control muscle movement and posture unconsciously. For instance, the stretch reflex, such as the knee j erk reaction demonstrated during a physical exam, involves the activation of muscle spindle receptors that initiate a contraction of the stretched muscle while inhibiting the antagonist muscles.