Final answer:
Muscle spindles are proprioceptive stretch receptors that detect muscle stretch or lengthening. Their activation is modulated by the state of the muscle.
Step-by-step explanation:
Muscle spindles are proprioceptive stretch receptors that detect the amount of stretch or lengthening of muscles. They are activated by muscle stretch, or the lengthening of muscles.
The activation of muscle spindles is modulated by the state of the muscle, meaning that the response of the muscle spindle can be influenced by factors such as muscle tension or contraction.