What is the role of receptors and effectors in the plant response?
A receptor is a structure which receives something, while an effector carries out a task. For example the photo receptors in the eyes receive photons of light. Stretch receptors in the muscles detect the amount of stretch in those muscles. In both these cases the receptors are neural and so the response is to fire an action potential. You can also have hormonal receptors eg glucose receptors on your islet cells in the pancreas.
The effectors are the organs which respond to the signals generally triggered by these receptors. eg when you hit your knee with a reflex hammer the stretch receptors in your quadraceps muscles detect the stretch, fire an action potential (the signal) to your spinal cord, where they synapse with motor neurons which run back to the stretched muscle, causing it to contract. In this case the effector is the quadriceps muscle since it is the thing that carries out the function triggered by the signal.
In the case of high blood glucose the receptors are glucose receptors on your beta islet cells in your pancreas, which then produce the signal (insulin). Body cells such as muscle cells or liver cells would be the effectors as they then receive the signal and act on it, ie absorb glucose.