In a homeostatic control mechanism, which component monitors the environment?

In the homeostatic control mechanism, there  is the receptor the integrating center and the effector. The receptor is the mechanism that receives the information about the environment ie things like the temperature, the humidity, the pH of liquids introduced into the digestive system. It then signals to the integrating system of the hypothalamus to trigger a response say to move a muscle which once moved or tightened or relaxed is then the effector.

In a homeostatic control mechanism, the component that monitor the environment is the RECEPTOR.
Receptors are usually located within and on the surface of cells, they are the sense organs of the cells and monitor what happen between the inside and the outside of the cells. They typically bind with signalling molecules and set up  cascade pathways.