Pain is a signal that protects us from harmful stimuli. An excess of any type of stimulus such as pressure, heat, cold, excessive mechanical stretch, and specific chemical compounds stimulates pain receptors. In the human body, pain receptors are the tips of certain sensory neurons found in almost every tissue. However, most internal organs are poorly supplied with pain receptors. For this reason, pain from internal structures is often difficult to locate. In fact, pain is often not projected back to the organ that is stimulated. Instead, it is referred to an area just under the skin that may be some distance from the organ involved. The area to which the pain is referred is connected to nerve fibres from the same level of the spinal cord as the organ involved.