There are millions of pain receptors distributed around the skin, muscle, bones, and internal organs. Some of these special nerve endings are more sensitive to pain than others.
The cornea of the eye, the thin transparent layer that covers the pupil, is the most sensitive part of the body. A nicked or torn cornea can be excruciatingly painful. We tend to develop protective calluses on the parts of the body we regularly put to rough use, such as the hands and feet. Thicker skin has fewer nerve endings and is less sensitive to pain.
The sensitivity of internal organs is somewhat curious. In the operating room they can be cut, crushed , or cauterized (burned) without causing a patient pain. But they are extremely sensitive to distension, spastic contraction, inflammation, obstructions, ruptures, and bleeding. Internal tumors tend to cause pain when they grow large enough to push organs out of shape.
The least sensitive organ of the body is the brain. Although the brain constantly processes pain signals, brain tissue itself has few pain receptors. However, the meninges, membranous sacs that enclose brain tissue, are highly sensitive to some kinds of stimulation, particularly pressure.