"For the fetus to respond to surgical damage, receptors in the affected tissue, such as skin and muscle, must signal the noxious stimulus or damage to the central nervous system. Nociceptors are sensory nerve terminals found in the skin and internal organs that convert tissue damage into electrical signals. The pattern and strength of these nociceptor signals is the first determining step in generating pain. If nociceptor activity is prevented, such as following local anaesthesia, then pain is blocked. Deep tissue damage, for example, that cuts through nerve bundles causes a brief burst of electrical activity in some of the cut nerve endings known as an injury discharge. The injured tissue, however, is now isolated from the central nervous system and, within a few minutes, the isolated tissue becomes ‘numb’ and pain free. Similarly, rare genetic defects that prevent all nociceptive signals result in a complete inability to sense pain. Anatomical studies of human fetal skin shows the presence of nerve terminals and fibres deep in the skin from 6 weeks of gestational age. These terminals are not nociceptors and are specialised for the processing of non-damaging sensations such as touch, vibration and temperature, rather than pain. From 10 weeks, nerve terminals become more numerous and extend towards the outer surface of the skin. The terminals closer to the surface are likely to be immature nociceptors, necessary for pain experience following tissue damage, but they are not unequivocally present until 17 weeks. In other mammals, newly formed fetal nociceptors are able to signal tissue damage but the intensity of their signals is weaker than in adults. The internal organs develop nerve terminals later than the skin, beginning to appear from 13 weeks and then increasing and spreading with age, so that the pancreas, for example, is innervated by 20 weeks."