Nociception (also nocioception or nociperception, from latin nocere 'to harm or hurt') is the encoding and processing of harmful stimuli in the nervous system, and, therefore, the ability of a body to sense potential harm. It is the afferent activity in the peripheral and central nervous systems produced by stimulation of specialized free nerve endings called nociceptors or "pain receptors" that only respond to tissue damage caused by intense chemical (e.g., chilli powder in the eyes), mechanical (e.g., pinching, crushing) or thermal (heat and cold) stimulation. Once stimulated, a nociceptor sends a signal along a chain of nerve fibers via the spinal cord to the brain. Nociception triggers a variety of autonomic responses and may also result in a subjective experience of pain in sentient beings. Nociceptive neurons generate trains of action potentials in response to intense stimuli, and the frequency of firing determines the intensity of the pain.