The large myelinated primary afferent non-nociceptive fibers traverse the top of the dorsal horn through Lissauer's tract and then ascend the spinal cord through the white matter of the dorsal column or decussate to the contralateral ventral spinothalamic tract. Nociception exists at the intersection of the nervous system, the immune system, and the endocrine system. Painful information is conveyed by nociceptors which detect a wide range of thermal, chemical, or mechanical perturbations through a variety of mechanisms, including bare nerve endings which either react to specific noxious stimuli or are polymodal and respond to a variety of noxious stimuli. Sensory information is relayed via sensory neurons. Primary sensory neurons can be divided into two main categories: those which relay innocuous non-painful information and those that relay painful nociceptive information. The primary afferent summarizes nociceptive information from the periphery and feeds this information centrally into the spinal cord.