specificity was backed mostly by physiologists and physicians, and the intensive theory was mostly backed by psychologists. However, after a series of clinical observations by Henry Head and experiments by Max von Frey, the psychologists migrated to specificity almost en masse, and by century's end, most textbooks on physiology and psychology were presenting pain specificity as fact. Modern Some sensory fibers do not differentiate between noxious and non-noxious stimuli, while others, nociceptors, respond only to noxious, high intensity stimuli. At the peripheral end of the nociceptor, noxious stimuli generate currents that, above a given threshold, send signals along the nerve fiber to the spinal cord. The "specificity" (whether it responds to thermal, chemical or mechanical features of its environment) of a nociceptor is determined by which ion channels it expresses at its peripheral end. Dozens of different types of nociceptor ion channels have so far been identified, and their exact functions are still being determined. The pain signal travels from the periphery to the spinal cord along A-delta and C fibers. Because the A-delta fiber is thicker than the C fiber, and is thinly sheathed in an electrically insulating material (myelin), it carries its signal faster (5–30 m/s) than the unmyelinated C fiber (0.5–2 m/s). Pain evoked by the A-delta fibers is described as sharp and is felt first. This is followed by a duller pain, often described as burning, carried by the C fibers. These A-delta and C fibers enter the spinal cord via Lissauer's tract and connect with spinal cord nerve fibers in the central gelatinous substance of the spinal cord. These spinal cord fibers then cross the cord via the anterior white commissure and ascend in the spinothalamic tract. Before reaching the brain, the spinothalamic tract splits into the lateral, neospinothalamic tract and the medial, paleospinothalamic tract. The neospinothalamic tract carries the fast, sharp A-delta signal to the ventral posterolateral nucleus of the thalamus. The paleospinothalamic tract carries the slow, dull, C-fiber pain signal. Some of the paleospinothalamic fibers peel off in the brain stem, connecting with the reticular formation or midbrain periaqueductal gray, and the remainder terminate in the intralaminar nuclei of the thalamus. Pain-related activity in the thalamus spreads to the insular cortex (thought to embody, among other things, the feeling that distinguishes pain from other homeostatic emotions such as itch and nausea) and anterior cingulate cortex (thought to embody, among other things, the affective/motivational