pathway of pain

A nociceptive neuron is one that transmits pain signals its cell body is in the dorsal root ganglion near the spinal cord,approximatly 25 percent of the myelinated A-delta and 50 percent of the unmyelinated C fibers contact nociceptors . once nociceptor is stimulated it release neuropeptide (substance P) that intiate electrical impulses along afferent fiber toward the spinal cord . substance P also serves as transmitter substance between first order afferent fiber and second order afferent fiber at the dorsal horn of the spinal column. the A-delta and C fibers which transmit pain and temperature have different diameters (A-delta are larger) and different conduction velocities (A-delta are faster) these difference resultin two qualitatively different type of pain termed fast and slow. fast pain is brief,well localized,and well matched to the stimulus. slow pain is an aching,throbbing,orburning sensation that is poorly localized and less related to the stimulus . fast pain is transmitted over the larger faster conducting A-delta afferent neuron and originate from receptor in the skin  . slow pain is transmitted by C fiber that originate from skin ,ligament and muscle .some of A-delta and Cfiber enter spinal cord through the dorsolateral tract of lissauer and synapse in the marginal zone(lamina 1) or substantia gelatinosa(lamina 2) with second order neuron . most of nociceptive second-order neuron ascend to the higher center along one of three tract (1)lateral spinothalamic tract (2) spinoreticular tract (3)spinoencephalic tract .with the remainder ascend along spinocervical tract or as projections to tha gracile and cuneate nuclie of the medulla , approximately 90 percent  of second-order afferent terminate in the thalamus. third order neuron project to the sensory cortex and other centers in the central nervous system where the pain is perceived