Transmission refers to the transfer of noxious stimuli from primary nociceptors in the periphery to cell bodies in the spinal cord dorsal horn. As described above, AS and C fibers are the axons of unipolar neurons that have distal projections known as nociceptive endings. After the synapse in the dorsal root, the second-order neurons send their signals contralater-ally and upward through the spinothalamic tract. The signals ofthe spinothalamic tract travel up the spinal cord through the medulla and synapse on neurons in the thalamus. Nerves from the thalamus then relay the signal to various areas of the somatosensory cortex, where pain perception takes place. Glutamate, the excitatory amino acid implicated in transmission from primary afferent nociceptors to dorsal horn neurons, has a number of receptors [amino-3-hydroxyl-5-methyl-4-propionic acid (AMPA), kainate, N-methyl-D-aspartate (NMDA), and metabotropic] it activates. The various combinations of these receptors exist on neurons in various laminae of the dorsal horn.
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