Nociceptive stimuli are transmitted through thinly myelinated or unmyelinated primary afferent fibers called nociceptors, which terminate mainly in the superficial dorsal horn of the spinal cord. While most nociceptive fibers terminate in the spinal segment of the entrance, (collateral) fibers may ascend and descend several segments upon their entry into the spinal cord, which is reflected in the receptive fields of central nociceptive neurons. In chronic pain states like inflammatory or neuropathic pain, the area of nociceptive activity may expand even further in rostrocaudal and mediolateral directions. Also, within minutes (inflammatory pain) or days (neuropathic pain), an increased sensitivity of peripheral and central nociceptive neurons will develop, which is referred to as sensitization. While anatomical, physiological, and psychophysical techniques have focused on one particular aspect of central sensitization at a time, functional imaging techniques like functional MRI, intrinsic optical imaging, and autofluorescent flavoprotein imaging (AFI) are able to capture both spatial and temporal dimensions of central sensitization simultaneously. AFI and other neuroimaging techniques may clarify fundamental aspects relating to the spread of nociceptive activity within the spinal cord and may thus provide a practical tool to test the efficacy of new analgesic drugs or procedures in animals and ultimately in humans.
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