2.1.1 The normal physiology of acute pain
Pain is a complex and multifaceted experience, intertwining sensory, affective, cognitive, and behavioral dimensions. Nociception is the sensory nervous system process of encoding noxious or tissue-damaging stimuli, involving the activation of peripheral nociceptors, which in turn communicate through synapses with the central nervous system16. Acute pain can produce autonomic responses, frequently leading to increases in blood pressure, cardiac output, respiratory and heart rate. Thus, the experience of acute pain involves a visceral sympathetic reaction that informs the organism it must avoid or withdraw from a source of discomfort or damage17. Ascending neural pathways carry sensory information from the periphery through the spine to corticolimbic regions; descending pathways, conversely, modulate the pain experience as they relay physical and emotional signals down the spinal cord18.