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The term pain threshold refers to the maximum intensity or duration of a sensory stimulus at which the body shuts down. In scientific literature the term is clearly differentiated from the term pain tolerance. Pain threshold is the minimum stimulus which elicits pain and involves measurement of stimulus intensity, whereas pain tolerance is the degree of pain which a subject can tolerate, and involves a measurement of a subject's response to pain.
Consider, as examples, the three potentially painful stimuli of heat applied to the skin, pressure on a shin bone, or interruption of blood supply to a muscle. When applied at low intensity, or for a short time, these events are recognized as non-painful sensations. By progressively intensifying or prolonging the stimulus, a stage is reached where the subject/person would say "that is painful", or, in the case of animals, when some observable response occurs which is presumed to be indicative of pain. The temperature, the pressure, or the duration of muscle ischaemia at which that change from ordinary sensation to pain occurs, is the pain threshold—it is a measure of the stimulus when the response occurs, but not a measure of the response. In the cases mentioned the pain threshold may be expressed as degrees Celsius, kilopascals, or seconds respectively. The use of such units clearly indicates the meaning of the term, as not a measurement of "hurt" or "suffering".
Increased and decreased pain threshold
Pain threshold is mostly a function of how healthy the pain nerves and central pain processing pathways are, and of the chemical milieu in the tissues which the nociceptors innervate. It is commonly reduced by inflammation in the vicinity of nociceptors, such that normal touch becomes painful in the area of inflammation (allodynia). In such circumstances, the use of anti-inflammatory medications or physical therapies such as cooling or mild heat may return the nerve sensitivity or threshold to normal - it does not increase the threshold to above normal.
The use of morphine-like drugs does elevate the pain threshold (as well as pain tolerance). The same is true of a diverse number of other centrally acting analgesic drugs affecting pain relay neurons. Local physical techniques such as electrical stimulation, cooling or heat may temporarily increase the pain threshold.
However, short of damaging pain nerves - such as may occur in nerve injuries or neuropathies - the effects of interventions (pharmacological or physical) used to increase the pain threshold do not last much longer than the duration of their use. Additionally, in normal healthy persons, it is not significantly changed by any mental or physical practice or training. In contrast, pain tolerance may be increased by medications as well as by physical, cognitive and affective interventions and training.
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