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sensation (“first pain”). This is followed by a diffuse burning sensation approxi-


mately 1 s later (“second pain”) (Price et al. 1977 ). The activation of the motor


withdrawal of the hand is a protective reflex to minimize potential tissue damage


from the insult and is not related to the conscious sensation. The sharp, pricking


“first”pain is related to information encoded by the small, myelinated Aδperipheral


nerves that provide information about temperature and touch as well as pain. These


nerves conduct impulses at >1 m/s. The diffuse, burning“second”pain is related to


information encoded by unmyelinated C peripheral nerves that provides informa-


tion about higher intensity temperature and pressure as well as pain. These nerves


transmit information at a slower rate, approximately 0.7 m/s. The 1-s delay in


“first”and“second”pain is related to the different amount of time it takes for the


information to travel from thefinger to the brain by these two nervefiber types.


There are other features of“first”and“second”pain that help us understand


factors associated with the onset and maintenance of persistent clinical pain. If a


healthy, pain-free subject rates the pain intensity of“second pain”in a series of heat


pulses (temperatures≥45 °C) of varying frequencies of stimulation to the hand or


foot, the pain experience changes with changes in frequency. If the heat pulses are


presented at 5 s intervals, the rated“second”pain intensity using VAS ratings is


unchanged and will be the same for thefirst and fourth stimulus. If the frequency of


stimulation increases to once every 2.5 s, the rated“second”pain intensity will
increase with each of the four stimuli.“First”pain does not increase with changes in


frequency. We call this enhanced increase summation of“second”pain with the


succession of pulses“windup”(Price et al. 1977 , 1994 ).


This phenomenon of“windup”occurs if we increase the number of pulses as


well. So, when a subject rates the pain intensity for each stimulus in a series of ten


pulses delivered at a frequency of one every 2 s, the judged pain intensity also


increases with each pulse. Subjects will also report that after the tenth pulse, there is


a lingering pain aftersensation at 15 s after stimulation is terminated that is gone by


120 s (Price et al. 1977 , 1994 ). The increased sensitivity to each successive stim-


ulus is referred to as hyperesthesia which is a common feature of persistent clinical


pain. However, in persistent pain patients, the magnitudes of these effects are


enhanced (Price et al. 1992 , 2002 ; Staud et al. 2001 ,2003a). Furthermore, in


clinical persistent pain (e.g.,fibromyalgia, irritable bowel syndrome, postherpetic


neuralgia or “shingles pain”, tempromandibular disorders), the enhanced pain


experience (lower pain threshold) is oftentimes associated with non-noxious stim-


ulation (allodynia) (Woolf 2011 ). For example, patients suffering from postherpetic


neuralgia afflicting the trunk will complain that the movement of their shirt or


blouse across the skin (light touch) is painful.


Pain patient ratings of“windup”and the lingering aftersensations have provided


insights into nervous system changes in these conditions. For example,


fibromyalgia patients and control subjects rated the pain intensities of each of ten


pressure pulses (1 kg/s and maintained for 2.5 s) delivered every 2 s and every 5 s


and their aftersensations at 15 and 120 s (Staud et al. 2007 ). Both patients and


controls rated the pain intensities of pulses delivered every 2 s as greater than those
delivered every 5 s. This indicated that“windup”was similar in both groups.


110 M.W. Heft and M.E. Robinson

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