PERIPHERAL ACUTE PAIN MECHANISMS

Many studies in several species, including humans, have identified a s ubset of primary afferent nerve fibres that are activated by potential or actual tissue-damaging stimuli. Discharge patterns of these nocice ptive afferents faithfully reproduce some aspects of the applied stimu li (e.g. shape of the stimulus-response function) but not others (e.g. time-course of a sustained stimulus). Since primary nociceptive affer ents provide the input to the central nervous system, their encoding p roperties have to be considered when studying central processing. On t he other hand, pain perception correlates with some aspects of nocicep tor discharges (e.g. fatigue with repetition of brief heat pulses), bu t not with others (e.g, absolute thresholds). Therefore, the painfulne ss of a stimulus cannot be deduced from nociceptor discharges alone; c entral processing needs to be taken into account, particularly central summation, In addition to the immediate responses of nociceptive affe rents to external stimulation, acute pain mechanisms also comprise the short-term plasticity of the nociceptive system as a consequence of p rolonged noxious stimulation