Peripheral afferents and spinal inhibitory system in dynamic and static mechanical allodynia
Mechanical allodynia is a debilitating pain from innocuous mechanical stimuli such as touch. To unravel its mechanism, one approach has been to focus on the fact that different afferent types mediate mechanical allodynia from stimuli “moving” across the skin (eg, brushing) and from stimuli “fixed” on a certain spot (eg, gentle pressure or weak punctate stimulation); the former type of allodynia being designated “dynamic” and the latter, “static”23,39 (these terms, however, differentiate mechanical allodynia by “stimulation mode,” not by afferent type). Thus, because the stimulation mode differs between dynamic and static mechanical allodynia, different afferent inputs are presumably generated and then differentially processed in the spinal cord. In this regard, recent literature suggests that inhibitory processing of afferent inputs by glycine- and γ-aminobutyric acid (GABA) transmission differs between dynamic and static mechanical allodynia, respectively.36,41 In addition to the dynamic/static stimulation mode, factors such as the size/depth of stimulated area (eg, large pressure probe vs von Frey filament) and the stimulation velocity/duration can influence mechanical allodynia because of spatial/temporal summation, counteraction between afferents,2,34 and afferents' adaptation properties.