Factors affecting cutaneous mechanoreceptor response. I. Constant-force versus constant-displacement stimulation.

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Abstract
Certain mechanical properties of mammalian skin were characterized and the role of these properties were examined in determining the threshold and suprathreshold responses to mechanicl stimuli of both slowly and rapidly adapting cutaneous mechanoreceptors of raccoon and cat glabrous skin. A mechanical stimulating system allowing the control of either stimulus force or displacement and the simultaneous monitoring of both was utilized. Single fibers of the median nerve were isolated by microdissection, and punctiform stimuli (ramp-and-hold waveforms) were applied normal to their receptive fields. Glabrous and hairy skin of raccoon, cat, dog, monkey and human all display properties of viscoelastic materials, including stress relaxation, creep, and creep recovery. A given displacement produces a greater reactive force in glabrous than in hairy skin, and a given applied force produces a greater displacement in hairy than in glabrous skin. In both glabrous and hairy skin, displacement is an approximately linear function of applied force immediately (0-300 ms) on stimulus application, but becomes a negatively accelerating function over time (up to 20 s later). Both absolute force and absolute displacement thresholds are comparable in rapidly adapting and in moderately and very slowly adapting type I glabrous skin mechanoreceptors, and force and displacement thresholds are positively correlated. Provided stimulus ramps are of sufficiently high velocity, adaptation of type I glabrous skin mechanoreceptors is more rapid with constant-displacement than with constant-force stimuli. Following lower ramp velocities, rates of adaptation to constant-force and constant-displacement stimuli may be comparable. Adaptation of type I mechanoreceptors to constant displacements does not parallel stress relaxation. Even after reactive force has become relatively stable, discharge rate drops to zero within 10-15 s in moderately slowly adapting units and continues to decline for many seconds or minutes in very slowly adapting units. For both categories of type I mechanoreceptor, a logarithmic function most appropriately describes the relationship between mechanical stimulus force and primary afferent discharge rate. Results are compared with previous findings in noncutaneous slowly adapting mechanoreceptors and are discussed in terms of the contributions of viscoelastic properties of skin to cutaneous mechanoreceptor activity. Mechanical events at the skin surface contribute to , but do not alone account for, the discharge properties of cutaneous mechanoreceptors.