Projected and receptive fields: a comparison of projected areas of sensations evoked by intraneural stimulation of mechanoreceptive units, and their innervation territories.

Microneurography and intraneural microstimulation were employed in awake human subjects to study the characteristics of cutaneous mechanoreceptive units and the sensations mediated by them. 172 units innervating the hand and forearm were identified as either PC, RA (FA in hairy skin), SA I or SA II. Analysis of action potential waveforms in a sample of units suggests that most recordings were from within the myelin sheath. Receptive fields of RA and SA I units were significantly smaller than those of PC and SA II units and showed a proximodistal size gradient, which the latter two did not. The quality of sensations evoked by intraneural stimulation was determined by the type of unit activated, except in the case of SA II units, and their magnitude could be influenced by mechanical coactivation of other sensory units. As a rule projected fields of evoked sensations were larger the further away they were from the limb tip. This grading indicates that inputs from single mechanoreceptive units are processed differently according to their source and sensory submodality, possibly as a result of unequal "resolving powers" of the corresponding cortical neurons.