Mechanoreceptive afferents in the human sural nerve.

Microneurography was used to characterize 104 low-threshold mechanoreceptive afferents in the human sural nerve. The afferents were readily classified into four types using criteria developed for the glabrous skin of the hand: SA I (31%), SA II (11%), FA I (49%), and FA II (9%). The distribution of fascicle fields and receptive fields of individual afferents on the lateral side of the foot indicates that the glabrous skin portion of the innervation territory of the sural nerve is more densely innervated than the non-glabrous skin portion. The different populations in the glabrous and non-glabrous skin regions were similar regarding proportion of unit types, receptive field sizes, and force thresholds. The receptive field sizes of the type I units of the present sample were about twice the size compared to those of the glabrous hand units, and the force threshold were at least three times higher for three of the unit types (SA I, FA I, and FA II). Given their receptive properties, it is likely that mechanoreceptive afferents in the sural nerve provide rich information about contact patterns between the foot and environment during stance and locomotion.