Figure 2. Rectal mechanoreceptors responded to focal transmural compression by von Frey hairs.

After identifying stretch-sensitive units, their distortion-sensitive sites were located with von Frey hairs and marked on the tissue. Repeated responses to hand-held von Frey hairs of a range of stiffnesses up to 1.1 mN were then recorded. Stiffer von Frey hairs often caused irreversible decreases in excitability. A, typical recording shows graded responses to increasing force. B, mean data from 8 units in 4 animals.