Figure 7.

A scheme of the pad's performance in the low- and high-frequency working ranges of the metatarsal vibration sensor. Blue line: typical threshold (tuning) curve, adapted and schematized from ref. [8], showing the displacement of the substrate (and tarsus) necessary to elicit a response of the sensory cells at different frequencies. At low frequencies below ca 40 Hz large displacements are required. According to our present results this is because much of the tarsal (T) displacement at its contact area with the pad (P) is used to compress the pad's hydrated and soft frontal part (upper inset). At high frequencies, however, minute displacements suffice to elicit a nervous response (lower inset). Then the epicuticle on the pad's front side undergoes glass transition, implying sufficient stiffness to transmit the mechanical stimulus to the slits (S) of the vibration receptor much more effectively than at low frequencies [15]. Arrows show load direction. Yellow lines indicate the contact area between the tarsus and the pad. T, tarsus; P, pad; S, slits of the vibration receptor; MT, metatarsus.