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Touch is the sense of mechanical contact and tissue deformation. Cutaneous mechanoreceptors (Meissner corpuscles, Pacinian corpuscles, Ruffini cells, and Merkel cells) respond to different aspects of deformation: static pressure, deformation velocity, acceleration, and vibration. Each type is a sensor specialized in a frequency band and type of deformation.

Hertz's law F = (4/3)E*R·^(3/2) describes the mechanics of spherical contact in soft tissue: the force grows with ^1.5 (non-linearly), which produces a more sensitive response to small indentations than a linear relationship. The threshold deflection of Meissner corpuscles is 1 m comparable to the roughness of very smooth surfaces detectable with the tip of the finger.

Pacinian corpuscles act as resonant mechanical oscillators: their resonance frequency f_res = (1/2)(k/m) 250 Hz coincides with the most informative vibration frequencies for fine texture and slip. Their high Q (quality factor) makes them very frequency selective but insensitive outside their band. They are responsible for the sensation of vibration and for detecting whether a surface is rough or smooth.

The shear wave velocity c_T = (G/) in skin (G 20 kPa, 1000 kg/m³) is c_T 47 m/s, very slow compared to sound in air. Tactile vibrations propagate through the body as low-speed shear waves, which explains the sensation of propagated vibration: the hum of a motor is felt throughout the hand even if it is only touched at one point.

The penetration depth _pen of the vibration in the viscoelastic tissue determines how many mechanoreceptors are activated simultaneously. At high frequency (large ) _pen decreases and only the most superficial receptors respond; At low frequency, the signal penetrates deeper and activates Ruffini receptors in the deep dermis. This frequency dependence is fundamental for the perception of texture.

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ID:(0, 575)