Mechanical transmission in a Pacinian corpuscle. An analysis and a theory

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Abstract
The main structural elements in force transmission through the corpuscle are the lamellae, their interconnexions, and the interlamellar fluid. The 2 former provide the elastic elements and constraint for the fluid; and the latter, the viscous elements. The mechanical equivalent incorporating these elements is a system of dashpots (the lamellar surfaces and the interlamellar fluid) and springs (the lamellae and their interconnexions); it is a mechanical filter which suppresses low frequencies. The dynamic and static patters of lamella displacements in the equivalent are in close agreement with those observed in Pacinian corpuscles. Under static compression, only elastic forces exist in the corpuscle. Analysis shows that such forces are transmitted poorly from periphery to centre through the lamellated structure. The compliance of the lamellar interconnexions is so high in relation to that of the lamellae themselves, that most of the pressure load is carried by the outer lamellae. As a result, only a small fraction of the steady-state pressure at the outer surface reaches the centre of the corpuscle where the sensory ending is located. Under dynamic compression, viscous forces develop in the corpuscle; and these account for most of the pressure at times too early for development of elastic deformations. Analysis shows that such forces are transmitted well. For example, if a typical corpuscle of 500 [mu] diameter is compressed by 20 [mu] linearly during 2 msec, the pressure differences near to the centre of the corpuscle are initially as high as at the periphery, and stay within the same order throughout the process of compression. Pressure at the centre increases steeply with velocity of compression which explains the marked velocity dependence of the generator response of the sensory ending. If, in the foregoing example, the 20 [mu] compression is held fixed after 2 msec, the pressure differences at the centre fall abruptly to near zero with the onset of the static phase. The duration of pressure transients at the centre approximates that of the ''active'' phase of the generator current of the sensory ending derived from experiments, as expected in a causal relationship: pressure difference[forward arrow] generator current. The mechanical filter action of the corpuscle is the rate-limiting factor in generator response adaptation. When the corpuscle is released and consumed in viscous flow. Thus, viscous pressure is produced anew. The magnitude of this pressure depends on the velocity of release. The pressure distribution is rotated by 90[degree] with respect to that in compression; i.e. during release, compression occurs once again, but this time at right angles to the direction of initial compression. Experiments show that the sensory ending does not discriminate such a rotation; the polarity and order of magnitude of the generator response to compression in one plane are the same as in another. Analysis shows that considerable pressure differences may be developed at the centre of the corpuscle during releases at physiological velocites. In a passive return from a compression of 20 [mu], the pressure difference at the centre (and the generator current) is of the same order of magnitude as that in a compression of the velocity in 4; accounting for the ''off''-response of the sensory ending in purely mechanical terms.