Pacinian Corpuscle

The pacinian corpuscle consists of the unmyelinated terminal part of an afferent nerve fiber that is surrounded by concentric lamellae formed by the membranes of numerous supporting cells. The axon terminal membrane is adapted in such a way that its ionic permeability increases when it is deformed by applied pressure. Although the permeability change appears to be nonspecific, the principal ion flux that occurs is an inflow of sodium ions (Na⁺) because of the great difference in the electrochemical potential of this ion on the two sides of the membrane. The Na⁺ influx causes a depolarizing current to flow through the axon terminal and the nearby nodes of Ranvier of the afferent fiber. The depolarization caused by this current comprises the generator potential. If the depolarization is great enough, it will produce an action potential at the point of lowest threshold, in this case, at the first node. This action potential then propagates along the afferent fiber to the central nervous system (CNS).

The pacinian corpuscle is specifically adapted to respond to rapidly changing mechanical stimulation. Experiments on isolated pacinian corpuscles have shown that this adaptation involves both the physical structure of the receptor and the properties of the action potential–generating mechanism.

When pressure is applied to an intact pacinian corpuscle, single-action potentials are evoked at the beginning and end of the pressure pulse. If action potentials are blocked by a drug such as tetrodotoxin, the generator potentials evoked by the pressure pulse can be recorded. In the intact pacinian corpuscle, these potentials consist of rapidly decaying depolarizations that occur at the beginning and end of the pulse.

If all the lamellae of the sheath, except the innermost, are dissected away, the response of the pacinian corpuscle to the pressure pulse is modified. The generator potential now decays slowly throughout the period of applied pressure, and no additional depolarization appears at the termination of the pulse. This finding indicates that the viscoelastic properties of the intact capsule dissipate applied pressure, which means that only sudden pressure changes can reach the membrane of the nerve terminal and produce a generator potential.

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