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Threshold

Action potentials

Threshold u

Receptor potential

Receptor potential

-<— Resting membrane potential receptor potential rises above the threshold level, the greater becomes the action potential frequency.

Receptor Potential of the Pacinian Corpuscle— An Example of Receptor Function

The student should at this point restudy the anatomical structure of the pacinian corpuscle shown in Figure 46-1. Note that the corpuscle has a central nerve fiber extending through its core. Surrounding this are multiple concentric capsule layers, so that compression anywhere on the outside of the corpuscle will elongate, indent, or otherwise deform the central fiber.

Now study Figure 46-3, which shows only the central fiber of the pacinian corpuscle after all capsule layers but one have been removed. The tip of the central fiber inside the capsule is unmyelinated, but the fiber does become myelinated (the blue sheath shown in the figure) shortly before leaving the corpuscle to enter a peripheral sensory nerve.

The figure also shows the mechanism by which a receptor potential is produced in the pacinian corpuscle. Observe the small area of the terminal fiber that has been deformed by compression of the corpuscle, and note that ion channels have opened in the membrane, allowing positively charged sodium ions to diffuse to the interior of the fiber. This creates increased positivity inside the fiber, which is the "receptor potential." The receptor potential in turn induces a local circuit of current flow, shown by the arrows, that spreads along the nerve fiber. At the first node of Ranvier, which itself lies inside the capsule of the pacinian corpuscle, the local current flow depolarizes the fiber membrane at this node, which then sets off typical action potentials that are transmitted along the nerve fiber toward the central nervous system.

Relation Between Stimulus Intensity and the Receptor Potential. Figure 46-4 shows the changing amplitude of the receptor potential caused by progressively stronger mechanical compression (increasing "stimulus strength") applied experimentally to the central core of a pacinian corpuscle. Note that the amplitude

Receptor potential

Action potential

Deformed area

Receptor potential

Action potential

Deformed area

Node of Ranvier

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Essentials of Human Physiology

Essentials of Human Physiology

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