Sa Sa

heat slow burning pain

FIGURE 1 Specialized somatic sensory nerve endings in the skin: (A) hairy skin such as the arm, and (B) glabrous skin such as the palm of the hand. Fibers are classified (type II, III, or IV) according to conduction velocity. All of the axons shown are covered with an intermittent, insulating wrapping of myelin except for the type IV (C) fibers. Rapidly adapting receptors (RAs) respond during the beginning and end of a movement but not during a steady stimulus. Slowly adapting receptors (SAs) respond throughout a stimulus. Not shown are the sensory neurons transmitting proprioceptive information from muscles and joints. These large (13-20 mm in diameter), heavily myelinated, and fast (80-120 m/s) axons are classified as type I fibers.

pressure on the surface of the skin, layers are compressed and the compression is transmitted to the underlying nerve terminal membrane, where mechanosensitive channels are triggered to open. Increasing the conductance of the membrane to sodium or to sodium and potassium leads to small depolarizations called generator potentials. These channels are not the type of voltage-gated sodium channels that directly produce an action potential; however, action potentials will result from a given stimulus if the generator potentials produced are able to summate and bring the cell to the threshold level for activating voltage-gated channels. The number of action potentials produced will be determined by the strength of the generator potential, which in turn reflects the degree of mechanical distortion.

The response of the Pacinian corpuscle is not sustained, however, because the encapsulating layers are covered with a viscous liquid that allows them to slip past each other and dissipate the pressure to surrounding tissues. When the pressure is relieved, slippage of the layers back

A. Nerve bundle containing thousands of axons +

external stimulus electrode external stimulus electrode external recording electrodes

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