these same recording techniques cannot distinguish easily between Ab and Ag fibers. Therefore, the following classification is frequently used by sensory physiologists:

Group la

Fibers from the annulospiral endings of muscle spindles (average about 17 microns in diameter; these are a-type A fibers in the general classification).

Group lb

Fibers from the Golgi tendon organs (average about 16 micrometers in diameter; these also are a-type A fibers).

Group ll

Fibers from most discrete cutaneous tactile receptors and from the flower-spray endings of the muscle spindles (average about 8 micrometers in diameter; these are b- and g-type A fibers in the general classification).

Group lll

Fibers carrying temperature, crude touch, and pricking pain sensations (average about 3 micrometers in diameter; they are 8-type A fibers in the general classification).

Group IV

Unmyelinated fibers carrying pain, itch, temperature, and crude touch sensations (0.5 to 2 micrometers in diameter; they are type C fibers in the general classification).

Transmission of Signals of Different Intensity in Nerve Tracts—Spatial and Temporal Summation

One of the characteristics of each signal that always must be conveyed is signal intensity—for instance, the intensity of pain. The different gradations of intensity can be transmitted either by using increasing numbers of parallel fibers or by sending more action potentials along a single fiber. These two mechanisms are called, respectively, spatial summation and temporal summation.

Spatial Summation. Figure 46-7 shows the phenomenon of spatial summation, whereby increasing signal strength is transmitted by using progressively greater numbers of fibers. This figure shows a section of skin innervated by a large number of parallel pain fibers. Each of these arborizes into hundreds of minute free nerve endings that serve as pain receptors. The entire cluster of fibers from one pain fiber frequently covers an area of skin as large as 5 centimeters in diameter. This area is called the receptor field of that fiber. The number of endings is large in the center of the field but diminishes toward the periphery. One can also see from the figure that the arborizing fibrils overlap those from other pain fibers. Therefore, a pinprick of the skin usually stimulates endings from many different pain fibers simultaneously. When the pinprick is in the center of the receptive field of a particular pain fiber, the degree of stimulation of that fiber is far greater than when it is in the periphery of the field, because the number of free nerve endings in the middle of the field is much greater than at the periphery.

Thus, the lower part of Figure 46-7 shows three views of the cross section of the nerve bundle leading from the skin area. To the left is the effect of a weak

stimulus stimulus stimulus

Figure 46-7

stimulus stimulus stimulus

Figure 46-7

Pattern of stimulation of pain fibers in a nerve leading from an area of skin pricked by a pin. This is an example of spatial summation.

stimulus, with only a single nerve fiber in the middle of the bundle stimulated strongly (represented by the red-colored fiber), whereas several adjacent fibers are stimulated weakly (half-red fibers). The other two views of the nerve cross section show the effect of a moderate stimulus and a strong stimulus, with progressively more fibers being stimulated. Thus, the stronger signals spread to more and more fibers. This is the phenomenon of spatial summation.

Temporal Summation. A second means for transmitting signals of increasing strength is by increasing the frequency of nerve impulses in each fiber, which is called temporal summation. Figure 46-8 demonstrates this, showing in the upper part a changing strength of signal and in the lower part the actual impulses transmitted by the nerve fiber.

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