تسجيل الدخول

تسجيل

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

المؤلف:  John E. Hall, PhD

المصدر:  Guyton and Hall Textbook of Medical Physiology

الجزء والصفحة:  13th Edition , p600

2026-06-07

14

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 trans mitted 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 1 shows the phenome non 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 fibers 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.

Fig1.  Pattern of stimulation of pain fibers in a nerve leading  from an area of skin pricked by a pin. This pattern of stimulation is  an example of spatial summation. 

Temporal Summation. A second means for transmit ting signals of increasing strength is by increasing the frequency of nerve impulses in each fiber, which is called temporal summation. Figure 2 demonstrates this phenomenon, showing in the upper part a changing strength of signal and in the lower part the actual impulses transmitted by the nerve fiber.

Fig2. Translation of signal strength into a frequency modulated series of nerve impulses, showing the strength of signal  (above) and the separate nerve impulses (below). This illustration is  an example of temporal summation.