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Intracellular fluid

FIGURE 8-15

The membrane potential of a cell can be depolarized by using a stimulating current generator, and the potential can be recorded by a pair of electrodes, one inside the cell and the other in the extracellular fluid, as in Figure 8-7.

(a) Membrane potential is closer to the resting potential with increasing distance from the depolarization site.

(b) Local current surrounding the depolarized region produces depolarization of adjacent regions.

Whenever a graded potential occurs, charge flows between the place of origin of the potential and adjacent regions of the plasma membrane, which are still at the resting potential. In Figure 8-15a, a small region of a membrane has been depolarized by a stimulus and therefore has a potential less negative than adjacent areas. Inside the cell (Figure 8-15b), positive charge (positive ions) will flow through the intracellular fluid away from the depolarized region and toward the more negative, resting regions of the membrane. Simultaneously, outside the cell, positive charge will flow from the more positive region of the resting membrane toward the less positive region just created by the depolarization. The greater the potential change, the greater the currents. By convention, the direction in which positive ions move is designated the direction of the current, although negatively charged ions simultaneously move in the opposite direction. In fact, the local current is carried by ions such as K+, Na+, CP, and HCO-T.

Note that this local current moves positive charges toward the depolarization site along the outside of the membrane and away from the depolarization site along the inside of the membrane. Thus it produces a decrease in the amount of charge separation (depolarization) in the membrane sites adjacent to the originally depolarized region.

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

Neural Control Mechanisms CHAPTER EIGHT

Neural Control Mechanisms CHAPTER EIGHT

Depending upon the initiating event, graded potentials can occur in either a depolarizing or hyperpo-larizing direction (Figure 8-16a), and their magnitude is related to the magnitude of the initiating event (Figure 8-16b). Moreover, local current flows much like water flows through a leaky hose. Charge is lost across the membrane because the membrane is permeable to ions, just as water is lost from the leaky hose. The result is that the magnitude of the current decreases with the distance from the initial site of the potential change, just as water flow decreases the farther along the leaky hose you are from the faucet (Figure 8-17). In fact, plasma membranes are so leaky to ions that local currents die out almost completely within a few millimeters of their point of origin. There is another way of saying the same thing: Local current is decremental; that is, its amplitude decreases with increasing distance from the site of origin of the potential. The resulting change in membrane potential from resting level therefore also decreases with the distance from the potential's site of origin (Figures 8-15a and 8-16c).

Because the electric signal decreases with distance, graded potentials (and the local current they generate) can function as signals only over very short distances (a few millimeters). Nevertheless, graded potentials are the only means of communication used by some neurons and, as we shall see, play very important roles in the initiation and integration of the long-distance signals by neurons and some other cells.