Graded Potentials

Graded potentials are changes in membrane potential that are confined to a relatively small region of the plasma membrane and die out within 1 to 2 mm of their site of origin. They are usually produced by some specific change in the cell's environment acting on a specialized region of the membrane, and they are called "graded potentials" simply because the magnitude of the potential change can vary (is graded). We shall encounter a number of graded potentials, which are given various names related to the location of the potential or to the function it performs: receptor potential, synaptic potential, and pacemaker potential (Table 8-3).

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FIGURE 8-14

Depolarizing, repolarizing, hyperpolarizing, and overshoot changes in membrane potential.

Graded Potential Distance
Distance
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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

TABLE 8-3 A Miniglossary of Terms Describing the Membrane Potential

Potential = potential difference

The voltage difference between two points.

Membrane potential = transmembrane potential

The voltage difference between the inside and outside of a cell.

Equilibrium potential

The voltage difference across a membrane that produces a flux of a given ion species that is equal but opposite the flux due to the concentration gradient affecting that same ion species.

Resting membrane potential = resting potential

The steady transmembrane potential of a cell that is not producing an electric signal.

Graded potential

A potential change of variable amplitude and duration that is conducted decrementally; it has no threshold or refractory period.

Action potential

A brief all-or-none depolarization of the membrane, reversing polarity in neurons; it has a threshold and refractory period and is conducted without decrement.

Synaptic potential

A graded potential change produced in the postsynaptic neuron in response to release of a neurotransmitter by a presynaptic terminal; it may be depolarizing (an excitatory postsynaptic potential or EPSP) or hyperpolarizing (an inhibitory postsynaptic potential or IPSP).

Receptor potential

A graded potential produced at the peripheral endings of afferent neurons (or in separate receptor cells) in response to a stimulus.

Pacemaker potential

A spontaneously occurring graded potential change that occurs in certain specialized cells.

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.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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Responses

  • Martin Schulz
    What uses graded potentials?
    7 years ago
  • andi
    What is the role of graded potentials?
    6 years ago
  • Asmarina
    How are graded potential created?
    7 years ago
  • SEMRAWIT
    Are graded potentials and local potentials the same thing?
    7 years ago
  • rose baggins
    Are action potentials the same thing as graded potentials?
    6 years ago
  • ute
    Why in graded potential magnitude decreases with distance?
    6 years ago
  • lisa b
    Why are graded potentials called local currents?
    6 years ago
  • IONA
    Where are graded potential found in the body?
    6 years ago
  • costanzo de luca
    What does graded potential function in the body?
    6 years ago
  • michael
    How graded receptor potentials depolarizing ?
    6 years ago
  • John
    How is graded potential related to post synaptic potentials?
    6 years ago
  • May
    How are the graded potential and action potential related?
    6 years ago
  • rachel
    How are graded potentials and action potentials related?
    6 years ago
  • Pamela
    How do graded potentials move?
    6 years ago
  • Patryk Marshall
    Where do graded potential occur in the body?
    5 years ago
  • Marcel
    What are the functions of graded potential?
    4 years ago
  • Ethan
    Are action potentials and graded potentials the same ething?
    3 years ago
  • Joanna
    Where do graded potentials occur?
    11 months ago

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