Graded Potential vs Action Potential
All the body cells show membrane potential, largely due to the uneven distribution of sodium, chloride, and potassium ions and also due to the permeability difference of the plasma membrane to these ions. This membrane potential results in positive and negative charges across the membrane. The neurons and muscle cells are two types of special cells that have developed a special use for the membrane potential. They can undergo transient, rapid fluctuations in their membrane potentials due to stimuli. These changes finally result in electrical signals. Neurons use these signals to receive, process, initiate, and transmit messages while the muscle cells use them to initiate contractions. There are two basic forms of electric signals, which the neurons use to transmit the messages, namely, graded potential and action potential.
Graded Potentials
Graded potential is a small transient change in the membrane potential that occurs in varying grades or degrees of magnitude or strength. The graded potentials are caused by the activation of a class of channel proteins called ‘gated ion channels’ and can be generated either in sensory or motor nerves and begin the process of transmittance. The gated ion channel selectively allows only certain ions to diffuse through it. When it allows diffusing, it is open, and when it does not allow, it is closed. Therefore, the gated ion channel behaves like a door that can be opened or closed.
The amount of responding ion channels varies depending on the strength of the stimulus; thus a strong stimulus causes more ion channels to open. If more ion channels open, more ions will diffuse across the plasma membrane, causing a larger change in the membrane potential.
Action Potentials
Action potentials are brief, rapid, large changes in the membrane potential and are produced in excitable cells (nerve and muscle) when the resting potential is altered. A single action potential involves only a small portion of a total excitable cell membrane and propagates throughout the remainder of the cell membrane without any reduction in the strength of the signal.
During an action potential, the membrane potential transiently reverses. When depolarization reaches the threshold potential, it will result in an action potential. The action potential is caused by a class of ion channels called voltage –gated ion channels. These ion channels are found in both the neurons and muscle cells. In neurons, two different voltage ion channels are used to create an action potential, namely, voltage-gated Na+ channels and voltage-gated K+ channels. These channels open and close in response to changes in the membrane potential, and they control the flow of the ions by selectively allowing them to move across them.
What is the difference between Graded Potential and Action Potential?
• Action potentials serve as long-distance signals whereas graded potentials serve as short-distance signals.
• The graded potentials are small changes in the membrane potential that can reinforce or negate each other. In contrast, the action potentials are large (100 mV) changes in the membrane potential that can serve as faithful long- distance signals.
• Activation of the gated ion channels causes the graded potential whereas the activation of the voltage-gated ion channels causes the action potential.
• Net movement of Na+, Cl–, or Ca2+ across the plasma membrane produces a graded potential. Sequential movement of Na+ into and K+ out of the cell across voltage-gated channels produces an action potential.
• The duration of the graded potential varies with the duration of the triggering event or the stimulus while the duration of the action potential is constant.
• The action potential occurs in the regions of the membrane with an abundance of the voltage-gated channels while the graded potential occurs in the regions of the membrane designed to respond to the triggering event.
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