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A graded potential in physiology, is described as local changes in membrane potential that occur in varying grades or degrees of magnitude or strength. When compared to graded potential, an action potential is described as brief, rapid, large (100mV) changes in membrane potential during which the potential actually reverses so that the inside of the excitable cell transiently becomes more positive than the outside. As with a graded potential, an action potential involves only a small portion of the total excitable cell.
Action potentials occur in several types of animal cells (excitable cells), which include neurons, muscle cells, and endocrine cells, as well as in some plant cells. In neurons, they play a central role in cell-to-cell communication. In other types of cells, their main function is to activate intracellular processes. Action potentials in neurons are also known as “nerve impulses” or “spikes”. A neuron that emits an action potential is often said to “fire”. Depending on the stimulus, graded potentials can be depolarizing or hyperpolarizing.
Action potentials always lead to depolarization of the membrane and reversal of the membrane potential.
Graded potentials amplitude is proportional to the strength of the stimulus. Amplitude is generally small (a few mV to tens of mV). The duration of graded potentials may be a few milliseconds to seconds. When compared to graded potentials, action potentials amplitude is all-or-none; strength of the stimulus is coded in the frequency of all-or-none action potentials generated (large amplitude). Nearly all cells from animals and plants function as batteries, in the sense that they maintain a voltage difference between the interior and the exterior of the cell, with the interior being the negative pole of the battery.
The voltage of a cell is usually measured in millivolts(mV), or thousandths of a volt. A typical voltage for animal cell is -70mV. Because cells are so small, voltages of this magnitude give rise to very strong electric forces within the cell. Action potential duration is relatively short; 3 to 5 milliseconds. An increase in permeability of sodium and potassium are responsible for the neuronal action potential. The ions involved are Na+ and K+ (for neuronal action potentials). In graded potentials, the ions involved are usually Na+, K+ or Cl-.
No refractory period is associated with graded potentials. When compared to graded potentials, absolute and relative refractory periods are important aspects of action potentials. Graded potentials can be summed over time (temporal summation) and across space (spatial summation). Graded potentials travel by passive spread (electronic spread) to neighboring membrane regions. Amplitude diminishes as graded potentials travel away from the initial site (decrement). Summation is not possible with action potentials (due to the all-or-none nature, and the presence of refractory periods).
Action potential propagation to neighboring membrane regions is characterized by regeneration of a new action potential at every point along the way. Amplitude does not diminish as action potentials propagate along neuronal projections (non-decremental). The time during which a subsequent action potential is impossible or difficult to fire is called the re Graded potentials are brought about by external stimuli (in sensory neurons) or by neurotransmitters released in synapses, where they cause graded potentials in the post-synaptic cell. Action potentials are triggered by membrane depolarization to threshold.
Graded potentials are responsible for the initial membrane depolarization to threshold. Action potentials occur in plasma membrane regions where voltage-gated Na+ and K+ channels are highly concentrated. In principle, graded potentials can occur in any region of the cell plasma membrane, however, in neurons, graded potentials occur in specialized regions of synaptic contract with other cells (post-synaptic plasma membrane in dendrites), or membrane regions involved in receiving sensory stimuli. Graded potentials occur in the membranes of many cell types, such as epithelial cells, gland cells, and a variety of sensory receptors.
They are often the trigger for specific cell functions. Similarly, a graded potential at the synaptic terminal is the trigger for the exocytosis of a neurotransmitter, such as ACh. The end plate supports the graded potentials, whereas the rest consists of excitable membrane. In animal cells, there are two primary types of action potentials, one type generated by voltage-gated sodium channels, the other by voltage-gated calcium channels. Sodium-based action potentials usually last for under one millisecond, whereas calcium-based action potentials may last for 100 milliseconds or longer.
Graded potentials are particularly important in neurons, where they are produced by synapses – a temporary rise or fall in membrane potential produced by activation of a synapse (postsynaptic potential). Neurotransmitters that act to open sodium channels cause the membrane potential to rise, while neurotransmitters that act on potassium channels cause it to fall. Because the membrane potential in a neuron must rise past the threshold value to produce an action potential, a rise in membrane potential is excitatory, while a fall is inhibitory.
In graded potential, a depolarization from the resting potential continues slowly to the threshold. The type of signal realized in graded potential is an input signal (short distance) and contiguous conduction signal in action potential which entails the spread of the action potential along every small area of membrane down the length of axon (long distance). In action potential, the length of time is constant. Compared to action potentials, if the stimulus is not constantly moving forward, the graded potential will die out. These are the comparisons of action potentials (all or none) to graded potentials.
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