Table 1. Features of graded potentials and action potentials. Note: The details of action potentials noted here refer to those of neuronal action potentials. As we will see throughout our study of physiology, other action potentials for example, in skeletal, cardiac, and smooth myocytes, and in some endocrine cells exhibit different features than those mentioned here.
Physiology Web at www. Test Questions. Daily Quiz. Physiology Tutor. Depending on the stimulus, graded potentials can be depolarizing or hyperpolarizing. Action potentials always lead to depolarization of membrane and reversal of the membrane potential. And let me just move it over here.
And let's say that this synaptic potential, or post-synaptic potential, is a depolarization. Let me say, right at this piece of membrane, we get about this size of a depolarization. As the depolarization spreads across the membrane, it's going to decay in size. So let's say, maybe, we check in with it here, at this piece of the membrane. Now it's a smaller size than it was when it started over here. And as it continues spreading across the membrane, maybe if we check in with it over here, it's now actually quite small.
So that by the time it gets to the trigger zone, where the decisions are made to fire an action potential or not, the depolarization that started way over here may not have much of an effect on the membrane at the trigger zone.
Similar to the concept of temporal summation is the concept of spatial summation-- that if two graded potentials happen far enough away from each other, they may have no effect on each other.
For example, let's say that there's another excitatory input way down here at this dendrite, that causes a depolarization. Just like this depolarization, as this spreads across the membrane, it's going to decay, so that it'll get smaller with distance. So that maybe by the time these two reach the trigger zone, they've decayed entirely so that they have no effect on each other.
But if, instead, you had two kinds of excitatory input very close to each other on the membrane, then those two depolarizations could have spatial summation.
They can add together in space. So that you could get a depolarization twice the size. The same would be true for hyperpolarizations. You can have temporal and spatial summation of hyperpolarizations, to get hyperpolarizations that are larger in size. So what would happen if you had an excitatory input and an inhibitory input at the same time and place?
Well, instead of getting both a depolarization and a hyperpolarization, what you may get is no change to the membrane potential. They may cancel each other out and leave the membrane potential at the resting potential. Now one effect of the fact that graded membrane potential changes decay with distance is that the closer an input is to the trigger zone, the greater effect it will have on the likelihood of an action potential being fired down the axon.
Because if a graded potential starts closer to the trigger zone, it will decay less by the time it gets there than a graded potential that starts farther away and decays more with greater distance. Therefore a synapse that's closer to the trigger zone will have a greater influence on the behavior of the neuron in terms of action potentials being fired, than the synapse that's farther away.
The representation on the right shows electrical movement away from rest. This movement is called hyperpolarization and we see that hyperpolarization moves farther from threshold rather than towards it. Now for some application. These changes in the resting potential come in two forms; as graded potentials or action potentials. Graded potentials always precede action potentials, so we'll address them first.
With graded potentials, the magnitude of the response is proportional to the strength of the stimulus. Hence, a strong stimulus might result in a 10mV change in the membrane potentials, while a weaker stimulus may produce only a 5mV change. A hyperpolarising graded potential is known as an inhibitory postsynaptic potential IPSP because the movement of ions that occur here prevent depolarisation which is characteristic of an action potential.
If graded potentials reaching the axon hillock depolarise the membrane to the threshold voltage or above, an action potential is initiated. A graded potential which is above the threshold voltage is known as suprathreshold graded potential and this generates an action potential.
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