If you're seeing this message, it means we're having trouble loading external resources on our website.

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content
Current time:0:00Total duration:9:48

Neuron graded potential description

Video transcript

in this video I want to describe the graded membrane potential changes that occur in neurons in response to input which we just call greeted potentials for short so I've drawn a blown-up neuron here we have a soma in red and an axon and green and two dendrites in blue and recall that resting neurons that is neurons that aren't receiving any input usually have a stable charge separation across the entire membrane where there are a layer of positively charged ions also called cations on the outside of the neuron membrane and a layer of negatively charged ions also called anions on the inside of the membrane and that we call the outside zero just to set it as a reference and that the resting membrane potential of neurons may vary but it's often around negative 60 millivolts so let me show that on a graph here let's say we're looking at this piece of membrane and on the x-axis we'll put time and on the y-axis will put the membrane potential in millivolts and so let me put right in the middle here this negative 60 millivolts it's a common neuron resting potential and that when the neuron is at rest without inputs most neurons just have a stable potential at their resting membrane potential where it's not changing over time without input now inputs from certain types of stimuli may increase or decrease the membrane potential of the neuron a small amount for a brief time before it returns back to the resting potential these transient membrane potential changes are called graded potentials and they tend to occur in the dendrites of the neuron and in the soma of the neuron and the size and the duration of the graded potentials is determined by the size and the duration of inputs both excitatory inputs and inhibitory inputs graded potentials do not pass into the axons of most types of neurons instead most axons have a different membrane potential change called an action potential action potential start at the area called the trigger zone which is the initial segment or the start of the axon and they start when the CIM find effect of the graded potentials at any moment in time brings the membrane of the trigger zone across a certain value called the threshold potential so let me just draw that with a little dashed line here and this threshold potential will vary between neurons but some are around negative 50 millivolts would be a common threshold potential so that if the membrane potential at the trigger zone can be moved from the resting potential which is often around negative 60 millivolts over the threshold potential which is often around negative 50 millivolts then a totally different potential change will happen called the action potential that will shoot all the way down the axon now this adding together of graded potentials is called summation and summation at the trigger zone is how neurons process information from their inputs most neurons respond to inputs from other neurons in the form of neurotransmitter molecules that are released at synapses so that if this is the axon terminal of another neuron it may release neurotransmitter at the synapse where these two neurons come together which will bind to little receptors on the membrane of this neuron in this case here on a dendrite and this will produce some kind of graded potential now we'll get into the details of this more in other videos but this is the most common type of input that a neuron will receive and depending on the neurotransmitter and depending on the receptor this may be an excitatory input or may be an inhibitory input now some other types of neurons and neurons like cells that are sensory receptors may also generate graded potentials from physical stimuli such as light or odorant molecules graded potentials produced from a synapse are called synaptic or postsynaptic potentials those generated by stimuli and sensory receptors are also called receptor potentials a graded potential like this one that moves the membrane potential to a less negative number or closer to zero is called a depolarization the polarization because now the membrane is less polarized it has less charge separation these are also called excitatory potentials because they move the membrane potential closer to the threshold so they increase the likelihood that an action potential will be started at the trigger zone a graded potential like this one that moves the membrane potential to a more negative number farther away from zero is called a hyperpolarization hyper-polarization because it's increasing the polarization or the charge separation of the membrane hyper polarizations are also called inhibitory potentials because by moving the membrane potential farther from the threshold they're decreasing the likelihood that an action potential will be started at the trigger zone two important properties of graded potentials are that they decay with both time and distance so that their effect is brief and local graded potentials decay with time just like I've drawn here the membrane potential changes for a brief time and then it returns to the resting potential unless there is more input and because graded potentials decay with time if two graded potentials happen that are separated by enough time they won't have any effect on each other for example let's say that this depolarization happens and is finished before a second depolarization over here occurs since this one was already done already fully decayed these two had no effect on each other but if two depolarizations happened right around the same time their effects can add together you have additive effects it can get a depolarization twice the size we call this process temporal summation or adding together of graded potentials in time graded potentials also decay with distance as well as with time so let's look at this depolarization and let me just move it over here let's say that this synaptic potential or postsynaptic 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 and at this piece of the membrane now it's it's a smaller size than it was when it started over here and as it continued 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'll be get smaller with distance so that maybe by the time these two reach the trigger zone 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 could add together in space so that you could get up depolarization twice the size the same would be true for hyper polarizations you can have temporal and spatial summation of hyper polarizations to get hyper polarizations that are larger in size so what would happen if you had an excitatory input and an inhibitory input at the same time in 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 in 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 of a graded potential starts closer to the trigger zone it will decay less by the time it gets there then 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 then the synapse that's farther away for example here way out at the end of a dendrite one last thing that I want to mention is that synaptic potentials like these tend to be quite small and and in fact I've drawn these too large because they're usually less than 1 millivolt in size therefore most neurons require the temporal and spatial summation of many synaptic potentials to move the 10 millivolts or so that usually separate a typical resting and a typical threshold potential for any particular neuron so that as all the different synapses that are connecting this neuron to lots of other neurons in its network are creating all these synaptic potentials the membrane potential of the dendrites and the soma is constantly moving around and wiggling around off the resting potential until there's enough excitatory potentials enough of these depolarizations that are being summed in space and time to cause an action potential to be fired down the axon so some very complex processing of information from all these inputs can occur because of these graded potentials