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

Overview of neuron function

This video introduces the function and functional types of neurons.  By Matt Jensen. Created by Matthew Barry Jensen.

Want to join the conversation?

  • leaf red style avatar for user Jack McClelland
    Wait...so action potentials happen faster with axons of a larger diameter? Is that it?
    (5 votes)
    Default Khan Academy avatar avatar for user
    • leaf green style avatar for user Brooke
      Resistance within an axon is inversely proportional to the axon's diameter. Because the resistance is lower, the larger the axon's diameter, the faster the action potential can be transmitted.

      A good way to think about it is to imagine water flowing through a pipe with a small diameter as compared to one with a large diameter. The water that is touching the side of the pipe experiences resistance (from friction) that slows it down. The water that isn't touching the sides of the pipe (so, the water in the middle) is not in contact with the pipe surface and so it does not experience that resistance from friction. Because it doesn't experience this resistance, it can travel at a faster speed.

      In a large pipe, while the water on the edges experiences the resistance, there is a lot of water in the middle that can travel at this faster pace. Contrast that with a small pipe, which has less water in the middle, and so it has less water that can travel at a faster rate. The same is true in large vs. small diameter axons, except that it is the cell membrane that is the cause of the resistance that can impede the movement of a charge down the axon.
      (36 votes)
  • piceratops sapling style avatar for user sarah.h0089
    Is the "trigger zone" being discussed at the same as the axon hillock? If so, are those two words interchangeable? If "trigger zone" and "axon hillock" are not interchangeable, could you please explain the difference? Thanks!
    (13 votes)
    Default Khan Academy avatar avatar for user
    • male robot hal style avatar for user Jared Trout
      The trigger zone is an area near the axon hillock that causes voltage gated channels within the cell to open and initiate the propagation of the cell's action potential.

      The axon hillock specifically refers to an area within the cell where the axon originates. It is cone shaped and is marked by a high density of intracellular microfilaments and a lack of Nissl bodies.
      (13 votes)
  • winston default style avatar for user David
    Are one of the physical stimuli what we see? Say we are told to press a button as soon as we see a dot appear... Do neurons, in this case, carry our signal?
    (3 votes)
    Default Khan Academy avatar avatar for user
  • old spice man green style avatar for user Uncle Bart
    I believe that this expression is somewhat problematic: " It is more negative inside the cell membrane whereas it is more positive outside the cell membrane". I believe that you should emphasize the fact that both outside and inside of the cell are positive(since both potassium and sodium are known as positive (+) ions). The difference is; inside the cell is LESS positive compared to outside of the cell.
    (3 votes)
    Default Khan Academy avatar avatar for user
  • duskpin ultimate style avatar for user Devesh Murali
    Is there Interneurons in the Peripheral Nervous System? If yes, why wasn't it added in the video?
    (4 votes)
    Default Khan Academy avatar avatar for user
  • aqualine ultimate style avatar for user varunananth1
    Does information have to be simultaneously inputted into the axon as graded potential to create an instant release at the trigger zone, or can graded potential be stored, and once past a certain limit, is released? (What I am asking is that if graded potential must be inputted at the instant before release, or can it be charged up?)
    (1 vote)
    Default Khan Academy avatar avatar for user
    • piceratops ultimate style avatar for user ILoveToLearn
      Well, electrostatic spread causes the signals to dissipate after a certain distance. That's why there are 2 different types of signal summation at the axon hillock: temporal and spatial. For simplicity here I'm going to describe EPSPs and ignore the IPSPs until the end. (EPSP and IPSP stand for excitatory and inhibitory postsynaptic potential.)
      Temporal summation occurs when the same excitatory fiber pulsates signals repeatedly to the same neuron. Each graded potential it produces is too small to break the threshold value, but when the charges are added together at the axon hillock, the threshold is broken and an AP is generated.
      Spatial summation: Two or more excitatory nerve fibers fire EPSPs, that, through synergistic interactions, sum and break threshold. An AP is generated and fired down the axon. (AP = action potential.)
      Anyway, I said I would mention IPSPs at the end. IPSPs (inhibitory postsynaptic potentials) can hyperpolarize the cell (make it more negative) which makes it much more difficult to depolarize the cell. Remember that depolarization is a rapid burst of positive charge. With inhibition, the threshold value for an AP to be fired remains the same; what changes is the "inner negativity" of the cell. It becomes much more negative than its extracellular environment. Because it's so much more negative than its environment, it would take a much larger (and likely huge) influx of positive ions to get the neuron past its threshold value.
      I hope all this made sense to you. If you would like further clarification, please ask! I love neurology, and I can think of few things more enjoyable than answering questions about the nervous system!
      Great questions! I realize this was a long-winded response that kind of rabbit-trailed from your direct question. So in short, no, charge is not stored up because the charges diffuse away into nothingness after a certain distance. You might enjoy Khan Academy's videos about nerve conduction and electrochemistry. I've attempted to describe the overview in short, sensible form here, but I understand it's a rather long discussion that I left here.
      Anyway, cheers!
      (7 votes)
  • leaf blue style avatar for user Gage J.
    So, if action potentials are always the same size and duration, will neurotransmitter release always be the same in quantity/magnitude?
    (3 votes)
    Default Khan Academy avatar avatar for user
    • male robot hal style avatar for user mortis
      Neurotransmitters are usually stored inside vesicles in terminal end of neurons. When action potential reaches terminal end, some of those vesicles are fused to cell membrane and transmitter of those vesicles are released to synaps gap. If there is a lot of action potentials without enough recovery time for neuron, terminal end can run out of transmitter-vesciles. That means that action potentials don't make terminal end release anymore neurotransmitters.
      In normal case, if neurons have time for recovery, the amount of neutrotransmitters released are functionally constant. That means, there is no functional chance between action potentials. Of course the action which neurotransmitters cause depend on many other things too. For example one neuron takes input for many other neurons and then calculates should it or should it not release action potential - one neuron doesn't command other neuron by itself.
      Also the action potentials of same neuron are not always same size and duration. Elctrical enviroment of a neuron makes an effect on neuron's function.
      (1 vote)
  • duskpin ultimate style avatar for user burzynska.adrianna
    "Input information usually comes in through the dendrites. Although less often, it'll come in through the soma or the axon." In what cases could the input skip dendrites and go straight into an axon or soma?
    (2 votes)
    Default Khan Academy avatar avatar for user
  • leaf green style avatar for user Rx.niteeshsinha
    How summation of signals occurs in a Neuron?
    (1 vote)
    Default Khan Academy avatar avatar for user
    • piceratops sapling style avatar for user Katherine Terhune
      At the dendrites, a neuron will receive signals from many different axons. Some of these are excitatory (makes the membrane potential more positive) and some of them inhibitory (makes the membrane potential more negative). These signals will travel through the dendrites and the soma to the axon hillock, where they add up to one membrane potential.
      (2 votes)
  • duskpin ultimate style avatar for user Piper76828
    Where do the neurotransmitters come from, the target cell or the terminal?
    (1 vote)
    Default Khan Academy avatar avatar for user

Video transcript

In this video, I want to provide an overview of neuron function, which I think of sort of like how a gun functions. And we'll go into a lot more detail on how a neuron functions in later videos. But in this video. I just want to give a bird's eye overview of it. The function of neurons is to process and transmit information. Without input, most neurons have a stable electrical charge difference across their cell membrane, where it's more negative inside the cell membrane and more positive outside the cell membrane. And we call this the resting membrane potential or just resting potential for short. And this resting potential is really how the neuron is going to be able to be excitable and respond to input. And I think of this as similar to loading a gun by putting a bullet in it. Neurons receive excitatory or inhibitory input from other cells or from physical stimuli like odorant molecules in the nose. Input information usually comes in through the dendrites. Although less often, it'll come in through the soma or the axon. The information from the inputs is transmitted through dendrites or the soma to the axon with membrane potential changes called graded potentials. These graded potentials are changes to the membrane potential away from the resting potential, which are small in size and brief in duration, and which travel fairly short distances. The size and the duration of a graded potential is proportional to the size and the duration of the input. Summation, or an adding together of all the excitatory and inhibitory graded potentials at any moment in time occurs at the trigger zone, the axon initial segment right here. This summation of graded potentials is the way neurons process information from their inputs. If the membrane potential at the trigger zone crosses a value called the threshold potential, information will then be fired down the axon. So I like to think of this process of summation of the excitatory and inhibitory graded potentials at the trigger zone as analogous to the trigger of a gun. In fact, that's why it's called the trigger zone. I think of the graded potentials as being like the finger on the gun, that may be squeezing a little harder or relaxing. But once the trigger of the gun is pulled back past a certain threshold distance, a bullet will be fired down the barrel of the gun, just like if the membrane potential of the trigger zone crosses a threshold value, information will be fired down the axon. The way information is fired down the axon is with a different kind of change to the membrane potential called an action potential. An action potential is usually large in size and brief in duration. But it's usually conducted the entire length of the axon, no matter how long it is, so that it can travel a very long distance, just like a bullet usually has no trouble making it down the barrel of the gun. And like a bullet traveling through the barrel of a gun, action potentials tend to travel very quickly down the length of the axon. Action potentials are different than graded potentials because they're usually the same size and duration for any particular neuron, as opposed to the graded potentials, whose size and duration depends on the size and the duration of the inputs. Action potentials are conducted faster along larger axons, axons with a larger diameter, and along axons that have a myelin sheath, that I've drawn in yellow here. When an action potential reaches the axon terminals at the end of the axon, information will then cross, usually a small gap, to the target cell of the neuron. And the way this happens for most synapses where an axon terminal makes contact with the target cell is by release of molecules called neurotransmitters that bind to receptors on the target cell and which may change its behavior. Neurotransmitter is then removed from the synapse. So it's reset to transmit more information. And I think of this part as similar to the bullet leaving the gun, to hit the target. The input information that was converted into the size and the duration of graded potentials is then converted into the temporal pattern of firing of action potentials down the axon. And this information is then converted to the amount and the temporal pattern of neurotransmitter release at the synapse. These steps are how neurons transmit information, often over long distances. This is the general way that neurons usually function. But there are multiple functional types of neurons. So let's take a look at some of those. Here I've drawn a few different neurons, with their somas in red, their axons in green, and their dendrites in blue. And I've drawn a line here to separate between the central nervous system on this side-- so I'll just write CNS for short-- and the peripheral nervous system on this side-- so I'll just write PNS for short. And there's some different ways we can categorize functional types of neurons. The first way is the direction of information flow between the CNS and the PNS. If a neuron like this pseudounipolar neuron right here brings information from the periphery in toward the central nervous system, we call that an afferent neuron. Afferent, meaning it's bringing information into the central nervous system. We can also call this type of neuron a sensory neuron because the information it's bringing into the central nervous system involves information about a stimulus. And a stimulus is anything that can be sensed in the internal or external environment, which is to say anything inside the body or anything outside the body. These neurons are carrying information away from the central nervous system out into the periphery. So instead of calling them afferent neurons, we call them efferent neurons. And there are two main kinds of efferent neurons. The first we call motor neurons. Motor, which means movement. These are efferent neurons that control skeletal muscle, the main type of muscle that's attached to our skeleton, that moves us around. These motor neurons are also called somatomotor neurons or neurons of the somatic nervous system. The other type of the efferent neurons are called autonomic neurons. And these neurons control smooth muscle, like the muscle around our blood vessels; cardiac muscle, the muscle of our heart; and gland cells, the cells of our glands that secrete hormones into the bloodstream. These autonomic neurons are also called visceromotor neurons or neurons of the autonomic nervous system. Most neurons of the central nervous system aren't any of these types of neurons, however. They're like this neuron, in that they connect other neurons together. So these are called interneurons, neurons between neurons. And there are many interneurons in the central nervous system, forming very complex pathways for information to travel. So that while an individual neuron is processing and transmitting information, these complex networks of neurons in the central nervous system are doing even more complex processing and transmitting of information.