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Action Potential terminology

  1. Mar 13, 2017 #1
    Hello,

    I noticed that in Wikipedia, action potential is not solely defined as a neural process, but rather as a process that happens between any cells that can release chemical signals and have target cells that can receive the signals.

    Is this true? Would we say that endocrine system creates a slow, wide-spread action potential throughout the body by releasing adrenaline?

    Similarly, could plants also have action potentials? Or does terminology change when we step out of the animal world?

    Additionally, does an action potential refer to solely one cell triggering the action potential, or can we say that an action potential could happen for example, in a specific domain of the nervous system, involving many neurons that are of same type and doing a similar function?

    Any thoughts appreciated!
     
  2. jcsd
  3. Mar 13, 2017 #2

    Ygggdrasil

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    Can you quote the specific passage you're referencing? In the first sentence of the English Wikipedia article states:
    "In physiology, an action potential is a short-lasting event in which the electrical membrane potential of a cell rapidly rises and falls, following a consistent trajectory. Action potentials occur in several types of animal cells, called excitable cells, which include neurons, muscle cells, and endocrine cells, as well as in some plant cells."​

    It does not suggest that action potentials can occur in any cells like those involved in typical endocrine signalling (action potentials from neurons can stimulate endocrine cells to release their hormone signals, but these hormones typically do not induce action potentials in their target cells).
     
  4. Mar 13, 2017 #3
    Yes, that was it, they reference the endocrine cells, also later, there is a whole paragraph with a title "Plant action potentials" that talks about "release of negative chloride ions".

    I realize that action potentials should not be confused with the actual release of hormones into the endocrine system or a similar processes in other living systems, as you pointed out @Ygggdrasil. Thanks for your response!
     
    Last edited: Mar 13, 2017
  5. Mar 13, 2017 #4

    BillTre

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    Like @Ygggdrasil said action potentials are rapid changes in membrane potential in a cell (depolarization followed by repolarization). They occur once the voltage has passed the threshold for the voltage channel to open. The repolarization results for the channel closing followed by events (pumping ions back out, sequestering ions, or movement of other ions) that restore the membrane potential. Another important feature of action potentials is that they are self propagating since the membrane change will evoke more membrane change in other areas that have not yet reached the threshold potential for the ion gates to open (thus spreading down an axon).

    Action potentials are distinguished from graded potentials which can be driven by a variety of different processes. They are not predominately driven by a voltage sensitive channel and do not have such a reproducible time course (rapid rise at a certain value followed by a fall to resting potential).

    The most frequently discussed action potentials (in neurons) result from Na+ ions entering through voltage gated Na+ channels.

    Action potentials can also work with Ca++ ions entering through voltage gated Ca++ channels. There are voltage sensitive Ca++ channels at (at least some) neuronal terminals (where vesicle release would occur) and muscles. Ca++ is important in muscle contraction and release of synaptic vesicles.
    For example:
    The membrane change of a Na+ action potential enters an axon terminal and triggers the voltage sensitive Ca++ channels there.
    This lets in a rapid burst of Ca++ which may trigger vesicle release. An action potential will help speed the process by getting more Ca++ in there more quickly.
    Ca++ action potentials are generally slower than Na+ action potentials.
    (Just because Nature wants to confuse you, there are many other ways Ca++ can be released inside cells to achieve similar ends.)

    It is also feasible that Cl- ions could be the basis of action potentials in plants (again not something I really know). Cl- is a common negative ion in cells.

    Action potentials may occur in endocrine cells to trigger their release of hormone. I don't know for sure, but it is feasible. However, the action potential would not be going directly from one cell to another.
    It might work like this:
    An action potential in one cell evokes transmitter release (via stimulated Ca++ increases in the cell).
    The released transmitter molecules stimulate receptors that result in ion channels opening in the downstream (post-synaptic) cell (making graded potentials).
    When the graded potentials in the second cell exceed a threshold, a voltage sensitive Ca++ channel opens and lets a lot of Ca++ in which then evokes release of hormone.
    Two separate action potentials in this case.

    Added Complexity: electrical synapses are known in the nervous system which can connect the insides of neighboring cells, allowing ions to flow between them This links their membrane potentials to some extent and could potentially allow an action potential to go from one cell to another directly. This is not the usual case and probably NOT what Wikipedia was referring to.
     
  6. Mar 13, 2017 #5
    Thanks for this response @BillTre! That really helps.

    Basically, an action potential can trigger an endocrine cell to release a hormone, but the actual endocrine molecules that get released into an organism's system and interact with other cells should never be confused with an action potential. I thin that is where my main confusion about this concept was lying.

    Also, action potential itself does refer usually to just one cell membrane's change in potential, or would it be ok to say that a batch of cells' membranes getting triggered with the same "stimulus" could be called an action potential? I think what I am asking is: can a "spike train" be called an "action potential" or is it a mistake to do this with many cells.

    Many thanks again!
     
  7. Mar 13, 2017 #6

    BillTre

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    Action potentials are also called spikes. A spike train is a series of action potentials occurring in a short period of time, in a single cell.
    Many action potentials in different cells are just a bunch of action potentials in different cells.
     
  8. Mar 13, 2017 #7
    Thanks @BillTre! That really helps!
     
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