Is action potential a signal transduction?

[icakeov]

This might be either an obvious one, or I might be totally off track.
I haven't seen, or been able to find neural action potentials being referenced as signal transductions.

Aren't they signal transductions? And if not (say if action potential is only referenced to the change in potential), is there at least a name to the synaptic signal transduction process that happen to neurotransmitters that creates action potentials?

Hope this is clear enough of a question.

 

[BillTre]

Transduction in general involves the changing of one kind of a signal (or change in a condition) into another.

In biology, transduction has several different uses.
In the nervous system it is usually meant to refer to some kind of change in condition (or a signal) into a neural response.
For example:

• light to a change in membrane potential of a photoreceptor
• a chemical interacting with a receptor to a change in membrane potential of a taste or olfactory (smell) receptor
• the sound or pressure induced movement of hair cells into changes in their membrane potentials
• external electrical field changes into changes in the membrane potential of specialized hair cells
• changes in pressure or temperature into changes in membrane potential of skin "touch" receptors
• The changes in tension in muscle or tendons into membrane potentials of neurons that sense these functions
• etc.

There are other meanings also, some associated with changing whatever might be activating a receptor into some molecular change inside a cell (like activating a second messenger system that can then have propagate a signal to have other molecular effects. Others having to do with putting DNA into cells to change their genetics. Could be others.

The way I think of an action potential is that it is (usually) the result of a neural cell integrating a variety of inputs it is getting (through changes in membrane potential). An action potential occurs when voltage sensitive channel proteins open because their opening threshold is exceeded, current flows into the cell and depolarizes neighboring areas of membrane that also contain voltage sensitive channel proteins which than open. This propagates the action potential further along the neuron (usually but not always down an axon). An action potential is usually triggered at a particular point in a neuron (classically the axon hillock) when the membrane potentials in the cell add up to a large enough membrane potential to exceed the activation threshold for the neuron. I am using Wikipedia here as a reference, but they are kind of thin on the diversity of neurons and action potentials that can actually be found in different nervous systems. There is an immense variety.

With respect to synaptic transmission, the synaptic receptors for the neural transmitters released at the synapse, bind the receptors on the post-synaptic (downstream) side of the synapse. The binding event is transduced into a neural change in membrane potential either directly through the opening of an ion channel or indirectly through a second messenger system. These results of individual receptor binding events usually produce small changes, but can be large in number. If there are enough of them in a short enough time, they can add up to exceed the activation threshold for the voltage sensitive ion channels that underlie the action potential.

Since these are changes only in membrane potential (going from one kind of membrane potential to another), I would not consider them transductions (not chnaging from something else into a neural signal), just processing or integrations, but some might disagree on that.
Here, Wikipedia uses transduction in the way you did in your question. I would argue that it is lumping too many individual steps together (transduction (into a change in membrane potential) and summation of membrane potential changes (to fire off an action potential)) and is therefore using the word incorrectly. I would say they can be the result of signal transduction, but are not the same thing.

Whatever way you use the word, it is always better to understand the underlying meaning and to be clear how you are using your words when talking with others about things in order to communicate clearly.

 

[icakeov]

Thank for such a detailed break down BillTre!

The main reason why this confused me is because of the signal transduction definition itself: "process by which a chemical or physical signal is transmitted through a cell as a series of molecular events” (at least on Wikipedia) which seems really broad and it only seemed logical to conclude that action potential would be part of that process in some way or another.

Perhaps the Wiki definition could be more precise?

Or as you pointed out, action potentials are only membrane potential changes, the membrane is not "a cell". But then again, the interaction between a ligand and a membrane receptor happens only on the membrane, yet it is still part of the signal transduction machinery. And after all, the neurotransmitters do go in and out of the cell, so the effect is produced by a "signal being transmitted through a cell".

Or perhaps because it is an electric build-up, it is neither “chemical" nor “physical”? But then again, every "molecular event" or a molecule confirmation change carries an "electrical potential change" of sorts I would imagine, so this is just another “electrical change” which just happens to involve a whole bunch of molecules building up to it.

Maybe it is more fair to say that an action potential is a part of a signal transduction as an energy change, but cannot be associated with the whole process?

Thanks again.

 

[Ygggdrasil]

It's not clear to me that the term signal transduction has a strict definition in biology. When biologists talk about signal transduction cascades, they are most often referring to the intracellular signaling pathways that are initiated by binding of a ligand to a receptor at the plasma membrane and cause various changes to cellular physiology (changes to gene expression, post-translational modification of proteins, etc). Most often, the term crops up among cell biologists and especially in cancer biology, where disruptions to these signal transduction cascades are thought to be important drivers of cancer (e.g. the PI3K-AKT pathway, the WNT signaling pathway, EGFR signaling, etc.).

Because neurobiology is such a specialized branch of biology, there is not as much crosstalk between general cell biologists and neurobiologists, so the signaling that occurs at neurological synapses probably doesn't get folded into the broader discussion of signal transduction cascades even though it probably fits under that term. There definitely are similarities (activation of receptors, generation/release of second messengers like calcium, intraceullular propagation of signals), though some of the processes occur very differently (as you note, though electrochemical means rather than protein-protein interactions and post-translational modification of proteins).

I'll echo BillTre's comment and say that you probably shouldn't worry too much about the exact usage of the terms. Unlike math or physics where certain terms have very precise meanings, many terms in biology refer to broad concepts and our classification of different phenomena under the umbrellas of these concepts is often artificial and arbitrary.

 

[icakeov]

Very helpful and clarified, thank you!