If nerve conduction is bidirectional, how does it work?

[scilearner]

When stimulated in the middle of axon, nerve conduction is bidirectioal. Now I know the andromic conduction stops at the first synapse. Is the reason for this, that there are no neurotransmitters in the body of a neuron, and more importantly no calcium channels there to activte them. Thanks

[SMF]

You are correct. Almost all neuron networks that transmit information over distance (not local interneurons) are polarized such that an action potential can only be initiated at one end (usually the cell body) and transmitted by way of a synapse at the other end. SM

[CharonY]

The reason why AP travel in a specific direction is due to the kinetics of the involved ion channels. Once they they have activated, they do not react for a given time (refractory period). This prevents the AP running into the opposite direction. However, if you stimulate the middle of the axon, for example, the channels in both directions can be opened. Hence, from the depolarized area the AP can move into both directions. Neurotransmitter are only relevant to eventually create AP in the postsynaptic cell.

[SMF]

CharonY, so, you don't think that a neurotransmitter would ever be relevant for creating an action potential in a presynaptic cell? SM

Edited February 18, 2011 by SMF

[CharonY]

What, precisely, do you think neurotransmitter are, and what is their role?

[SMF]

I know what a neurotransmitter is, and I know that they frequently initiate action potentials in both the cell that is presynaptic, and the cell that is postsynaptic, to a specified synapse. I don't believe-- "Neurotransmitter are only relevant to eventually create AP in the postsynaptic cell" as you said. What is important to Scilearner's question is that synapses cannot transmit information in both directions while axons can, and this polarizes the direction of travel in most normally functioning nervous tissues. SM

[CharonY]

Now this is rather non-canonical. I still wonder, how does presynaptic AP generation by their own NT work?

Also, do you mean that within a neuron the AP movement is normally bi-directional (e.g. if we disregard the synaptic cleft)?

[SMF]

CharonY. I can't respond to your specific questions because I am having a problem understanding what you are saying relative to the previous posts. In my experience this means that we are both misunderstanding each other, so I will restate what I know in hopes that this will correspond to what you are about:

 

Presynaptic neurons release a neurotransmitter onto postsynaptic neurons in a synapse. This results in transferring excitation, in the form of generating an action potential, from the pre- to postsynaptic neurons. This is a polarized event because a synapse usually only works in one direction (presynaptic neurotransmitter release and binding of the neurotransmitter to postsynaptic receptors). So, even though an axon is capable of transmitting an action potential in both directions the polarization of synapses in both upstream and downstream synapses insures that information, in the form of action potentials, is only transmitted in one direction.

 

There may be instances in which antidromic action potentials in axons are normally used by the nervous system. I say this because I have given up trying to outguess the evolutionary process, but generally antidromic conduction mostly only occurs in the lab. I have recorded both extracellular and intracellular action potentials from neuron cell bodies when I have stimulated their axon, electrically, to generate antidromic conduction. SM

[CharonY]

Ok, I had problems with

I know what a neurotransmitter is, and I know that they frequently initiate action potentials in both the cell that is presynaptic, and the cell that is postsynaptic, to a specified synapse.

as together with your previous post it implied that NT are also capable of creating AP in the presynaptic cell. This is normally not the case. The last post is accurate, however (an, to me, oddly different from what you posted earlier).

 

To summarize synaptic events are polarized, and also normal AP event along axons as they are generally generated at the soma and due to the ion channel kinetics it can only proceed in one direction. I.e. we have directionality within as well as between cells.

[SMF]

I think I was clear, but....

 

Just to be accurate, most action potentials are initiated in the axon hillock, not the cell body, and continue forward down the axon, and backward into the cell body. It gets even more complicated with dendrodendritic synapses. SM

[sound]

this is why science is beautiful

[SMF]

Sound, I agree. I have not been active in neurophysiological research for more than 15 years, but a quick search revealed that in addition to retrograde action potentials invading dendrites there are a variety of neural systems that have retrograde axonal action potentials normally, and neurotransmitter release in a synapse from a postsynaptic cell to affect the presynaptic cell has been observed (retrograde synaptic transmission). I am glad that I put in the qualifier regarding evolution above. Amazing! SM

Edited February 19, 2011 by SMF