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Medical Chemical and Electrical transmission

  1. Nov 3, 2005 #1
    Signal propagation along motor nerves or fibres of muscles is electrochemical (or electrical)l, whereas transmission at the neuromuscular junction is chemical, Why is that?
     
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  3. Nov 3, 2005 #2

    somasimple

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    The subject is polemical. The transmission is electrical since it involves charged particles = ions but there is no wires only tubes and ions cross the membrane in both directions (that does not happen in an electrical circuit).


    BTW, gap junctions are electrical synapes (allowing ions flow between cells) and highly used in brain > 70%

    Chemical is used to code the message, ions to send it.
     
  4. Nov 3, 2005 #3
    I know the question asks why the nerve transmission is electrical, but I don't think it wants 'because its due to charged particles'. It's asking for adv. and disadv between the two.

    I've had discussions with others - but an electrical homeostatic system can work but we just haven't evolved one.
     
  5. Nov 3, 2005 #4

    somasimple

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    Transmitting a message along axon that have a meter length is a huge affair.
    Trying to make it with chemical needs a carrier and you haven't one.
    Using charged particles is like using a modem. You code your message with a good noise immunity and re-code it at the end with chemical.
    you get an analogical system with many functions and chemicals. It is more powerful.
     
  6. Nov 3, 2005 #5

    somasimple

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    We have an electrotonic one but an electrical one is a non sense. Trying to create such electrical circuit in a body that contains such electrolytes is the best way to carry... short circuits!
     
  7. Nov 3, 2005 #6

    DocToxyn

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    The two systems complement each other in that one, the electrical is fast and relatively simple, and the other, the chemical is slower, but more amenable to control. Electrical signals carried along axons are basically an all-or-nothing process. Once the signal is sent, it carries on without any particular flavor to it. Once it gets to a nerve terminal where the chmical synapse resides, this is where a refinement of that signal can occur to provide it with some sort of purpose. The chemicals released at the synapse, neurotransmitters (NTs) (or neuropeptides, but we'll ignore those for now) are defined by the cell type, ie, a dopaminergic cell has dopamine, a gabaergic cell has GABA. The dopamine acts interacts with receptors at the postsynaptic membrane (the cell across the synapse from the one receiving the electrical signal) and depending on what those receptors are, it will provide a stimulatory or an inhibitory response. GABA is predominantly an inhibitory NT, glutamate is excitatory. This distinction in cell type/NT characteristics allows the simple "on" signal from the axon to be modulated to a stimulatory or inhibitory signal. It gets further complicated the more one travels along the path and the signal gets parcelled out into, or along, other nuclei (groups of cells) which can also modify its character. The chemical synapse also allows a termination of signal to be achieved by diffusion, metabolism or re-uptake of the released NT.

    If we want to think of it in more familiar terms think about the electricity coming into your house. As it is the electricity can't really do anything for you (except perhaps make your hair stand up:surprised ), you need something like a toaster to translate it into heat, or a lamp to create light. These would be analagous to the chemical synapse providing functional use to the incoming electrical signal. Carry it further and think of your TV which is powered by the electricity but needs more information via the cable to actually provide you with that quality programming you need. It's like the brain nuclei receiving multiple inputs to ultimatley output a response or action.
     
  8. Nov 3, 2005 #7
    Transmission of a message electrically can work if we put extra myelin sheath around axons.

    So you are saying chemical transmission represents an analogical system?

    I can see your point. But we have electrical synapses in the brain, so why won't it work for the rest of the body, presumbly you just need better and more types of ion channels etc.
     
  9. Nov 3, 2005 #8
    Is chemical coordination more complex then? I've always though an electrical system is more recent and complex (which is why we have it in the brain).

    Why do you need chemical transmission at the neuromuscular junction though? We have current neurotransmitters which bind to AChR sites for example. Surely it is equally possible to use ions as transmitter substances, and we can also use receptor channels which bind ions. Also, instead of hydrolysing acetylcholine using acetylcholinesterase, why not use an ion pump which pumps ions back to the pre-synaptic terminal?

    We have many neurotransmitters, as you have stated, which is fine. Surely we already have chloride ions which hyperpolarise the post-synaptic cells (I am not entirely sure), so acting as an inhibitory signal?

    The only reason I can think of is that different types of transmitters are macromolecules so are extremely complex - so there is no overlap of functions, which may happen if you use simple ions like sodium/potassium/chloride.
     
  10. Nov 3, 2005 #9

    DocToxyn

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    If you think of the entire process of synthesis, release, metabolism, re-uptake, re-packaging, etc that is involved in chemical neurotransmission, I think you could argue that it is more complex (some electrophysiologists would probably argue with me). I think it is a more flexible system. You have the ability through different receptors, eg. synthesis/release modifying autoreceptors to control specific aspects of NT function. One neurotransmitter can have many different receptors on the postsynaptic membrane which then will dictate that cells response, dopamine has at least 5identified receptors that are either exitatory or inhibitory in their own way. You can also get activation/inactivation of other second messengers within the cell that carry out other processes, adenylate cyclase in the case of dopamine, calcium in the case of glutamate acting at NMDA channels. All these aspects of chemical signalling lend a plasticity to the system that is critical to carrying out the complex higher functions found in the brain of "higher" animal species.
    I think this is where the distinction lies, I have heard between 50 and 100 different NT (or NT-like) substances are created in the body thus this diverse array speaks to the need for varied function that could not be handled by a handfull of ions and their associated channels. If you tried to create a totally electrical/biological system there would be too much shorting and cross-talk once you tried to progress beyond a certain level of complexity.
     
  11. Nov 3, 2005 #10
    Surely such biological systems with only electrical connections would still work if you prevent cross-interference between signals (we already have those in nerve fibres - signals travel in opposite directions but the signals are insulated by myelin). So why can you not extend this network to create an electrically based biological system?

    Oh and something about all-or-none. This is true only for electrochemical signals in action potentials - it's the frequency that matters. If the system is not 'all-or-none' would the signals themselves be meaningless and carry no information? But then how come chemical coordination is not all-or-none?
     
  12. Nov 3, 2005 #11
    In this way a small presynaptic nerve terminal can affect many membrane channels in a large portion of the postsynaptic membrane. This lead to huge signal amplification: the small current through the membrane of the nerve terminal results in a much larger current through the membrane of the sarcolemma.
     
  13. Nov 3, 2005 #12

    somasimple

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    Just a silly theory for me. Some axons have more than 200 turns of meylin and it has nothing to do with electricity. An unique turn would be sufficient.

    Electricity travels via conductosr with an average speed of 300,000,000 ms-1. The fastest neuron whe know is only 250 ms-1, just a little difference. :rolleyes:

    Gaps junctions are called electrical synapes but it is their medical denomination but we know that they are just tubes connecting/enabling ions flux between cells.

    Yes, nervous transmission is analogic!
    it uses a frequency to quantal transmission and uses 80 known neuro transmitters at once (in brain). A single neuron is able to make...20,000 different peptides.
     
  14. Nov 3, 2005 #13

    somasimple

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  15. Nov 4, 2005 #14
    I did not mean the signal can travel in both directions - i meant APs in opposite directions without crossing each other.
     
  16. Nov 4, 2005 #15
    But wnat you just said does not argue against a purely electrically based system. Axons travel much slowly because it's not a current (well except for passive conduction) but is achieved electrochemically.
     
  17. Nov 4, 2005 #16
    but why can't you have electrical transmission at the neuromuscular junction?
     
  18. Nov 4, 2005 #17

    somasimple

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    I did too! but such process is often done in laboratory!

    passive conduction will exist every time you enable an electrical circuit within electrolytes. It doesn't tell us that it exists!

    Muscle isn't an electrical engine, so... You may activate it with electricity because it will move ions and engages a reaction but its normal functioning isn't electrical.
     
  19. Nov 4, 2005 #18
    because it could not achieve the huge amplification
     
  20. Nov 4, 2005 #19

    DocToxyn

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    To your first part, yes, there are ways to prevent cross-talk, but the larger and more complex your system gets the more difficult it will be to facilitate its function with a bunch of bulky wires running from place to place. It's more efficient and flexible (adaptable) when you have a mix of chemical and electrical. Gerben brings up another great point about chemical synapses, the signal can be amplified greatly, a single synaptic vesicle (and nerve terminals have a lot of these) can release thousands of NT molecules which then reach the postsynaptic membrane and can potentially depolarize it.
    To your second part, the action potential along the axon is termed "all or none" because the cell is encountering many different signals some excitatory and some inhibitory and when excitatory outweighs the inhibitory enough the potential reaches the critical threshold, the signal is generated and it moves along the axon. There is either a signal or there isn't. Perhaps in specific ases one might be able to argue that chemical synapses do act in a all-or-none-type fashion. At the NMJ, the signal comes down the axon, triggers the NT release at the verve terminal, NT activates the postsynaptic membrane and the musle cell contracts, it either happens or it doesn't (it's not quite that simple, but for arguments sake we'll go with it). However the chemical transmission can go much further than that, activating receptors that don't necessarily immediately depolarize the postsynatic cell, but rather subtly modify it's function. Modulation of enzyme phosphorylation, activation of second messengers systems, depression or potentiation of the cell, all these things can be achieved through the varied interaction of the numerous different NT with their numerous different receptors. It takes signal transduction many steps beyond a "simple" alteration in membrane potential.
     
  21. Nov 13, 2005 #20
    Is it correct to say:
    signal along muscle is electrical because we want a 1:1 process i.e. 1AP=1contraction, if grading of AP occured then the system would not work as you cannot have a half contracting muscle

    but at the neuromuscular junction, you either get a contraction, or you don't; and this is still done by chemical not electrical?
     
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