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How does impulse travel in our nerves

  1. Nov 12, 2003 #1
    hi all,

    I dontn know much in biology. The way messages are transmitted in our nerves has always amazed me. Can anyone explain how does impulse travel in our nerves.

    -benzun
    All For God
     
  2. jcsd
  3. Nov 12, 2003 #2

    adrenaline

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    My memory is vague but I remember the general principle.

    You are talking about how a nerve generates an action potential. How an impulse travels within our nerves.

    The intracelluar milieu of nerve cells contain high concentrations of organic anions and inorganic cations mainly high K+ and a lower concentration of N+ and CL-. In the extracellular fluid outside the nerve cell, the concentrations of K+ are low and those of Na+CL- are high....the opposite of the intracellular environment. The result is a difference in potential across the membrane called resting potential with the inside of the cell being strongly negative compared with the fluids outside the cell.

    During impulse transmission, there is a change in the resting potential and a flow of electric current across the membrane. This is the action potential. It is essentially a very rapid depolarization (decrease in negativity of the inside relative the positive outside) and a somewhat slower repolarization to the resting potential.

    At the peak of the action potential the inside of the cell becomes positive with respect to the outside. How does this happen? First, there is an initial inward current due to the influx of Na+ and subsequently an outward current due to migration of the K+ from inside to the outside. This initial depolarization causes Na+ conductance to begin to increase, which depolarizes the membrane further and this increases Na+ conductance and so on. Repolarization then occurs when the K+ efflux restores the internal negativity. These passages of Na+ and K+ are thought to occur through seperate channels in the cell membrane. Other ions cross the membrane during the action potential such as Ca++ etc. but that gets more dettailed. Of course, I did not get into the transmission across synapses.

    That is a basic description without all the different equations Nernst equation, Donnan equillibrium, crap that I can't remember.
     
    Last edited: Nov 12, 2003
  4. Nov 12, 2003 #3

    iansmith

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    Adrenaline you got most of the stuff rigth. K+ and Na+ are actively pump across the membrane against their concentration gradient which requires a huges amount of ATP. 2K+ in 3NA+ out for 1 ATP. The pump runs continuously and spend 70-80% of the neurons energy. K+ diffuse out of the of the cell and Na+ diffuse back in. Overall, there is more are greater K+/Na+ concetration going out then going in. Therefore the outside of the cell become positive and the inside has a negative charge in part due to the protein content. There is an equilibrium that is reach and the resting membrane potential (RMP) is -70 mV.

    Then come the action potential. The first step is excitation. This trigger the action potential. The voltage rise from -70 mV to -50 mV. -50mV is the threshold limit. This is due to an increase Na+ in the cell. When the threshold is reach, depolarization occures. It goes from -50mV to +30mV. The Na+/K+ pumps stop, Na+ gates open, Na+ rushes in. Then the cell repolarize. It goes from +30 mV to -85 mV. Voltage sensitive K+ gates open, K+ rushes out of the cell. At midpoint Na+ gates close and Na+/K+ pumps restarts. The last step is Hyperpolarization. Voltage rise from -85 mv to -70 mV. This is the slowesst phase. K+ gates close, Na+/K+ pumps are running and RMP is restore.

    Action potential is all or none. Once it is initiate is goes all the way to the end of the neuron. The Action potential propagates along the nerve cell because Na+ spread in the cell, some Na+ spreads down the cell and ajacent membrane are less +ve/-ve and it reach RMP.

    Action potential also spread faster in myelenated fibers. Also large diameter fibers can fire up every 0.4 msec (2500 action potential per sec. Small diameter fiber is every 4 msec (250/sec)
     
  5. Nov 17, 2003 #4
    I have 2 questions-

    1)how fast does this impulse travel?

    2)is it possible to convert electric impulse into nerve impulse?

    -Benzun
    All For God.
     
  6. Nov 19, 2003 #5

    iansmith

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    Dependent on the nerve fiber type
    A Fiber:
    largest diameter, all myelinated
    Fastest fiber - 125 m/sec

    B fiber:
    Mid size fiber, myelinated
    speed up to 10m/sec

    C fiber:
    smallest fiber, unmyelinated
    speed up to 0.5m/sec
     
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