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    Synapses are one way, but in an unmyelinated axon, "Local Circuits" are set up to ensure impulses flow one way, but what are these local circuits, I presume they are not physical electron based ones? Do Na+ ions diffuse to resting areas, triggering a depolarisation? This is the only explanation I understand, is it right?

    PS If my understanding is right, then there will be no difference in myelinated and unmyelinated speed of conduction, since it only depends on speed of diffusion of Na+. So what I said must be wrong? Anyone know the "real" explanation?
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    An action potential can only be formed where the axon is unmyelinated. If it is myelinated the impulse is forced to 'jump' between the nodes of Ranvier, meaning that fewer action potentials are needed and so speeding up conduction.

    As far as I understand it synapses are the most importannt thing in ensuring an impulse is only transmitted in one direction - as a neurotransmitter can only be released at one end on the neuron an impuulse can only be passed one way.

    EDIT: I'd also assume that the fact that the p.d. across the membrane dips below resting after an action potential has been formed would prevent re-stimulation of that section of membrane, so Na+ ions diffuse both ways and would, if it was isolated, trigger action potentials both ways. The 'dip', however, prevents re-stimulation backwards (if that makes sense) so the impulse only travels one way.

    Hope that helps,
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    Yep, Na+ ions in depolarised regions of the axon will flow to the neighbouring parts of the axon still at resting potential, (-65mV) but not to those parts in their refractory period, (< -65mV).

    Speed of impulse is much, much quicker in myelinated neurones, since with a myelin sheath present, depolarisation of the membrane can ONLY occur at the nodes of Ranvier. Hence local circuits are set up between neighbouring nodes, and the impulse 'jumps' from one node to the next. It is the size of the action potential that never changes, and only the frequency of the action potential can change, (dependent on the intensity of the stimulus).

    Transmission is only one way since only the post-synaptic membrane at a synapse has receptor proteins complimentary to the neurotransmitter substance.
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    thanx my notes do not really give a good account of whats happening
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    yeah, like eggnog said,

    the Na+ diffuses down the neurone,

    this causes the action potential to become less negative

    this causes the threshold level to be reached

    therefore causing a nerve impulse to be generated further down the neurone
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    (Original post by eggnog)
    Yep, Na+ ions in depolarised regions of the axon will flow to the neighbouring parts of the axon still at resting potential, (-65mV) but not to those parts in their refractory period, (< -65mV).
    Those part that are in the refractory period will have a positive voltage hence will have a potential of >-65mV not <-65mV...
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    The refractory period is when the potential difference across the membrane drops below -65mV, hence the membrane is hyperpolarised, and lasts until the resting potential is reached.

    http://upload.wikimedia.org/wikipedi...-potential.png
 
 
 
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