active_or_passive - When do I use active properties and...

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When do I use active properties and when do I use passive? This can be one of the most difficult and confusing things in this class. Two general principles can help with this. (1) At every part of a neuron there is a competition between active and passive properties In reality both passive and active properties are present all the time at all locations. However for passive properties to play a role current needs to be flowing towards the section of neuron we are looking at (either through injection from an electrode, or via propagation from another part of the neuron via current propagation [see (2) below]). If in the part of the neuron you are looking at lots of channels are open, in that part the active properties (active because they use the concentration gradients generated by the pumps) dominate and the passive properties are not as important (2) Voltage really does not ‘move’ down a cell. What moves are charged ions , and moving charge is current. The voltage changes as a function of distance and we can calculate this by looking at how current travels down a cell. When we say the voltage ‘propagates’ we are really talking about calculating the voltage change due to current propagating. The other property that is important is that passively propagating current decreases as it encounters internal resistance or flows out through low membrane resistance as shown in Fig. 1 below. Now let’s look at three examples that might be confusing some of you Δ V=IR Δ V=IR Δ V=IR Δ V=IR Δ V=IR Δ V=IR The low membrane resistance here (schematized by big hole in membrane) causes most of the flowing current to leave. Also because the resistance is so low here the voltage change is very small. Distal end of dendrite Current is lost as you go down the dendrite so voltage change across membrane becomes smaller Figure 1
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Example 1: Stimulate a dendrite to cause the axon to fire action potentials Stimulate dendrite Record from axon at spike initiating zone (SIZ) 100 0 100 0 time (msec) I Δ V Applied current and recorded voltage change at dendrite 0 time (msec) V AP Threshold RMP Grey = Time neuron would be above AP threshold if only passive properties Recording from SIZ if only passive properties operate (for example if you apply TTX and TEA) 0 time (msec) V AP Threshold RMP Actual recording from SIZ 100 100 0 time (msec) V AP Threshold RMP Result if no change in membrane resistance; action potential would ride on (add to) passive voltage 100 If the action potential added to the passive properties (as shown to the
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active_or_passive - When do I use active properties and...

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