We want to talk a little bit about the cable properties of cells. Specifically, we want to focus on neurons, but these are true, These cable properties are true of any cell. And what that means is that when these cells are especially when they're in the extended membrane projections, like the axons or the dendrites, they act a bit like electrical cables and they demonstrate very passive electrical properties, meaning that they don't involve any particular functions of the membrane or the cytopla
sm to do this other than the existence of a membrane potential. So and this is important because it is one of the mechanisms involved in how neurons convey electrical signals. The conductance of what's called electrotonic current or passive current conduction along these cell processes like the axon or the dendrites are essential to neuron function. And part of what we'll talk about later action potentials as well as how the dendrites work. So how what are these cable properties? How do they wor
k? Well, if we have a stimulus at a particular location and actually change the membrane potential. Remember, membrane potential is at rest negative on the inside and positive on the outside. If we were to open a channel and we can use sodium as an example here and allow sodium influx to occur, it's going to move down its electrochemical gradient and enter the cell. What happens is we now have a high density of positive charge in that one location. Well, Voltage is a separation of charge densiti
es is what drives current, and that's really what we want to focus on here is the current flow along this axon or dendrite. Because we let sodium in at this one spot, that is going to be different in its electrical potential. The density of charge at that location relative to an adjacent location. And so we're going to get cytoplasmic current. Now, an important thing to point out is that cytoplasmic current would go the other way as well. And there's no directionality to it. We're just sort of f
ocusing on this left or right movement. And what's going to happen is this electrical current is going to take essentially the path of least resistance. And there's there are two pathways here that each have a a resistance. The membrane itself has a resistance to current flow, as does the cytoplasm. And what's going to happen here is we're going to get greater current flow in one direction or the other based upon the balancing between those resistances. So in this case, we have the thicker arrow
here sort of flowing down the cytoplasm. And in terms of neuronal function, that's ideal. We want most of the current to travel down the cell and not leak out. Notice some of it will leak out. We don't have perfect resistance to current flow here at the membrane, but what's going to happen is we're going to lose some of our current flow along that pathway. It continues down, lose some more, it continues down. And you notice our arrow in here is sort of shrinking in size because our voltage chan
ge is getting smaller as we go from left to right here. We're we're not changing the membrane potential as much as we did at this spot over here because we're losing current flow to ion flux across the membrane. At some point down the cell, we're going to have no change in the membrane potential Vm, and that's because we will have dissipated this change of membrane potential due to the leak. We call that voltage decay over distance. That's what's shown graphically down here. Now, the purple and
the blue are just showing two different initial changes in the voltage of the cell here. How big was this ion flux? if it was bigger we start here. If it was smaller, we start here. In either case, we're going to get this sort of exponential decay in the change in membrane potential...here to here to here, the farther we move along that cell. OK, now this is important because as I said before, it's a component of how electrical current travels along dendrites as well as how it travels along axon
s. Now in dendrites, it is usually the only mechanism of conveying electrical signals. It's not always true, whereas in axons it is one part of how electrical signals are conveyed. And we'll have to show you additional components to travel over greater distances. And as I've said, cable properties of cells are really pretty lousy. And so electrical current usually travels effectively over very small distances... millimeters, while axons can often have lengths that are significantly longer than t
hat. And so we're going to have to have another mechanism in there to maintain transmission of a signal over a greater distance.
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