Читать книгу Neurobiology For Dummies - Frank Amthor - Страница 113

A dendrite can be modeled (electrically) as an insulating membrane that separates an inner conductive core from the outer, extracellular fluid (which is also conductive). The membrane has resistance, which is normally very high except where ion channels and capacitance exist.

Capacitance occurs when a thin insulator separates two conductors, such as the neural membrane separating the conductive fluids inside and outside the cell. Normally, no current will flow across an insulator between two conductors. But, if the insulator is thin, and its area is large, transient current can flow due to a redistribution of charges as positive charge on one side of the insulator attract to negative charges on the other side (or vice versa).

To understand what membrane resistance means to spreading synaptic current, think of a water pipe. Suppose a water pipe is standing straight up and connected to a faucet at the bottom that is turned off. If you suddenly turn on the faucet, water will gush out of the top of the pipe almost immediately. The same idea applies to a neuron for a synaptic input on a dendrite with very high membrane resistance (no leakage) and no membrane capacitance between the synapse and another location on the dendrite. Now suppose the pipe has many small holes all along its length. When the faucet is turned on, some water will exit the end of the pipe almost immediately. But a lot of water will escape through the holes along the way, so the force won’t be as strong when the water gushes out of the top. In other words, the holes, like low membrane resistance, will weaken the “signal” reaching the top of the pipe from the opened faucet at the bottom.

The water pipe idea can also help us to understand membrane capacitance. Suppose the pipe is not a stiff metal one, but a very stretchy rubber hose. If you suddenly turn on the faucet, the water flow creates a bulge at the end of the hose near the faucet. The water travels down and creates another bulge, and so on, until water finally begins to leave the open end of the hose. Eventually, the flow out the end of the pipe will be equal to the flow into the pipe at the faucet. Membrane capacitance works in the same way, by delaying and soothing sharp inputs at one point on a dendrite while they’re on their way to other dendritic locations.

The highly stretchable hose is like membrane capacitance. Even if the end of the hose — which you can think of as the energy storage mechanism — farthest from the faucet were closed, opening the faucet would allow some water flow until the force stretching the hose was exactly equal to the force where the water enters the faucet. This flow is always transient, however, just like the flow of current through a capacitor, which has a low resistance to changing voltage but blocks constant voltage.

The combination of low membrane resistance (holes in the pipe) and high membrane capacitance (stretchable pipe) means that synaptic input currents in one dendritic location may be severely weakened, delayed, and smoothed when they arrive at some other location. Figure 3-3 is a model that represents both continuous membrane resistance (due mostly to ion channels) and membrane capacitance.

Figure 3-3: Electrical model of the distributed membrane resistance and capacitance in a neural process such as a dendrite or axon. In thin processes, the axial resistance r_a can become significant.

In real neurons, synaptic signals are so reduced over distances larger than about 200 micrometers that they become ineffective (low amplitude and slow time course). To communicate synaptic inputs over larger distances, neurons require active propagation, which I discuss in the next section.