Читать книгу Neurobiology For Dummies - Frank Amthor - Страница 114

Most neurons have dendritic trees with diameters of less than a few hundred micrometers. This is the limit for passive signal propagation from the most distal dendrites to the soma. Larger neurons need to overcome the problems of weakened signals and signal smoothing if they are to communicate over distances larger than a few dendritic trees.

In the earlier section titled “Cable properties,” I mention that the analysis of dendritic signal propagation uses cable theory, derived from undersea telegraph cable technology. An undersea telegraph cable is a conductor such as a copper wire, insulated, and surrounded by seawater, which itself is conductive. This is like a neural process such as a dendrite immersed in conductive extracellular fluid (which, by the way, is somewhat similar in ionic composition to seawater).

When testing began for the first undersea telegraph cables, it was found that signals (telegraph pulses) applied at one end were lost through the insulating layer and smeared out and delayed over time (due to the capacitance of the cable) when they were received some distance from the source. The solution for telegraph cables was placing repeaters along their length that boosted the signal back to its original strength before it was lost.

Just like telegraph cables, neurons also use repeaters. Neuronal repeaters are the voltage-dependent sodium channels (we discuss them earlier in this chapter) working with the action potential. Voltage dependent-sodium channels cause further depolarization, which amplifies the depolarization signal in the membrane. This amplification in axons allows neurons to send pulses over arbitrarily long distances, exceeding meters.

The first long-distance neural signal amplification system to evolve was continuous in unmyelinated axons, with voltage-dependent sodium channels placed continuously.

Neuroscience research is full of surprises. One from the last two decades is that neurons use signal amplification not only in their long axons, but also in their dendritic trees. This signal amplification can be done with either voltage-dependent sodium channels or voltage-dependent calcium channels, often located at branch points in the dendritic tree. These channels not only amplify, but also participate in the computing that takes place in the neuron’s dendritic tree. This is because the dendrite can use amplification to control the flow of information throughout the dendritic tree. The neuron’s dendritic tree can be divided into many subunits that interact and make complex processes — such as recognizing a particular pattern of action potentials on the dendritic tree — happen.