Читать книгу Neurobiology For Dummies - Frank Amthor - Страница 112

Neurobiologists continue to unveil new complexities about neurons, while continuously proving that we have much more to learn. Creating an accurate model of the activity of just one neuron — the complex, time-varying changes in its thousands of synapses and millions of voltage-dependent membrane ion channels — can take 100 percent of the processing power of a quite large computer.

A major reason why the interactions among inputs to a neuron can be so complicated is that neural dendrites have what are called cable properties for transmitting signals. This means that the way synaptic signals on different points on a dendrite interact depends on the structure of the dendritic tree between the two points. This includes whether the inputs are on the same or different dendritic branches. The reason the dendritic branch structure makes so much difference is that electrical parameters of the dendrites — such as membrane resistance, membrane capacitance, and dendritic axial resistance — (which we discuss in a moment) are distributed along the dendrite or dendritic tree. Understanding how synaptic inputs interact within the dendritic tree is modeled using cable theory, which was originally developed for transoceanic submarine telegraph cables. The application of this theory to neurons was championed by Wilfred Rall, who made influential contributions to our undertanding of dendritic integration.

Synaptic input current is typically divided into passive versus active conduction properties. Passive properties are those in the absence of voltage-dependent ion channels that themselves cause currents to flow through the membrane in response to synaptic input currents (and other voltage-dependent channels). Active properties involve voltage-dependent ion channels such as the voltage-gated sodium channel that can act to amplify signals.