Introduction to Cellular Biophysics, Volume 2: From membrane transport to neural signalling by Armin Kargol

Author:Armin Kargol
Language: eng
Format: epub
ISBN: 9781643277561
Publisher: IOP Publishing
Published: 2019-12-12T16:00:00+00:00

This type of an equivalent circuit can be used to characterize an entire cell, provided the cell is sufficiently small, compact, and with simple enough geometry, so that any variations of electrical properties over its surface can be neglected. They are called electrically small cells (Weiss 1996), and the quantities like the conductance, capacitance, currents, membrane potential, etc., represent the averaged properties of the entire cell. On the other hand, in this chapter we concentrate on neurons that may have very long processes, like axons, and there is no reason to expect that any membrane parameter (e.g. conductance) or electrical variable (e.g. membrane potential) would be constant everywhere on the surface of the cell. In fact, the opposite is true—these quantities vary from location to location along neural processes, and these variations are crucial to neural signaling. For such cells, dubbed the electrically large cells, the diagram in figure 4.3 may represent only a small patch of the cell and the entire cell can be viewed as composed of such patches coupled together through external and internal conducting solutions. Figure 4.4 shows an equivalent circuit diagram for a fragment of an electrically large cell.

Figure 4.4. An equivalent circuit diagram for an extended cell or a cylindrical cellular process, such as an axon. Each segment with parallel resistor–capacitor connections represent a small patch of the membrane. The patches are coupled by extracellular and intracellular solutions—conductors with resistance re and ri per unit length. Here, the transmembrane current is assumed flowing into the cell, while the longitudinal currents in the external and internal solutions go from left to right.

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