• Document: BIOELECTRICITY. Chapter 1. Electrical Potentials. Electrical Currents
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Chapter 1 BIOELECTRICITY This chapter introduces the basic concepts used in making electrical measurements from cells and in describing instruments used in making these measurements. Electrical Potentials A cell derives its electrical properties mostly from the electrical properties of its membrane. A membrane, in turn, acquires its properties from its lipids and proteins, such as ion channels and transporters. An electrical potential difference exists between the interior and exterior of cells. A charged object (ion) gains or loses energy as it moves between places of different electrical potential, just as an object with mass moves "up" or "down" between points of different gravitational potential. Electrical potential differences are usually denoted as V or ∆V and measured in volts; therefore, potential is also termed voltage. The potential difference across a cell relates the potential of the cell's interior to that of the external solution, which, according to the commonly accepted convention, is zero. Potential differences between two points that are separated by an insulator are larger than the differences between these points separated by a conductor. Thus, the lipid membrane, which is a good insulator, has an electrical potential difference across it. This potential difference ("transmembrane potential") amounts to less than 0.1 V, typically 30 to 90 mV in most animal cells, but can be as much as 150 - 200 mV in plant cells. On the other hand, the salt-rich solutions of the cytoplasm and blood are fairly good conductors, and there are usually very small differences at steady state (rarely more than a few millivolts) between any two points within a cell's cytoplasm or within the extracellular solution. Electrophysiological equipment enables researchers to measure potential (voltage) differences in biological systems. Electrical Currents Electrophysiological equipment can also measure current, which is the flow of electrical charge passing a point per unit of time. Current (I) is measured in amperes (A). Usually, currents measured by electrophysiological equipment range from picoamperes to microamperes. For AXON GUIDE 2 / Chapter one instance, typically, 104 Na+ ions cross the membrane each millisecond that a single Na+ channel is open. This current equals 1.6 pA (1.6 x 10-19 coul/ion x 104 ions/ms x 103 ms/s). Two handy rules about currents often help to understand electrophysiological phenomena: (1) current is conserved at a branch point (Figure 1-1); and (2) current always flows in a complete circuit (Figure 1-2). In electrophysiological measurements, currents can flow through capacitors, resistors, ion channels, amplifiers, electrodes and other entities, but they always flow in complete circuits. I total I1 I2 I total = I 1 + I 2 Figure 1-1. Conservation of Current Current is conserved at a branch point. I I Microelectrode Battery I = I1+ I2 Cell I1 I2 Capacitor I ELECTRONIC INSTRUMENT I Figure 1-2. A Typical Electrical Circuit Example of an electrical circuit with various parts. Current always flows in a complete circuit. Bioelectricity / 3 Resistors and Conductors Currents flow through resistors or conductors. The two terms actually complement one another  the former emphasizes the barriers to current flow, while the latter emphasizes the pathways for flow. In quantitative terms, resistance R (units: ohms (Ω)) is the inverse of conductance G (units: siemens (S)); thus, infinite resistance is zero conductance. In electrophysiology, it is convenient to discuss currents in terms of conductance because side-by-side ("parallel") conductances simply summate (Figure 1-3). The most important application of the parallel conductances involves ion channels. When several ion channels in a membrane are open simultaneously, the total conductance is simply the sum of the conductances of the individual open channels. G G G total = 2G γ γ G total = 2 γ

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