Lipid bilayer capacitance, voltage

The frequency response of various chemical constituents of nerve membrane was studied. Biological membranes in general consist of lipids and proteins. Firstly, impedance characteristics of artificial lipid bilayer membranes are examined using lecithin-hexadecane preparations. It was observed that the capacitance of plain lipid membranes was independent of frequency between 100 Hz and 20 KHz. Moreover, application of external voltages has no effect up to 200 mV. Secondly, membrane capacitance and conductance of nerve axon were investigated. There are three components in nerve membranes, i.e., conductance, capaci-... 

Initial measurements on valinomycin-doped membranes (13, 14) showed that lipid bilayers, which provide cells with an effective permeability barrier, are equilibrium objects by this criterion. Within an accuracy of several percent, the experimentally obtained values of the spectral density of voltage noise showed agreement with those calculated from relation 1. Figure 1 illustrates this agreement for three valinomycin concentrations. For the valinomycin-K+ system chosen for these experiments and for the frequency range used in measurements, the dispersion in membrane impedance was caused only by geometrical capacitance of the bilayer the characteristic times of the transport process itself were too small to influence impedance in this range. 

Ehospholipid Bilaver Membranes. A lipid bilayer supported on an aperture between the LAPS and the controlling electrode can be represented by adding the equivalent circuit for a lipid bilayer (5) in series with the LAPS, as in Figure 3. As a thin insulating film, the membrane has a resistance (Rm), and a capacitance (Cm) associated with it. In addition, ion gradients across the membrane may cause trans-membrane potentials. These potentials are represented by the voltage source v in the equivalent circuit. It is assumed that no low resistance pathways around the membrane are present. We would like to be able to measure Rm, Cm, and Vm. 

The equivalent circuit model of the membrane is best visualised schematically where capacitance represents the insulating myocardial lipid bilayer and each conductor-voltage (battery) combination represents a type of ionic channel. 

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