Membrane noise

Electrical noise in biological membrane systems is explained in terms of opening and closing of ionic channels (for two useful review see De Felice (1981) and Frehland (1982)). The qualitative behaviour of membrane activity strongly depends on the density of membrane channels (Holden, 1981 Holden Yoda 1981). Integrating numerically the celebrated Hodgkin-Huxley equations (Hodgkin Huxley, 1952) it was demonstrated that the number of channels is a bifurcation parameter. The fact that channel 

The kinetic model adopted by Hill Chen (1972), (but see Chen, 1978), assumed that the channels are independent and that each one consists of x independent subunits. Each subunit can be in either one of two configurations. The state of the channel is given by the number of subunits in a particular configuration. A channel is thought to be nonconducting unless all X subunits are in the appropriate configuration. For the particular case of X = 2 the kinetic scheme is 

Often the current spectrum is measured. Theoretically, the spectrum of component fluctuation is 

More complex models exist to describe different types of membrane noise. 

Noise analysis is a method of differentiating between concurrent transport mechanisms (Fig. 5.4). A particular problem, transmitter-induced membrane noise, will be studied in Chapter 7. 

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Most review papers on the subject of biological membrane noise have been devoted primarily to conductance noise that arises from channel switching between different conductance states. Several excellent reviews that vary in volume and in mathematical level of presentation of material are available (3-9). 

DeFelice, L. J. Introduction to Membrane Noise Plenum New York, 1981. 

A. Verveen and L. De Felice, Membrane Noise, Prog. Biophys. Molec. Biol 28, 189-265 (1974). 

Other phenomena are interesting from the noise point of view. They related to ion transport across membranes/ " equilibrium and nonequilibrium kinetic systems/ nerve membrane noise, " and membrane current fluctuations from ionic channels (Na channels and K channels in axons) in stationary or nonstationary states.Some of these studies have been described in extended reviews. 

H. Lecar and R. Nossal, Theory of Threshold Fluctuations in Nerves I. Relationships Between Electrical Noise and Fluctuations in Axon Firing, Biophys. J. 11, 1048-1067 (1971) II. Analysis of Various Sources of Membrane Noise, Biophys. J. 11, 1068-1084 (1971). 

Fluctuation phenomena occurring in consequence of transport processes through biological membranes can be readily studied. Membranes are two-dimensional, and 10 molecules are contained in a 500 pm membrane surface, which is available to experimental techniques (Neher Stevens, 1977). (Membrane noise can be analysed by using the language of stochastic kinetics, as will be shown in Subsection 5.5.3, as well as in Chapter 7.)... 

The possibility of the measurement of postsynaptic membrane noise (Katz Miledi, 1972 De Felice, 1981) gives a deeper insight into the details of the chemical mechanism of transmitter-receptor interaction. 

De Felice, L. J. (1981). Introduction to membrane noise. Plenum Press, New York. 

Erdi, P. Ropolyi, L. (1979). Investigation of transmitter-receptor interactions by analyzing postsynaptic membrane noise using stochastic kinetics. Biol. Cyb., 32, 41-5. 

A less direct estimate of and is obtained from the amplitude of the power spectra of current fluctuations. Basically, power spectra contain more information than the simple variance, as shown by eqn.(7). The additional information is very useful to ascertain to what extent the measured current fluctuations can be attributed to the flickering of ion specific channels between open and closed states rather than to other noise sources. This control was particularly desirable in the early studies of nerve membrane noise, which revealed the presence of large 1/f spectral components of still unclear origin (16). Indeed, the first unequivocal characterizations of sodium and potassium channel noise in the squid axon membrane (13) and of sodium channel noise in frog nodes (14,15) were obtained from the fitting of measured spectra with the superposition of Lorentzian-like spectra plus 1/f components. From the low frequency asymptote and the cut-off frequency of the Lorentzian components estimates of Y =12 pS and were obtained for the ionic channels of quid... 

KATZ,B. MILEDI,R. (1970). Membrane noise produced by acetyl-... 

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