Changes in membrane

Resting potential is a stable membrane potential in nonexcitable cells, or the most stable membrane potential between Action Potentials in excitable cells. In some excitable tissues it is impossible to define a resting potential because of continuous change in membrane potential. 

Many of the physical changes in membrane structure of cells are reversible and species differences in the degree of disruption of dry membranes may relate to differences in composition, protective mechanisms or to additional damage occurring during desiccation (see below). 

Changes in membrane structure (eg caused by ischemia) can affect water balance and ion flux and therefore every process within the cell. Specific deficiencies or alterations of certain membrane components lead to a variety of diseases (see Table 41-5). In short, normal cellular function depends on normal membranes. 

Voltage-gated Open in response to a change in membrane potential, eg, Na+, Kk and Ca + channels in heart. 

A breakthrough in cell modelling occurred with the work of the British scientists. Sir Alan L. Hodgkin and Sir Andrew F. Huxley, for which they were in 1963 (jointly with Sir John C. Eccles) awarded the Nobel prize. Their new electrical models calculated the changes in membrane potential on the basis of the underlying ionic currents. 

Other ion channels are closed at rest, but may be opened by a change in membrane potential, by intracellular messengers such as Ca + ions, or by neurotransmitters. These are responsible for the active signalling properties of nerve cells and are discussed below (see Hille 1992, for a comprehensive account). A large number of ion channels have now been cloned. This chapter concerns function, rather than structure, and hence does not systematically follow the structural classification. 

Figure 11.5 Chloride distribution and the GABAa response. The change in membrane voltage (Fm) that results from an increase in chloride conductance following activation of GABAa receptors is determined by the resting membrane potential and the chloride equilibrium potential (Fci)- (a) Immature neurons accumulate CF via NKCC, while mature neurons possess a Cl -extruding transporter (KCC2). (b) In immature neurons GABAa receptor activation leads to CF exit and membrane depolarisation while in mature neurons the principal response is CF entry and h5q)erpolarisation. This is the classic inhibitory postsynaptic potential (IPSP)...

L-type Ca channels are primarily regulated by voltage and are thus opened and closed in response to changes in membrane potential, but in addition, they are regulated by receptor-dependent processes involving protein phosphorylation and G-proteins (Fig. 2). Many lines of evidence support the hypothesis that Ca channels are regulated by phosphorylation by several protein kinases, in particular by... 

FIGURE 30.3 Changes in membrane potential cp of a cell membrane occurring upon application of depolarizing current pulses of different amplitude / (a,b) below threshold (c) excitation of the membrane during an above-threshold pulse. 

BE Schaeffer, JA Zadunaisky. (1979). Stimulation of chloride transport by fatty acids in corneal epithelium and relation to changes in membrane fluidity. Biochim Biophys Acta 556 131-143. 

Graded potentials are short-distance signals (see Table 4.1). They are local changes in membrane potential that occur at synapses where one neuron... 

Figure 4.1 Types of changes in membrane potential. The resting membrane potential in a typical neuron is -70 mV. Movement of the membrane potential toward zero (less negative) is referred to as depolarization. The return of the membrane potential to its resting value is referred to as repolarization. Movement of the membrane potential further away from zero (more negative) is referred to as hyperpolarization.

Sodium channels open more rapidly than K+ channels because they are more voltage sensitive and a small depolarization is sufficient to open them. Larger changes in membrane potential associated with further cell excitation are required to open the less voltage-sensitive K+ channels. Therefore, the increase in the permeability of K+ ions occurs later than that of Na+ ions. This is functionally significant because if both types of ion channels opened concurrently, the change in membrane potential that would occur due to Na+ ion influx would be cancelled out by K+ ion efflux and the action potential could not be generated. 

Grinvald A, Hildesheim R, Farber IC, Anglister L (1982) Improved fluorescent-probes for the measurement of rapid changes in membrane-potential. Biophys J 39(3) 301—308... 

Ion channel A pore located in the centre of a voltage-operated or receptor-operated ion channel protein which, when opened by a change in membrane potential or the binding of a neurotransmitter, allows ions to enter or leave the cell. 

---