Neuroscience/Cellular Neurobiology/Neuronal Membrane

Membrane Potentials

Membrane Properties

The membrane, just like in any other cell, is a phospholipid bilayer separating two regions of ion-filled, water-based solutions. The separation of various ions across this membrane creates the potentials that form the basis of the electrical communication in neurons.

Chemical Background

Proteins are located throughout the membrane and function to regulate the concentrations of various ions on either side of the membrane. Many of these proteins function as ion channels-that is, they control the passage of ions across the membrane. Based on the construction of a particular ion channel, it will be opened by different stimuli, electrical or chemical, and when open, permeable to a specific ion. The ions that commonly have channels are the monovalent potassium, sodium and chlorine and the divalent calcium. There are also important ion channels with no gate, referred to as leak channels because the ions will 'leak' across the membrane through these channels down their electrochemical gradient. Another set of membrane proteins serve to force ions across the membrane against their chemical gradient. These are known ion pumps, and the most common one is the sodium potassium ion pump that, at the cost of one molecule of ATP(Adenosine triphosphate), forces three sodium ions out of the cell and two potassium ions in.

Electrical Background

Because opposite charges attract, the separation of positive and negative ions (ex. Na+ and Cl-) across the membrane creates a potential for charges to move toward each other. When these charges do move relative to other nearby charges, this movement is referred to as electrical current. By convention a positive current refers to the movement of positive charge. The electrical potential exerted on these charges is a measurable force called voltage measured in volts. Another relevant characteristic of the movement of charge is the ease with which the charges or ions move across a barrier such as the membrane. This is measured with two inversely related quantities, conductance (g) and resistance (R). the relation between the two is R=1/g. To tie everything together is Ohm's Law, which says that current (I) is equal to the conductance (g) multiplied by the electrical force (V).

Measurement

The membrane potential can be measured experimentally with an electrode inserted through the membrane of the cell. The electrical force is then measured, and most cell register at about -65 millivolts, usually stated as Vm = -65 mV. This potential is necessary for the function of a neuron.

Composition of the Resting Potential

This measure of -65mV is a composite of the potentials of K+, Na+, Cl- and Ca2+ ions. To determine what a particular ion, for example, K+, contributes to the composite, an experiment can be setup in which a potassium salt is placed in solution on one side of the membrane. The salt dissociates, leaving K+ and A- (the anion) in solution. The membrane, being permeable to K+ only, prevents the anion from crossing. Diffusion forces K+ to spread out, so K+ crosses the membrane without A-. There is also a force acting against diffusion, the electrical attraction between the negative anion and the positive K+, and the balancing of these two forces is referred to as an ionic equilibrium. Because this equilibrium involves the separation of opposite charges, it has a potential associated with it. K+'s equal. Potential is about -80mV. Each of the ions associated with membrane potentials have a characteristic equilibrium potential. The composite will be the overall resting membrane potential, and because this composite is a compromise between the ionic equilibrium potentials of the ions permeable to the membrane, none of the ions are at their ionic equilibrium. The difference between an ion's ion equilibrium potential and the overall resting potential is the force on that ion to move toward its equilibrium. This force is referred to as the ionic driving force.

Mechanisms

Driving Force

If an ion is not at equilibrium it has a potential to move. This is a force on the ion. The total driving force is a sum of the electrical potential and the chemical gradient potential. (A negative current is an inward current; a positive current, out of the cell. [DANGER: most Neuroscience websites and books mention that inward current is positive current and outward current is negative current. The opposite of what is here.]) K+'s force is -80mV. Resting potential is -60mV. Thus, there is a net positive force of 20mV on the cell. This pushes positive ions out of the cell. If one opens Na and K channels (the same number), Na generates more current because its electrical force is higher than K. Na will then enter the cell and K will exit the cell. For Na, the outside concentration is much higher than the concentration inside, and there is a negative charge inside that pulls Na inside. Cl- equilibrium potential is just below resting potential, so there is some movement when permeability is higher.

Ion Pumps

It is essential to maintain the concentration gradients to keep electrical forces. K continues to leak out and Na in, until conc.s across membrane are equal. The membrane compensates by pumping these ions against their conc. gradient. Requires ATP. Three Na ions bind to protein on the inside of the membrane, and when ATP binds, the protein changes conformation to release Na on the outside. K binds and that cause the protein to change back. This causes a positive current out of the cell. Ca2+ is used differently, generally herded elsewhere. Ca has low intracellular conc.

Resources

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