Resting Membrane Potential In nerve cells, potassium ions (K+) are at higher concentration inside the membrane than outside whereas the opposite is true for sodium (Na+), calcium (Ca2+), and chloride (Cl–) ions (Fig. 1.9-1). The bulk solutions on either side of the membrane are electrically neutral, with most of the intracellular negative charge being contributed by large organic anions (acids and proteins). The differential distribution of ions across neuronal membranes results in part from the action of membrane pumps that use energy from adenosine triphosphate (ATP) to drive ions against a concentration gradient into or out of the cell. The best characterized pump is the Na+-K+ adenosine triphosphatase (ATPase) that transports 3 Na+ out of and 2 K+ into the cell during each cycle. Because an unequal amount of charge is moved during each cycle, the pump is electrogenic and produces an electrochemical potential across the membrane that makes the inside of the membrane negative with respect to the outside. Na+-K+ ATPase activity is a major contributor to brain energy utilization, with as much as 40 percent of brain oxygen consumption resulting from pump activity required to reestablish ionic homeostasis following action potential firing and synaptic transmission. The cardiac glycosides digoxin (Lanoxin) and ouabain are effective inhibitors of Na+-K+ ATPase in the heart and improve myocardial contractility by depolarizing cardiac myocytes and increasing intracellular Ca2+.
[...] Na+ and Ca2+ channel opening has the opposite effect, making the inside of the cell less negative (depolarization). At any time, the membrane potential is a weighted average of the equilibrium potentials of the ions to which the membrane is permeable. Passive Membrane Properties To understand how ion concentration gradients, electrical gradients, ion channels, and the distribution of charges across the membrane are related, it is helpful to describe the cell membrane as an electrical circuit consisting of resistors (conductors), batteries, and capacitors. [...]
[...] The bulk extracellular and intracellular solutions are electrically neutral and the charge separation that produces the membrane potential occurs in the immediate vicinity of the membrane. The number of ions needed to change the membrane potential is very small relative to concentrations in the bulk solutions. For example, a potential change of 100 mV across a 1 cm2 area of membrane requires the movement of only about 10–12 moles of a monovalent ion. By comparison, Na+ and are present at about M in the extracellular and intracellular fluids, respectively. [...]
[...] Note that capacitive current flows only when the membrane potential is changing (i.e., there is some change in voltage as a function of time [dt]. The total current flowing across a membrane at any given time is a sum of Icap and Iionic. One of the major tools used by physiologists to study ionic currents is a voltage clamp (or more recently a patch clamp). These techniques employ electrical devices to keep the membrane potential constant and eliminate the contribution of capacitive currents during physiological studies, thus making it possible to measure ionic currents directly. [...]
[...] Action Potential Conduction in Axons Action potentials are typically generated in the neuronal cell body or in the initial segment of the axon (also called the axon hillock) where Na+ channels are densely collected. Because action potentials are generated at a distance from the nerve terminals where neurotransmitters are released, an important question concerns how action potentials are transmitted to the synaptic terminals. In a strictly passive nerve fiber, leakage of current across the membrane results in decremental conduction with the signal fading over a distance that is determined by the longitudinal (axial) resistance of the fiber, the membrane capacitance, and the transmembrane resistance. [...]
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