Membrane potential circumstances

Ions which affect the membrane potential directly will produce an apparent increase in activity of the ion to which the electrode nominally responds. In these circumstances the cell potential is more accurately given by the expression... 

Of the plasma total concentration of calcium (around 2.5 mmol/1), approximately half is bound to albumin. The unbound fraction is physiologically active in roles such as clotting, in regulating neuromuscular membrane potential and of course for bone formation. There exists an equilibrium between the bound and free fractions, so the albumin can be seen as a buffer able to release or take up calcium as circumstances... 

For the calculation of membrane phenomena as diffusion through membranes, membrane potentials, electrical resistance, transference numbers during electrodialysis, concentration profiles in the membrane under different circumstances, the flux equations have to be solved with the appropriate boundary-conditions. 

Only when the membrane potential is reduced to -120 mV, are all. ionic channels nearly completely closed, and the membrane becomes really passive. Under these circumstances, the membrane will be tightly packed and proteins as well as lipids will be locked into a tightly constrained state leading to a small capacitance. Figure 6 shows the membrane capacitances at various potentials. 

The motor unit has four components a motor neuron in the brain or spinal cord, its axon and related axons that comprise the peripheral nerve, the neuromuscular junction, and all the muscle fibers activated by the neuron. Like other cells, nerve and muscle cells have an external membrane that separates the inner fluids from those on the outside. The fluid on the inside is rich in potassium (K), magnesium (Mg), and phosphorus (P), whereas the fluid on the outside contains sodium (Na), calcium (Ca), and chloride (Cl). When all is quiet, the internal chemical composition of both nerve and muscle cells is remarkably constant and is called resting membrane potential. A primary reason for this constancy lies in the cells ability to regulate the flow of sodium— thanks to an enzyme in the membrane called Na+/K+ ATP-ase. Because the inside of the cell has less sodium than the outside, there is a negative potential (like a microscopic battery) of 70-90 mV. Under ordinary circumstances, the interior of the cell is 30 times richer in potassium than the extracellular fluid and the sodium concentration is 10-12 times greater on the outside of the cell. At rest, sodium tends to flow into cells and potassium oozes out. 

In a number of cases, data taken from earlier experiments were misrepresented in purporting to show effects in completely different circumstances, e.g. the traces showing changes in intracellular calcium concentration of pulmonary artery cells in response to ryanodyne and hypoxia were used again but claimed to show membrane potential changes in cerebral arterial myocytes induced by IP3 and heparin. 

Under ordinary circumstances in mammalian brain, the extracellular Na" " and K" " concentrations are 145 and 3 mM, respectively the intracellular concentrations 12 and 155 mM. However, because the resting membrane potential is a composite of a variety of factors (see earlier section), neither the Na nor the K" " gradient is at equilibrium with the electrical potential spanning the membrane. Hence, even at rest there is a small inward drift of Na+ and a small outward drift of. The Na + -K+ ATPase pump therefo -e maintains this gradient by utilizing energy derived from oxidative phosphorylation of the neuron. 

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