5 Membrane Potentials and Action Potentials Molecular Gradients

5 Membrane Potentials and Action Potentials

Molecular Gradients inside outside (in m. M) Na+ K+ Mg 2+ Ca 2+ H+ HCO 3 Cl. SO 42 PO 3 - 14 140 0. 5 10 -4 (p. H 7. 2) 10 5 -15 2 75 142 4 1 -2 (p. H 7. 4) 28 110 1 4 protein 40 5

Basic physics of membrane potentials • Membrane potentials caused by diffusion Remember: sodium is pumped out of the cell, potassium is pumped in. . .

Resting membrane potential of nerves Active Transport of Sodium and Potassium Ions Through the Membrane Na (outside) = Na (inside) = 142 m. Eq/L 14 m. Eq/L K (outside) K (inside) 4 m. Eq/L 140 m. Eq/L = =

Relation of the diffusion potential to the concentration difference The Nernst Equation EMF (millivolts) = - 61 log Conc. inside Conc. outside Calculation of the diffusion potential when the membrane is permeable to the Several Different Ions

The Goldman-Hodgkin-Katz equation

Measuring the Membrane Potential

Origin of the Normal Resting membrane potential

Resting membrane potentials in nerve fibers

Contribution of the potassium diffusion potential = -94 MV Contribution of Sodium diffusion through the nerve membrane = +61 MV Contribution of the Na-K pump = -4 MV

Nerve Action potential

• Polarization Stage. = -90 mv (Resting Stage) • Depolarization Stage. = +35 mv Action Stage • Repolarization Stage. = • Hyperpolarization Stage. = 0 -90 mv -100 mv

Activation of the Sodium and Potassium Channels

Roles of Other Ions During the Action Potential Calcium Ions (keep the resting membrane potential) Threshold for the initiation of the Action Potential

Propagation of the Action Potential • Direction of propagation • All-or-Nothing principle