Membrane Potentials


Diffusion and equilibrium potentials:

• diffusion potential is the potential difference generated across the membrane due to concentration difference of a ion that is permeable to the membrane.
• amount of diffusing potential depends upon the amount of concentration gradient across the membrane.
• the charge of diffusion potential depends upon the charge on the ions that are diffusing.
• equilibrium potential is the potential when the equilibrium is achieved across the membrane so no more ion diffusion occurs.

Resting membrane potential:

• it is the potential difference across the membrane at rest.That is negative inside than outside.
• it is established by diffusion potential.
• this potential difference is mainly produced by the ions that are more permeable to the membrane.
• for example k+ ions are more permeable than Na+ ions at rest.so resting membrane potential of a nerve membrane is -70mV that is near to the K+ ions equilibrium potential of -85 mV.
• So the resting membrane potential is very sensitive to the extracellular K+ ion concentration.
• Increased extracellular K+ concentration will reduce the efflux of K+ ions causing depolarization.
• decreased extracellular K+ concentration will accelerate the efflux of K+ ions causing hyperpolarization.

Action Potential:

• it is the electrochemical fluctuation in the membrane of excitable cell and which rapidly propagate.

Phases of Action potential:

1. Depolarization:conduction of a signal across the membrane is done by a rapid membrane depolarization that changes the normal resting negative potential to a positive membrane potential.It depolarizes the membrane toward the threshold.Depolarization causes rapid opening of the activation gates of the Na+ channel.The Na+ is driven toward the Na+ equilibrium potential  of +65mV.
2. Repolarization of action potential:depolarization closes the Inactivation gates of the Na+ channel that results in closure of Na+ channels and Na+ conductance stops.depolarization slowly opens K+ channels and increases K+ conductance.

Refractory periods:

• Absolute refractory period:during this period no matter how how strong the stimulus,it can not induce second action potential.This is due to voltage inactivation of Na+ channels.
• Relative refractive period:during this period a greater than normal stimulus is required to induce a second action potential
• Accommodation:it occurs when the cell membrane is held at depolarized level such that the threshold potential is passed without firing an action potential e.g in hyperkalemia.

Propagation of action potentials:there are some factor that can affect conduction velocity.Most important factors are:

• size of an action potential
• cell diameter
• myelination
• demyelination