ACTION MEMBRANE POTENTIAL.

Action potential

Definition:

The action potential is a rapid and reversible reversal of the electrical potential difference across the plasma membrane of excitable cells such as neurons, muscle cells and some endocrine cells. 

In a neuronal action potential, the membrane potential rapidly changes from its resting level of approximately -70 mV to around +50 mV and, subsequently, rapidly returns to the resting level again. 

The neuronal action potential forms an important basis for information processing, propagation, and transmission. 

In muscle cells, the action potential precedes, and is necessary to bring about, muscle contraction.

 Some endocrine cells also exhibit action potentials, where the excitation leads to hormone secretion.

The action potential is also referred to as the electrical impulse or nervous impulse.

Resting potential across the membrane.

RESTING MEMBRANE POTENTIAL (RMP) : CURVE, GENERATION & MAINTENANCE

DEFINITION

• Resting Membrane Potential (RMP) is the voltage (charge) difference across the cell membrane when the cell is at rest.
• The resting membrane potential represents an equilibrium situation at which the driving force for the membrane-permeant ions down their concentration gradients across the membrane is equal and opposite to the driving force for these ions down their electrical gradients.
• When two electrodes are connected through a suitable amplifier and placed on the surface of a single axon, no potential difference is observed. However, if one electrode is inserted into the interior of the cell, a constant potential diff erence is observed, with the inside negative relative to the outside of the cell at rest.

GENERATION OF RMP

A membrane potential results from separation of positive and negative charges across the cell membrane.

In order for a potential difference to be present across a membrane lipid bilayer, two conditions must be met.

• First, there must be an unequal distribution of ions of one or more species across the membrane (i.e., a concentration gradient).
• Second, the membrane must be permeable to one or more of these ion species. The permeability is provided by the existence of channels or pores in the bilayer; these channels are usually permeable to a single species of ions.

The resting membrane potential represents an equilibrium situation at which the driving force for the membrane-permeant ions down their concentration gradients across the membrane is equal and opposite to the driving force for these ions down their electrical gradients.

In neurons, the resting membrane potential is usually about –70 mV, which is close to the equilibrium potential for K + .

Resting membrane potential is generated & maintained by following considerations :

1. Na-K ATPase (Na+-K+ Pump) – Active Transport of Sodium and Potassium Ions Through the Membrane

• In neurons, the concentration of K + is much higher inside than outside the cell, while the concentration of Na+ is much higher outside than inside the cell. This concentration difference is established by Na-K ATPase since this is an electrogenic pump because more positive charges are pumped to the outside than to the inside (three Na+ ions to the outside for each two K+ ions to the inside), leaving a net deficit of positive ions on the inside; this causes a negative potential inside the cell membrane.

2. Na+ & K+ Leak Channels – Leakage of Potassium Through the Nerve Membrane

• The outward K + concentration gradient results in passive movement of K + out of the cell when K + -selective channels are open. Similarly, the inward Na + concentration gradient results in passive movement of Na + into the cell when Na + -selective channels are open.
• Because there are more open K + channels than Na + channels at rest, the membrane permeability to K+ is greater. Consequently, the intracellular and extracellular K + concentrations are the prime determinants of the resting membrane potential, which is therefore close to the equilibrium potential for K+ .
• Steady ion leaks cannot continue forever without eventually dissipating the ion gradients. Th is is prevented by the Na , K ATPase, which actively moves Na + and K + against their electrochemical gradients.