- Resting Membrane Potential Definition: The resting membrane potential is the voltage difference across a cell membrane when the cell is at rest, typically ranging from -60mV to -100mV.
- Action Membrane Potential: Action membrane potential occurs when the cell membrane’s permeability changes, leading to a positive potential inside the cell due to sodium ion influx.
- Ion Movement Across Membrane: Ion diffusion across the cell membrane creates the membrane potential, with potassium ions moving out and sodium ions moving in during action potential.
- All or Nothing Law: The action potential’s value is consistent regardless of how the cell is stimulated, demonstrating the all or nothing law.
- Resting and Action Potential in Biomedical Instrumentation: Understanding these potentials is crucial for biomedical instruments that measure and interpret neural and muscular activity.
The cell membrane separates extra-cellular fluid (outside) and intra-cellular fluid (inside). Extra-cellular fluid has more sodium and chloride ions, but fewer potassium ions. Intra-cellular fluid has more potassium ions. Neurons send electrochemical messages, producing electrical signals. These electrically charged chemicals are called ions. Sodium and potassium ions have one positive charge, calcium ions have two, and chloride ions have one negative charge. The cell membrane is semi-permeable, allowing some ions to pass while blocking others.
Resting Membrane Potential
Diffusion moves substances across the cell membrane, creating membrane potential. When a cell is not signaling, it is at “resting state”, with a negative inside and positive outside. This allows potassium (K+) and chloride (CL–) ions to enter, but blocks sodium (Na+) ions. Thus, sodium ion concentration is lower inside the cell. This ion difference creates the resting membrane potential, ranging from -60mV to -100mV. This value remains stable until disturbed, keeping the cell polarized.
For example, in blood plasma, high sodium levels can cause renal damage and dehydration, while low levels can lead to renal failure and adrenocortical hypofunction. High potassium levels can cause shock and acidosis, where the patient may lose consciousness and experience tachycardia, lowering blood pressure. Increased chloride ions can cause respiratory issues.
Action Membrane Potential
When an ionic current or external energy excites the cell membrane, its permeability changes. Sodium ions flow inside the cell, creating an ionic current and reducing the membrane barrier. This influx of sodium ions balances with the outside ions, while potassium ions flow outside. This imbalance creates a positive potential inside and a negative potential outside, known as Action Membrane Potential, with a value of 20mV. At this point, the cell is depolarized.
When sodium ions stop flowing into the cell, the ionic current reduces the membrane barrier, allowing the cell to return to its polarized state. In the resting state, sodium ions move outside the cell using the Sodium Pump.
In nerve and muscle, cell repolarisation occurs fast after depolarization. Action potential appears as a spike for one millisecond. In heart muscle, an action potential occurs for 150 to 300 milliseconds. Therefore, repolarization occurs slowly in the heart.
All or Nothing Law
The value of action potential remains the same irrespective of the method of excitation of a cell. It does not depend on the stimulus intensity. This is all or nothing law.
Absolute Refractory Period: It is the period when the cell is non-responsive to any stimulus. It is 1 millisecond for nerve cell.
Relative Refractory Period: A new action potential occurs in this period. This requires higher stimulus value to re-initiate the action potential.
