- 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.
Resting potential and action potential are the voltages across an excitable cell membrane. The 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 as electrical signals. Those 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, so some ions pass while others are held back.
Resting Membrane Potential
Diffusion and the sodium-potassium pump set the voltage across the cell membrane. In the “resting state” a quiet cell is negative inside and positive outside. At rest the membrane is more open to potassium (K+) and chloride (CL–) than to sodium (Na+). Sodium ion concentration therefore stays lower inside the cell. That ion difference is the resting membrane potential, often about -60 mV to -100 mV in many nerve and muscle cells. The value stays near that range until a stimulus depolarizes the cell.
Plasma sodium, potassium and chloride must stay in a narrow band for the same reason. High or low sodium can harm the kidney and fluid balance. High potassium can disturb heart rhythm and acid-base state. A large chloride shift can disturb breathing.
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. That sodium influx is later matched by potassium ions flowing outside. The inside of the cell then goes positive relative to the outside. That voltage is the action membrane potential; the overshoot is often tens of millivolts, and a textbook example uses 20 mV. At this point, the cell is depolarized.
When sodium channels close, potassium outflow returns the cell toward its polarized state. In the resting state, the sodium-potassium pump also moves sodium out of the cell.
In nerve and skeletal muscle, repolarisation after depolarization is fast. The action potential appears as a spike of about one millisecond. In heart muscle, an action potential lasts about 150 to 300 milliseconds, so repolarization is slower.
All or Nothing Law
Once a patch of membrane is above threshold, the action potential amplitude at that site stays much the same whatever the method of excitation. Stimulus intensity is not coded in that peak voltage. The all or nothing law describes that stereotyped spike for a given axon.
Absolute Refractory Period: During this period the cell does not fire another action potential, no matter how large the stimulus. For a nerve cell that window is about 1 millisecond.
Relative Refractory Period: A new action potential can occur in this period, but only if the stimulus is larger than the usual threshold.
