The Diffusion Potential and Membrane Potential
The diffusion of ions across the cell membrane due to their concentration gradients creates the diffusion potential. The potassium ions are of higher concentration inside the cell membrane in comparison to the outside; they diffuse outside due to this concentration gradient leaving the negatively charged ions behind.
This causes electropositive charges to accumulate outside and electronegativity develops inside the membrane. The diffusion potential then develops across the membrane which prevents further efflux of potassium ions from inside despite the existence of a high concentration gradient. The value of diffusion potential is -94 mV in a nerve fiber of mammalian origin. In case the membrane is selectively permeable only to sodium ions, the diffusion of sodium ions will occur towards the inside of the membrane since the concentration of sodium ions is much more in the ECF than in the ICF. This diffusion of positive ions inside will cause electropositivity inside and electronegativity on the outer side of the cell membrane thereby diffusion potential will rise to +61 mV which will block the further influx of sodium ions. The diffusion potential at which the net diffusion of a particular ion across a membrane is prevented is known as the Nernst potential which can be determined using the Nernst Equation as given
As this value increases, it indicates a greater tendency for the ion to diffuse which means a greater Nernst potential is required to prevent further diffusion of ions.
When the membrane is permeable to more than one ion then the diffusion potential relies on several parameters; the charge of individual ions, their concentrations on either side of the membrane, and the membrane permeability to these ions. The membrane potential on the inside of the cell membrane can then be calculated by applying the Goldman equation also known as the Goldman-Hodgkin-Katz equation as given.
The ions involved are sodium, potassium, and chloride. These three ions are predominantly involved in the maintenance of the membrane potential of muscle fibers and nerve fibers.
The selective permeability of the membrane to particular ions determines the membrane potential. If the membrane is selectively permeable only to sodium ions and impermeable to the other ions, i.e., potassium and chloride, the membrane potential will be determined by the sodium ion concentration gradient and will be equal to the sodium ion Nernst potential. The same principle applies to other permeant ions as well.
3.1.1 Ionic Basis of Resting Membrane Potential (RMP)
Before understanding the action potential, it is very important to have an idea about the resting membrane potential. The cell membrane of most cells maintains an ionic concentration difference operated by the "'Na' K' pump’” and the potassium “leak” channel systems. The "'Na'K' pump” actively pumps out sodium ions from the intracellular fluid to the extracellular fluid and draws in potassium ions against the concentration gradient. This pump exchanges three sodium ions for every two potassium ions and thus develops a negative potential inside the cell membrane. A large difference in ionic concentration also develops across the membrane in the resting stage by the ”Na'K' pump.” The concentration of sodium and potassium ions inside the cell membrane is 14 mEq/L and 140 mEq/L while their concentration outside is 142 mEq/L and 4 mEq/L, respectively. The potassium “leak channel” functions opposite to the "'Na'K' pump” as this ion channel favors the movement of sodium ions into the ICF and potassium ions into the ECF by following the chemical gradient but are 100 times more permeable to potassium ions than to sodium ions. These two systems work together to maintain a steady state across membranes. This net results in a positive charge outside the cell membrane and a negative charge on the inner side. This potential difference (denoted as Vm) in the inside and outside the cell membrane at rest is known as the resting membrane potential (RMP). The magnitude of the RPM can be determined by the Nernst equation as given:
where [ion]o and [ion]i are the concentration of particular ions in the ECF and ICF, respectively.
In the nerve cell, the RMP is -70 mV.
The voltmeter is used to measure the membrane potential. It is a highly sophisticated instrument through which minute changes in potential difference across the membrane can be detected.
3.2