Question

Define resting membrane potential and action potential (1 point). Why is the resting membrane potential negative and close to the K+ equilibrium potential when cells are at rest, and then positive and closer to the equilibrium potential of Na+ when activated (4 points).

Answer 1

Resting Membrane Potential:
Definition : - It is the membrane potential at rest when stimulus is not applied to the cell.

3 factors decides the resting membrane potential are :
(1) the polarity of the electrical charge of each ion

(2) the permeability of the membrane (P) to each ion

(3) the concentrations (C) of the
respective ions on the inside (i) and outside (o) of the membrane.

- The following formula, called the
Goldman equation, or the Goldman-Hodgkin-Katz equation, gives the calculated membrane potential on
the inside of the membrane when two univalent positive ions, sodium (Na+) and potassium (K+), and one univalent negative ion, chloride (Cl–), are involved.

- First, sodium, potassium, and chloride ions are the most important ions involved in the development of membrane potentials in nerve and muscle fibers, as well as in the neuronal cells in the nervous system. The concentration gradient of each of these ions across the membrane helps determine the voltage of the membrane potential.

- the degree of importance of each of the ions in determining the voltage is proportional to the membrane permeability for that particular ion. That is, if the membrane has zero permeability to both potassium and chloride ions, the membrane potential becomes entirely dominated by the concentration gradient of sodium ions alone, and the resulting potential will be equal to the Nernst potential for sodium. The same holds for each of the other two ions if the membrane should become selectively permeable for either one of them alone.

- a positive ion concentration gradient from inside the membrane to the outside causes electronegativity inside the membrane. The reason for this is that excess positive ions diffuse to the outside when their concentration is higher inside than outside. This carries positive charges to the outside but leaves the nondiffusible negative anions on the inside, thus creating electronegativity on the inside. The opposite effect occurs when there is a gradient for a negative ion. That is, a chloride ion gradient from the outside to the inside causes negativity inside the cell because excess negatively charged chloride ions diffuse to the inside, while
leaving the nondiffusible positive ions on the outside.

- As at rest, the membrane is more leaky to k+ ions hence it diffuses out of the cell making the membrane potential more negative till the membrane potential come close to quilibrium potential of K+.

- During deolarization, due to opening of voltage gated sodium channels with rapid influx of sodium ions into the cell makes membrane potential positive to a value near to the equilibrium potential of Na+.