Question

1. An electrophysiologist is studying the electrical properties of an isolated squid neuron placed in normal extracellular fluid. She places a recording electrode inside the cell to record the potential difference across the membrane. She records a resting membrane potential of -70 mV.

1. Name three factors that contribute to this resting membrane potential and explain “how” they contribute to this negative value

Answer 1

three factors that 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, and

(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.

The importance and the meaning of this equation.

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.

Second, 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.

Third, 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.