MEMBRANE POTENTIALS CAUSED BY ION CONCENTRATION

Differences Across a Selectively Permeable Membrane

 In Figure 1A, the potassium concentration is great inside a nerve fiber membrane but very low outside the membrane. Let us assume that the membrane in this instance is permeable to the potassium ions but not to any other ions. Because of the large potassium concentration gradient from inside toward outside, there is a strong tendency for extra numbers of potassium ions to diffuse outward through the membrane. As they do so, they carry positive electrical charges to the outside, thus creating electropositivity outside the membrane and electronegativity inside because of negative anions that remain behind and do not diffuse outward with the potassium. Within a millisecond or so, the potential difference between the inside and outside, called the diffusion potential, becomes great enough to block further net potassium diffusion to the exterior, despite the high potassium ion concentration gradient. In the normal mammalian nerve fiber, the potential difference is about 94 millivolts, with negativity inside the fiber membrane.

Fig1. A, Establishment of a diffusion potential across a nerve  fiber membrane, caused by diffusion of potassium ions from inside  the cell to outside through a membrane that is selectively permeable  only to potassium. B, Establishment of a diffusion potential when the  nerve fiber membrane is permeable only to sodium ions. Note that  the internal membrane potential is negative when potassium ions  diffuse and positive when sodium ions diffuse because of opposite  concentration gradients of these two ions. 

Figure 1B shows the same phenomenon as in Figure 1A, but this time with a high concentration of sodium ions outside the membrane and a low concentration of sodium ions inside. These ions are also positively charged. This time, the membrane is highly permeable to the sodium ions but is impermeable to all other ions. Diffusion of the positively charged sodium ions to the inside creates a membrane potential of opposite polarity to that in Figure 1A, with negativity outside and positivity inside. Again, the membrane potential rises high enough within milliseconds to block further net diffusion of sodium ions to the inside; however, this time, in the mammalian nerve fiber, the potential is about 61 milli volts positive inside the fiber.

Thus, in both parts of Figure 1, we see that a con centration difference of ions across a selectively permeable membrane can, under appropriate conditions, create a membrane potential. Later in this chapter, we show that many of the rapid changes in membrane potentials observed during nerve and muscle impulse transmission result from the occurrence of such rapidly changing diffusion potentials.

The Nernst Equation Describes the Relation of Diffusion Potential to the Ion Concentration Difference Across a Membrane. The diffusion potential level across a membrane that exactly opposes the net diffusion of a particular ion through the membrane is called the Nernst potential for that ion, a term that was introduced in Chapter 4. The magnitude of the Nernst potential is determined by the ratio of the concentrations of that specific ion on the two sides of the membrane. The greater this ratio, the greater the tendency for the ion to diffuse in one direction, and therefore the greater the Nernst potential required to prevent additional net diffusion. The following equation, called the Nernst equation, can be used to calculate the Nernst potential for any univalent ion at the normal body temperature of 98.6°F (37°C):

where EMF is electromotive force and z is the electrical charge of the ion (e.g., +1 for K+).

When using this formula, it is usually assumed that the potential in the extracellular fluid outside the membrane remains at zero potential, and the Nernst potential is the potential inside the membrane. Also, the sign of the potential is positive (+) if the ion diffusing from inside to outside is a negative ion, and it is negative (−) if the ion is positive. Thus, when the concentration of positive potassium ions on the inside is 10 times that on the outside, the log of 10 is 1, so the Nernst potential calculates to be −61 millivolts inside the membrane.

The Goldman Equation Is Used to Calculate the Diffusion Potential When the Membrane Is Permeable to Several Different Ions. When a membrane is permeable to several different ions, the diffusion potential that develops depends on three factors: (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. Thus, 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.

Several key points become evident from the Goldman 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 con centration gradient of each of these ions across the mem brane helps determine the voltage of the membrane potential.

Second, the quantitative importance of each of the ions in determining the voltage is proportional to the mem brane permeability for that particular ion. That is, if the membrane has zero permeability to 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 phenomenon is that excess positive ions diffuse to the outside when their concentration is higher inside than outside. This diffusion 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.

Fourth, as explained later, the permeability of the sodium and potassium channels undergoes rapid changes during transmission of a nerve impulse, whereas the permeability of the chloride channels does not change greatly during this process. Therefore, rapid changes in sodium and potassium permeability are primarily responsible for signal transmission in neurons, which is the subject of most of the remainder of this chapter.

مواضيع ذات صلة

MEASURING THE MEMBRANE POTENTIAL

ACTIVE TRANSPORT” OF SUBSTANCES THROUGH MEMBRANES

The Cell Membrane Consists of A Lipid Bilayer With Cell Membrane Transport Proteins

Apoptosis—Programmed Cell Death

Cilia and Ciliary Movements

تركيب الخلية Structure of the cell

LYSOSOMES

السيطرة على انقسام الخلية

الظاهرة الأسموزية

طرق انتقال المواد من وإلى الخلية من خلال الغشاء

انتقال المواد الغذائية من وإلى الخلية عبر غشاء الخلية

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