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The protoplasm of nerve and muscle cells contains between 30 and 50 times as many potassium ions, eight to ten times fewer sodium ions, and 50 times fewer chlorine ions as does the extracellular fluid.

Quick levelling out of the difference in concentrations is hampered by the extremely thin plasmatic membrane (about 100 angstroms) covering living cells.

Conceptions of the structure of this membrane are based on data obtained by electron microscopy, X-ray diffraction, and chemical analysis. It is supposed that it consists of a double layer of phospholipid molecules lined on the inside with a layer of protein molecules and on the outside with a layer of molecules of compound carbohydrates — mucopolysaccharides. This three-layer structure is shown in the diagram in Fig. 114.

In the cell membrane there are minute channels, or pores, a few angstroms in diameter, through which molecules of water and other substances, and ions of dimensions corresponding to the diameter of the pores pass in and out of the cell.

To the structural elements of the membrane are bound various ions which lend a particular electric charge to the walls of its pores, thereby impeding or facilitating the passage of other ions. It is supposed, for example, that the presence of dissociated phos-

fic. 114. Scheme of the molecular structure of a membrane showing the bimolecular lipid layer Z (circles designate polar groups of phospholipids) and two non-llpid monolayers: external (mucopolysaccharide) X; Internal (protein) У (after Robertson)

J

fic. 115. The appearance of a potential difference on an artificial membrane separating KjSO, solutions of different concentrations (C_t and CJ. The membrane is selectively permeable to K* cations (small circles) and impermeable to SO< anions (large circles)

I and 2 — electrodes immersed in the solution; 3 — electrical measuring instrument

phate and carboxyl groups is the reason why the membrane of nerve fibre is much less permeable to anions than to cations.

Permeability to different cations also varies, and changes regularly in the different functional conditions of the tissue. At rest the permeability of nerve-fibre membrane to potassium ions is between 20 and 100 times that to sodium ions, whereas in an excited state the ratio is significantly reversed.

An experiment illustrating from the standpoint of the Bernstein-Hodgkin theory how the resting potential of a membrane arises is as follows. The right half of a vessel (Fig. 115) divided by an artificial semi-permeable membrane whose pores easily allow positively charged K’ ions to pass and are impenetrable to negatively charged SOJ ions, is filled with a concentrated K₂SO₄ solution, and the left half is filled with a less concentrated K₂SO₄ solution. Owing to the existence of a concentration gradient, potassium ions begin to diffuse through the membrane, primarily from the right half of the vessel (in which their concentration equals C\) into the left (with a concentration of C₂). Correspondingly the negatively charged anions of SO4, to which the membrane is impermeable, concentrate at the membrane surface in the right half of the vessel. Their negative charge keeps potassium ions electrostatically on the membrane surface at left which results in polarization of the membrane, a potential difference arising between its two surfaces.