Resting Potential

A resting nerve cell is one that is not being stimulated to send a nerve impulse. Under resting conditions, non-gated (always open) channels determine the membrane permeability to sodium and potassium ions.

Sodium and potassium ions follow the laws of diffusion described in chapter 3 (p. 82) and show a net movement from areas of high concentration to areas of low concentration across a membrane as their permeabilities permit. The resting cell membrane is only slightly permeable to these ions, but the membrane is more permeable to potassium ions than to sodium ions. Also, the cytoplasm of these cells has many negatively charged ions, called anions, which include phosphate (PO4-2), sulfate (SO4-2), and proteins, that are synthesized inside the cell and cannot diffuse through cell membranes (fig. 10.12a).

If we consider an imaginary neuron, before a membrane potential has been established, we would expect potassium to diffuse out of the cell more rapidly than sodium could diffuse in. This means that every millisecond (as the membrane potential is being established in our imaginary cell), a few more positive ions leave the cell than enter it (fig. 10.12a). As a result, the outside of the membrane gains a slight surplus of positive charges, and the inside reflects a surplus of the impermeable negatively charged ions. This creates a separation of positive and negative electrical charges between the inside and outside surfaces of the cell membrane (fig 10.12fo). All this time the cell continues to expend metabolic energy in the form of ATP to actively transport sodium and potassium ions in opposite directions, thus maintaining the concentration gradients for those ions responsible for their diffusion in the first place.

Protein

Gatelike mechanism

Protein

Gatelike mechanism

(a) Channel closed (b) Channel open

Figure 10.11

A gatelike mechanism can close (a) or open (b) some of the channels in cell membranes through which ions pass.

Cell membrane

Cell membrane

(a) Channel closed (b) Channel open

Figure 10.11

A gatelike mechanism can close (a) or open (b) some of the channels in cell membranes through which ions pass.

Shier-Butler-Lewis: I III. Integration and I 10. Nervous System I: I I © The McGraw-Hill

Human Anatomy and Coordination Basic Structure and Companies, 2001

Physiology, Ninth Edition Function

Nerve fiber

Cell membrane

Intracellular fluid

© 0 0 Q Na+ diffusion High K+ Low Na+ © Q Na+ diffusion

© 0 0 Q Na+ diffusion High K+ Low Na+ © Q Na+ diffusion

Essentials of Human Physiology

Essentials of Human Physiology

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