life11e_ch44

key concept 44.2 Neurons Generate and Transmit Electric Signals

Various ion pumps create ion concentration gradients across animal cell membranes. For example, Na⁺ concentrations are higher outside cells and K⁺ concentrations are higher inside. The positive charges of these ions are balanced by negatively charged ions both inside and outside the cell. But, across the cell membrane there is an electric charge difference, with the inside of the cell being negative relative to the outside. This electrical charge is due to the cell membrane being differentially permeable to certain ions. For neurons at rest, their membranes are mostly permeable to K⁺, and that permeability is primarily responsible for the electrical charge across their membranes.

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The membrane at rest is close to the equilibrium potential for K⁺ due to the selective permeability to K⁺ through “leak” channels open at rest.
When gated ion channels are activated by electrical, chemical, or mechanical stimuli, permeability of the membrane to the respective ions changes, resulting in a change in membrane potential.
Potential changes, through both inhibitory and excitatory inputs, are summed and result in the membrane potential deviating from the resting potential.
Rapid activation and inactivation of voltage-gated Na⁺ channels are responsible for action potentials.
Action potentials are all-or-none, self-regenerating events that are conducted rapidly and unidirectionally along axons. Myelination of axons results in saltatory conduction, which is faster than conduction in nonmyelinated axons.

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Because the concentration of K⁺ is higher inside the cell than in the extracellular fluid, K⁺ diffuses out of the cell down its concentration gradient. But when K⁺ leaks out of the cell, it leaves behind an unbalanced negative electric charge that tends to pull K⁺ back into the cell. An equilibrium is reached when the tendency for K⁺ to diffuse out is countered by the electric charge pulling K⁺ back in. The resulting charge difference across the membrane is called the membrane potential, with the inside of the cell negative relative to the outside. Membrane potentials exist in all cells.

Membrane potentials exist in all cells. In neurons the steady-state membrane potential is called the resting potential. Stimuli that cause changes in the permeability of the cell membrane cause local changes in that membrane potential. These local changes can generate the large electrical signals called action potentials (APs) that are conducted along the axons and convey information about the stimulus that caused the initial small change in membrane potential. The AP is a sudden, large, transient change in membrane potential generated by sudden changes in permeability to Na⁺ ions. Before describing the mechanisms of these permeability changes and the resulting APs in detail, a review of some simple concepts of electricity may be useful.