Definitions

a. Depolarization makes the membrane potential less negative (the cell interior becomes less negative). Hyperpolarization makes the membrane potential more negative (the cell interior becomes more negative).

b. Inward current is the flow of positive charge into the cell. Inward current depolarizes the membrane potential.

c. Outward current is the flow of positive charge out of the cell. Outward current hyperpolarizes the membrane potential.

d. Action potential is a property of excitable cells (i.e., nerve, muscle) that consists of a rapid depolarization, or upstroke, followed by repolarization of the membrane potential. Action potentials have stereotypical size and shape, are propagating, and are all-or-none.

e. Threshold is the membrane potential at which the action potential is inevitable. Inward current depolarizes the membrane. If the inward current is not sufficient to depolarize the membrane to threshold, it does not produce an action potential. If the inward current depolarizes the membrane to threshold, it produces an action potential.

2. Ionic basis of the nerve action potential (Figure 1-6)

a. Resting membrane potential

- is approximately -70 mV, cell negative.

- is the result of the high resting conductance to K+, which drives the membrane potential toward the K+ equilibrium potential.

- At rest, the Na+ channels are closed and Na+ conductance is low.

b. Upstroke of the action potential

(1) Inward current depolarizes the membrane potential to threshold.

(2) Depolarization causes rapid opening of the activation gates of the Na+ channel, and the Na+ conductance of the membrane promptly increases.

(3) The Na+ conductance becomes higher than the K+ conductance, and the membrane potential is driven toward (but does not quite reach) the Na+ equilibrium potential of +65 mV. Thus, the rapid depolarization during the upstroke is caused by an inward Na+ current.

(4) The overshoot is the brief portion at the peak of the action potential when the membrane potential is positive.

(5) Tetrodotoxin (TTX) blocks these voltage-sensitive Na+ channels and abolishes action potentials.

c. Repolarization of the action potential

(1) Depolarization also closes the inactivation gates of the Na+ channel (but more slowly than it opens the activation gates). Closure of the inactivation gates results in closure of the Na+ channels, and Na+ conductance returns toward zero.

Relative refractory period

Relative refractory period

' K+ conductance

■ Na+ equilibrium potential

Time

Figure 1-6. Nerve action potential and associated changes in Na* and K* conductance.

■ Na+ equilibrium potential

' K+ conductance

----Resting membrane potential

K+ equilibrium potential

Time

Figure 1-6. Nerve action potential and associated changes in Na* and K* conductance.

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