Sequential Opening and Closing of Voltage Gated Na and K Channels Generate Action Potentials

The cycle of membrane depolarization, hyperpolarization, and return to the resting value that constitutes an action potential lasts 1-2 milliseconds and can occur hundreds of times a second in a typical neuron (see Figure 7-30). These cyclical changes in the membrane potential result from the sequential opening and closing first of voltage-gated Na+ channels and then of voltage-gated K+ channels. The role of these channels in the generation of action potentials was elucidated in classic studies done on the giant axon of the squid, in which multiple microelectrodes can be inserted without causing damage to the integrity of the plasma membrane. However, the same basic mechanism is used by all neurons.

Voltage-Gated Na+ Channels As just discussed, voltage-gated Na+ channels are closed in resting neurons. A small depolarization of the membrane causes a conformational change in these channel proteins that opens a gate on the cy-tosolic surface of the pore, permitting Na+ ions to pass through the pore into the cell. The greater the initial membrane depolarization, the more voltage-gated Na+ channels that open and the more Na+ ions enter.

As Na+ ions flow inward through opened channels, the excess positive charges on the cytosolic face and negative charges on the exoplasmic face diffuse a short distance away from the initial site of depolarization. This passive spread of positive and negative charges depolarizes (makes the inside less negative) adjacent segments of the plasma membrane, causing opening of additional voltage-gated Na+ channels in these segments and an increase in Na+ influx. As more Na+ ions enter the cell, the inside of the cell membrane becomes more depolarized, causing the opening of yet more voltage-gated Na+ channels and even more membrane depolarization, setting into motion an explosive entry of Na+ ions. For a fraction of a millisecond, the permeability of this region of the membrane to Na+ becomes vastly greater than that for K+, and the membrane potential approaches ENa, the equilibrium potential for a membrane permeable only to Na+ ions. As the membrane potential approaches ENa, however, further net inward movement of Na+ ions ceases, since the concentration gradient of Na+ ions (outside > inside) is now offset by the inside-positive membrane potential ENa. The action potential is at its peak, close to the value of ENa.

Figure 7-33 schematically depicts the critical structural features of voltage-gated Na+ channels and the conforma-tional changes that cause their opening and closing. In the resting state, a segment of the protein on the cytosolic face— the "gate"—obstructs the central pore, preventing passage of ions. A small depolarization of the membrane triggers movement of positively charged voltage-sensing a helices toward the exoplasmic surface, causing a conformational change in the gate that opens the channel and allows ion flow. After about 1 millisecond, further Na+ influx is prevented by movement of the cytosol-facing channel-inactivating segment into the open channel. As long as the membrane remains depolarized, the channel-inactivating segment remains in the channel opening; during this refractory period, the channel is inactivated and cannot be reopened. A few milliseconds after the inside-negative resting potential is reestablished, the channel-inactivating segment swings away from the pore and the channel returns to the closed resting state, once again able to be opened by depolarization.

Voltage-Gated K+ Channels The repolarization of the membrane that occurs during the refractory period is due largely to opening of voltage-gated K+ channels. The subsequent increased efflux of K+ from the cytosol removes the excess positive charges from the cytosolic face of the plasma membrane (i.e., makes it more negative), thereby restoring the inside-negative resting potential. Actually, for a brief instant the membrane becomes hyperpolarized, with the potential approaching EK, which is more negative than the resting potential (see Figure 7-30).

Opening of the voltage-gated K+ channels is induced by the large depolarization of the action potential. Unlike voltage-gated Na+ channels, most types of voltage-gated K+ channels remain open as long as the membrane is depolarized, and close only when the membrane potential has returned to an inside-negative value. Because the voltage-gated K+ channels open slightly after the initial depolarization, at the height of the action potential, they sometimes are called delayed K+ channels. Eventually all the voltage-gated K+ and Na+ channels return to their closed resting state. The only open channels in this baseline condition are the nongated K+ channels that generate the resting membrane potential, which soon returns to its usual value (see Figure 7-32a).

The patch-clamp tracings in Figure 7-34 reveal the essential properties of voltage-gated K+ channels. In this experiment, small segments of a neuronal plasma membrane were held "clamped" at different potentials, and the flux of electric charges through the patch due to flow of K+ ions through open K+ channels was measured. At the depolarizing voltage of —10 mV, the channels in the membrane patch open infrequently and remain open for only a few milliseconds, as judged, respectively, by the number and width of the "upward blips" on the tracings. Further, the ion flux through

Exterior

Cytosol

Repolarization of membrane, displacement of channel-inactivating segment, and closure of gate (slow, several ms)

Ion-selective pore

Ion-selective pore

Exterior

Cytosol

Voltage-Channel-inactivating sensing segment a helix

Closed Na+ channel

Outer vestibule

Depolarized membrane

Outer vestibule

Depolarized membrane

Inner vestibule

Open Na+ channel

Inner vestibule

Open Na+ channel

Initial depolarization, movement of voltage-sensing a helices, opening of channel (<0.1 ms)

Inactive Na+ channel (refractory period)

Return of voltage-sensing a helices to resting position, inactivation of channel (0.5-1.0 ms )

▲ FIGURE 7-33 Operational model of the voltage-gated Na+ channel. Four transmembrane domains in the protein contribute to the central pore through which ions move. The critical components that control movement of Na+ ions are shown here in the cutaway views depicting three of the four transmembrane domains. |1| In the closed, resting state, the voltage-sensing a helices, which have positively charged side chains every third residue, are attracted to the negative charges on the cytosolic side of the resting membrane. This keeps the gate segment in a position that blocks the channel, preventing entry of Na+ ions. |2| In response to a small depolarization, the voltage-sensing helices rotate in a screwlike manner toward the outer membrane surface, causing an immediate conformational change in the gate segment that opens the channel. |3| The voltage-sensing helices rapidly return to the resting position and the channel-inactivating segment moves into the open channel, preventing passage of further ions. |4|Once the membrane is repolarized, the channel-inactivating segment is displaced from the channel opening and the gate closes; the protein reverts to the closed, resting state and can be opened again by depolarization. [See W. A. Catterall, 2001, Nature 409:988; M. Zhou et al., 2001, Nature 411:657; and B. A. Yi and L. Y Jan, 2000, Neuron 27:423.]

them is rather small, as measured by the electric current passing through each open channel (the height of the blips). Depolarizing the membrane further to +20 mV causes these channels to open about twice as frequently. Also, more K+ ions move through each open channel (the height of the blips is greater) because the force driving cytosolic K+ ions out ward is greater at a membrane potential of +20 mV than at — 10 mV. Depolarizing the membrane further to +50 mV, the value at the peak of an action potential, causes opening of more K+ channels and also increases the flux of K+ through them. Thus, by opening during the peak of the action potential, these K+ channels permit the outward movement of K+

Two channels open

Two channels open

2QQ ms

▲ EXPERIMENTAL FIGURE 7-34 Probability of channel opening and current flux through individual voltage-gated K+ channels increases with the extent of membrane depolarization. These patch-clamp tracings were obtained from patches of neuronal plasma membrane clamped at three different potentials, +50, +20, and -10 mV. The upward deviations in the current indicate the opening of K+ channels and movement of K+

Ions outward (cytosolic to exoplasmlc face) across the membrane. Increasing the membrane depolarization (I.e., the clamping voltage) from -10 mV to +50 mV increases the probability a channel will open, the time it stays open, and the amount of electric current (numbers of ions) that pass through it. [From B. Pallota et al., 1981, Nature 293:471, as modified by B. Hille, 1992, Ion Channels of Excitable Membranes, 2d ed., Sinauer Associates, p. 122.]

ions and repolarization of the membrane potential while the voltage-gated Na+ channels are closed and inactivated.

More than 100 voltage-gated K+ channel proteins have been isolated from humans and other vertebrates. As we discuss later, all these channel proteins have a similar overall structure, but they exhibit different voltage dependencies, conductivities, channel kinetics, and other functional properties. However, many open only at strongly depolarizing voltages, a property required for generation of the maximal depolarization characteristic of the action potential before repolarization of the membrane begins.

Your Heart and Nutrition

Your Heart and Nutrition

Prevention is better than a cure. Learn how to cherish your heart by taking the necessary means to keep it pumping healthily and steadily through your life.

Get My Free Ebook


Post a comment