Sodium channel Sodium channel jjlijl jll frfrrt

Potassium channel

Sodium channel Sodium channel

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Sodium ion, Na+

Potassium ion, K+

Sodium ion, Na+

Potassium ion, K+

I During an action potential, sodium channels open, allowing sodium ions to move into the axon.

Axon (enlarged)

Sodium Potassium Q channel S channel^Q

Potassium channel figure 49-4

The presynaptic neuron releases neurotransmitter molecules into the synaptic cleft. These molecules bind to receptor proteins in the postsynaptic membrane, opening ion channels. Positive ions entering through these channels cause the membrane potential of the postsynaptic neuron to become more positive. If the membrane potential becomes sufficiently positive, the postsynaptic neuron will generate an action potential, continuing the signal.

Voltage-gated channels exist along the length of the axon. As a neuron is stimulated and the first small segment of the axon becomes more positively charged, the change in voltage opens channels in the next segment of axon membrane. As before, Na+ ions enter, driving the voltage in a positive direction and opening channels in the next segment of axon. In this way, positive charges pass down the axon membrane like dominoes falling in a row. Axon potentials usually travel in one direction only—away from the cell body, where they begin, and toward the axon terminal.

Shortly after they open, the voltage-gated channels for Na+ ions close. Then, voltage-gated channels for K+ ions open. The result is an abrupt outward flow of K+ ions. The exterior of the cell again becomes positively charged with respect to the interior of the cell. This change in charge signals the end of the action potential. However, the neuron cannot generate another action potential until resting potential is restored. This period, during which the neuron cannot send a signal, is called the refractory period.

After the action potential, the concentration of Na+ ions inside the cell is higher than when the cell is at rest, and the concentration of K+ ions inside the cell is lower. Ion channels and the sodium-potassium pump help reestablish the resting concentrations of Na+ ions and K+ ions. Na+ ions are moved out across the cell membrane, while K+ ions are moved in across the membrane. Once the original ion concentrations are restored, the neuron is ready for the next action potential. Restoration of resting potential comes at a price. Neurons need a continuous supply of ATP to keep the sodium-potassium pump operating. In fact, neurons consume a great deal of the body's daily energy.

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