Which statement correctly describes what causes the second voltage-gated Na+ channel to open?

A. As Naions enter the cell through the first channel, they spread out from the channel. When these Na+ ions reach the second channel, it opens.

B. As Na+ ions enter the cell through the first channel, Na+ ions outside the cell move toward the open Na+ channel. When the concentration of Na+ ions near the second channel becomes low enough, the second channel opens.

C. After the first channel opens, the movement of Na+ ions (both inside and outside the cell) alters the Na+ ion distribution across the membrane near the second channel, causing it to open.

D. After the first channel opens, the movement of many types of ions (both inside and outside the cell) alters the distribution of charges near the second channel, causing it to open.

Diagram showing ions that form the membrane potential

The correct answer is D. After the first channel opens, the movement of many types of ions (both inside and outside the cell) alters the distribution of charges near the second channel, causing it to open.

Our nervous system contains cells that send information to and from the brain and spinal cord. These nerve cells that transmit information are known as neurons.

Sensory nerves receive stimuli from receptors and then transmit this information from cell to cell to the central nervous system. A response is then sent back along motor nerves to a gland or muscle to bring about an appropriate response.

Such cells work by a change in voltage or membrane potential that occurs across the plasma membrane. At rest when no impulse is being transmitted the voltage difference is -70mV

This voltage difference is determined by the relative concentration of potassium and sodium ions inside compared with outside of the nerve cell.

When a nerve stimulus arrives at the dendrites of a neuron, this triggers the opening of voltage-gated sodium channels. Sodium rushes into the cell across the membrane which causes a change in the membrane potential and voltage changes to 30mV.

Eventually, these channels close and the voltage-gated potassium channels open causing a flow of potassium ions out of the cell.

It is the relative changes in the concentrations of these different ions across the cell membrane of the nerve cell that causes the action potential to be propagated.

The neuron

The nervous system consists of cells that transmit impulses, called neurons, and various supporting cells that assist these cells. The supporting cells include various neuroglia such as Schwann cells and oligodendrocytes.

Information from sensory receptors in the body sends signals along sensory nerves (neurons) to the brain where the information is integrated and interpreted. A response is then sent back along motor nerves (neurons) to an effector organ such as a gland or a muscle.

A neuron consists of extensions called dendrites and axons, and a cell body with a nucleus. The nerve impulse enters the dendrites, travels through the cell body and along the axon to axon terminals at the ends of the cell.

The impulse then moves from these terminals across a synaptic cleft which is the gap where another nerve cell or an effector cell is present.

Membrane potential and voltage

The nerve impulse is also called an action potential because it involves a change in voltages. The nerve cell is said to be at rest when no impulse is being transmitted, and at this time the membrane potential is negative.

The potential is the difference in charges that exists between the outside and inside of the neuron. This charge difference is due to the concentrations of different ions in the cell versus out the cell.

At rest, there are usually more sodium ions compared with potassium outside the cell than inside, while there are more potassium ions inside the neuron compared with sodium.

This difference in these two ions creates a chemical gradient and determines the voltage change across the membrane.

It is important to realize that each ion carries a charge and thus the relative concentrations of these charged ions will, of course, impact the overall charge across the cell membrane.

The movement of ions

Ions are able to move across a plasma membrane by passing through special integral proteins that are embedded in the membrane. These proteins act as channels that can open and close to carefully regulate the flow of ions.

There are different types of ion channels, some remain open but others only open when they receive a signal.

At rest, the difference in charge due to the relative concentrations of potassium and sodium ions is -70mV, which means the outside is more negative than inside the cell. At this time many of the ion channels are closed.

This difference is maintained by a sodium-potassium pump which works to maintain the resting membrane potential. It does this by pumping potassium in and at the same time, pumping sodium out of the cell.

It pumps two potassium ions for every three sodium ions. This is an energetically costly process that does need ATP to work.

Besides the sodium-potassium pumps, other polypeptides form channels that are voltage-gated protein channels. There are both sodium and potassium voltage-gated channels present in the membrane.

A nerve action potential

The action potential is the signal that is transmitted along the neuron, and it is how information flows to and from our central nervous system (brain and spinal cord).

At rest, the voltage-gated sodium channels are closed. When a stimulus is received these channels are activated to open, which allows the movement of ions across the plasma membrane.

It is important to realize that various triggers can act as stimuli to open sodium channels. Stimuli such as sound, light or even temperature changes can activate our sensory nerve cells to transmit a signal which can trigger different types of sodium channels to open.

Sodium Ions

If a large enough stimulus is received then this triggers the voltage-gated sodium channels to open. Sodium ions then move into the cell down their concentration gradient.

This is because there are more sodium ions outside than inside the cell and the natural tendency is for substances to move from high to a low concentration.

The movement of these ions continues until a voltage charge of 30 mV is reached, which then stimulates these channels to close.

Voltage-gated potassium channels now open and these ions travel across the membrane and pass out of the cell down their gradient from high to low.

The sodium-potassium pump continues to function and eventually the resting membrane potential is reached again.


  1. Editors of Encyclopedia Britannica (2018). Action potential. Retrieved from Britannica.com.
  2. RL Dorit, WF Walker, RD Barnes (1991).  Zoology. Philadelphia: USA, Saunders College Publishing.
  3. Editors of Encyclopedia Britannica (2018). Neuron. Retrieved from Britannica.com.
  4. OFG Kilgour (1982). Mastering Biology.London: UK, MacMillan Press.
  5. C Rye, R Wise, V Jurukovski, J DeSaix, J Choi, Y Avissar (2017). Biology. Houston: USA, Rice University.


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