action potential
The action potential causes a change in potential as ions are temporarily exchanged in the cell membrane when muscles and nerves are excited.
Na+ and K+ ions present in the cell membrane differ in their ionic composition in and out of the cell by the action of the pump.
Due to this compositional difference, measuring the inside of the cell membrane with a voltmeter shows a potential of -60 to -90mV, that is, a resting potential.
Then, when cells are excited in nerves and muscles, the polarity inside and outside the cell membrane is changed, and the intracellular potential changes to a positive potential of +30~+40mV.
The figure below is a typical cell membrane action potential graph.
Now let's see how the ion channels change when stimulated.
Stage 1: Depolarization by a stimulus of sufficient magnitude raises the cell membrane voltage above a threshold.
Step 2: At the membrane voltage above the threshold, a voltage-gated Na+ channel opens and sodium (Na+) enters the cell through this channel and an inward current is generated, which leads to rapid depolarization wake up Voltage-gated Na+ channels open at a very high rate by stimulation above the threshold, creating the rapid depolarization seen in action voltages.
Step 3: When the action voltage reaches the peak, the voltage-gated Na+ channel is inactivated and the voltage-gated K+ channel is slowly opened to generate an outward current. As it is created, repolarization occurs.
Step 4: As the membrane voltage returns to the stable membrane voltage by repolarization, both the voltage-dependent sodium channel and the voltage-dependent potassium channel close, and the ion distribution across the cell membrane returns to its original state.
That is, the action voltage is basically a phenomenon that occurs when the voltage-gated Na+ channel and the voltage-gated K+ channel open and close according to the change in membrane voltage due to stimulation. In some types of cells, a voltage-gated Ca2+ channel is opened by a stimulus that causes depolarization to generate an inward current and an action voltage is initiated.
refractory period
In the steady state, cells do not respond to stimuli that can generate an action voltage, and this period is called the refractory period. This is because the voltage-gated Na+ channel is inactivated at a voltage that has a positive value than the resting membrane voltage. In the absolute refractory period, there is no response at all to the stimulus, and in the relative refractory period, the A small-scale reaction appears.
Question
A medical student is studying human physiology. She learns that there is a membrane potential across cell membranes in excitable cells. The differential distribution of anions and cations both inside and outside the cells significantly contributes to the genesis of the membrane potential. Which of the following distributions of anions and cations best explains the above phenomenon?
A. High concentration of Na+ outside the cell and high concentration of K+ inside the cell
B. High concentration of K+ outside the cell and low concentration of K+ inside the cell
C. High concentration of Ca2+ outside the cell and high concentration of Cl- inside the cell
D. Low concentration of Cl- outside the cell and high concentration of Cl- inside the cell
E. Low concentration of K+ outside the cell and high concentration of Ca2+ inside the cell
Explanation:
Correct answer A: A high concentration of Na+ outside the cell and a high concentration of K+ inside the cell play an important role in the genesis of the membrane potential.
In a typical excitable cell, the intracellular concentrations of Na+, Ca2+, and Cl- ions are low while the intracellular concentration of K+ ions is high. On the other side of the membrane, the concentration of K+ ions outside the cell is low while the concentrations of Na+, Ca2+, and Cl- ions outside the cell are high.
Intracellular K+ ions tend to move outside along the concentration gradient while extracellular K+ ions tend to move into the cell along the electrical gradient. Therefore, an equilibrium is reached, where the tendency of K+ ions to move outside is balanced by the tendency of K+ ions to move into the cell. At this equilibrium, there is a slight excess of cations and anions outside and inside the cells, respectively.
This condition is also maintained by the Na+/K+ ATPase pump in the cell membrane which utilizes the energy of ATP to move 3 Na+ ions outside the cell and 2 K+ ions inside their cell, against the concentration gradient. The activity of the Na+/K+-ATPase pump contributes to the genesis of the membrane potential.
Option B: In excitable tissues, there is a high concentration of K+ ions inside the cell and a low concentration of K+ ions outside the cell.
Option C: Although it is true that the concentration of Ca2+ ions is high outside the cell and the concentration of Cl- ions is low inside the cell, this is not the main reason for a membrane potential.
Option D: Normally, the concentration of Cl- ions is low inside the cell, while the concentration of Cl- ions is high outside the cell.
Option E: Normally, there is a low concentration of K+ ions outside the cell but there is a low concentration of Ca2+ ions inside the cell.
Learning objective: In a typical excitable cell, the intracellular concentrations of Na+, Ca2+, and Cl- ions are low, while the intracellular concentration of K+ ions is high. However, outside the cell, the concentration of K+ ions is low while the concentrations of Na+, Ca2+, and Cl- ions are high. The Na+/K+-ATPase pump contributes to the genesis of the membrane potential by moving Na+ and K+ ions against their concentration gradients.
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| Book References: First Aid for the USMLE Step 1 (2020, 30th ed): 456, 457 First Aid for the USMLE Step 1 (2019, 29th ed): 448, 449 First Aid for the USMLE Step 1 (2018, 28th ed): 446 First Aid for the USMLE Step 1 (2017, 27th ed): 433 |
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