Dr Gawad Course Online Exam - Excitable - Lec 2 (Action Potential, Compound Action Potential, All Or None Rule)

The main cause of depolarization of a nerve is the influx of sodium ions (Na+). When a nerve is at rest, there is a higher concentration of sodium ions outside the cell and a higher concentration of potassium ions (K+) inside the cell. However, when a nerve is stimulated, sodium channels open, allowing sodium ions to rush into the cell, leading to depolarization. This influx of sodium ions changes the electrical charge inside the cell, allowing the nerve impulse to propagate along the nerve.

During repolarization, the cell membrane returns to its resting state after depolarization. This is achieved by the outflow of potassium ions (K+) from the cell, which restores the negative charge inside the cell and brings it back to its resting membrane potential. The outflux of K+ is crucial for repolarization as it counteracts the influx of sodium ions (Na+) during depolarization. Therefore, the correct answer is "out flux of K."

After hyperpolarization is caused by the delayed closure of voltage-gated potassium (K+) channels. During an action potential, these channels open to allow potassium ions to leave the cell, repolarizing it. However, after the action potential, these channels remain open for a short period, causing an excessive outflow of potassium ions, leading to hyperpolarization. This hyperpolarization is responsible for resetting the membrane potential and ensuring that the cell is ready for the next action potential.

The whole skeletal muscle is not applying the all or none rule. The all or none rule states that when a nerve impulse reaches a muscle fiber, the fiber will contract completely or not at all. However, in the case of a whole skeletal muscle, not all muscle fibers may be activated at the same time. Different muscle fibers within a skeletal muscle can be activated to varying degrees, resulting in graded contractions and allowing for fine motor control. Therefore, the whole skeletal muscle does not follow the all or none rule.

The ascending limb of a nerve action potential refers to the depolarization phase, where the membrane potential becomes more positive. This depolarization is primarily caused by the movement of sodium ions (Na+) into the cell. Voltage-gated channels are responsible for allowing the flow of Na+ ions into the cell during this phase, as they open in response to the depolarization of the membrane. Therefore, the correct answer is voltage-gated channels.

The statement that the larger the diameter, the longer the duration of the spike is false. In reality, the larger the diameter of the nerve fiber, the faster the conduction velocity and the shorter the duration of the spike. This is because a larger diameter allows for faster propagation of the action potential.

During the stage of complete depolarization of the action potential, the potential difference between the outer and inner surface of the cell membrane is zero. This is because the influx of sodium ions and the efflux of potassium ions cause a balanced charge distribution, resulting in no net potential difference across the membrane. This is a crucial point in the action potential where the cell is in its refractory period and unable to generate another action potential until repolarization occurs.

After depolarization refers to the period following the rapid depolarization phase of an action potential. During this stage, the membrane potential returns to its resting state after being briefly depolarized. This is considered a representation of a hypopolarized state because the membrane potential is still slightly elevated compared to the resting potential. It is important to note that hypopolarization is a relative term and refers to a less negative membrane potential compared to the resting state, but it is not as positive as during depolarization.

The statement that the outer membrane is more negative than the inner membrane is true. In a cell, the outer membrane is generally more negatively charged compared to the inner membrane. This is due to the distribution of ions across the cell membrane. The outer membrane has a higher concentration of negatively charged ions, such as chloride ions, while the inner membrane has a higher concentration of positively charged ions, such as potassium ions. This difference in charge creates an electrical potential across the cell membrane, with the outer membrane being more negative.