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

The main cause of depolarization of a nerve is the influx of sodium ions (Na+). When a nerve is stimulated, the membrane becomes permeable to sodium ions, allowing them to enter the cell. This influx of positive ions leads to a change in the electrical charge across the membrane, resulting in depolarization. This depolarization is an essential step in the transmission of nerve impulses.

The ascending limb of nerve action potential is due to the movement of Na+ ions through voltage-gated channels. These channels open in response to a change in membrane potential, allowing Na+ ions to flow into the cell, resulting in depolarization and the generation of an action potential. Leak channels allow the passive movement of ions across the membrane, but they do not play a significant role in the ascending limb of the nerve action potential. Ligand-gated channels open in response to the binding of specific molecules, but they are not responsible for the ascending limb of the action potential either.

During repolarization, the cell membrane returns to its resting state after depolarization. This is achieved by the outflux of potassium ions (K+) from the cell. As the potassium channels open, potassium ions move out of the cell, restoring the negative charge inside the cell and bringing it back to its resting membrane potential. Therefore, the outflux of K+ ions is responsible for repolarization of the nerve.

After hyperpolarization refers to a period of hyperpolarization following an action potential, where the membrane potential becomes more negative than the resting potential. This is caused by the delayed closure of voltage-gated potassium (K+) channels. These channels open during the depolarization phase of the action potential, allowing K+ ions to leave the cell and repolarize the membrane. However, they do not immediately close after repolarization, leading to an efflux of K+ ions and further hyperpolarization. Therefore, the delayed closure of voltage-gated K+ channels is the main cause of after hyperpolarization.

The correct answer is that the outer membrane is more negative than the inner membrane. This is because the outer membrane of a cell typically has a negative charge due to the presence of negatively charged molecules, such as phospholipids and proteins. On the other hand, the inner membrane usually has a positive charge. This difference in charge between the outer and inner membranes creates a polarity, with the outer membrane being more negative.

The larger the diameter of a nerve, the faster the velocity of conduction because a larger diameter allows for a greater surface area for ion channels to be present, resulting in faster transmission of the action potential. Myleinated (medullated) nerves are indeed faster than non-myleinated nerves due to the presence of myelin sheath, which acts as an insulator and speeds up conduction. However, the statement that the larger the diameter, the longer the duration of the spike is false. The duration of the spike is not dependent on the diameter of the nerve but rather on other factors such as the properties of the ion channels involved and the refractory period of the nerve.

At the stage of complete depolarization of the action potential, the potential difference between the outer and inner surface is zero. This is because during depolarization, the membrane potential becomes more positive, reaching a peak value. At this point, the electrical charge inside and outside the cell equalizes, resulting in zero potential difference. This is a characteristic feature of the depolarization phase of the 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 fully. In the case of a whole skeletal muscle, it is made up of multiple muscle fibers that can contract individually. Therefore, the contraction of the whole skeletal muscle is not dependent on the all or none rule, as not all fibers may contract simultaneously.

After depolarization refers to the stage that occurs immediately after depolarization, where the membrane potential returns to its resting state. During depolarization, the membrane potential becomes more positive, and after depolarization, it returns to its negative resting state. This is a representation of a hypopolarized state because the membrane potential is lower than its resting potential.