Question

Once the threshold level is reached
A. The Na⁺ gates open
B. An action potential will be generated
C. The membrane is depolarised
D. The refractory period is over

Answer 1

Hint: As the membranes of the neurons are in a polarised state, these cells are excitable and different types of channels are present on the membranes of these nerve cells.

Complete Step by Step Answer:
The resting state is that phase when the neurons are not transmitting any nerve impulse and during this state the axonal membrane is more permeable to the potassium ions and almost impermeable to the sodium ions. Also, the axonal membrane is also impermeable to the negatively charged proteins present in the axoplasm and thus contains a higher concentration of K+ and negatively charged proteins and low concentration of Na+. Now this creates a concentration gradient across the membrane and 2 potassium ions enter the cell whereas 3 sodium ions exit the cell via the active transport. For this transportation sodium-potassium pump is used. This movement of ions results in the positively charged outer membrane and negatively charged inner membrane. This is the polarised state.

When a stimulus is applied to a specific region of the polarised membrane, it turns freely permeable to the sodium ions as a result of which sodium ions enter the cell. This results in the positively charged inner membrane and negatively charged outer membrane. This state is called the depolarised state and this establishes an electrical potential difference. This potential difference at the plasma membrane is called the nerve impulse or action potential. When the threshold level is reached in a depolarised state a new action potential will be generated.

Thus, the correct answer is option B:- An action potential will be generated.

Note: This depolarised membrane acts as a stimulus for the adjacent region of the membrane. The sodium ions exit the previous membrane and becomes repolarised. This causes the conduction of the nerve impulse.