In the context of cell signaling and action potentials, what type of ion channel opens first in response to a threshold stimulus?

• A. Ligand-gated chloride (Cl\(^-\)) channels
• B. Voltage-gated potassium (K\(^+\)) channels
• C. Calcium (Ca\(^{2+}\)) channels
• D. Voltage-gated sodium (Na\(^+\)) channels

Answer: (D)

Verified step by step guidance

1. Understand the context of the problem: The question is about cell signaling and action potentials, which are processes that occur in excitable cells like neurons and muscle cells. Action potentials are rapid changes in membrane potential that propagate along the cell membrane.
2. Recall the sequence of events in an action potential: When a threshold stimulus is reached, specific ion channels open to initiate the depolarization phase of the action potential. This is a key concept in neurobiology.
3. Identify the type of ion channel involved: Voltage-gated sodium (Na\(^+\)) channels are responsible for the initial depolarization phase. These channels open in response to the threshold stimulus, allowing Na\(^+\) ions to flow into the cell, making the inside of the cell more positive.
4. Compare the other options: Ligand-gated chloride (Cl\(^-\)) channels are typically involved in inhibitory signaling, not the initiation of action potentials. Voltage-gated potassium (K\(^+\)) channels open later in the action potential to repolarize the membrane. Calcium (Ca\(^{2+}\)) channels are involved in processes like neurotransmitter release but are not the first to open during an action potential.
5. Conclude the reasoning: Based on the sequence of events in an action potential, the correct answer is Voltage-gated sodium (Na\(^+\)) channels, as they are the first to open in response to a threshold stimulus, initiating the depolarization phase.