In an action potential, which event occurs first after a neuron reaches threshold?

• A. Neurotransmitter is released into the synaptic cleft
• B. Voltage-gated Na\(^+\) channels open, causing Na\(^+\) influx
• C. The membrane potential becomes more negative (hyperpolarization)
• D. Voltage-gated K\(^+\) channels open, causing K\(^+\) efflux

Answer: (B)

Verified step by step guidance

1. Step 1: Understand the concept of an action potential. An action potential is a rapid change in membrane potential that occurs when a neuron sends a signal. It involves specific ion channels and changes in ion movement across the membrane.
2. Step 2: Recognize the threshold concept. The threshold is the critical level of membrane depolarization required to trigger an action potential. Once the threshold is reached, voltage-gated ion channels are activated.
3. Step 3: Identify the sequence of events in an action potential. The first event after reaching the threshold is the opening of voltage-gated Na⁺ channels. This allows Na⁺ ions to flow into the neuron, causing depolarization (a more positive membrane potential).
4. Step 4: Compare the options provided. The correct sequence begins with the opening of voltage-gated Na⁺ channels and Na⁺ influx. Other events, such as neurotransmitter release or hyperpolarization, occur later in the process.
5. Step 5: Relate this to the physiological process. The influx of Na⁺ ions is crucial for the rapid depolarization phase of the action potential, which is necessary for the propagation of the signal along the neuron.