Chapter 12 - Section 12.4 - Study Guide - Assess Your Learning Outcomes - Page 471: 5

The mechanism of an action potential is a complex electrochemical process that involves changes in the flow of ions across the neuronal membrane. This process is tightly regulated by the action of various types of membrane channels. Let's explore the mechanism of an action potential, how it relates to ion flows and membrane channels, and the concepts of depolarization and repolarization: **1. Mechanism of an Action Potential:** The mechanism of an action potential can be summarized in several key steps: **a. Resting Membrane Potential:** Neurons typically have a resting membrane potential of around -70 millivolts (mV). This resting state is maintained by the differential distribution of ions across the neuronal membrane, primarily sodium (Na+), potassium (K+), chloride (Cl-), and negatively charged proteins (A-). **b. Depolarization:** The action potential begins when the neuron receives a stimulus, which can be excitatory. If the stimulus is strong enough to reach the threshold potential (typically around -55 to -50 mV), it triggers the opening of voltage-gated sodium channels at the trigger zone (axon hillock). - Sodium Influx: The opening of voltage-gated sodium channels allows a rapid influx of sodium ions into the neuron, leading to a rapid increase in the membrane potential (depolarization). - Positive Feedback: The influx of sodium ions further depolarizes the membrane, creating a positive feedback loop. More sodium channels open, and the membrane potential rapidly becomes more positive. **c. Rising Phase:** As sodium ions continue to flow into the neuron, the membrane potential rises towards a positive value, usually around +30 mV. **d. Repolarization:** To prevent over-excitability and maintain proper signaling, the neuron must repolarize. This is achieved by the opening of voltage-gated potassium channels. - Potassium Efflux: Voltage-gated potassium channels open in response to the depolarization. This allows potassium ions (K+) to move out of the neuron, restoring the negative membrane potential. - Closing Sodium Channels: Voltage-gated sodium channels become inactivated or close during this phase, preventing further sodium influx. **e. Falling Phase:** The membrane potential rapidly falls back toward the resting level due to the efflux of potassium ions. **f. Hyperpolarization (Undershoot):** In some cases, the membrane potential may briefly overshoot the resting level, creating a hyperpolarized state. This occurs because potassium channels may remain open for a short time before closing. **2. Ion Flows and Membrane Channels:** - **Sodium Channels:** Voltage-gated sodium channels are responsible for the rapid depolarization phase. They open in response to depolarization and allow sodium ions to enter the neuron. - **Potassium Channels:** Voltage-gated potassium channels are crucial for repolarization and the restoration of the resting membrane potential. They open in response to depolarization and allow potassium ions to exit the neuron. - **Sodium-Potassium Pump:** After an action potential, the sodium-potassium pump (Na+/K+ pump) helps restore the ion concentration gradients by actively transporting sodium ions out of the neuron and potassium ions back into it. **3. Depolarization and Repolarization:** - **Depolarization:** Depolarization refers to the process in which the membrane potential becomes less negative (more positive) than the resting membrane potential. This is primarily caused by the influx of positively charged ions, such as sodium, into the neuron. - **Repolarization:** Repolarization is the process by which the membrane potential returns to its resting level (approximately -70 mV) after depolarization. This is primarily due to the efflux of positively charged ions, such as potassium, out of the neuron. In summary, the mechanism of an action potential involves changes in ion flows across the neuronal membrane through the action of voltage-gated sodium and potassium channels. Depolarization is the process of membrane potential becoming more positive due to sodium influx, while repolarization is the process of returning the membrane potential to its resting state through potassium efflux. These electrochemical processes underlie the generation and propagation of action potentials in neurons, allowing them to transmit electrical signals.

The mechanism of an action potential is a complex electrochemical process that involves changes in the flow of ions across the neuronal membrane. This process is tightly regulated by the action of various types of membrane channels. Let's explore the mechanism of an action potential, how it relates to ion flows and membrane channels, and the concepts of depolarization and repolarization: **1. Mechanism of an Action Potential:** The mechanism of an action potential can be summarized in several key steps: **a. Resting Membrane Potential:** Neurons typically have a resting membrane potential of around -70 millivolts (mV). This resting state is maintained by the differential distribution of ions across the neuronal membrane, primarily sodium (Na+), potassium (K+), chloride (Cl-), and negatively charged proteins (A-). **b. Depolarization:** The action potential begins when the neuron receives a stimulus, which can be excitatory. If the stimulus is strong enough to reach the threshold potential (typically around -55 to -50 mV), it triggers the opening of voltage-gated sodium channels at the trigger zone (axon hillock). - Sodium Influx: The opening of voltage-gated sodium channels allows a rapid influx of sodium ions into the neuron, leading to a rapid increase in the membrane potential (depolarization). - Positive Feedback: The influx of sodium ions further depolarizes the membrane, creating a positive feedback loop. More sodium channels open, and the membrane potential rapidly becomes more positive. **c. Rising Phase:** As sodium ions continue to flow into the neuron, the membrane potential rises towards a positive value, usually around +30 mV. **d. Repolarization:** To prevent over-excitability and maintain proper signaling, the neuron must repolarize. This is achieved by the opening of voltage-gated potassium channels. - Potassium Efflux: Voltage-gated potassium channels open in response to the depolarization. This allows potassium ions (K+) to move out of the neuron, restoring the negative membrane potential. - Closing Sodium Channels: Voltage-gated sodium channels become inactivated or close during this phase, preventing further sodium influx. **e. Falling Phase:** The membrane potential rapidly falls back toward the resting level due to the efflux of potassium ions. **f. Hyperpolarization (Undershoot):** In some cases, the membrane potential may briefly overshoot the resting level, creating a hyperpolarized state. This occurs because potassium channels may remain open for a short time before closing. **2. Ion Flows and Membrane Channels:** - **Sodium Channels:** Voltage-gated sodium channels are responsible for the rapid depolarization phase. They open in response to depolarization and allow sodium ions to enter the neuron. - **Potassium Channels:** Voltage-gated potassium channels are crucial for repolarization and the restoration of the resting membrane potential. They open in response to depolarization and allow potassium ions to exit the neuron. - **Sodium-Potassium Pump:** After an action potential, the sodium-potassium pump (Na+/K+ pump) helps restore the ion concentration gradients by actively transporting sodium ions out of the neuron and potassium ions back into it. **3. Depolarization and Repolarization:** - **Depolarization:** Depolarization refers to the process in which the membrane potential becomes less negative (more positive) than the resting membrane potential. This is primarily caused by the influx of positively charged ions, such as sodium, into the neuron. - **Repolarization:** Repolarization is the process by which the membrane potential returns to its resting level (approximately -70 mV) after depolarization. This is primarily due to the efflux of positively charged ions, such as potassium, out of the neuron. In summary, the mechanism of an action potential involves changes in ion flows across the neuronal membrane through the action of voltage-gated sodium and potassium channels. Depolarization is the process of membrane potential becoming more positive due to sodium influx, while repolarization is the process of returning the membrane potential to its resting state through potassium efflux. These electrochemical processes underlie the generation and propagation of action potentials in neurons, allowing them to transmit electrical signals.