Nervous system 5

• What is an action potential (AP)?

  An action potential (AP) is a rapid, temporary change in the membrane potential of a neuron. It is the way neurons communicate information by reversing the electrical charge across their membrane for a short time (about 1 millisecond)
• What are the stages of the action potential?

  the main stages of an AP are:

  • Depolarization: The membrane potential becomes less negative (more positive) as Na+ ions enter the cell.

  • Repolarization: The membrane potential becomes more negative again as K+ ions leave the cell.

  • Hyperpolarization: The membrane potential becomes more negative than the resting potential, briefly, before returning to the resting membrane potential (RMP).
• What is the ionic basis of the action potential?

  the AP is caused by changes in the permeability of the cell membrane to sodium (Na+) and potassium (K+) ions.

  • Increased Na+ permeability: Sodium channels open, allowing Na+ to flow into the cell, causing depolarization.

  • Increased K+ permeability: Potassium channels open later, allowing K+ to flow out, causing repolarization and the return to the RMP.
• What triggers an action potential?

  an action potential is triggered when the membrane potential reaches a certain level called the threshold. This occurs due to:

  • Inputs from other neurons or sensory activation.

  • The membrane must depolarize by 15-30 mV above the resting membrane potential (RMP) to reach the threshold and trigger the AP.
• What happens at the threshold of an action potential?

  at the threshold:

  • The membrane potential rapidly reverses from -60 mV to +40 mV.

  • Sodium channels open, allowing Na+ ions to rush into the cell.

  • When the membrane potential approaches +30 mV, sodium channels close (inactivate) and potassium channels open.
• How does an action potential return to rest?

  the membrane potential returns to rest through:

  • K+ efflux (K+ leaving the cell), driven by the electrochemical gradient, causing repolarization.

  • A brief period of hyperpolarization occurs (the membrane potential becomes more negative than RMP).

  • Sodium-potassium pump restores the proper ion concentrations to their resting levels.
• What is the refractory period (RP)?

  the refractory period is a time after an action potential during which the neuron is unable (or less able) to fire another AP. It includes:

  • Absolute Refractory Period (ARP): From the opening of sodium channels until they inactivate. During this period, the neuron cannot generate another AP, no matter how strong the stimulus is.

  • Relative Refractory Period (RRP): Occurs after the ARP, when sodium channels return to their resting state. During this time, a stronger-than-normal stimulus can trigger an AP.
• Why is the refractory period important?

  The refractory period ensures that action potentials:

  • Do not overlap and are generated in a controlled manner.

  • Only move in one direction along the axon (orthodromic propagation), from the axon hillock to the axon terminals.

  • Prevents retrograde propagation (movement backward), which is blocked by the ARP.
• How is the action potential propagated along the axon?

  the action potential is propagated by local currents. As one part of the membrane depolarizes, it causes the neighboring membrane region to depolarize, spreading the AP. In myelinated axons, this occurs at the nodes of Ranvier in a process called saltatory conduction—the AP jumps from node to node, speeding up transmission.
• How does the frequency of action potentials code information?

  information in the nervous system is frequency-coded, meaning that the number of action potentials per second (AP frequency) encodes the strength of the stimulus. A stronger stimulus generates more frequent action potentials.
• What is the role of voltage-gated sodium and potassium channels in the action potential?

  voltage gated sodium channels open at the threshold, allowing Na+ to enter the neuron, causing depolarization.

  • Voltage-gated potassium channels open later, allowing K+ to leave the neuron, causing repolarization and the return to the resting membrane potential.
• What is saltatory conduction, and where does it occur?

  saltatory conduction occurs in myelinated axons, where the action potential jumps from one node of Ranvier to the next, speeding up the transmission of the signal along the axon.
• Why is the concept of the refractory period crucial in ensuring proper action potential propagation?

  the refractory period ensures that the action potential can only travel in one direction, from the cell body to the axon terminals, and prevents the neuron from firing multiple action potentials at once, maintaining clear and controlled communication.
• How does the sodium-potassium pump contribute to the action potential?

  after the action potential, the sodium-potassium pump restores the correct ion balance by pumping Na+ out of the cell and K+ back in, helping return the membrane to its resting state and ready for the next action potential.