Action Potentials

Action Potentials

Lecture Objectives

• Explain how and where action potentials can be triggered in a typical neuron.

• Describe the key events (changes in ion permeability, transmembrane ion movement, and membrane potential) that occur during an action potential in a neuronal membrane.

• Explain how action potentials are used to send signals between multiple neurons in a pathway.

• Explain the differences between the propagation of action potentials along unmyelinated versus myelinated axons.

Outline

• Action potential initiation.

• Key Events of an action potential.

• Conduction of action potentials in unmyelinated vs myelinated axons.

Membrane Potential

• The membrane potential (Vm) is a compromise between the equilibrium potentials of all permeable ions.

• $Vm$ is always closest to the equilibrium potential of the ion species to which the cell is most permeable.

• At rest, potassium permeability ($P<em>K$) is much greater than sodium permeability ($P</em>{Na}$), therefore $Vm$ is close to the equilibrium potential for potassium ($E_K$).

• Changes in ion permeability alter membrane potential.

• Increase in sodium permeability ($P_{Na}$) leads to depolarization.

• Increase in potassium permeability ($P_K$) leads to repolarization or hyperpolarization.

Withdrawal Reflex

• The withdrawal reflex is used to understand the role and physiology of action potentials.

• Components:

  • Afferent (sensory) neuron.

  • Efferent (motor) neuron.

  • Interneuron (in the spinal cord).

  • Effector (muscle).

Initiation of Action Potentials

• Touch-sensitive neurons are activated by mechanical force.

• Opening of mechanically gated ion channels alters membrane ion permeability.

• Main effect: increase in $P_{Na}$, leading to depolarization.

• Mechanically gated sodium-permeable channels are located on dendrites.

• Increased $P_{Na}$ triggers a depolarizing local potential (receptor potential).

• Receptor potential spreads to the initial segment.

• Mechanically-gated channels are also known as stretch-gated channels.

Local Potentials

• Small deviations in resting membrane potential that occur in response to the opening or closing of mechanically-gated or ligand-gated ion channels.

• Degree of deviation depends on the strength of the stimulus, which is affected by many factors.

Threshold

• At the initial segment, a depolarizing local potential triggers the opening of voltage-gated $Na^+$ channels if $Vm$ goes above the threshold for that channel.

• Threshold is typically around -65 mV.

Action Potentials Characteristics

• Large in magnitude (>100 mV).

• Standard size (all-or-none).

• No size change as they spread (actively propagated).

• Useful over long distances.

• Sequence of depolarization and repolarization along an axon to send the signal to the axon terminal.

Action Potential Triggering

• An action potential (AP) can be triggered by a depolarizing local potential reaching the initial segment if it is large enough.

Sequence of Events in an Action Potential

1. Resting State:

   • Voltage-gated $Na^+$ channels are closed.

   • Membrane potential is at resting level (e.g., -70 mV).

2. Depolarization:

   • A triggering event causes the membrane potential to reach threshold.

   • Voltage-gated $Na^+$ channel activation gates open quickly.

   • $Na^+$ ions rush into the cell, causing rapid depolarization.

3. Repolarization:

   • $Na^+$ inactivation gates close shortly after activation gates open (intrinsic property).

   • Voltage-gated $K^+$ channels open more slowly.

   • $K^+$ ions flow out of the cell, causing repolarization.

4. Hyperpolarization:

   • $K^+$ channels remain open for a short time, causing the membrane potential to dip below the resting level.

   • $Na^+$ inactivation gate opens, and the activation gate closes.

5. Return to Resting State:

   • $K^+$ channels close.

   • Membrane potential returns to resting level.

Phases of Action Potential

• Rising Phase:

  • Caused by $Na^+$ influx (increase in $P_{Na}$).

• Falling Phase:

  • Caused by $K^+$ efflux (increase in $P_K$).

Importance of Ion Permeability

• Changes in ion permeability ($P<em>{Na}$, $P</em>K$) explain the different phases of the action potential.

Action Potential Propagation

Unmyelinated Axons

• $Na^+$ entry results in a depolarizing local potential.

• AP at one point triggers AP in the next section.

• AP conduction velocity is slow (e.g., 0.1 - 2 m/s).

• Due to the refractory period, action potentials cannot change directions.

Myelinated Axons

• Glial cells (Schwann cells in the peripheral nervous system and oligodendrocytes in the central nervous system) provide myelin coats to some axons.

• APs occur only at Nodes of Ranvier: saltatory conduction.

• AP conduction velocity is rapid (e.g., 100 m/s).

• Due to the refractory period, action potentials cannot change directions.

• Example of significance: multiple sclerosis (where myelin is damaged).

Continuous vs. Saltatory Conduction

• Continuous Conduction (Unmyelinated Axons):

  • Current flow due to the opening of $Na^+$ channels.

  • Action potential propagates along the entire axon.

• Saltatory Conduction (Myelinated Axons):

  • Current flow due to the opening of $Na^+$ channels only at the Nodes of Ranvier.

  • Action potential jumps from node to node, increasing conduction velocity.