9.1 Membrane excitability and action potentials

Neurons communicate through electrical signals called action potentials. These rapid changes in membrane voltage are crucial for transmitting information in the nervous system. Understanding the mechanisms behind action potentials is key to grasping how neurons function and interact.

Action potentials rely on the coordinated opening and closing of ion channels. These protein structures allow specific ions to flow across the cell membrane, creating electrical currents that drive the different phases of an action potential. Voltage-gated sodium and potassium channels play starring roles in this process.

Ion Channels in Action Potentials

Role of Ion Channels

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• Ion channels are integral membrane proteins that allow the selective passage of specific ions across the cell membrane, creating electrical currents
• Voltage-gated ion channels open or close in response to changes in the membrane potential, allowing ions to flow down their electrochemical gradients
• The opening and closing of voltage-gated sodium (Na+) and potassium (K+) channels are primarily responsible for generating and propagating action potentials in excitable cells (neurons, muscle cells)
• Ligand-gated ion channels, such as those activated by neurotransmitters (acetylcholine, GABA), can also contribute to the generation of action potentials by altering the membrane potential

Types of Ion Channels Involved

• Voltage-gated sodium (Na+) channels rapidly open during the rising phase of the action potential, allowing Na+ influx and driving depolarization towards the Na+ equilibrium potential
• Voltage-gated potassium (K+) channels open with a delay during the falling phase, allowing K+ efflux and driving repolarization towards the K+ equilibrium potential
• The Na+/K+ ATPase pump maintains the resting ion concentrations by actively transporting Na+ out of the cell and K+ into the cell, setting the stage for the next action potential
• Other ion channels, such as calcium (Ca2+) channels, can also contribute to action potential generation and modulation in some cell types (cardiac muscle cells)

Action Potential Phases and Mechanisms

Phases of an Action Potential

• Resting phase occurs when the cell is at its resting membrane potential, typically around -70 mV, due to the unequal distribution of ions across the membrane maintained by the Na+/K+ ATPase pump and the selective permeability of the membrane to K+ ions
• Rising phase (depolarization) is initiated when the membrane potential reaches the threshold value, causing voltage-gated Na+ channels to open rapidly, allowing Na+ ions to flow into the cell and driving the membrane potential towards the Na+ equilibrium potential
• Peak of the action potential occurs when the membrane potential reaches a maximum value, typically around +40 mV, due to the influx of Na+ ions
• Falling phase (repolarization) begins with the inactivation of voltage-gated Na+ channels and the delayed opening of voltage-gated K+ channels, allowing K+ ions to flow out of the cell, driving the membrane potential back towards the K+ equilibrium potential
• Hyperpolarization phase occurs when the membrane potential briefly overshoots the resting potential due to the continued efflux of K+ ions before the voltage-gated K+ channels close, and the Na+/K+ ATPase pump restores the resting ion concentrations

Refractory Periods

• Absolute refractory period is the time during which a second action potential cannot be initiated, regardless of the stimulus strength, due to the inactivation of voltage-gated Na+ channels
  1. Occurs during the peak and early falling phase of the action potential
  2. Prevents the action potential from propagating backward along the axon
• Relative refractory period is the time during which a stronger-than-normal stimulus is required to initiate another action potential, as the membrane potential is closer to the resting potential and some voltage-gated Na+ channels have recovered from inactivation
  1. Occurs during the late falling phase and hyperpolarization phase
  2. Limits the maximum frequency of action potential firing

Membrane Potential and Ion Channels

Relationship between Membrane Potential and Ion Channel States

• The state of voltage-gated ion channels (open, closed, or inactivated) is determined by the membrane potential and the channels' specific voltage-dependent conformational changes
• Voltage-gated Na+ channels have three main states: closed (resting), open (activated), and inactivated
  1. At the resting membrane potential, most Na+ channels are closed
  2. When the membrane potential reaches the threshold, the channels rapidly open, allowing Na+ influx
  3. After a brief period, the channels inactivate, preventing further Na+ influx
• Voltage-gated K+ channels have two main states: closed (resting) and open (activated)
  1. At the resting membrane potential, most K+ channels are closed
  2. During the action potential, the channels open with a slight delay compared to Na+ channels, allowing K+ efflux and contributing to repolarization

Voltage-Dependent Activation and Inactivation

• The membrane potential at which half of the channels are activated is called the half-activation voltage, which differs for various types of voltage-gated ion channels
• The steepness of the voltage-dependent activation and inactivation curves determines the sensitivity of the channels to changes in membrane potential
  1. A steeper activation curve indicates that a small change in membrane potential can lead to a large change in the probability of the channel being open
  2. A steeper inactivation curve indicates that the channel is more likely to inactivate rapidly after opening
• The voltage-dependent properties of ion channels shape the action potential waveform and determine the threshold potential for action potential initiation

Action Potential Propagation Factors

Myelination and Saltatory Conduction

• Myelination, the insulation of axons by myelin sheaths formed by Schwann cells (in the peripheral nervous system) or oligodendrocytes (in the central nervous system), increases the speed and efficiency of action potential propagation by enabling saltatory conduction
• In myelinated axons, action potentials are regenerated at the nodes of Ranvier, where there is a high density of voltage-gated Na+ channels, allowing the action potential to jump from one node to the next (saltatory conduction)
  1. Saltatory conduction reduces the capacitance and increases the resistance of the axonal membrane, allowing for faster and more energy-efficient propagation
  2. Myelination enables faster conduction velocities (up to 150 m/s) compared to unmyelinated axons (up to 2 m/s)

Axon Diameter and Ion Channel Density

• The diameter of the axon affects the speed of action potential propagation, with larger diameter axons having lower resistance and faster conduction velocities
  1. Larger axons have a greater cross-sectional area, which reduces the internal resistance to current flow
  2. In unmyelinated axons, conduction velocity is proportional to the square root of the axon diameter
• The density and distribution of voltage-gated ion channels along the axon influence the speed and efficiency of action potential propagation
  1. A higher density of channels allows for faster depolarization and repolarization
  2. Non-uniform distribution of ion channels, such as clustering at the nodes of Ranvier, facilitates saltatory conduction in myelinated axons

Temperature and Refractory Periods

• The temperature of the environment can affect the speed of action potential propagation, with higher temperatures generally increasing the rate of ion channel kinetics and thus the conduction velocity
  1. A 10°C increase in temperature can lead to a 1.5 to 2-fold increase in conduction velocity
  2. Temperature changes can also influence the duration of action potentials and refractory periods
• The refractory periods limit the maximum frequency at which action potentials can be generated and propagated, as a new action potential cannot be initiated until the ion channels have recovered from their inactivated states
  1. The absolute refractory period sets the upper limit for the maximum firing rate of an axon
  2. The relative refractory period influences the probability of action potential initiation in response to subsequent stimuli