BS2015 block 2 lexture 3

Introduction

  • Purpose of the session: Encourage students to reflect on what they hope to learn and come up with questions during the lecture, fostering active learning and critical thinking.

  • Use of Post-It notes: Implement a method for active engagement during the lecture, allowing students to anonymously pose questions or note key takeaways that can be addressed.

Action Potentials
Definition and Mechanism

  • Action Potential (AP): A rapid, transient, and self-propagating change in the membrane potential of an excitable cell (like neurons or muscle cells), initiated when the membrane depolarizes to a critical threshold potential. This change is primarily due to the sequential, voltage-gated movement of ions, generating electrical current.

  • Mechanism: It involves a rapid influx of positive ions (typically Na+) causing depolarization, followed by an efflux of positive ions (typically K+) causing repolarization, and often a subsequent hyperpolarization.

  • Propagation: An action potential is self-propagating along the axon without decrement, due to the regeneration of the signal at successive points along the membrane as it moves.

Characteristics of Action Potentials

  • All-or-None Principle: Action potentials either occur fully if the threshold is reached, or not at all. Subthreshold stimuli produce only local, graded potentials.

  • Uniformity in Amplitude: The amplitude of action potentials (typically around +30 to +50 mV from resting potential) remains consistent throughout the entire length of the axon, regardless of transmission distance. This ensures signal integrity over long distances.

  • Example Measurement: When measuring the action potential at different points along an axon, both recording points will exhibit the same amplitude and similar waveform, demonstrating faithful propagation.

  • Shape of Action Potential: While the amplitude is constant, the duration and specific shape of action potentials can vary significantly across different cell types (e.g., shorter in the SA node compared to ventricular muscle cells), reflecting differences in the types and kinetics of ion channels expressed.

Ionic Conductance During Action Potentials
Sodium Conductance

  • Activation: Voltage-gated sodium channels possess two gates: an activation gate and an inactivation gate. In response to depolarization reaching the threshold, the activation gates open rapidly, allowing a massive influx of sodium ions (Na+) into the cell. This inward current is driven by both the electrochemical gradient (high extracellular Na+, negative intracellular potential), which further depolarizes the membrane (rising phase of the AP).

  • Inactivation: Almost immediately after opening, the inactivation gates of the sodium channels close. This rapidly blocks further Na+ influx and is a crucial event for the repolarization phase and the absolute refractory period.

  • Transient Nature: The action potential's rising phase is characterized by this very rapid and transient increase in sodium conductance, leading to a swift membrane depolarization.

Potassium Conductance

  • Delayed Activation: Following the opening of sodium channels and the rising phase of depolarization, voltage-gated potassium channels begin to open, but with a significant delay compared to sodium channels.

  • Repolarization Mechanism: Once open, these channels allow potassium ions (K+) to exit the cell, driven by their electrochemical gradient (high intracellular K+, positive intracellular potential during depolarization). This outward positive current causes the inside of the cell to become more negative, leading to membrane repolarization (falling phase of the AP).

  • Hyperpolarization: Due to their slower closing kinetics, potassium channels often remain open briefly after the membrane has repolarized to the resting potential, causing a transient hyperpolarization (undershoot) where the membrane potential becomes even more negative than the resting potential.

Refractory Periods
Absolute Refractory Period

  • Definition: A period during which no additional action potential can be generated, regardless of the strength of the stimulus. This is primarily due to the inactivation of voltage-gated sodium channels, making them unresponsive to further depolarization.

  • Significance: This period ensures that action potentials propagate in only one direction along the axon (unidirectional propagation) and limits the maximum frequency at which a neuron can fire.

Relative Refractory Period

  • Definition: A phase immediately following the absolute refractory period, during which it is possible to trigger another action potential, but only with a stronger than normal stimulus. This is because some sodium channels have recovered from inactivation, but many potassium channels are still open (leading to hyperpolarization and increased K+ conductance), thus making the cell harder to depolarize to threshold.

  • Recovery Phase: As sodium channels recover from inactivation and the membrane repolarizes (and hyperpolarization subsides), the cell gradually regains its normal excitability, preparing for the next potential action potential.

Experimental Evidence and Observations
Current Injection Experiment

  • Instrumentation: Sophisticated use of microelectrodes to inject precise amounts of current into a neuron and simultaneously record the resultant changes in membrane potential.

  • Graph Observations: Subthreshold current injections may cause local, graded potentials that wax and wane (exhibiting an exponential rise and fall). However, once the injected current depolarizes the membrane to the threshold potential, a full action potential is triggered, characterized by its rapid rise and fall.

  • Threshold Behavior: The relationship between injected current and resultant voltage is typically linear for subthreshold potentials. However, once the threshold is reached, the all-or-none behavior of voltage-gated sodium channels dictates that a full action potential will fire, regardless of how much stronger the stimulus is beyond the threshold.

Measurement Techniques

  • Control of Membrane Potential (Voltage Clamp): Experimental setups, particularly the voltage-clamp technique, allow researchers to hold the membrane potential at a known voltage (e.g., resting potential of -70 mV). From this baseline, researchers can precisely assess the behavior of ion channels and the resulting currents as the voltage is stepped to different potentials. This separates ionic currents from changes in membrane potential.

  • Current Clamp: This technique involves injecting a constant current and observing the resulting changes in membrane potential, which is useful for studying action potential firing properties.

  • Capacitance Considerations: The cell membrane, acting as a capacitor, requires brief, initial capacitive currents to change its potential without immediately activating or inactivating ion channels. These currents must be accounted for during measurements.

  • Current Directionality: By convention, an increase in inward currents (positive ions flowing into the cell, like Na+ or Ca2+) is shown as a downward deflection, while outward currents (positive ions flowing out of the cell, like K+ or Cl-) are shown as an upward deflection on current recordings.

Signal Propagation in Neurons
Mechanism of Signal Transmission

  • Local Depolarization: Following a stimulus and the initial influx of sodium at one point, the local positive charge spreads passively to adjacent segments of the membrane (known as local current flow or cable properties). This depolarization effectively brings the neighboring membrane to its threshold, triggering the opening of voltage-gated sodium channels there and initiating a new action potential.

  • Contiguous Conduction: This process is similar to a domino effect, where depolarization in one area sequentially influences and activates the next segment, maintaining the signal flow along the unmyelinated neuron. The absolute refractory period ensures that the propagation is unidirectional, preventing the action potential from traveling backward.

Relevance in Real-World Applications
Importance of Understanding Action Potentials

  • Clinical Implications: Detailed knowledge of action potential dynamics is absolutely crucial in understanding the normal physiological function of the cardiac system (e.g., EKG interpretation, arrhythmias), the nervous system (e.g., sensory perception, motor control), and muscle contraction. It is also vital for comprehending the pathophysiology of diseases affecting ion channels (known as channelopathies) such as certain forms of epilepsy, cardiac arrhythmias, and muscular disorders like periodic paralysis. This understanding informs the development of targeted pharmacological therapies.

Conclusion

  • The continuous input of sodium during the rising phase and the subsequent efflux of potassium during repolarization orchestrate the precise waveform of the action potential. The refractory period plays a critical role in ensuring action potentials propagate in only one direction, reinforcing the unidirectional flow of signals essential for efficient neural and muscular communication.

  • Understanding these detailed biophysical principles is crucial for integrating this knowledge into clinical practice, advancing neuroscience research, and developing novel treatments for neurological and cardiac disorders.