Overview
This lecture covers the key mechanisms and properties of action potentials in nerve and muscle cells, including the roles of ion permeability, voltage-gated channels, myelination, and related clinical correlations.
Resting Membrane Potential & Ion Permeability
- Resting membrane potential (RMP) is typically around -90 mV.
- Increased potassium (K⁺) permeability shifts RMP towards K⁺ equilibrium potential (Eₖ).
- Increased sodium (Na⁺) permeability shifts RMP towards Na⁺ equilibrium potential (Eₙₐ).
Key Terms in Membrane Potentials
- Depolarization: membrane potential becomes less negative (closer to 0).
- Hyperpolarization: membrane potential becomes more negative (further from 0).
- Overshoot: membrane potential exceeds 0 into the positive range.
- Repolarization: returns membrane potential towards RMP after depolarization.
- Threshold: minimum membrane potential needed to trigger an action potential.
- Excitability: ability of a membrane to depolarize and generate action potentials.
Action Potential: Definition & Properties
- An action potential is a regenerating depolarization that propagates along excitable membranes.
- Propagation occurs without decrement (no decrease in amplitude).
- Action potentials are all-or-none events: if threshold is reached, a full action potential occurs.
- Amplitude of action potentials remains constant regardless of stimulus strength.
Mechanism of Action Potential Generation
- At rest, Na⁺ channels' activation gate is closed; inactivation gate is open.
- Depolarization opens activation gate, increasing Na⁺ permeability and causing the upstroke.
- Inactivation gate then closes (slower), contributing to downstroke (repolarization).
- K⁺ channels open later, increasing K⁺ conductance and aiding repolarization and after-hyperpolarization.
- Voltage-gated ion channels control the sequence of ionic permeability changes.
Propagation & Directionality
- Action potentials propagate in both directions from stimulus but do not travel backward due to inactivation of Na⁺ channels.
- Inactivation gates prevent reopening until RMP is restored.
Myelination & Saltatory Conduction
- Larger diameter and myelinated fibers conduct action potentials faster.
- Myelin increases efficiency; breaks called nodes of Ranvier contain high densities of Na⁺ channels.
- Saltatory conduction: action potentials appear to "jump" from node to node, increasing speed.
- Demyelination (e.g., in multiple sclerosis) disrupts propagation, causing neurological symptoms.
Clinical Correlation: Local Anesthetics
- Local anesthetics (e.g., lidocaine) block voltage-gated Na⁺ channels from inside the nerve, preventing action potential generation.
- Active fibers are blocked more quickly by anesthetics due to frequent channel opening.
Key Terms & Definitions
- Resting Membrane Potential (RMP) — baseline electrical charge difference across the membrane.
- Depolarization — movement of membrane potential toward zero.
- Hyperpolarization — movement of membrane potential further from zero.
- Threshold — critical potential needed to trigger an action potential.
- All-or-none principle — action potentials occur fully or not at all.
- Saltatory conduction — apparent "jumping" of action potentials across nodes of Ranvier in myelinated fibers.
- Node of Ranvier — gap in myelin with concentrated voltage-gated Na⁺ channels.
Action Items / Next Steps
- Review any provided practice questions or case studies.
- Study mechanisms of voltage-gated channel operation and myelination effects.
- Prepare for upcoming content on electrotonic potentials.