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Action Potential Overview

  • Focuses on the period of latent addition in action potentials.

Setting Up The Axon Model

  • Establish a vertical axis and a zero point at 0 millivolts.

  • Use increments of 10 millivolts from -80 to +40 millivolts.

  • Dotted line at -60 millivolts represents resting membrane potential.

  • Dotted line at +40 millivolts denotes peak action potential.

  • Threshold for firing set at -40 millivolts.

Resting Membrane Potential

  • Resting membrane potential varies by axon; common value set at -70 mv, though -60 mv is chosen for variety.

  • Peak action potential (~+40 mv) can also vary between axons.

  • Threshold can vary; -40 mv is a possible but less commonly used number.

Inhibitory and Excitatory Postsynaptic Potentials

  • Inhibitory postsynaptic potential (IPSP):

    • Forces negatively charged ions (anions) into the axon, leading to hyperpolarization.

    • Hyperpolarization decreases likelihood of action potential generation.

  • Excitatory postsynaptic potential (EPSP):

    • Forces positively charged ions (cations) into the initial segment, leading to depolarization.

    • Effectiveness depends on size of influx.

Threshold Concept

  • Threshold is a point where there is a 50% chance of generating an action potential.

  • Subthreshold depolarizations may trigger action potential but with reduced probability.

Additivity of Postsynaptic Potentials

  • Postsynaptic potentials are additive.

  • Multiple influences can significantly shift internal state towards or away from threshold.

  • Algebraic summation example: -10 (hyperpolarizing) + 15 (depolarizing) = +5 mv (net effect).

Probability and Action Potential

  • Action potentials are probabilistic; more cation influence increases likelihood of triggering an action potential (e.g., 1% to 80% likelihood).

  • Initial segment of action potential generation relies on cumulative effects of excitatory and inhibitory stimuli.

Period of Latent Addition

  • Only evident during early action potential generation in the initial axon segment.

  • Subsequent action potential phases are not influenced by latent addition.

Misnomers in Terminology

  • Depolarization: Movement towards zero voltage (less negative), misinterpreted as becoming more polarized when crossing zero.

  • Absolute Refractory Period: Time during which sodium channels are open; cannot open further irrespective of stimulus.

  • Relative Refractory Period: Sodium channels reset but potassium efflux is happening, allowing for potential new action potentials given strong enough stimuli.

Hyperpolarization

  • Described incorrectly as excess potassium leaving; it's an artifact of measurement.

  • Potassium ions' position post-exit creates a temporary apparent increase in negativity until they drift away.

Conduction of Action Potential

  • Involves minimal numbers of ions moving down the axon during action potentials (7 sodium ions).

  • Myelinated vs. Nonmyelinated Axons:

    • Nonmyelinated: Ion channels evenly spread, action potentials propagate through each segment.

    • Myelinated: Ion channels concentrated at nodes of Ranvier, allowing swift, saltatory conduction.

Orthodromic vs. Antidromic Conduction

  • Orthodromic Conduction: Action potentials follow the correct direction from soma to bouton; enabled by absolute refractory periods.

  • Antidromic Conduction: Action potentials that travel opposite direction (e.g., bouton to soma) only under unusual circumstances.

Properties of Action Potentials

  1. Threshold: Specific point where action potential likelihood reaches 50%.

  2. All or Nothing: Action potential occurs fully or not at all, no variations in size.

  3. Self-Propagating: Each active membrane segment triggers the next segment.

  4. Nondecremental: Maintains consistent magnitude despite distance traveled, unlike ripples in water.

Sodium-Potassium Pump

  • Critical for maintaining water balance in neurons by moving sodium out and potassium into cells.

  • Hypothetical scenarios do not prevent generation of action potentials, showcasing pump’s supporting but not essential role in action potential generation.