action potential podcasts

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.