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
Threshold: Specific point where action potential likelihood reaches 50%.
All or Nothing: Action potential occurs fully or not at all, no variations in size.
Self-Propagating: Each active membrane segment triggers the next segment.
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.