The Nervous System, Part 2 - Action! Potential!: Crash Course Anatomy & Physiology #9

Action Potential: The Neuron's Electrical Signal
📌 Neurons communicate via an action potential, a single, uniform electrical impulse that zips down the axon, similar to a single "ping" from an app.
💡 The brain deciphers signals not by signal strength, but by the frequency or number of pulses transmitted, akin to reading binary code.
⚡ Every action potential involves a membrane potential that cycles from a resting state of approximately -70 mV up to a peak of +40 mV during depolarization.

Cellular Electricity and Membrane Potential
🔋 A neuron functions like a tiny battery, maintaining a resting membrane potential due to separated positive and negative charges across its membrane.
⚙️ The sodium-potassium pump actively maintains this potential by pumping out three $\text{Na}^+$ ions for every two $\text{K}^+$ ions pumped in, creating an electrochemical gradient.
🚪 Ion movement across the membrane is controlled by channels that open based on voltage (voltage-gated), chemical binding (ligand-gated), or physical stretch (mechanically gated).

The Mechanism of Action Potential Firing
🛑 The signal requires the membrane potential to reach a crucial threshold of -55 mV; below this, the neuron returns to rest (a graded potential).
🌊 Reaching the threshold causes voltage-gated $\text{Na}^+$ channels to open, leading to rapid influx of positive ions and massive depolarization up to $+40 \text{ mV}$.
📉 Following depolarization, voltage-gated $\text{K}^+$ channels open, allowing $\text{K}^+$ ions to flow out, initiating repolarization, which often overshoots to hyperpolarization (around $-75 \text{ mV}$).
🚫 During the refractory period, the section of the axon cannot respond to new stimuli, ensuring signals travel only in one direction down the axon.

Signal Transmission Speed and Strength
💨 The strength of a sensation (e.g., paper cut vs. crushing a can) is encoded by the frequency of action potentials, not their magnitude, which remains constant.
💨 Conduction velocity varies; signals are fastest in reflex pathways, largely determined by the presence of a myelin sheath.
🏃 Myelinated axons utilize saltatory conduction, where the impulse "leaps" between the Nodes of Ranvier, increasing transmission speed significantly.

Key Points & Insights
➡️ Neurons communicate through a constant-strength impulse (action potential); variable information (intensity) is conveyed through the frequency of firing.
➡️ The resting state of a neuron is established by the sodium-potassium pump maintaining a resting membrane potential of approximately -70 mV.
➡️ For an action potential to occur, the membrane potential MUST hit the -55 mV threshold; this is an all-or-nothing event.
➡️ Speed of signal transmission is maximized by myelination and saltatory conduction, allowing the current to jump gaps along the axon.

📸 Video summarized with SummaryTube.com on Feb 01, 2026, 08:40 UTC