🚪 3. Tap the story open
Open one piece at a time. Each section gives the simple story first, then the college wording.
🧪 Part A — Ions, gradients, and voltage
Like you're 5: Na⁺ and K⁺ are tiny charged particles. A gradient means there is more here than there, or the charge is different here than there.
College level: The course defines ions as atoms or molecules with a charge. A concentration/chemical gradient is a difference in concentration across space. An electrical gradient is a difference in charge across space.
Why it matters: Gradients create forces. Channels determine whether ions can actually cross the membrane.
🔋 Part B — Resting potential = prepared, not inactive
Like you're 5: The neuron looks quiet, but it is ready. Sodium is mostly outside. Potassium is mostly inside. The inside is more negative.
College level: Resting membrane potential is about −70 mV. The membrane is selectively permeable.
💪 Part C — Sodium–potassium pump
Like you're 5: The pump is a worker spending energy to keep the ion crowds in the right places.
College level: The pump actively transports 3 Na⁺ out and 2 K⁺ in, maintaining the concentration differences used by the neuron.
Memory line: 3 sodium OUT · 2 potassium IN
⚖️ Part D — Sodium vs potassium forces
Na⁺ at rest
Concentration gradient: IN
Electrical gradient: IN
Both favor inward movement.
K⁺ at rest
Concentration gradient: OUT
Electrical gradient: IN
The forces oppose each other.
📉 Part E — Hyperpolarization vs depolarization
Hyperpolarization: more negative than rest.
Depolarization: less negative, toward zero.
Threshold: the level of depolarization needed to trigger an action potential.
🎯 Part F — Threshold → Na⁺ IN → rising phase
Like you're 5: A small knock may fade. A strong enough knock reaches the GO line.
College level: Once threshold is reached, voltage-gated Na⁺ channels open and Na⁺ rushes in, causing rapid depolarization.
Professor quiz value: threshold is tested at about −55 mV.
🎆 Part G — The action potential is the WHOLE spike
Important: the action potential is not just Na⁺ entering. It is the rapid voltage event that includes the rise and the fall.
🧹 Part H — Peak → K⁺ OUT → falling phase
Near the peak, Na⁺ entry stops/inactivates and K⁺ channels open. K⁺ moves out, bringing the membrane voltage back down: repolarization.
K⁺ channels remain open briefly, which produces the undershoot / hyperpolarization.
🛣️ Part I — Propagation down the axon
Like you're 5: One patch fires, then the next patch fires, then the next — like a moving wave.
College level: The action potential is regenerated along the axon.
⚡ Part J — Myelin + saltatory conduction
Like you're 5: Myelin is the fast lane. The electrical event is regenerated at the exposed gaps.
College level: Those gaps are Nodes of Ranvier. Myelinated conduction is saltatory conduction.
🔒 Part K — Refractory period
Absolute refractory: about 1 ms; another action potential cannot be produced.
Relative refractory: about 2–4 ms; another action potential is possible, but requires stronger stimulation.
