Neuron action potential - physiology

Neuron Structure and Signal Reception
📌 Neurons consist of dendrites (receive signals), the soma (cell body with organelles), and the axon (transmits signals).
💬 Signals are received via neurotransmitters binding to receptors, converting the chemical signal into an electrical signal by opening ion channels.
⚡ A neuron "fires" when the combined effect of multiple inputs triggers an action potential, an electrical signal that travels down the axon up to $100 \text{ meters per second}$.

Resting Potential and Depolarization
📊 The cell maintains a resting membrane potential of approximately $-65 \text{ mV}$ due to differential ion concentrations ($Na^+, Cl^-, Ca^{2+}$ outside; $K^+, A^-$ inside).
🌊 Neurotransmitter binding opens ligand-gated ion channels, causing either an Excitatory Postsynaptic Potential (EPSP) (net positive influx, depolarization) or an Inhibitory Postsynaptic Potential (IPSP) (net negative influx, repolarization).
💥 When cumulative EPSPs reach the threshold, typically around $-55 \text{ mV}$ at the axon hillock, voltage-gated $Na^+$ channels open, causing rapid depolarization up to $+40 \text{ mV}$.

Action Potential Propagation and Refractory Periods
🛑 Depolarization ends when voltage-gated $Na^+$ channels inactivate. During this time, the cell enters the absolute refractory period.
⤴️ Slow-responding voltage-gated $K^+$ channels open, leading to $K^+$ efflux, causing repolarization and a brief period of hyperpolarization (relative refractory period).
🔄 The sodium-potassium pump actively moves three $Na^+$ ions out for every two $K^+$ ions moved in, restoring the resting potential.

Myelination and Conduction Speed
⚡ Fatty myelin insulates the axon, and voltage-gated channels only exist at the gaps called nodes of Ranvier.
🏃‍♀️ The action potential propagates by the charge "jumping" from node to node in a fast process called saltatory conduction.

Key Points & Insights
➡️ The fundamental process of neural signaling involves converting chemical input (neurotransmitters) into electrical propagation (action potential) within the cell.
➡️ Depolarization occurs when the membrane potential moves toward zero (e.g., reaching $-55 \text{ mV}$ from $-65 \text{ mV}$), initiated by $Na^+$ influx.
➡️ The interplay between inactivated $Na^+$ channels and open $K^+$ channels establishes the absolute refractory period, ensuring unidirectional signal flow.
➡️ Myelin dramatically increases signal speed via saltatory conduction, bypassing the uninsulated segments of the axon.

📸 Video summarized with SummaryTube.com on Mar 01, 2026, 10:01 UTC