201. Ion Channel Dynamics

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Ion Channel Dynamics

This simulation models voltage-gated ion channels using the Hodgkin-Huxley formalism, demonstrating how activation and inactivation gates control ion flow in response to changes in membrane potential.

Hodgkin-Huxley Model

• Na⁺ Channels (m³h): Three rapid activation gates (m) and one slower inactivation gate (h)
• K⁺ Channels (n⁴): Four identical activation gates (n), no inactivation
• Gate Variables: Dimensionless probabilities (0-1) representing gate open probability

Activation vs Inactivation

• Activation Gates: Open probability increases with depolarization (fast)
• Inactivation Gates: Open probability decreases with depolarization (slower)
• Result: Na⁺ current is transient - channels activate then inactivate

Voltage Sensors (S4 Segments)

Each channel contains four voltage-sensing S4 segments, which are α-helices with positively charged residues. Upon depolarization, these segments move outward, triggering conformational changes that open the channel pore.

Gating Currents

The movement of S4 voltage sensors constitutes a small "gating current" that can be measured experimentally. This current precedes ion flow and reflects the charge movement within the membrane during gate transitions.

Rate Constants

The transition rates α (opening) and β (closing) are voltage-dependent functions that determine how quickly gates respond to voltage changes. Temperature affects these rates through Q₁₀ factors.

Channel States

• Closed: Activation gates closed, inactivation gate open (resting)
• Open: All gates open (activated)
• Inactivated: Inactivation gate closed (refractory)