204. Calcium Waves

Calcium Waves

This simulation models intracellular calcium wave propagation, a fundamental signaling mechanism in many cell types. Calcium waves coordinate cellular responses by spreading information across the cell.

Key Components

• IP₃ Receptors (IP₃R): ER channels opened by IP₃ and Ca²⁺ (biphasic - activated at low Ca²⁺, inhibited at high Ca²⁺)
• Ryanodine Receptors (RyR): Enable calcium-induced calcium release (CICR)
• SERCA Pumps: ATP-driven pumps that refill the ER with Ca²⁺
• Endoplasmic Reticulum: Intracellular Ca²⁺ store (~500 µM vs ~0.1 µM cytosolic)

CICR - Calcium-Induced Calcium Release

The positive feedback mechanism where Ca²⁺ released from one receptor cluster activates neighboring receptors, creating a self-propagating wave. This is analogous to the regenerative depolarization in action potentials.

Wave Propagation

• Initiation: Local IP₃ or Ca²⁺ increase triggers release from ER
• Propagation: Diffusing Ca²⁺ activates neighboring IP₃Rs and RyRs
• Termination: Ca²⁺-dependent inactivation and SERCA pumping
• Recovery: ER refilling restores release capability

Biphasic IP₃R Response

IP₃R open probability depends on both IP₃ and Ca²⁺ concentrations. Low Ca²⁺ potentiates opening (co-agonism), while high Ca²⁺ causes inactivation. This creates the characteristic oscillatory behavior seen in calcium signaling.

Physiological Roles

• Egg activation at fertilization
• Cardiac muscle contraction (excitation-contraction coupling)
• Neurotransmitter release
• Gene expression regulation
• Cell death (apoptosis) signaling