Calcium enters through NMDA receptors, voltage-gated calcium channels, and IP3 receptors on the ER, rising to micromolar concentrations on millisecond timescales. Intracellular calcium activates CaMKII, calcineurin, and PKC, driving plasticity. Buffering proteins and mitochondrial sequestration determine duration and spatial spread.
Measure [Ca2+] using fluorescent indicators. Map calcium-dependent enzyme activation during plasticity.
[Ca2+] is uniform—it forms nanodomains near sources. All calcium is toxic—the right amount drives plasticity.
Calcium is not just a structural ion — inside neurons, it is one of the most versatile signaling molecules in biology. When a neuron is strongly activated, calcium concentrations in the dendrite can rise from tens of nanomolar at rest to micromolar or higher within milliseconds. That rapid rise triggers enzymes, shapes synaptic strength, and ultimately governs whether a synapse becomes stronger (LTP) or weaker (LTD). Understanding calcium signaling means understanding a major part of how the brain learns.
Calcium enters postsynaptic neurons through several routes. NMDA receptors are the most important: they are both ligand-gated (requiring glutamate) and voltage-dependent (requiring concurrent depolarization to expel a Mg2+ block), making them a coincidence detector for pre- and postsynaptic activity. Voltage-gated calcium channels (VGCCs) open in response to membrane depolarization alone. And IP3 receptors on the endoplasmic reticulum membrane respond to IP3 (produced when metabotropic receptors activate phospholipase C) by releasing calcium from internal stores. These three routes have different kinetics and can be activated independently or together depending on the pattern of activity.
A critical point — and a common misconception — is that calcium does not simply diffuse uniformly through the cell once it enters. Calcium forms nanodomains: zones of very high concentration immediately adjacent to open channels, which fall off steeply within tens to hundreds of nanometers due to rapid buffering by proteins like calbindin and calretinin. This means enzymes sitting close to channel mouths experience much higher calcium than those farther away, enabling spatial specificity. The machinery of synaptic plasticity is localized to the postsynaptic density precisely to exploit this proximity.
Once calcium rises, it activates a set of calcium-sensing effectors. CaMKII (calcium/calmodulin-dependent protein kinase II) is activated first, phosphorylating AMPA receptors to increase their conductance and traffic more to the synapse — the core of LTP. Calcineurin (a phosphatase) is activated by more modest calcium rises and reverses many of these phosphorylations, contributing to LTD. PKC (protein kinase C) is also calcium-sensitive and participates in both plasticity and regulation of channel expression. The same ion thus pushes synapses in opposite directions depending on the amplitude, duration, and spatial pattern of the calcium signal.
After the signal, calcium is rapidly cleared by several mechanisms: plasma membrane Ca2+ ATPases pump it out of the cell; NCX (Na+/Ca2+ exchangers) also export it; cytoplasmic buffers absorb it temporarily; and mitochondria sequester large loads during intense activity. The speed of clearance determines how long signaling continues and whether enzymes remain activated long enough to drive lasting structural changes. Plasticity, in this view, is not just about calcium arriving — it is about the neuron's ability to decode the shape of the calcium transient and convert it into durable changes in synaptic strength.