Questions: Calcium Signaling in Neurons

Question 1

Which statement best describes how calcium is spatially distributed inside a neuron during active signaling?

• A. Calcium floods the entire cytoplasm uniformly within milliseconds of channel opening
• B. Calcium forms high-concentration nanodomains near open channel sources, with rapid falloff at distance
• C. Calcium enters only at the soma and diffuses slowly outward to dendrites
• D. Calcium concentration is homogeneous across compartments because buffering proteins distribute it evenly

Answer: (B)

Calcium does not spread uniformly — it forms steep concentration gradients called nanodomains immediately adjacent to open channels, because buffering proteins and diffusion limit spread. [Ca2+] can reach hundreds of micromolar within nanometers of a channel mouth while remaining near baseline just micrometers away. This spatial confinement allows different calcium-dependent processes to be triggered selectively based on proximity to the source, not just the average cytoplasmic concentration.

Question 2

Elevated intracellular calcium in a postsynaptic neuron is generally a sign of pathological stress and should be minimized for normal neural function.

• T. True
• F. False

Answer: False

Calcium is a critical second messenger at physiological concentrations. Calcium influx through NMDA receptors and VGCCs activates CaMKII, calcineurin, and PKC, which in turn drive synaptic plasticity (LTP and LTD), gene expression changes, and structural remodeling. It is the *magnitude, duration, and spatial pattern* of the calcium signal that determines whether the result is plasticity or toxicity — not the presence of calcium per se.

Question 3

Name two distinct sources from which intracellular calcium can rise in a postsynaptic neuron following strong synaptic activation, and identify one downstream protein each source can activate.

Model answer: NMDA receptors on the plasma membrane allow Ca2+ entry from the extracellular space and activate CaMKII, which phosphorylates AMPA receptors to strengthen the synapse. IP3 receptors on the endoplasmic reticulum release Ca2+ from internal stores and can activate calcineurin (protein phosphatase 2B), which dephosphorylates substrates involved in LTD and gene expression.

Neurons have both plasma membrane channels (NMDA receptors, voltage-gated Ca2+ channels) and intracellular store release (IP3R, ryanodine receptors on the ER) as calcium sources. These sources have different activation thresholds and kinetics, allowing graded calcium responses that differentially activate CaMKII (favors LTP), calcineurin (favors LTD), or PKC depending on concentration and context.