Ca2+homeostasis and its correlation with weakly and strongly buffered motoneurons under physiological and pathophysiological conditions. (A) The Ca2+ buffering capacity (KS) of a cell, reflecting relative fraction of bound versus free Ca2+, can be calculated by using the ‘added buffer’ approach by linear one-compartment model. The recovery time of [Ca2+]i elevations (τ) depends on the amount of endogenous buffer (S; denotes Ca2+-binding proteins), the amount of exogenous buffer (B; i.e. Fura-2) and the transport rate (γ) of Ca2+ across cellular membranes. KB indicates the buffer capacity of the exogenous buffer (i.e. Fura-2). (B) Ca2+ homeostasis in weakly and strongly buffered MNs. The amplitude of Ca2+ transients is several times larger in weakly buffered cells (e.g. HMNS and SMNs) than in strongly buffered cells (e.g. oculomotor neurons), and the recovery time is significantly accelerated (τ). (C) Low Ca2+ buffering in ALS-vulnerable HMNs exposes mitochondria to higher Ca2+ loads compared to high-buffered cells. Under normal physiological conditions the neurotransmitter opens glutamate, NMDA and AMPA receptor channels along with VDCC with high glutamate release and reuptake by EAAT1 and EAAT2. This results in a small rise in intracellular calcium that can be buffered by the cell. In ALS disorder, the glutamate receptor channels possess high calcium conductivity and thereby high Ca2+ loads; increase the risk for mitochondrial damage. This triggers mitochondrial production of reactive oxygen species (ROS), which then inhibit glial EAAT2 function. This leads to further increase in glutamate concentrations in the synapse and further rises in postsynaptic calcium levels which contributes to the selective vulnerability of MNs in ALS. Low cytosolic Ca2+ buffering capacity promotes Ca2+ accumulation and formation of subcellular domains around influx sites (red), and thus facilitates the interaction of elevated calcium levels with intracellular organelles such as mitochondria (modified from refs. [62, 63, 73, 94, 118]).