The Role of Calcium

The entrance of an action potential at a nerve terminal causes voltage-dependent calcium channels at the active zone to open after ~300µs. The driving force for calcium entry is extremely high as of the large concentration gradient; the free Ca²⁺ concentration at rest in a terminal is 100 nM while the external concentration is about 1 mM. In spite of this the presence of diffusion barriers and calcium buffers in the terminal limit the rise in calcium concentration to within 50 nm of the channel mouth. The [Ca²⁺] within 10 nm of the channel mouth increases to between 100 and 200 µM, that matches the half-maximal concentration of Ca²⁺ for glutamate release.