Neurons are extremely sensitive cells, whose function, like that of all cells, can be influenced by changes in their environment. Using pumps to regulate the internal and external electrolyte milieu, neurons keep toxic calcium ions outside the cell but allow the cell membrane to transmit signals electrically. If changes in the environment damage the membranes or if the energy-driven pumps fail, calcium ions can enter the neuron and permanently disable it. Local oxygen deprivation, such as that which occurs during ischemic stroke, can lead rapidly to transient or permanent injury of neurons by affecting the cells' energy requirements, pump function, . . .