The brain of a healthy person functions by generating electrical activity, much like a computer. For the brain’s computer to work, brain tissue is made up of billions of batteries - in reality nerve cells and their supporting cells ("astrocytes") - to store electrical and chemical energy. The energy stored in the nerve cells is then used to generate current, which can be seen by making brain wave recordings, as from the scalp. However, when some of the brain’s circuits get damaged by a head injury or stroke, chemicals get released that cause the nerve cells/batteries to short circuit. Then the batteries get drained, like when the lights are left on in a car, and the brain can no longer generate electrical patterns. Unlike the batteries of all the cars in a parking lot, however, the brain’s billions of batteries are connected to each other. So when one group of batteries short circuits, it causes all the neighboring batteries to short circuit as well. This loss of brain activity spreads in a wave, sometimes like a ripple in a pond originating from the injured brain region, sometimes also cycling around this region. Scientists call this ‘spreading depression’ because brain activity is decreased, or ‘depressed’.
Fortunately, brain tissue can usually recover from spreading depression by using sugar and oxygen to recharge its batteries, but this is an exhausting process. If it happens too often, as recharging a car battery, or if all the sugar and oxygen get used up, as in a stroke, the batteries won’t work any longer. Hopefully, scientists will discover ways to isolate damaged brain tissue with drugs, so that when part of it gets injured, the rest of it won’t get short-circuited as a result.