How much of the human brain do we actually use?

Three to a hundred percent.

   Most of us have heard that we only use 10% of our brain, which leads to vast discussions about the great things we might be able to if able to harness the other 90 per cent.
   Frankly, all of the human brain is used every once in a while. On another note, a Peter Lennie of the new york University Center for Neural Science indicates that the human brain should optimally have up to 3 per cent of its neurones firing at any one time, any more and the energy needed to 'reset' each neurone after it fires becomes too much for the brain to handle.
   The central nervous system consists of the brain and spinal chord. It's only made up of only two kinds of cells; neurones and glia. Neurones are basically the processors of information, receiving and sending input and output between each other. Input is received through the neurone's dendrites (a branch like structure), whilst output leaves through the cable-resembling axons.
   A single neurone may have as many as 10,000 dendrites, but no more than one axon. The axon may be thousands of times longer than the cell body of its neurone. The largest axon, found on a Giraffe is 15 feet (4.5 metres) long.
   The convergence between axons and dendrites are know as synapses. It's where electrical impulses are transposed into chemical signals. The synapses link  neurones to one another, operating as a switch and making the brain into a very convoluted network that can be turned on and off between each impulse.
   Glia cells accomodate the structural framework of the brain. They act as the caretaker of the brain, removing the debris of cadaverous (dead) neurones. For every one neurone, there are approximately fifty glia.
   There are nearly three million miles (nearly 5,000,000 km) of axons, up to two hundred billion neurones and one quadrillion(1,000,000,000,000,000) synapses in the human brain. if they were to be spread  out side by side, they would cover 25,000 square metres; the size of four football fields.
   There are more ways that information is exchangeable in the brain than the number of atoms in the universe so the brain, if exercised daily, clearly has high potential for greatness. It's up to the possessor to choose how they use theirs.