Though it has not been actually proven that this myth of having access to only ten percent of our brains, is actually a myth, enough significant research has been revealed to note how massively unlikely it is that it is true. For example, it is known that each portion of the brain is responsible of different tasks, such as the motor cortex which controls body movement. In addition, the text "Scientific Perspectives on Pseudoscience and the Paranormal" by Timothy J. Lawson mentions that muscles "atrophy' after lack of use for an extended period of time. In other words, if we did not use the other ninety percent of our brains, then it would die out and we would be left with ONLY the ten percent.
Overall, the actual truth can not yet be determined, but the facts dramatically tip the scale in favor of this myth remaining so.
Lawson, Timothy. J. (2007) Scientific Perspectives on Pseudoscience and the Paranormal. Upper Saddle River, New Jersey. Pearson Education.