The brain is very efficient when it comes to task sharing. Different parts of the brain are responsible for different functions like vision, intelligence, emotions or breathing. The part of the brain which is responsible for speaking is the Broca’s area. Broca’s area is located in the frontal part of the left half of the brain.
If some parts of the brain happen to have insufficient blood flow, the cells in those parts of the brain may die. This results to what we know as "stroke". A stroke may affect the Broca’s and when this happens, the individual may also be unable to speak following the stroke.