The parts of the brain that decrease in volume as senescence begins are called:

Its called gryi and sulci. Senescene is the process by which cells irreversibly stop dividing and enter a state of permanent growth without undergoing cell death. The brain cerebral cortex consists of gyri that are typically surrounded by one or more sulci. Gyri and Sulci give the brain its wrinkled appearance. Cerebral cortex is also the most developed area of the brain and is responsible for higher brain functions such as thinking, planning and decision making.