What is Polymicrogyria? Let’s break this big word down into it’s basic parts. “Poly” means many, “micro” means very small, and “gyria” is the medical term for the wrinkles in the brain. So we have “many very small wrinkles in the brain”. During the baby’s development in the uterus, the nerve cells divide and make up the layers of the brain and make the wrinkles (gyri) of the brain. With PMG, that process is altered and it causes the brain to form improperly. It forms small wrinkles which don’t process the information it receives like a normally formed brain. This causes problems with functioning in the body much like a stroke can affect the brain. Depending on what part of the brain is involved and how severe the deformity is, results in the impairments that we see with PMG.