What Causes Pain At The Back Of Neck?

My son had a very bad fall on Saturday night. He is in the hospital in ICU. They are keeping a neck brace on him. Tomorrow they are going to do an MRI. He is complaining of pain in the back of his neck and although he can move his head from side to side he can not put his chin on his chest. The Dr. said they wanted to rule out an injury that affects the arteries in the neck. I can t remember the name he said. Do you have any suggestions what it might be. My EMail is YYYY@YYYY