My wife and I have been arguing about this today. She thinks the brain is a muscle for some reason. I don't because I don't think the brain is made of muscle tissue and lacks the qualification to be called a muscle. Which of us is right?
Anatomically the brain is not a muscle. She is likely referring to the assertion that the brain follows the same rules as muscles do in the sense that it atrophies when left inert and continues to grow and strengthen the more it is stressed and supported with healthy diet and sleep.