There's definitely a host of differences between the brains of fluent speakers and us stutterers, especially when it comes to motor control. Blocks are often a response to motor inaccuracies in speech. (I know this begs a lot of questions. I'm writing a book about stuttering at the moment, and I have several chapters on the neuroscience of stuttering.) Here's a [paper](https://scholar.google.com/scholar?hl=en&as_sdt=0%2C22&q=Kinematic+Analyses+of+Speech%2C+Orofacial+Nonspeech%2C+and+Finger+Movements+in+Stuttering+and+Nonstuttering+Adults&btnG=) on some of the differences in motor control. It's a bit technical. (Click on the [PDF] link on the right side to get the complete paper.)