Brains are meat. Complex meat, but ultimately they are made of cells, fed by oxygenated, glucose-rich blood and as susceptible to damage or training as many other cell types. For example, when you train a muscle to stretch further, more than simply training the muscle fibers to extend further, you are training the stretch reflex in the brain to allow a deeper stretch. If the brain doesn't adapt at the same time as the muscle then you behave as though the muscle is shorter.
The mind is, according to many, harder to pin down, but I think it irrefutable that the brain is the substrate for the mind. It is easier, when practicing psychotherapy, for example, to consider the mind independently of the brain, but brain damage and psychopharmacology draw us back to the physicality of the brain, and the effect that changes to the brain have upon the mind. I think the converse is also true.*
The state of a mind affects the brain's behavior, that much is clear. When we're distracted, worried, or stressed, typically seen as conditions of the mind, we perform certain tasks less well; our brains work less well. I contend that the mind affects brain (re)development in concrete biological ways. I think I'm living evidence that this is so, as is anyone who recovers from brain damage, whether from stroke, concussion, shrapnel, gunshot or any other cause.
There is a difficult line for neurologists to tread. On the one hand, recovery is possible in most cases if and only if a patient's mind believes that it is possible. On the other, even the most optimistic patient still has to put in a lot of effort over a long time to see results. I have tried to remain resolute, but I had little idea two years ago just how long it would take to recover. I think I have a better idea now, and it's a long time.
Perhaps it is particular to stroke, or maybe it is just any brain damage that involves older people, but there is an infectious attitude of defeat to overcome. Add to that the depression that is a common result of brain damage and slow recovery, and it is difficult to break out of the pernicious state of mind that inhibits recovery.
The good news is that minds can, with difficulty, be controlled. We are conscious. We can make choices. We can recognize that the default despair is counter-productive. Above all, we can 'fake it, until we make it.' By doing so, we use our minds to control our actions until our brains encode that behavior as automatic. Anyone can do this. Those of us with brain damage must do this.
* There is no inconsistency here with the belief of many that the mind has something like extra going on, call it a soul, divine spark, or what you will. I don't believe in anything supernatural, but my belief on the matter doesn't preclude that possibility.