Read any article on reality based martial arts or street fighting or any current self defense course and you'll likely read about how people's fighting skills degrade under stress. Stress can cause the loss of fine motor skills. Stress and adrenaline and some other brain/body processes can also cause shaking, sweating, tunnel vision and auditory exclusion. As a result, many instructors are spending a lot more time teaching and practicing self defense techniques utilizing gross motor skills. They are also raising their student's stress levels through a variety of methods such as exercise, blindfolds and dynamic scenario based training. I think this is a great thing and should be included in any serious training program.
What you'll often read in conjunction with these articles is that much of the more fine motor skill based techniques currently being taught in most dojos or schools are of limited or no value. Some will say they are a complete waste of time.
While I plan to write more on stress and it's affects on combat, I'd like to mention the following:
It is my belief that if you train realistically and with the proper mindset for long enough, your stress response to violence can greatly reduce or even disappear.
There is much truth to the saying that "You fight like you train". If your mind is in the right place and you train realistically and with focus, you may not lose your fine motor skills at all. Proper training and mindset can essentially inoculate you from violence. Not all people can reach this level, but many can.
Stressful situations can cause drastically different responses in different people, and I find this very interesting. Some people can remain calm and some people just fall apart.
Let me ask this. If you knew that someone posed you no real threat and that you could easily defend yourself against them with a minimum of effort, how stressed would you be if they approached you and wanted to fight? Probably not too stressed. Proper training can get you into this state. After all, if you've been through the same type of incident dozens or hundreds of times in your training, the real event isn't going to rattle you very much.
You can't necessarily change how stress and adrenaline affects you, but you can reduce or eliminate the amount or the severity of the stress you experience in the first place through effective and consistent training.