As buzrw's first comment states, most nervousness is primarily a sympathetic nervous system response.
There are also neurotransmitter imbalances involved with anxiety, especially in anxiety disorders. The NT are dopamine, serotonin, and norepinephrine.
I want to clarify buzrw's comment on nerve cell regeneration, as I am unsure if he is stating it's proven to exist or to not exist. "Primary neurons are not readily mitotic..." makes me think it's the latter.
Neuroregeneration is proven. Mayo Clinic has good information here and this is a well-phrased simplified answer here.
It is more commonly seen in peripheral nervous system, as the central nervous system has limited capabilities to regenerate. Mostly it is regrowth of severed axons of cells, and differs between different nerve cell types. There are other types of regeneration as well. They involve many physiological processes we're only starting to understand, such as the role of neuroligin. Without getting too technical, there is a lot that can be done to help stimulate and guide the regrowth of some neural damage, but there are still chasms in our knowledge in the fields of neurology and psychiatry.