Is nervous an adjective

A nerve is a noun, a cell or collection of cells (especially sensory cells) that communicate with other cells through electrical impulses.

Beyond that, any Word prefaced by an article (a man, a theory) is a noun, because articles only modify nouns. When a verb follows an article, it must express the act of performance of the verb.

E.g.

go -> a go (a try)

see -> a see (a look)

jump -> a jump (a hop)