twitch vs fibrillate

twitch is a noun but fibrillate is not a noun.

Word	Noun	Adjective	Verb	Adverb
twitch	Yes	No	Yes	No
fibrillate	No	No	Yes	No

As verbs, fibrillate is a hyponym of twitch; that is, fibrillate is a word with a more specific, narrower meaning than twitch:

twitch (noun)	fibrillate (noun)
a sudden muscle spasm; especially one caused by a nervous condition

twitch (verb)	fibrillate (verb)
make an uncontrolled, short, jerky motion	make fine, irregular, rapid twitching movements
squeeze tightly between the fingers
move or pull with a sudden motion
toss with a sharp movement so as to cause to turn over in the air
move with abrupt, seemingly uncontrolled motions