startle vs reflex action

startle and reflex action both are nouns.

Word	Noun	Adjective	Verb	Adverb
startle	Yes	No	Yes	No
reflex action	Yes	No	No	No

As nouns, reflex action is a hypernym of startle; that is, reflex action is a word with a broader meaning than startle:

startle (noun)	reflex action (noun)
a sudden involuntary movement	an automatic instinctive unlearned reaction to a stimulus

startle (verb)	reflex action (verb)
to stimulate to action
move or jump suddenly, as if in surprise or alarm