Need a synonym for "action" in a specific context

Question

I need a synonym for the word "action". Here is the sentence where I need a synonym:

When an event occurs, an action is taken.

For example:

When you read this question, you write a comment.

Answer 1

When an event occurs, a reaction follows.

You could also say: "an event here will certainly trigger a reaction."

• reaction - (noun) - "the way someone acts or feels in response to something that happens, is said, etc." Merriam-Webster "a response to a stimulus" TFD

Another choice would be: "a response"

Answer 2

"Trigger" (verb meaning to cause) is a word we commonly use in software engineering.

The occurrence of event X triggers action Y

or

The occurrence of event X triggers event Y

"reaction" is another synonym, although it has a more general purpose and does not necessarily imply a procedural standard action taking place. It is also widely used to refer to natural reaction or instinct.

Answer 3

A response is a general term

A reaction to something:

an extended, jazzy piano solo drew the biggest response from the crowd

an honors degree course in Japanese has been established in response to an increasing demand

Oxford Dictionaries Online