What is the origin of the idiom "hit a nerve" and when to use it?

The exact origin of the phrase "hit a nerve" can be traced back to the medical understanding of how physical stimulation of a nerve can elicit a strong, often painful, response. It is also used in social or political discussions when a statement or action deeply affects individuals or groups involved. In these situations, the phrase highlights that a particular subject has struck a sensitive spot, revealing underlying tensions, unresolved issues, or deeply held beliefs.