Mine has never been normal since I got sick, but it is variable and mixed. These days, the most common response is what most people here are reporting - an exaggerated startle response. However, sometimes I'll have none at all (at least not outwardly) in response to a stimuli that should trigger it. I think a few people above mentioned the patellar reflex - the knee jerk thingie with the hammer. In the early years my docs were always surprised that I had no response at all to this.
Quite frequently, I'll
feel a startle response - it travels down my spine and rattles in my mind - but no overt movement accompanies it. If anything I go a bit slack. This brings to mind Polyvagal theory, which if I recall* posits 3 branches to the ANS, the third being a freeze/"play possum" response.
*(it's been a long time since I last read about it)
"play possum" in conjunction with hibernation/torpor/dauer is intriguing to think about. I wonder if some of us have conserved pseudo-vestigial genes and pathways for entering these states, but - over millennia of disuse - have lost the genes for exiting them. ...I suppose this isn't an optimistic theory... Perhaps the "switches" are
both conserved and a treatment would be to find the correct epigenetic modifier to trigger transcription of the "exit torpor" instructions. Entirely speculative. (and possibly this last paragraph belongs in a different thread? I'm new here

...will go poke around with the search function in a mo)
I upped my NAC intake about 5-fold starting 3 weeks ago. I feel like my startle response has reduced since then, but it's too early to say with any certainty.
Does anyone else experience what I mentioned above? An
internally felt startle response but with no accompanying movement?