Heightened arousal, involves another group of posttraumatic stress symptoms, which can be manifested through sleep disturbances, increased feelings of anger and irritability, concentration difficulties, startle reactions
, and hypervigilance.
Detailed measurements of facial muscles during the startle reaction
suggest that it is probably a reflex, according to a report in the November JOURNAL OF PERSONALITY AND SOCIAL PSYCHOLOGY.
Sudden shocks - either building emotional terrors or simple startle reactions
, cause stress hormones to pour out, messing with the heart's rhythm, putting more stress on certain parts of the arteries or muscles, or poisoning the muscles entirely.