The Startle Response is also known as the startle reflex and the alarm reaction.
The alarm reaction is a completely natural, involuntary reaction to a stimulus such as a flash of light, a sudden threatening movement or loud noise. The startle reflex is considered to be innate, being found in newborn babies, in which case it is known as the Moro reflex.
The alarm reaction is brought about by the activation of the Sympathetic Nervous System, SNS, and still occurs even when the stimulus is anticipated and people are trying to stop their reaction, although the strength of the response can be modified.
Research has shown that the startle reaction takes place within milliseconds of the stimulus, in a way that is designed to protect the body from attack. Frank Pierce Jones (1951) showed that the reflex starts with the head which jerks as the neck muscles contract and the eye muscles tighten and blink. Then the response moves down into the torso which flinches; the shoulders raise and arms stiffen, the abdominal muscles contract and the chest flattens, then the knees flex - all this in around one second. Alongside these external changes, breathing and blood pressure levels change and the heart rate accelerates. Interestingly, 'the response begins with extension' and immediately changes to flexion.
Whilst the muscular changes that take place in the startle reaction can return to normal fairly quickly if the danger recedes, breathing and the vascular system take rather longer to calm down, as the Parasympathetic Nervous System, PNS, begins to take over to bring back a state of calm to the whole system. If the perceived danger continues, then the fight/flight response may develop.
People can also react with the startle response to other milder stimuli, such as a phone ringing. The more anxious and stressed a person is, the more frequently they tend to over-react to situations with this pattern of flinching and contracting. This is very much the case in people who suffer from Post-traumatic Stress Disorder, PTSD and they can display an exaggerated startle response, or hyperekplexia.
Habituation can lessen the strength of the startle response but the more familiar this pattern is, the harder it can be for the body to let go afterwards and some people habitually live with their muscles held in a contracted manner, echoing their previous responses to alarming situations. These habitual patterns can begin to be addressed in Alexander lessons, which focus on the head neck back relationship, which encourages a freeing-up and lengthening of the musculature.
As Frank Pierce Jones put it 'The Startle Pattern may be taken as a paradigm of malposture in general'... when 'muscles in various combinations and degrees of tension have shortened displacing the head or holding it in a fixed position.... The procedures used in the Alexander Technique establish a new dynamic balance among the forces acting on the head so as to allow more of the postural work to be done by discs and ligaments and by muscles acting at their optimal length'.
(The Alexander Technique: Body Awareness in Action ~ Frank Pierce Jones 1976).