Something drops on the floor. A door suddenly slams. Someone honks unexpectedly beside us.

Before we have time to work out what happened, we may blink, flinch or feel our whole body jump for a moment.

This is the startle response — a rapid, involuntary reaction to an unexpected and intense stimulus.

Why do we jump when we hear a loud sound?

The brain does not need to wait for a conscious decision before reacting.

Auditory information can activate neural circuits involved in the startle response extremely quickly. The reaction commonly includes an eye blink and brief contractions of muscles in different parts of the body.

In other words, the body can begin reacting before we have fully worked out what we have heard.

What happens to our heart and breathing?

The response is not limited to the muscles.

The startle reflex can be accompanied by temporary changes in heart rate, blood pressure and breathing.

In an experimental study in humans, a sudden loud sound produced a short-lived increase in blood pressure and heart rate during the first few seconds after the stimulus.

This does not mean that everyone experiences exactly the same response every time. The magnitude can vary depending on the stimulus and the individual.

Why do we blink?

Blinking is one of the most consistent features of the human startle response.

It can be thought of as part of a very rapid defensive reaction: the body responds automatically to something that appeared suddenly and may require our attention.

Why can we be startled even when we know the sound is harmless?

The startle response is different from consciously deciding whether something is dangerous.

We may jump at a sudden noise and only a moment later realise that it was simply an object falling to the floor.

The physical reaction has already started before the brain has completed its assessment of the situation.

Why do some people startle more easily?

The response is not identical in everyone or in every situation.

The intensity and suddenness of the stimulus, the surrounding context and the person's state can influence the response. Research also shows that startle can be modified by emotional and contextual factors.

When is it worth paying attention?

Being startled by a sudden or loud sound is a normal response.

If someone experiences unusually intense or frequent reactions to sounds, particularly when they are accompanied by other symptoms or significantly affect everyday life, it is worth discussing the experience with a healthcare professional.

The takeaway

When a sudden loud sound startles us, the body can react almost instantly. We may blink, contract our muscles and experience temporary changes in heart rate, blood pressure and breathing.

We may not even have time to think, “What was that?” — our body has already made the first move. https://healthpont.com/sudden-loud-sound-startle-response/