When we are threatened, our body springs into action. Our brain sends signals to the body causing a variety of physiological changes that prepare us to face the challenge or to run away.
Activating a response
Have you ever been startled by a garden hose, only to realise it is not a snake and completely harmless? Before we re even consciously aware of a threat, our brain activates the nervous system, which causes the release of hormones from the adrenal glands. Meanwhile, the information also travels the longer route to our cortex where conscious brain regions can analyse whether the threat is genuine. If not, it will calm down the physical reaction.
Modern stress tends to be very different to the type encountered by our ancestors – our stressors often overstay their welcome, and can’t be dealt with by fighting or fleeing. Stress is helpful in the short-term, but continued stress negatively affects your health, causing headaches and illness.
In times of high stress you may experience tunnel vision, in which you don’t notice anything around you.
- Brain activity – The amygdala signals the body to take action before the fearful stimulus has even been recognized by the visual cortex – this happens when you jump at shadows. Then, the visual cortex fully analyzes the image to check if the threat is real, and your physical reactions adjust accordingly. Your cortex also consults memories stored in the hippocampus to check if the threat was faced in the past.
- Alternating pathways – Signals from the brain are sent to the body via nerves, and also by hormones released from the pituitary gland. The nervous signals travel faster than the hormones, so they kick-start hormone production in the adrenal glands.
- Hormone producer – The adrenal glands that sit on top of the kidneys produce more adrenaline and cortisol in response to the nervous signals and hormones sent by the pituitary gland. This heightens the physical effects of stress.
- Short-term effects – Within seconds, heart rate and breathing increase to boost oxygen circulation. Blood vessels close to the skin constrict, leaving you pale, and your bladder muscles relax, possibly leading to embarrassing accidents.
- Long-term effects – Over minutes and hours, signals from the adrenal glands continue to cause a cascade of reactions. Blood sugar rises and fat stores are metabolised for your energy so your muscles continue to work a their full potential. Non-vital processes, such as immune system activities, are shut down to conserve energy.