Such a tendency is understandable. Humans evolved in a hostile, wild environment with dangers at every turn. Those humans who developed a healthy paranoia and jumped at shadows (that genuinely may have had teeth) survived long enough to pass on their genes. As a result, when presented with any conceivable threat or danger, the modern human has a suite of (mostly unconscious) response mechanisms providing a reflex that enables them to deal better with said threat, and this reflex is still very much alive and kicking (as are humans, thanks to it). This reflex is the fight-or-flight response, which is a great name as it concisely but accurately describes its function. When presented with a threat, people can either fight it or run away.
The fight-or-flight response starts in the brain, as you’d expect. Information from the senses reaches the brain and enters the thalamus, which is basically a central hub for the brain. If the brain were a city, the thalamus would be like the main station where everything arrives before being sent to where it needs to be.21 The thalamus connects to both the advanced conscious parts of the brain in the cortex and the more primitive “reptile” regions in the midbrain and brainstem. It’s an important area.
Sometimes the sensory information that reaches the thalamus is worrying. It might be unfamiliar, or familiar but worrying in context. If you’re lost in the woods and you hear a growl, that’s unfamiliar. If you’re home alone and you hear footsteps upstairs, that’s familiar, but in a bad way. In either case, the sensory information reporting this is tagged “This isn’t good.” In the cortex, where it’s processed further, the more analytical part of the brain looks at the information and wonders “Is this something to worry about?” while checking the memory to see if anything similar has occurred before. If there’s not enough information to determine that whatever we’re experiencing is safe, it can trigger the fight-or-flight response.
However, as well as the cortex, the sensory information is relayed to the amygdala, the part of the brain responsible for strong emotional processing, and fear in particular. The amygdala doesn’t do subtlety; it senses something might be amiss and initiates a red alert straight away, a response far faster than the more complex analysis in the cortex could ever hope to be. This is why a scary sensation, like a balloon popping unexpectedly, produces a fear response almost instantly, before you can process it enough to realize it’s harmless.22
The hypothalamus is then signaled. This is the region right under the thalamus (hence the name), and is largely responsible for “making things happen” in the body. To extend my earlier metaphor, if the thalamus is the station, the hypothalamus is the taxi stand outside it, taking important things into the city where they get stuff done. One of the roles of the hypothalamus is triggering the fight-or-flight response. It does this by getting the sympathetic nervous system to put the body effectively at “battle stations.”
At this point you may be wondering, “What’s the sympathetic nervous system?” Good question.
The nervous system, the network of nerves and neurons spread throughout the body, allows the brain to control the body and the body to communicate with and influence the brain. The central nervous system—the brain and the spinal cord—is where the big decisions are made, and as such these areas are protected by a sturdy layer of bone (the skull and the spinal column). But many major nerves branch out from these structures, dividing and spreading further until they innervate (the actual term for supplying organs and tissue with nerves) the rest of the body. These far-reaching nerves and branches, outside the brain and spinal cord, are referred to as the peripheral nervous system.
The peripheral nervous system has two components. There’s the somatic nervous system, also known as the voluntary nervous system, which links the brain to our musculoskeletal system to allow conscious movement. There’s also the autonomic nervous system, which controls all the unconscious processes that keep us functioning, so is largely linked to internal organs.
But, just to make it more complicated, the autonomic nervous system also has two components: the sympathetic and parasympathetic nervous systems. The parasympathetic nervous system is responsible for maintaining the more calm processes of the body, such as gradual digestion after meals or regulating the expulsion of waste. If someone were to make a sitcom starring the different parts of the human body, the parasympathetic nervous system would be the laidback character, telling people to “chill out” while rarely getting off the couch.
In contrast, the sympathetic nervous system is incredibly highly strung. It would be the twitchy paranoid one, constantly wrapping itself in tinfoil and ranting about the CIA to anyone who’ll listen. The sympathetic nervous system is often labeled the fight-or-flight system because it’s what causes the various responses the body employs to deal with threats. The sympathetic nervous system dilates our pupils, to ensure more light enters our eyes so we can better spot dangers. It increases the heart rate while shunting blood away from peripheral areas and non-essential organs and systems (including digestion and salivation—hence the dry mouth when we’re scared) and towards the muscles, to ensure that we have as much energy as possible for running or fighting (and feel quite tense as a result).
The sympathetic system and parasympathetic systems are constantly active and usually balance each other and ensure normal functioning of our bodily systems. But in times of emergency, the sympathetic nervous system takes over and adapts the body for fighting or (metaphorical) flying. The fight-or-flight response triggers the adrenal medulla (just above the kidneys) as well, meaning our bodies are flooded with adrenalin, which produces many more of the familiar responses to a threat: tension, butterflies in the stomach, rapid breathing for oxygenation, even relaxing of the bowels (you don’t want to be carrying unnecessary “weight” while running for your life).
Our awareness is also turned up, making us extra sensitive to potential dangers, reducing our ability to concentrate on any minor issues we were dealing with before the scary thing happened. This is the result of both the brain being alert to danger anyway and by the adrenalin suddenly hitting it, enhancing some forms of activity and limiting others.23
The brain’s emotional processing also steps up a gear,24 largely because the amygdala is involved. If we’re dealing with a threat, we need to be motivated to take it on or get away from it asap, so we rapidly become intensely fearful or angry, providing further focus and ensuring we don’t waste time with tedious “reasoning.”
When faced with a potential threat, both brain and body rapidly shift to a state of enhanced awareness and physical readiness to deal with it. But the problem with this is the “potential” aspect. The fight-or-flight response kicks in before we know whether it’s actually needed.