all about mindset. But much like the outmoded concept of nature versus nurture, assuming that stress must be either a wholly physical or a wholly mental condition creates a false dichotomy between the two. Your mind and body are not separate parties ruling over you through coalition; they are very much linked. Those seemingly superficial cues — the sweating, the trembling, the rattling breath — betray a deeper and far more complex physiological response, one that engages an elaborate series of glands, nerves, and neurons called the hypothalamic-pituitary-adrenal (HPA) axis.
The HPA axis consists of three segments: the hypothalamus, the pituitary gland, and the adrenal glands. The hypothalamus is a tiny part of the brain — in humans, the entire thing is roughly the size of an almond — located just above the brain stem. Despite its small size, the hypothalamus’s duties are manifold, though its most vital function is facilitating communication between the nervous system and the endocrine system. The nervous system links the brain to the rest of the body, allowing it to move muscles and receive sensory input through the transmission of electrochemical signals. The endocrine system comprises a series of glands that secrete hormones responsible for regulating a number of important bodily functions, including internal temperature, hunger, thirst, circadian rhythm (or sleep cycle), and, most germane to our current discussion, response to stress.
The HPA axis is itself an endocrine system, which is why its first junction is the hypothalamus. When our body acquires sensory data (captured by our eyes, ears, nose, tongue, and sense of touch) indicating a potential threat, the hypothalamus takes that information — brought to it by the nervous system in the form of electrochemical impulses — and translates it into a chemical-based language that the HPA axis can understand. It does this by secreting a hormone called “corticotropin-releasing hormone,” or CRH, which travels from the hypothalamus to the nearby pituitary gland, where it stimulates the release of — you guessed it — corticotropin. However, this was apparently too self-evident for anatomists, who reissued corticotropin a clunkier and less intuitive name: adrenocorticotropic hormone, or ACTH.
The pituitary gland is even smaller than the hypothalamus: about the size of a pea in humans and weighing less than a gram. Like the hypothalamus, its duties are manifold. Though most renowned for its role in human sexual development, the pituitary gland is also a vital part of our stress response system. As we’ve mentioned, it secretes the hormone ACTH, which travels through the bloodstream to a pair of glands perched atop the kidneys. Here, at the adrenal cortex, we have reached the final stop along the HPA axis. This is where we truly get down to business.
Upon receiving a dose of ACTH, the adrenal cortex instructs its glands to secrete a pair of hormones responsible for implementing our physiological stress response. The first of the two is adrenaline, a name you’ve probably heard before. We attribute to adrenaline the “rush” we feel in the face of danger, the jackhammering heart and sweaty palms and manic burst of wide-eyed energy. Yet adrenaline is only one half of an integral stress response partnership. It’s other half is a far less celebrated hormone, but one that is no less important: cortisol.
Cortisol’s main role in responding to stress is redistributing energy (in the form of glucose, or sugar) to the parts of the body that need it most in a time of crisis — the brain for making snap decisions, the major muscle groups to carry those decisions out, and the heart to provide the muscles with the surplus oxygen they need to work at full capacity. Cortisol acts as a kind of override switch, drawing power from other areas of the body (such as digestion, immune system response, and gamete production) and allowing us to make full use of our adrenaline. Adrenaline rushes headlong into battle, ramping up our heart rate and boosting our muscle tone and readying our fight-or-flight response, which is why it tends to get all the glory. But cortisol is no less vital. It provides a critical support role in the stress response system, overseeing our glucose supplies and keeping the troops well-nourished for the duration of the battle.
It’s easy to see how a stress response system can come in handy. When fleeing from a sabre-tooth tiger or defending your village from a warring tribe, the ability to dip into the reserves and muster up that extra bit of energy could make the difference between life and death. But eventually the threat passes, at which point our energy is no longer well-spent on muscle tone and a hammering heart. The surplus glucose needs to be returned to the systems from which it was initially drawn, or else we’ll have escaped from whatever triggered our stress response system in the first place only to die from malnutrition or disease, as our digestive and immune systems failed to shift back into gear. Fortunately, our HPA axis has a nifty way of returning to business as usual. Once cortisol reaches a certain concentration in the bloodstream, it makes its way up to the hypothalamus and pituitary gland, where it dampens the production of CRH and ACTH, respectively. As our stress hormone levels drop, the amount of cortisol in the bloodstream gradually decreases until the body returns to its original, pre-stress state. This is called a feedback loop, and it’s an incredibly valuable trait. In essence, it’s like having your bathwater reach up and shut off the tap before the tub gets too full and floods the washroom.
Thanks to the feedback loop, there’s nothing unhealthy about experiencing occasional bouts of stress. This is a good thing, as it’s a very fortunate few who can live their entire lives without succumbing to feelings of anxiety, frustration, or fear. When we do experience these unpleasant sensations, our body provides us with the extra energy needed to respond to them, then dials us back down to a state of hormonal equilibrium called homeostasis. Homeostasis is our body’s natural resting point, where internal temperature, pH, and hormone levels coexist in perfect harmony. An organism in homeostasis is functioning optimally — in utilitarian terms, it is expending the least possible amount of energy to perform the most work. Less waste means a more efficient conversion of food into energy, greater production of gametes (or sex cells), a more disciplined and well-staffed immune system, and a smarter, sleeker, swifter brain. An organism in homeostasis is, in short, better equipped to survive, which is why our bodies have a number of mechanisms in place designed to make homeostatic harmony sturdy and, in the event of a shakeup, easy to re-obtain. The HPA cortisol feedback loop is one of them.
Think of cortisol as a kind of hormonal ballast, keeping our bodies on an even keel in calm or choppy waters. Only sometimes the waters are more than just choppy. Sometimes life steers us into the arms of a raging hurricane. If the waves of stress and fear batter us too intensely, allowing no chance for us to rest and recalibrate, our ballast may shift. At this point, our body is no longer able to manage itself effectively. With our homeostatic equilibrium compromised, our beleaguered ship begins to sail at an uneven keel. Stress sloshes onto our decks, and our poor vessel is suddenly at risk of sinking. The captain calls all hands on deck in a desperate attempt to keep our ship afloat. The cook stops cooking. The navigator abandons his charts. The ship doctor closes his clinic. Our body’s many systems, suppressed by cortisol in a desperate attempt to weather the storm, stop working. Our immune system shuts down, increasing our risk of contracting diseases and reducing our ability to fight them. Our digestion and sleep schedules become interrupted, weakening us further still.
This is bad enough in adulthood. During early childhood, when the bulk of our energy should be directed toward our physical and mental growth, excessive stress can have disastrous lifelong consequences. Chronic exposure to high levels of stressful stimuli — called toxic stress — among children corrodes the very foundation of human development. And when the groundwork is poorly set, everything built atop it risks collapse. Problems we associate with adulthood or even old age — heart disease, diabetes, alcoholism, etc. — all have their roots in early childhood.
We should clarify that toxic stress is very different from the mild anxieties, injuries, and upsets that comprise a normal and healthy childhood. Your child stubbing their toe or being scolded for colouring on the walls or throwing a tantrum in the supermarket are not signs of toxic stress sinking its poisonous fangs into their defenceless minds. A little stress isn’t a bad thing; in fact, it can actually be good from a developmental standpoint.
Stress, as defined by experts in child trauma, comes in three levels. The first, called positive stress, is a normal part of growing up. Every child gets stressed out at some point, and moments of mild frustration, anger, or anxiety