INSIDE THE BRAIN – Part 4: The Brain That Keeps Us Alive
- Marcela Emilia Silva do Valle Pereira Ma Emilia
- há 3 dias
- 17 min de leitura
The Balance We Cannot See
Throughout this series, it has become clear just how essential the brain is to the functioning of the human body.
The organ that has been more closely understood through its failures than through its normal functioning still hides many mysteries that are yet to be discovered.
After exploring the brain when it fails — the lesions, the diseases, the syndromes — there is also the other side of that story: the journey through cerebral robustness.
Beneath everything that is conscious, there is a brain that never rests. That never stops. That works in silence, without one even being "aware" of it.
The brain works independently by itself. It is an imperfect yet complex command center. It is autonomous in its capacity for reconfiguration, by altering its own physical structure (synapses) based on lived experiences and stimuli — that is, continuously adapting.
And do you know what is most fascinating? The entire brain not only transforms according to experience or stimulus, but also prepares the body for it. At the same time, it is responsible for a succession of functions that keep the organism alive — regulating temperature, blood pressure, heart rate, blood sugar levels, and breathing.
And it does all of this with extraordinary precision, twenty-four hours a day, seven days a week, for an entire lifetime.
So, unlike the brain that fails, here we will explore when it does not think — but keeps all of us alive.
🧠 The Maintenance of Life
The concept of homeostasis, fundamental to the maintenance of life, was coined by Walter Bradford Cannon in 1926, from the Greek homos (similar) and stasis (stability). But the idea behind the concept is much older — it goes back to the French physician Claude Bernard, who in the 19th century described the "milieu intérieur": the idea that living organisms maintain a relatively constant internal environment, regardless of variations in the external environment.
Homeostasis is, in essence, the capacity of the organism to keep its physiological variables within narrow and precise limits, even when the external environment changes constantly.
Body temperature must be 37ºC, even when it is cold outside. Blood pressure stable, even when running. Glucose balanced, even hours after a meal. Blood pH maintained between 7.35 and 7.45 (here a minimal variation can be fatal).
But this does not happen by magic, nor through conscious control.
Homeostasis is, then, a capacity of the nervous system to maintain a stable internal environment necessary for the ideal functioning of neurons, despite external variations. It is the brain regulating the most diverse bodily activities through feedback mechanisms to maintain survival, whilst also having to maintain itself.
This control is also not something that operates with exact numbers or processes. It is constantly adapting to what the person is experiencing. The balance is dynamic, where parameters may vary within a safe and healthy range.
There can also be failures in its regulation, and when this occurs, it can lead to neurological and endocrine diseases (most commonly), the most common being chronic elevation of cortisol (chronic stress) and associations with anxiety and depression. In severe cases, it can be fatal.
Even so, the brain is more phenomenal than that, and this responsibility for regulating life is coordinated by a region the size of an almond, weighing only around 4 grams.
✨ The body is not static — it is in constant adjustment. Homeostasis is not a fixed state, but a dynamic and continuous process of correction and rebalancing.
🧠 The Hypothalamus — The Control Centre of Life
That region, the size of an almond, weighing around 4 grams, has a name: the hypothalamus.
This small structure is located in the diencephalon, below the thalamus — and is probably the most powerful and important region in terms of survival. It is not the largest, nor the most sophisticated, but it is what ensures that everything else continues to function.
And it is not just about the brain — it is about the entire body.
Such a small structure manages to coordinate various functions simultaneously according to the way it communicates with the rest of the organism.
First, it is necessary to understand two terms that appear frequently in neuroscience, as they explain exactly how this communication works.
Afferent and Efferent Connections — the Language of the Hypothalamus
The hypothalamus is connected to many structures of the Central Nervous System (CNS). It is a heterogeneous area that functions as a bidirectional command center. On one hand, it receives information — about the variable values of the external and internal environment — on the other, it sends orders — stimulating other parts of the CNS to repair whatever is necessary.
It is important to highlight that the hypothalamus is a chemosensitive organ. This means it "reads" the blood directly rather than simply "listening" to the neurons.
Afferent connections are those that arrive at the hypothalamus — they are the signals that inform it about the state of the organism as a whole. Blood temperature, glucose levels, osmotic pressure, circulating hormones, sensory information from the environment. It is through these signals that the hypothalamus "knows" what is happening inside and outside the body.
The main "entry" pathways of afferent connections are:
Neural Connections – "Electrical wires" (axons coming from the rest of the brain)
Humoral Connections – "Chemical sensors" (hypothalamic neurons that "read" the blood to measure nutrients and hormones)
Intrinsic Receptors – "Integrated thermometers" (sensitive cells that respond to sensory information)
Now, efferent connections are those that depart from the hypothalamus — orders it sends to the rest of the organism in response to what it received from the afferents. It is through these that it activates perspiration, speeds up the heart, stimulates hunger, triggers the stress response, and regulates sleep.
The efferent pathways of the hypothalamus follow very similar routes to the afferent inputs, but here the objective is the execution of the task, and they are also divided into three main routes:
Connections with the Limbic System (Behaviour) – fundamental for the formation of memories and emotional processing, as well as sending signals to influence decision-making based on biological needs
Endocrine Control (Hormonal Pathway) – when the hypothalamus commands the pituitary gland (hypophysis) to release hormones into the bloodstream (we will discuss this pathway further ahead)
Control of the Autonomic Nervous System (ANS) – sending axons to the brainstem and spinal cord to control involuntary functions (we will discuss this further in the next section of the article)
In short, the efferent are how the hypothalamus transforms a "feeling" (such as thirst, cold, or emotion) into a biological action resulting in a biological or behavioural reaction.
It is this capacity to receive and send information simultaneously, and to do so continuously, automatically, and with incredible precision, that makes the hypothalamus so extraordinary.
The Functions of the Hypothalamus Go Far Beyond What One Might Imagine
When thinking about bodily regulation, obvious functions such as temperature, breathing, the heart, or hunger tend to come to mind. But the hypothalamus goes far beyond that.
Its functions cover practically everything the organism does autonomously — that is, without having to think about it to happen:
Regulation of body temperature
The hypothalamus is the body's thermostat. When temperature rises, it activates perspiration and vasodilation. When it falls, it triggers muscular shivering and vasoconstriction.
Regulation of appetite and thirst
It contains specific nuclei that detect glucose, leptin and ghrelin levels in the blood, generating the sensations of hunger, satiety and thirst, respectively.
Regulation of sleep and wakefulness
It coordinates the circadian rhythm together with the suprachiasmatic nucleus, determining the cycles of sleep and activity.
Cardiovascular regulation
It influences heart rate and blood pressure through the autonomic nervous system.
Endocrine regulation
It commands the pituitary gland, which in turn regulates the thyroid, adrenals, gonads and growth.
Regulation of emotional and sexual behaviour
It is closely linked to the limbic system, participating in stress response, aggression, reproductive behaviour, and fear and pleasure responses.
Regulation of the immune system
Through the HPA axis and connections with the autonomic nervous system, the hypothalamus influences the inflammatory and immune response of the organism.
And yes — even the blinking of eyes has hypothalamic influence. The blinking reflex is controlled primarily by the brainstem, but its frequency and regulation are influenced by the hypothalamus, which adjusts this reflex according to the state of wakefulness, hydration and alertness of the organism. When drowsy, the eyes blink more slowly. When at maximum alertness — in a stressful or dangerous situation — blinking may almost stop.
Nothing is by chance.
✨ It is almost impossible to name a function of the organism that the hypothalamus does not touch, directly or indirectly.
The HPA Axis — When the Brain Speaks to the Body
To regulate the internal environment, the hypothalamus uses two major systems in parallel: the autonomic nervous system (which we will discuss shortly) and the endocrine system — the hormonal communication system of the organism.
There is still some controversy about the findings, but in a certain sense the hypothalamus is considered an endocrine gland. It is known that it produces two polypeptides: the hormones vasopressin (antidiuretic hormone) and oxytocin (the love hormone).
One of the most important axes in human physiology arises from the endocrine version of the hypothalamus: the HPA axis — hypothalamus-pituitary-adrenal:
The hypothalamus, upon receiving afferent information and detecting a threat or imbalance — whether physical stress, such as a drop in glucose, or psychological stress, such as a dangerous situation — sends a chemical signal called CRH (corticotrophin-releasing hormone) to the pituitary gland, a structure the size of a pea hanging just below the hypothalamus.
The pituitary gland, upon receiving this signal, releases ACTH (adrenocorticotrophic hormone) into the bloodstream. The ACTH stimulates the adrenal glands to produce more cortisol to balance the level of stress in the body. Its task is, then, to "travel" to the adrenal glands, located at the top of the kidneys, and stimulate the production and release of cortisol to balance and regulate stress levels through negative feedback.
Cortisol, which is often portrayed as the villain, is presented here in acute and punctual doses as an essential hormone. It mobilizes energy, reduces inflammation, sharpens attention and prepares the organism to respond to a challenge. Cortisol only becomes a problem in this scenario when this system is activated chronically, without rest.
But the HPA axis has a self-regulation mechanism: when cortisol levels rise sufficiently to contain the stress, the hypothalamus and pituitary gland themselves receive this signal and reduce the production of CRH and ACTH. This response is known as negative feedback — that is, the system itself signaling that it is enough. The fact is that the hypothalamus also knows how harmful cortisol can be, which is why this negative feedback is so essential.
✨ It is precisely this capacity for self-regulation that makes the HPA axis so elegant — and it is precisely when this self-regulation fails that stress becomes disease.
The hypothalamus also commands the pituitary gland in the regulation of virtually the entire endocrine system: the thyroid (metabolism), the gonads (reproduction), growth, water balance. It is, in fact, the hormonal maestro of the entire organism.
And, beyond all this hormonal orchestration, the hypothalamus coordinates yet another response system — faster, more direct, more immediate. A system that requires neither hormones nor bloodstream to act.
It is the autonomic nervous system.
⚡ The Autonomic Nervous System — The Body's Autopilot
The nervous system is the body's communication and control network, responsible for perceiving the world around it, making internal decisions and coordinating functions — from the most complex to the simplest. It is what keeps the organism in survival mode.
The nervous system is divided, so to speak, into two major "matrices" that always work together:
– The Central Nervous System (CNS)
– The Peripheral Nervous System (PNS)
The CNS, already introduced earlier in this series, is the processing center where everything that is perceived and interpreted. It translates information, clarifies images, and decodes sounds. It is composed of the encephalon — brain, cerebellum and brainstem — and the spinal cord — the "super expressway" of information that runs down the back, connecting the encephalon to the rest of the body.
The PNS comprises every nerve and ganglion that lies outside this central axis of the CNS, but which is responsible for carrying orders from the CNS to the muscles and peripheral organs and making the return journey.
The PNS is further divided in an interesting way:
Somatic – Voluntary actions (arms, legs, hands, feet)
Autonomic – Actions on autopilot, occurring independently of any voluntary action.
The autonomic nervous system (ANS) is the division of the PNS responsible for controlling the involuntary functions of the organism — those that occur without having to think: heartbeat, digestion, breathing, pupil dilation, gland secretion.
It is called "autonomic" precisely because it operates independently of consciousness. But this does not mean it is immune to the brain's influence — quite the contrary. It is not isolated; it is in constant communication with the hypothalamus, which — as seen — functions as the great maestro of this orchestra.
The ANS is divided into two main branches, with opposing and complementary functions: the sympathetic nervous system and the parasympathetic nervous system. Most organs receive influence from both simultaneously — and the state of the body at any given moment is the result of the balance between them.
It is almost like a permanent tug of war — but a healthy tug of war, where neither side should win forever.
In fact, it is a common mistake to think that when one "switches on" the other "switches off" completely. What actually happens is that both systems are always active, operating in a minimal and continuous state called tone. And the body's health has as its indicator the Heart Rate Variability, which is precisely the healthy "tug of war" between the two.
🔴 The Sympathetic Nervous System — Fight or Flight
Imagine crossing a road and a car appearing out of nowhere at high speed. In fractions of a second — before you can even think — the heart races, breathing quickens, muscles tense, pupils dilate. And without realizing it, the body is already acting, fleeing the dangerous situation.
This, ladies and gentlemen, is the sympathetic nervous system in action.
The sympathetic system is the system of response to danger, stress, and immediate action.
When activated, it prepares the organism for a rapid and intense response — what physiologist Walter Cannon, the same person who coined the term homeostasis, called the "fight or flight" response. Stay and face it, or flee and take cover.
When the sympathetic system is activated, a series of changes occurs in cascade, and all of them have a single objective: to put the body into survival mode.
In this case, the expected response is for the heart to speed up to pump more blood
into the muscles. Breathing to become faster and deeper to increase oxygen supply. Pupils to dilate to widen the visual field. Blood flows to be redirected from the digestive organs to the skeletal muscles — because digestion is definitely not a priority at that moment. The liver to release glucose into the bloodstream, immediate fuel for action. Perspiration to increase to cool the body during exertion. And the adrenal glands to release adrenaline and noradrenaline — the hormones of action.
The response spreads rapidly to various organs in a systemic manner. This is because the neurons of the sympathetic system have a "transfer station" called the Sympathetic Ganglion Chain. This runs along both sides of the spinal column, like a power rail through which the signal enters and spreads rapidly.
However, all of this stimulation can also cause the system's "bug".
The thing is, the sympathetic system serves to fight or flee, but it can become more complicated when it involves a biological response to a more frightening danger. And the reaction to this fear can be called Freeze (Freezing).
There are three well-accepted reasons to explain how a person can become paralyzed, without reacting, even in the face of imminent danger with a body full of adrenaline:
Information Overload:
+ If the stimulus is too terrifying or unexpected, the amygdala (the center responsible for fear) "hijacks the moment", interrupting all processing through the correct pathways, and sends its signals directly to the hypothalamus, bypassing the prefrontal cortex (which plans the reaction), thus causing a "short circuit". At the same time, the prefrontal cortex is trying to understand what is happening.
+ Result: The brain cannot decide which motor command to send, and so the body freezes due to subconscious analytical indecision.
The Polyvagal Theory:
+ This is a highly respected theory (by Stephen Porges) which states that animals have a defensive response even older than the sympathetic system — the Dorsal Vagal. In this theory, this system is the trigger for tonic immobility, that is, a way of "playing dead" in order to survive or to reduce pain should the worst occur.
+ This is a protective paralysis, as the brain perceives that the threat is so great that fighting or fleeing is futile, and therefore activates this emergency "circuit breaker".
Command Conflict:
+ Sometimes the sympathetic and parasympathetic systems are triggered almost simultaneously and "BAM", they clash, and the two commands collide with equal force.
+ While the sympathetic says "Run", the parasympathetic says "Stay still", and hence the imbalance.
+ The body enters a state of rigid hyperactivation — extremely tense, but motionless. Like pressing the accelerator of a car and it not moving.
🧠 But breathe — regardless of the response, whether fighting, fleeing or freezing, any one or all three of these tactics were essential for the survival of the species. In an evolutionary context, it was they that allowed fleeing from a predator, fighting for territory or going unnoticed by a predator. And they still do!!!!
In seconds, the organism went from rest to maximum response capacity — without needing to think, without needing to decide.
And here comes one of the most interesting — and most relevant — ironies of modern neuroscience:
✨ The sympathetic system does not distinguish between a lion and an urgent email from the boss. For the primitive brain, stress is stress — and the biological response is exactly the same.
The problem is that Freezing is becoming increasingly normalized in modern human beings and unfortunately manifests in unexpected ways. These are sensations of freezing in the face of an important presentation, or in a real risk situation in which control of the danger lies in the emotional state of a third party.
The problem is not the sympathetic system itself — for it is essential and saves lives. The problem is when it is activated repeatedly, without the body having time to recover.
But that is where the other side of the system comes in.
🔵 The Parasympathetic Nervous System — Rest and Digest
If the sympathetic system is the accelerator, the parasympathetic nervous system is the brake.
But not an emergency brake — a smooth, gentle adjustment, the kind that tells the body: "the danger has passed, you can relax now."
It is the system of recovery, rest, digestion, regeneration. Some researchers call it the "rest and digest" response — in direct opposition to the sympathetic "fight or flight".
When the parasympathetic system takes control, the body does the reverse. The heart calms. Breathing becomes slower and deeper. Digestion is reactivated — blood flow returns to the gastrointestinal tract. The pupil’s contract. The salivary and lacrimal glands are activated. And the organism enters repair and cellular regeneration mode — it is in this way and in this state that the body recovers, repairs itself and prepares for the next challenge.
So, whilst the sympathetic system spends energy recklessly to save the body from danger, the parasympathetic steps in to clean up the mess, replenish the energy stores and ensure that the "machine" keeps functioning for many more years to come.
Just like the sympathetic, it spreads throughout the entire body, but the parasympathetic ganglia are located very close to or sometimes even within the organs they control, in order to have a more focused response. The origin of the nerves is slightly different — they have a more localized distribution. Either it is superior (cranial) — four nerves that emerge from the brainstem (with particular note of the Vagus Nerve) — or it is inferior (sacral) — emerging from the vertebrae at the base of the spine (S2 and S4).
As highlighted, the great protagonist of the parasympathetic system is the vagus nerve — the longest cranial nerve in the human body. It runs from the brainstem to the abdominal organs, innervating the heart, lungs, stomach and intestine. It is literally the conductor of relaxation in the body.
🧠 The vagus nerve is today one of the most studied targets in modern neuroscience. Its stimulation — whether through slow deep breathing, meditation, singing, or even implantable medical devices — has demonstrated significant effects in reducing inflammation, regulating mood and treating conditions such as depression and epilepsy.
You know that feeling of relief that comes after a long, deep breath? It is not imagination. It is THE PARASYMPATHETIC!
In fact, it is through the parasympathetic system that deep relaxation experienced after a meal or following an exhausting day result. It is the release of acetylcholine (the neurotransmitter of calm).
Quick aside! The neurotransmitter acetylcholine, incidentally, is also used by the sympathetic system to induce perspiration (leaving a "parasympathetic scent in the air"), even though almost the entire sympathetic system uses noradrenaline to communicate with the organs. (Aside closed, back to the parasympathetic)
In this wave of calm, the parasympathetic also proves essential for human connection. It is what relaxes the facial muscles, maintains eye contact and modulates the voice in a friendly manner with others. For when under its effect, one is more open to learning and socialization, in contrast to the sympathetic, which is in survival mode.
✨ Taking a deep breath is a direct activation of the parasympathetic system, through the vagus nerve. The body knows what it is doing — sometimes one simply needs to let it.
⚖️ The Balance Between the Two
In healthy life, the sympathetic and parasympathetic are not rivals — they are partners. They alternate in a fluid and constant manner: the sympathetic activates in the face of a challenge, the parasympathetic takes over when the challenge passes. It is a natural rhythm that the body maintains on its own, without requesting permission.
As mentioned before, most organs receive influence from both systems simultaneously, the state of the body at any given moment is simply the result of which of the two is "speaking louder", and the balance between the two sides is exactly what keeps everything in place.
Heart Rate Variability (HRV) is considered one of the best physiological indicators of cardiovascular health and stress resilience. A high HRV indicates that the two systems communicate well and alternate efficiently. A low HRV may be a sign that the sympathetic has been dominant for too long.
✨ Neither system is better or worse — they are complementary. The problem arises whenever one dominates the other for too long.
🌍 Conclusion
Folks, with this post, I reach the end of this 4-part series on the Brain and the Nervous System, the main subjects of study in Neuroscience. And throughout this series, it has been a journey that few stop to imagine.
It began with anatomy — the lobes, the neurons, the synapses, the extraordinary architecture of an organ that cannot fit in the palm of evolution's hand. It then expanded to understand its failures — the lesions, the diseases, the syndromes that, paradoxically, revealed more about the brain than any healthy brain ever could have imagined. And now, in this fourth and final part, the discovery of how the brain works in silence, never resting, never thinking, yet maintaining life — regulating, adjusting, balancing and beginning again every single day.
And perhaps this is the most surprising lesson of the entire series:
🧠 The brain does not exist merely to think. It exists, first and foremost, to keep the organism alive.
Every thought, every emotion, every memory, every decision one has — depends on a stable biological substrate that the brain works incessantly to preserve. Homeostasis, the hypothalamus, the autonomic nervous system, the sympathetic and the parasympathetic — these are not merely neuroscience concepts. They are what makes us wake up in the morning, breathe without thinking, digest without effort, adapt without noticing.
And how to understand this changes — or at least should change — the way we relate to self-care.
Sleep, movement, nutrition, stress management, human relationships, conscious breathing — these are not "healthy habits" in the superficial sense of the word. They are real variables that the brain uses, every day, to maintain the balance that makes life possible.
✨ Taking care of the body is not separate from taking care of the brain. They are exactly the same thing.
It has been an immense pleasure to explore and share these themes in this series, part by part, layer by layer. The brain is — and will continue to be — one of the greatest mysteries that science has already uncovered, is trying to uncover, and still has much to uncover. And the more one learns about it, the clearer one thing becomes:
We are, above all, our brain. And it deserves to be treated as such.
📚 References and Essential Reading
Cannon, W. B. (1932). The Wisdom of the Body. W. W. Norton & Company.
Bernard, C. (1865). Introduction à l'étude de la médecine expérimentale. J. B. Baillière.
Porges, S. W. (2011). The Polyvagal Theory: Neurophysiological Foundations of Emotions, Attachment, Communication, and Self-regulation. W. W. Norton & Company.
Sapolsky, R. M. (2004). Why Zebras Don't Get Ulcers. 3rd ed. Holt Paperbacks.
Bear, M. F., Connors, B. W., & Paradiso, M. A. (2016). Neuroscience: Exploring the Brain. 4th ed. Wolters Kluwer.
Kandel, E. R., Schwartz, J. H., & Jessell, T. M. (2013). Principles of Neural Science. 5th ed. McGraw-Hill.
Jänig, W. (2006). The Integrative Action of the Autonomic Nervous System. Cambridge University Press.
Manes, F., & Niro, M. (2015). Usar el Cerebro. Planeta.
Nolte, J. Neuroscience. Original edition translation. Elsevier, 2008.
Cosenza, R. M. Fundamentos de Neuroanatomia. 4th ed. Guanabara Koogan, 2021.
👉 Further Reading
NIH — How the Brain Works: https://www.nimh.nih.gov/health/educational-resources/brain-basics
Porges, S. W. — The Polyvagal Theory (overview): https://www.stephenporges.com
American Institute of Stress — What is stress?: https://www.stress.org/what-is-stress
Walker, M. — Sleep is your superpower (TED Talk): https://www.ted.com/talks/matt_walker_sleep_is_your_superpower
Cleveland Clinic — Vagus Nerve: https://my.clevelandclinic.org/health/body/22279-vagus-nerve
Comentários