The Hour Between Dog and Wolf: Risk-taking, Gut Feelings and the Biology of Boom and Bust. John Coates

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СКАЧАТЬ style="font-size:15px;">      CONTROLLING OUR INTERNAL WEATHER

      For body and brain to be unified in this way, they must conduct a non-stop dialogue, a process, mentioned above, called homeostasis. Oxygen levels in the blood must be maintained within tight bands, and are kept so by a largely unconscious modulation of our breathing, as must heart rate and blood pressure. Body temperature too must be maintained within a degree or two of 37 degrees Celsius. Should it drop, say, below this band, the brain instructs our muscles to shiver and adrenal glands to raise our core temperature. Blood sugar levels too must be reported and then maintained within narrow bands, and should they fall, bringing on symptoms of low blood sugar, the brain promptly responds with a number of hormones, including adrenalin and glucagon, which liberate glucose stores for release into the blood. The amount of bodily signals being processed by the brain, coming as they do from almost every tissue, every muscle and organ, is voluminous.

      Much of this bodily regulation is a job allotted to the oldest part of the brain, known appropriately as the reptile brain, and specifically to a part of it called the brain stem (see fig. 3). Sitting on top of the spine and looking like a small, gnarled fist, the brain stem controls many of the automatic reflexes of the body – breathing, blood pressure, heart rate, sweating, blinking, startle – plus the pattern generators that produce unthinking repetitive movements like chewing, swallowing, walking, etc. The brain stem acts as the life-support system of the body; other, more developed parts of the brain, ones responsible for, say, consciousness, can be damaged, leaving us ‘brain dead’, as they say, yet we can live on in a coma as along as the brain stem continues to operate. However, as animals evolved, the nervous circuitry linking their visceral organs such as the gut and the heart to the brain became more sophisticated. From amphibians and reptiles through mammals, primates and humans, the brain grew more complex, and with it came an expanded capacity for regulating the body.

      An amphibian such as a frog cannot prevent the uncontrolled evaporation of water from its skin, so it must remain in or close to water at all times. Reptiles can retain water, and therefore can live in both water and desert. But they, like amphibians, are cold-blooded, and that means they depend on the sun and warm rocks for their heat, and become all but immobile in cool weather. Because they do not take responsibility for controlling their body temperature, amphibians and reptiles have relatively simple brains.

      Mammals, on the other hand, took on far greater control of their bodies, and therefore needed more brainpower. Most notably, they began to control their internal temperature, a process called thermoregulation. Thermoregulation is metabolically expensive, requiring mammals to burn a lot of fuel to generate body heat, to shiver when cold and sweat when hot, and to grow fur in autumn and moult in the spring. An idling mammal burns about five to ten times the energy of an idling reptile, so it needs to store a lot more fuel. As a result mammals had to develop greatly increased metabolic reserves; but once equipped with them they were free to hunt far and wide. The advent of mammals revolutionised life in the wild, and could be likened to the terrifying invention of mechanised warfare. Mammals, like tanks, could move a lot farther and a lot faster than their more primitive foes, so they proved unstoppable. But their mobility required more carefully managed supply lines, something that was accomplished by more advanced homeostatic circuitry.

      Humans in turn took on even more control over their bodies than lower mammals. This development is reflected in a more advanced nervous system and a more extensive and animated dialogue between body and brain. We find some evidence for this process in studies comparing the brain structures among animals and humans. In one noteworthy study of comparative brain anatomy, a group of scientists looked at differences in the size of various brain regions (size is measured as a percentage of total brain weight) among existing primates to see which regions correlated with life span, a measure they took as a proxy for survivability. Their study showed that in addition to the neo-cortex and cerebellum, two other brain regions grew relatively larger in humans, most notably two regions playing a role in the homeostatic control of the body – the hypothalamus and the amygdala (fig. 3).

      The hypothalamus, a brain region found by projecting lines in from the bridge of your nose and sideways from the front of your ears, regulates our hormones, and through them our eating, sleeping, sodium levels, water retention, reproduction, aggression and so on. It acts as the main integration site for emotional behaviour; in other words it coordinates the hormones and the brain stem and the emotional behaviours into a coherent bodily response. When, for example, an angry cat hisses, and arches its back, and fluffs its fur, and secretes adrenalin, it is the hypothalamus that has assembled these separate displays of anger and orchestrated them into a single coherent emotional act.

      Fig. 3. Basic brain anatomy. The brain stem, often called the reptile brain, controls automatic processes such as breathing, heart rate, blood pressure, etc. The cerebellum stores physical skills and fast behavioural reactions; it also contributes to dexterity, balance and coordination. The hypothalamus controls hormones and coordinates electrical and chemical elements of homeostasis. The amygdala processes information for emotional meaning. The neo-cortex, the latest evolved layer of the brain, processes discursive thought, planning and voluntary movement. The insula (located on the far side and near the top of the illuminated brain) gathers information from the body and assembles it into a sense of our embodied existence.

      The amygdala assigns emotional significance to events. Without the amygdala, we would view the world as a collection of uninteresting objects. A charging grizzly bear would impress us as nothing more threatening than a large, moving object. Bring the amygdala online, and miraculously the grizzly morphs into a terrifying and deadly predator and we scramble up the nearest tree. The amygdala is the key brain region registering danger in the outside world and initiating the suite of physical changes known as the ‘stress response’. It also registers signs of danger inside the body, such as rapid breathing and heart rate, increased blood pressure, etc., and these too can trigger an emotional reaction. The amygdala senses danger and rouses the body to high alert, and is in turn alarmed by our body’s arousal, this reciprocal influence of body on amygdala, amygdala on body, occasionally feeding on itself to produce runaway anxiety and panic attacks.

      Some of the most important research showing that connections between brain and body became more elaborate in humans is that conducted by Bud Craig, a physiologist at the University of Arizona. He has mapped out the nervous circuitry responsible for a remarkable phenomenon known as interoception, the perception of our inner world. We have senses like vision, hearing and smell that point outwards, to the external world; but it turns out we also have something very like sense organs that point inwards, perceiving internal organs such as the heart, lungs, liver, etc. The brain, being incurably nosey, has these listening devices – receptors that sense pain, temperature, chemical gradients, stretching tissue, immune-system activation – throughout the body, and like agents in the field they report back every detail of our viscera. This internal sensation can be brought to consciousness, as it is with hunger, pain, stomach and bowel distension, but most of it, like sodium levels or immune-system activation, remains largely unconscious, or inhabits the fringes of our awareness. But it is this diffuse information, flowing in from all regions of the body, that gives us the sense of how we feel.

      Interoceptive information is collected by a forest of nerves that flow back from every tissue in the body to the brain, travelling along nerves that feed into the spinal cord or along a superhighway of a nerve, called the vagus nerve, that travels up from the abdomen to the brain, collecting information from the gut, pancreas, heart and lungs. All this information is then channelled through various integration sites – regions of the brain that collect disparate individual sensations and assemble them into a unified experience – ending up in a region of the cortex called the insula, where something like an image of the overall state of the body is formed. Craig has looked at the nerves connecting body and brain in various animals, and has concluded that the pathways leading to the insula are present only in primates, and further that an awareness of the overall state of our body may be found uniquely in humans.

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