Vasoconstriction, metabolism, chills and hypothermia: the systems implemented on the basis of the messages that reach the brain from the skin receptors
Our thermal sensitivity depends on two organs: The skin, which has two distinct sensory pathways, one for cold and one for heat, and the brain, explains the neurophysiologist Matthew Cerri, author of The cure for the cold (Einaudi). The skin – he says – measures our temperature in the arms, legs and ears, which is often closer to the ambient temperature. Sending to the brain how cold and how hot it is is through i thermal receptors or skin thermoreceptors that do not work like a thermometer: they do not measure the temperature precisely, but are sensitive to its variations.
The information
The molecules that act as thermoreceptors are the result of recent discoveries for which David Julius won the Nobel Prize for Medicine in 2021 together with Ardem Patapoutian. They belong to a large family of substances (channel proteins) called Trp (Transient Receptor Potential). Each of its members specializes in sensing a particular thermal range. At any moment, therefore, the temperature that our brain perceives the symphony that results from the activation of different thermoreceptors, as if they were the instruments of an orchestra. The thermal information that the skin sends to the brain arrives in an area called the preoptic area. It is located inhypothalamus, a region in the center of the brain which supervises our survival, both in terms of maintaining vital functions and activating the behaviors necessary for the survival of the individual and of the species, continues the expert. The preoptic area, thanks to specific thermoreceptors, also measures our central body temperature which is constant (between 36.5 and 37.5), but only in that region of the body called Core which consists of the brain and thoracic and abdominal organs . As soon as the peripheral skin thermoreceptors communicate whether the environment is cooling down (or warming up), the brain activates the Core’s temperature defense mechanisms before it cools down (or warms up). The mechanism is called predictive homeostatic response.
The reactions
The first anti-cold weapon vasoconstriction. The brain deploys its defenses, with a hierarchical order, through the activation of a branch of the autonomic nervous system called sympathetic nervous system. The first defense is isolation. The skin vessels reduce their diameter and less blood reaches the extremities (vasoconstriction; that’s why we look pale in the cold). In this way, the heat loss is lowered and this may be sufficient. a very cheap defense because it does not require energy expenditure, but its effectiveness is limited, the expert points out. The second weapon the metabolism. If vasoconstriction is not enough, the brain orders certain organs to increase the metabolism: The thyroid gland will produce more thyroid hormone, the adrenal gland more adrenaline and cortisol, the so-called stress hormone, and the brown adipose tissue will start activating. This particular type of fabric specializes in consuming energy to produce heat. In obese subjects, however, it has stopped working and this is one of the possible causes of the obesity epidemic in Western countries, Cerri points out. The location of brown fat is strategic, being aimed at warming the blood going to the brain and spinal cord. In fact, it is found around the large arterial vessels of the neck and mediastinum, the median space of the thoracic cavity between the lungs, and close to each vertebra. The white adipose tissue instead has two other purposes: a deposit of energy and increases the body’s insulation from the outside. A little of this fabric (not too much) is necessary in order to have an energy reserve to draw on during the day. . The subsequent defense, however, is charged to the muscles which can help the thrill mechanism. The muscle in this circumstance is active rapid contractions whose purpose is not to make us move, but to get heat. Finally, even if this weren’t enough because the cold is too intense, as happens for mountaineers or shipwrecked people, then the brain changes its strategy and adopts the fourth option, concludes the expert. He switches off all energy-consuming activities and lets himself cool down. In this way he tries to buy time in the hope that external help will arrive or that the environmental conditions will change: it is about the‘accidental hypothermia which unfortunately still causes deaths even in the West.
December 18, 2022 (change December 18, 2022 | 15:21)
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