The brain is constantly receiving information about conditions both inside and outside the body. It interprets and uses this information to maintain a stable environment inside the body despite changes outside. This complex process – known as homeostasis, is essential for survival and for keeping the body’s tissues in working order.
The human body, just like plants or bacteria, can only thrive in certain conditions. Our basic survival requirements are a minimum of warmth, oxygen, water and food, but we also have less obvious needs: mental stimulation, attachment to others, amusement and novelty. Deny us the basics, and we will soon die; take away the other needs, and our health – mental and physical – will be seriously undermined.
Homeostasis – keeping the body stable
However, being in our environment it does not give us the things we need at precisely the time we need them. For example, we can’t immediately match our energy output, calorie by calorie, to the food we eat or replace every drop of heat we lose with a sip of water. Nor can we ensure that we are always in an environment that is a comfortable temperature. Homeostasis ensures that the body’s needs are met – that we maintain a store of nutrients to fuel action and that cells retain water even when none is available from outside. That core body temperature remains at an opti mum level despite external fluctuations.
Homeostasis is a whole-body mechanism that works by the influence of mental processes within certain organs and the controlling influence of consciousness and unconscious areas of the brain. Lt combines information from the senses – the sight of water, say – with self-generated knowledge, like feeling thirsty, to produce an appropriate response. Water levels in body cells, for example, are partly controlled by the kidneys, which absorb water from the blood, expelling it as you’re in it when levels are high and conserving it when they are low. A low water level in cells is signalled to the brain via nerve cells called osmoreceptors, which activate parts of the hypothalamus, producing feelings of thirst. The conscious areas of the brain then cake over, directing you to find a drink. The same process occurs with food: when you have eaten a big meal, nerve endings in the stomach wall send signals to another part of the hypothalamus, which creates the feeling of being satisfied. And when blood sugar levels fall again, yet another part of the hypothalamus produces a feeling of hunger and prompts you to find food.
Hormones and bonding
Similar processes ensure that our social, intellectual and emotional needs are fulfilled. Attachment and mental stimulation, for example, are essential for development and survival, especially in infancy, and the human need for contact with others continues throughout life. Intimate bonding behaviour (such as sex and breastfeeding) is associated with increased levels of a hormone called oxytocin, which creates feelings of satisfaction and serenity. These pleasant feelings, in turn, encourage us to seek out the people or situations that produce them, pulling us into ever-closer relationships with those we depend on or who depend on us. However, perpetual cuddling would make us vulnerable, so oxytocin-induced relaxation is countered by bursts of exploratory, outward-looking behaviour mediated by neurotransmitters like noradrenaline. Hence, the balance is maintained between the need for social bonding and the need to explore the wider environment.
APPETITE RESEARCH
For the first time in history, there are now more overfed people in the world than there are underfed. Surveys have shown that obesity is on the increase, including among children, and researchers have been trying to find drugs that might help to combat this new epidemic. At the other end of the scale, eating disorders such as bulimia and anorexia nervosa are also on the increase: studies in the 1990s showed that at least two per cent of females in the USA suffered from bulimia or anorexia – and the numbers are rising.
Controlling hunger signals
Research efforts to counteract some of these problems have focused mainly on examining the chemical messengers in the brain that signal hunger and satiety. Low serotonin levels are known to trigger binge eating, and drugs that increase this brain chemical are sometimes prescribed for bulimia. Other molecules that control the intake of food include the hormone leptin, which dampens appetite, and substances ailed orexins, which stimulate it. The search is on for drugs that will stimulate the appropriate brain chemicals to make disorders such as obesity and bulimia a thing of the past.
