Chapter 1
Chapter 2
Chapter 3
Chapter 4
Chapter 5
Chapter 6
Chapter 7
Chapter 8
Chapter 9
Chapter 10
Chapter 11
Chapter 12
Chapter 13
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Suppose we were to measure the heart rate and blood pressure of several people: one had just been told he had lost his job, the other was being attacked by a mugger in the street, a third had just lost a lot of blood from a bleeding vein, a fourth was exploring a rather dark, strange place that he found frightening. In all cases we are likely to find that the pulse quickens, the blood pressure rises, the
face may grow pale and, were we
to measure it, blood levels of adrenaline, the hormone secreted from
the inner part of the adrenal gland, had gone up. These changes
in the body, all the result of the increasing activity of the
sympathetic nervous system, would not tell us which person was which.
The sympathetic nervous system is a network of nerves that pass from
the brain and spinal cord to activate heart, blood vessels, lungs
and gut – it’s our general emergency
system. The chemical that is released by the sympathetic nerves is noradrenaline (called norepinephrine in the US). Here is our chemical code……..
Noradrenaline does more than simply wake you up, increase the power of your senses or make you anxious. One of the things any animal needs to do is to learn fast about situations or circumstances that can spell danger. If you’ve just seen a bear in a particular part of the forest, you won’t go there again in a hurry…….
There is a game I like to play with my students. Cambridge University is difficult to get into. After a rather prolonged process, the day comes when the letters are posted to all applicants telling them whether they have a place. Ask any student, even two or more years later, where he/she was when the letter arrived and they will tell you, and exactly what happened in the few minutes afterwards. The event is printed on their brain like a little movie; they can relive the moment. They may remember it for the rest of their lives, for all I know.
You will have similar memories; and people often share them - for example, in the UK, they remember where they were when they heard about the death of Princess Diana in 1997. In the US, it’s the World Trade Center disaster. Incidentally, the brain can also store very bad personal memories in perhaps the same way, and one result may be post-traumatic stress disorder (PTSD), but that’s another story (see Chapter 12).
Our lives are filled not so much with acute events (though they do occur) but rather with persistent stress. A job that is demanding, but over which we feel we have little control; an unsatisfactory relationship, from which there seems no resolution or escape; a loved one who is ill or otherwise endangered; a bereavement; the loss of a friend; all these are the stuff of life’s slings and arrows. We need a way of coping with these sorts of stress as well as with sudden emergencies…
… Luckily we
have one. It’s back to the adrenal gland, but this time to the
outer ball of cells that make up the adrenal cortex. This is a
classical hormone-producing gland, and it secretes several
steroid hormones. The one we are interested in is cortisol (also
called hydrocortisone); in the rat, cortisol is replaced by
corticosterone,….The adrenal gland is highly responsive to stress. A job interview, an aggressive encounter, the strain of looking after a dependent relative, an illness – all these stressors result in very marked increases in cortisol levels. Unlike noradrenaline, the cortisol response can last for days or weeks. This is an essential part of coping….
But there is increasing evidence that persistently high cortisol may be bad for the brain.
Imagine walking along a cliff path, when a sudden gust of wind blows you towards the edge. Whether you go over or not depends on several things – including the strength of the gust, but also how near you are to the edge at the time. Now apply this to the brain – or, rather, to some agent (a toxin, a lack of oxygen as in a stroke) to the brain. Whether your brain is damaged depends also on many things- the severity of the attack and so on. But high cortisol seems to push the brain nearer the brink: it’s called ‘endangerment’ . Raised cortisol levels mean that agents that would be resisted now provoke damage, or a small amount of damage becomes larger. Cortisol pushes the brain nearer the edge.
Scientific and literary quotes .