These scanning techniques have revealed which parts of the brain are associated with which functions. MRI scans, computed tomography ( CT) scans and diffusion tensor images (DTI) use the magnetic signatures of different tissues, X-ray absorption, or the movement of water molecules in those tissues, to image the brain. Functional magnetic resonance imaging ( fMRI) or positron emission tomography ( PET) monitor blood flow. Other ways of imaging brain activity are indirect. Some researchers think that synchronising the activity in different brain regions is important in perception. When this activity goes awry, it is called a seizure. These signals have wave-like patterns, which scientists classify from alpha (common while we are relaxing or sleeping), through to gamma (active thought). Collectively, this electrical activity can be detected outside the scalp by an electroencephalogram (EEG). Within individual neurons, signals are formed by electrochemical pulses. Similarly, a deficiency in serotonin, used by regions involved in emotion, can be linked to depression or mood disorders, and the loss of acetylcholine in the cerebral cortex is characteristic of Alzheimer’s disease. It can also increase susceptibility to addiction because it mediates our sensations of reward and pleasure. For example, a loss of dopamine in the basal ganglia, which control movements, leads to Parkinson’s disease. These neurochemicals are so important that deficiencies in them are linked to certain diseases. Others also spread their influence more widely, like a radio signal, making whole brain regions more or less sensitive. Some neurochemicals work in the synapse, passing specific messages from release sites to collection sites, called receptors. Signals pass between them by the release and capture of neurotransmitter and neuromodulator chemicals, such as glutamate, dopamine, acetylcholine, noradrenalin, serotonin and endorphins. The neurons in our brains communicate in a variety of ways. Exercising the brain and giving it the right diet can be just as important as it is for the rest of the body. How a brain ages also depends on genes and lifestyle too. The most notable bursts are in the first two or three years of life, during puberty, and also a final burst in young adulthood. The brain has bursts of growth and then periods of consolidation, when excess connections are pruned. Only relatively recently it was discovered that new brain cells are being born throughout our lives – a process called neurogenesis. There are many different types of neuron, only one of which is unique to humans and the other great apes, the so called spindle cells.īrain structure is shaped partly by genes, but largely by experience. Once thought to be support cells, they are now known to amplify neural signals and to be as important as neurons in mental calculations. The grey matter is the cell bodies of the neurons, while the white matter is the branching network of thread-like tendrils – called dendrites and axons – that spread out from the cell bodies to connect to other neurons.īut the brain also has another, even more numerous type of cell, called glial cells. While people often speak of their “ grey matter“, the brain also contains white matter. It is in these changing connections that memories are stored, habits learned and personalities shaped, by reinforcing certain patterns of brain activity, and losing others. The pattern and strength of the connections is constantly changing and no two brains are alike. Our brains form a million new connections for every second of our lives. Each neuron can make contact with thousands or even tens of thousands of others, via tiny structures called synapses. The complexity of the connectivity between these cells is mind-boggling. This jelly-like mass of tissue, weighing in at around 1.4 kilograms, contains a staggering one hundred billion nerve cells, or neurons. It produces our every thought, action, memory, feeling and experience of the world.
The brain is the most complex organ in the human body. (Image: Mehau Kulyk / Science Photo Library) A false-colour Magnetic Resonance Image (MRI) of a mid-sagittal section through the head of a normal 42 year-old woman, showing structures of the brain, spine and facial tissues