In all vertebrates and most invertebrates, the brain is the centre of the nervous system. It allows them to collect information (sensory system), act on that information (motor system) and store the result for future reference (memory), thus effectively making life possible.
The human brain is perhaps the most complex living structure known in the universe. Although it has the same general structure as the brains of other mammals, is over three times as large as the brain of a typical mammal with equivalent body size, and much more complex.
The adult human brain weighs on average about 1.5 kg (3lbs), and is about the size of a small head of cauliflower. It is very soft (having a consistency similar to soft gelatine or firm tofu) and, despite being referred to as “grey matter”, the live brain is actually pinkish-beige in colour (although it may turn grey after death) and slightly off-white in the interior. The interior white matter provides most of the brain’s structure and communications, while the grey matter that surrounds the white matter provides most of the actual computation and thinking functions (although this is, of course, a simplification).
Almost 80% of the brain consists of water (mainly in the cytoplasm of its cells), with a further 10-12% being fatty lipids and 8% protein. Although it accounts for just 2% of body weight, it uses fully 20-25% of the body’s oxygen supply, nutrients, and glucose (as fuel), all of which are supplied by constant blood flow. It is protected by the thick bones of the skull, suspended in cerebrospinal fluid, and isolated from the bloodstream by the blood-brain barrier, but the delicate nature of the human brain nevertheless makes it susceptible to many types of damage and disease.
It is a hugely complex organ, with an estimated 100 billion neurons passing signals to each other via as many as 1,000 trillion synaptic connections. It continuously receives and analyzes sensory information, responding by controlling all bodily actions and functions. It is also the centre of higher-order thinking, learning and memory, and gives us the power to think, plan, speak, imagine, dream, reason and experience emotions.