Human Brain Anatomy: Lobes, Cerebellum and Brainstem in 3D

A small organ with a big appetite

The picture shows the brain inside the skull. An adult brain weighs about 1.3 to 1.4 kilograms, roughly the same as a bag of sugar, and has around 86 billion neurons, the nerve cells that carry signals. There are about as many supporting cells beside them. The brain is only about 2 percent of the body's weight, yet it burns about 20 percent of its energy, which is why you feel tired after hard thinking. The wrinkled outer layer is the cortex, folded like paper in a bag to fit a large area inside a small skull.

Left, right, and four lobes

A deep groove splits the brain into two hemispheres, and each hemisphere has four lobes. The frontal lobe, behind the forehead, plans, decides and sends orders to the muscles. The parietal lobe, at the top, handles touch and the sense of where your body is in space. The temporal lobe, by the ears, deals with hearing and memory. The occipital lobe, at the back, processes what the eyes see. Tap a lobe in the sandbox and it lights up with its job. Each hemisphere mostly controls the opposite side of the body: the left one moves your right hand.

Jobs are shared, not boxed in

It is tempting to think each lobe works alone, like rooms in a house. In reality the lobes are busy partners. Reading a word uses the occipital lobe to see it, the temporal lobe to recognise it, and the frontal lobe to speak it aloud. In most people language is handled mainly by the left hemisphere, but nobody has a purely 'left-brained' or 'right-brained' personality. That popular idea is a myth. Both halves work all the time, and brain scans show them chatting constantly.

The little brain at the back

Under the back of the cerebrum sits the cerebellum, whose name means 'little brain'. It is only about a tenth of the brain's volume, yet it holds roughly four out of five of its neurons. The cerebellum does not start movements. It fine-tunes them, comparing what you meant to do with what the body is actually doing, and correcting the difference. That is what keeps you balanced on a bicycle and lets you touch your nose with your eyes shut. Practised skills such as typing or playing the piano depend on it.

The brainstem keeps you going

At the base of the brain, the brainstem connects the brain to the spinal cord. It has three parts: the midbrain, the pons and the medulla. The brainstem runs the jobs you never have to think about. It sets your breathing rhythm, adjusts your heartbeat and blood pressure, and manages swallowing, coughing and the sleep and wake cycle. Almost every signal travelling between the body and the brain passes through it, like a busy railway junction. Because it matters so much, it is tucked deep and well protected, at the very base of the skull.

The great bridge

The last moment lifts the two hemispheres apart. Between them lies the corpus callosum, a thick band of roughly 200 million nerve fibres that lets the halves share information, so that the picture from your left eye and your right eye becomes one scene. And now a famous myth: that people use only 10 percent of their brain. It is false. Scans show activity all over the brain across a day, and damage to almost any small area causes some loss. Even in sleep, much of the brain stays busy. Tap the lobes again and see for yourself.

The brain and its regions