The brain and spinal cord

Protecting the CNS

Your brain has about the consistency of firm jelly, weighs about 1.4 kg and uses roughly a fifth of all the energy your body burns at rest. It is protected by:

• the skull, a case of fused bones
• three membranes called the meninges; meningitis is an infection of these
• cerebrospinal fluid, which surrounds the brain and spinal cord, cushions them from knocks and carries nutrients.

The spinal cord is a bundle of nervous tissue about 45 cm long, running down a tunnel through the vertebrae of the backbone. It carries messages between the brain and the rest of the body and handles many reflexes itself (4B). Spinal nerves branch off it between each pair of vertebrae.

Parts of the brain

The largest part is the cerebrum. It is split into a left and a right hemisphere, joined by a thick band of about 200 million axons. Strangely, each hemisphere mostly controls and feels the opposite side of the body, so a stroke on the left side of the brain usually affects the right arm and leg. The wrinkled outer layer of the cerebrum, the cerebral cortex, is where most thinking happens. Its folds pack a large surface area into the skull. Each hemisphere is divided into four lobes.

The brain is not neatly divided like a map, though. Most tasks use several regions working together. Reading this sentence uses your occipital lobe to see the shapes, your temporal lobe to understand the words, and your frontal lobe to keep your attention on it.

How do we know?

For a long time, the only way to learn what a brain region did was to study people whose brains had been damaged, like Phineas Gage, and then look at their brains after they died. In the 1860s the French doctor Paul Broca studied a patient who could understand speech but could only say one word. After the patient died, Broca found damage in a small area of the left frontal lobe, now called Broca's area.

Today, scanners let scientists watch living brains at work. MRI uses strong magnets and radio waves to make detailed pictures. Functional MRI (fMRI) shows which areas are using more oxygen while a person does a task, such as looking at faces or tapping a finger. EEG records electrical activity through small sensors on the scalp.

A brain that rewires itself

Your brain is not fixed. Every time you practise a skill, the synapses between the neurons involved get stronger, and new connections form. This ability to change is called neuroplasticity. It is how you learn, and it is why practice makes things easier. It also lets people recover after a stroke: with therapy, undamaged areas can gradually take over some of the work of damaged ones. Teenage brains are especially good at rewiring, because the connections in the frontal lobe are still being built and pruned until the mid-20s.

Spinal cord injury

If the spinal cord is damaged, messages can't pass the injury. Everything below it may lose movement and feeling, while everything above still works. The higher the injury, the more of the body is affected.

• Paraplegia is loss of movement and feeling in the legs and lower body, from an injury in the chest or lower back.
• Tetraplegia (also called quadriplegia) affects the arms as well, from an injury in the neck.

The nerves that control the diaphragm leave the cord at the 3rd, 4th and 5th neck vertebrae (C3–C5). A person with an injury above this level can't breathe on their own. Unlike skin or bone, neurons in the CNS hardly regrow, which is why these injuries are usually permanent. Most spinal injuries in Australia come from road crashes and falls, and many happen to young people. That is one reason Victoria became the first place in the world to make bicycle helmets compulsory, in 1990.