Nervous system: brain and spinal cord

The brain and the spinal cord together form the central nervous system, the control center of the body. An adult brain weighs about 1.4 kilograms (3 pounds), yet it uses around 20 percent of the energy you take in. Everything you think, feel, remember, and do depends on it.

The brain has three main regions. The cerebrum is the large, folded top part, split into two halves called hemispheres. It handles thinking, memory, language, senses, and voluntary movement. Its wrinkled outer layer, the cerebral cortex, is folded so more surface area fits inside the skull. In general, the left hemisphere controls the right side of the body and the right hemisphere controls the left. The cerebellum sits at the back, underneath. It coordinates movement and balance, which is why you can walk without thinking about each step. The brainstem connects the brain to the spinal cord. It controls automatic functions such as breathing, heart rate, and swallowing, which keep going when you sleep. Deep inside, a small structure called the hypothalamus helps control body temperature, hunger, thirst, and sleep, and it links the nervous system to the hormones you will study soon.

The brain is soft, so it is carefully protected. The skull encloses it. Three layers of membranes, the meninges, wrap it, and cerebrospinal fluid cushions it and carries nutrients. The spinal cord is a thick bundle of nerve tissue about 45 centimeters (18 inches) long that runs down inside the vertebrae of your spine. It carries messages between the brain and the rest of the body, and it also handles simple reflexes by itself, without waiting for the brain.

Here is how the pieces work together. If you touch something hot, sensory neurons send a signal to the spinal cord. For a reflex, the cord can send a motor command back immediately. At the same time, signals travel up to the brain, which makes you aware of the pain and decides what to do next.

Myth to avoid: "we only use 10 percent of our brains." Brain scans show activity throughout the whole brain, and different regions are active at different times. Another mistake is to think the brain works in separate parts; its regions are always sending signals to each other. Next: the nerves that reach the rest of the body.