31 You've Got a Lot of Nerve!-Mid

You've Got a Lot of Nerve! FOCUS Question How does the brain control behavior?

Getting Connected Have you thanked your nervous system lately? You owe your best ideas—and even your survival—to your nervous system. It controls everything. Your nervous, system is made up of hundreds of billions of cells called neurons, or nerve cells. Neurons gather and transmit messages throughout your body. A neuron has three main parts. The cell body contains the nucleus and other tiny parts. The axon carries messages over long distances to other cells. Fingerlike dendrites receive messages from other neurons. Axons send messages to the dendrites of other neurons across tiny gaps called synapses. Synapses are where chemicals trigger electrical pulses. When neurons "fire," electrical pulses zip away.

The brain and spinal cord make up the central nervous system. The spinal cord is a bundle of nerves that travels from the brain down through the backbone. Spinal nerves fan out to the rest of the body. These side branches form the peripheral (puh-RIF-er-uhl) nervous system. There are two kinds of nerve cells in the peripheral nervous system. Sensory neurons gather information from the senses. They carry messages to the central nervous system. Motor neurons send messages from the central nervous system to muscles. All vertebrates—animals with backbones—have similar nervous systems. For example, both frogs and humans have a brain, spinal cord, and peripheral nerves. Word Wise The word peripheral means "to the side."

The Great Brain The brain has many important jobs. How does it keep track of them all? Different areas of the brain take care of each job. The brain stem and cerebellum are two key areas. They control autonomic (AH-tow-NAH-mick) functions. These functions include breathing, balancing, and other important functions that we don't have to think about. The cerebrum is another key area of the brain. Each of the five senses has its own region in the cerebrum: sight, hearing, smell, taste, and touch. In human brains, there are also well-developed regions for communication and higher-level thinking.

Every animal's brain is different. Each brain is developed for the actions and senses that are important for each animal. For example, bats make sound waves and listen for the echoes. This is called echolocation. A bat's brain has a larger region for hearing than a human's brain. Bats survive by using echolocation to find and capture food. Humans survive because our cerebrum allows us to think and communicate complex thoughts. Math Moment Your brain uses about 20% (or .20) of your energy. If you eat 2,000 calories in a day, how much energy goes toward "feeding" your brain?

Reflex Action Some actions are so basic and important that the brain doesn't need to be involved at all. Suppose you accidentally touched the handle of a hot pan. Without thinking, you'd pull back your hand. The sensory nerves in your hand would sense that the pan was hot, and an electric pulse would flash through your neurons to your spinal cord. Motor neurons would respond, and muscles would pull back your hand. The whole process happens in an instant. That's a reflex. Reflexes help animals to avoid danger and survive.