The Nervous System

In this topic you will learn about how the nervous system controls your ability to hear, smell, touch, and think.
Electrical signals, or impulses, are sent through nerve cells. Impulses flash along individual nerve cells, or neurons. Each neuron has three main parts—a cell body, dendrites, and an axon. Dendrites are branching nerve fibers that carry impulses toward the cell body. An axon is a nerve fiber that carries impulses away from the cell body.
When an impulse reaches the tip of an axon, another nerve cell can't pick it up directly. That's because there is a gap between the neurons. This gap is called a synapse, and it is only a fraction of an inch wide. When an impulse reaches the tip of an axon, the axon releases chemical droplets. The chemical connects the neurons by carrying the impulse across the gap to the dendrites on the next nerve cell.
There are three kinds of neurons—sensory, motor, and associative. The function of the sensory neuron is to collect information from stimuli (singular, stimulus) and send it to the brain and spinal cord. A stimulus is anything that causes a response. Motor neurons carry commands from the brain and spinal cord to the muscles and glands. For example, when you make a muscle, your brain sends impulses along motor neurons to your biceps muscle. Other motor neurons can speed up your heartbeat and help move food through your intestines. Associative neurons pass impulses from sensory to motor neurons. Most of the neurons in your brain are associative. They are found only inside the brain and the spinal cord.
Your brain and your spinal cord are your central nervous system. All other nerves are your outer, or peripheral, nervous system.
The bones running down the middle of your back are vertebrae. Stacked together, your vertebrae make up your backbone, or spine. Running through the holes in the center of your spine is your spinal cord. Your spinal cord is the biggest and thickest bundle of nerve cells in your body. Your spinal cord helps protect you when you are in danger. It can make you do something automatically. Such an action is called a reflex.
The three main parts of your brain are the cerebrum, cerebellum, and medulla. The cerebrum is the largest part of your brain. It has over 10 billion neurons. A deep groove separates the right half, or hemisphere, from the left. Both hemispheres of your cerebrum contain control centers for your senses. Each half also sends commands that allow you to control your skeletal muscles. Your cerebellum coordinates commands to your skeletal muscles and helps you keep your balance. The medulla works automatically to control your heart beat, your breathing, your blood pressure, and the muscles in your digestive system.