听力原文：To quickly review the main points covered in Chapter 7, the most important part of the system of nerves is the brain. The brain controls most of the body activities. Messages between the brain and the spinal cord are carried by nerves, which are made up of special cells called nerve cells. Nerve cells are found in the brain, spinal cord, spinal nerves and in the organs we use to see, hear, smell, taste and touch. Nerve cells have long string-like fibres that carry messages. These fibres have branched ends to send and receive messages. Remember that nerve cells are so small that they cannot be seen without a microscope. But one fibre from the nerve cell may stretch as long as three feet. There are many kinds of nerve cells and this chapter deals mainly with three kinds. The sensory nerve cells carry messages of heat, light, sound, and pressure from the skin, muscles and organs inside the body. Motor nerve cells carry messages to the muscles and this causes the muscles to move. Connecting nerve cells go between the sensory nerve cells and the motor nerve cells to the muscles. All these different nerve cells help us see, hear, taste, smell and feel. What is the main topic of this talk?