23.1–23.3: What is the nervous system?

Huskies that work as sled dogs—which are descended from lineages that were crossed with sighthounds—have excellent vision.

Figure 23.1: Avoiding danger. The sensation of pain gives us important information about the world.

Figure 23.2: Overview of nervous system functions.

Imagine a world without pain. Think of all that you could achieve. You could work harder, run farther, and just plain feel better. Maybe that’s why so many products are marketed as “painkillers.”

But would it really be a better world? Think again.

The case of Gabby Gingras tells us that we wouldn’t really be happier in a pain-free world. Gabby feels no pain. She was born that way. At first it seemed like a blessing: just after birth, when nurses drew blood from her, she did not cry but instead slept peacefully through the procedure. When, as a toddler, she fell down, she didn’t cry. But rather than being a gift that makes her life easier, Gabby’s inability to feel pain is a crippling curse. As she has grown older, some unexpected consequences of her condition make this clear. First, she inadvertently damaged her eyes—permanently scratching one of her corneas—by poking her fingers into them. Later, chewing on plastic toys, she cracked most of her teeth. Another girl, also born without the ability to feel pain, encountered even more severe problems. Holding her hand over a hot stove, she was severely burned—moving her hand only when she smelled the burning flesh. This condition is called “heritable sensory autonomic neuropathy,” and it reveals that, no matter how unpleasant pain is to experience, it does have a very large benefit. The pains we feel alert us to the need to extricate ourselves from a dangerous situation and can prevent much greater suffering in the long run (FIGURE 23-1).

The survival and well being of organisms depends on their awareness of the world around them and their ability to limit exposure to physically harmful situations. Our nervous system accomplishes these tasks by letting us see, hear, feel, taste, smell, remember (and forget!), think about, act on, and react to various events and stimuli around us. Present in all multicellular animals other than sponges (and not found in plants), the nervous system is a network of cells that collects information about an organism’s internal and external environments, processes that information, and sends signals to effectors, which are muscles and glands that are capable of responding to the information.

Nervous systems have three critical features (FIGURE 23-2).

• 1. They receive input (stimuli) from the surrounding world.
• 2. They process that information.
• 3. They initiate responses to the internal and external environments, when necessary.

In this chapter, we examine the organization of and structures found in nervous systems and the diversity of nervous system specializations that have evolved in the animal world—including specializations that allow humans to discern light from dark, to identify subtle differences in touch, temperature, and color, and to use language and abstract reasoning to perform and coordinate complex social behaviors. We also explore how the nervous system, interacting with the muscular and skeletal systems, can generate movement. And we examine how various drugs (legal and illegal) affect, often adversely, the functioning of the nervous system.