23.2: Neurons are the building blocks of all nervous systems.

The neuron is a type of cell that is specialized for generating and conducting electrical impulses and is found in all animals, with the exception of sponges. It is the building block of all nervous systems (FIGURE 23-3). No matter how complex or simple that system is, it consists of neurons. Each neuron is very small and has but a single option at any point in time: “fire” or “don’t fire” (meaning that the neuron either generates an electrical signal that can convey information to nearby cells, or does not). But, put together a few hundred billion neurons, and things start to get interesting. Taken together, groups of neurons bundled together with connective tissue—into structures called nerves—connect us to our world by enabling us to sense light, sound, touch, tastes, and smells and to respond to all of that sensory information. The accumulated information carried by all of our neurons—whether they are firing or not firing—is also responsible for information storage and retrieval and all thought in our brain.

Neurons are powerful cells, but they are also fragile. In the time it takes you to read this sentence, one of your neurons will die. If you read it again, another one will die. And neither will be replaced; unlike most cells in your body, very few neurons are able to replace themselves. This need not be cause for alarm, though. Even with about 9,000 neurons dying on a good day—significantly more if you consume any alcohol or happen to inhale any gasoline fumes—the 100 billion to one trillion you were born with are more than enough for you to keep your wits about you.

Even though they perform such varied and significant functions, neurons are relatively standard eukaryotic cells. Each has a cell body that contains a nucleus, mitochondria, endoplasmic reticulum, and so on. But neurons also have two specialized structures that make them unusually adept at interacting with the external environment and with other cells.

The first of these two important specializations is the dendrite. A dendrite is the part of a neuron that is like an antenna: it senses and responds to stimulation from outside the cell and sends that information toward the cell body of the neuron. Numerous dendrites may branch out from the neuron cell body.

The neuron’s second important specialization is the axon. An axon can transmit a signal, much as an electrical wire does, over great distances. Like dendrites, the axon is an extension of the plasma membrane from the main cell body. Sometimes very long, the axon transmits the signals picked up by the dendrites to the rest of the organism’s body. The end of the axon is specially modified in a way that allows it to transmit the signals to another cell. The longest cells in the world are neurons. Some neurons are several feet long. The sciatic (sigh-at-ick) nerve, for example, runs from your spinal cord all the way to the tips of your toes. Most of this distance is covered by a single axon.

Although neurons do all the actual work of the nervous system, they do it with a lot of help from other cells. Throughout the nervous system, large numbers of non-neuronal cells, called glial cells, function like a support staff to protect, insulate, and nourish the neurons. In the human brain, there are approximately nine times as many glial cells as there are neurons. And unlike most neurons, glial cells regularly divide. Not surprisingly, then, virtually all brain tumors in adults are formed from glial cells, the supporting cells of the brain.

Just as corporations keep their big computers locked up in super-clean, secure rooms to keep them functioning at an optimal level, the brain is protected from potentially harmful molecules in the blood. Glial cells line the blood vessels that supply the brain, forming a semi-permeable barrier. Called the “blood-brain barrier,” it allows essential nutrients and gases to pass through, while barring harmful molecules such as metabolic wastes produced throughout the body. The blood-brain barrier is not perfect, though. Many small molecules, including anesthetics and alcohol, can make it through to influence the brain. The barrier also can be broken down by hypertension, radiation, and a variety of infectious organisms.

To carry out the activities of the nervous system, there are three functional types of neurons (FIGURE 23-4).

• 1. Sensory neurons collect information from an animal’s environment and have dendrites modified to respond directly to internal and external stimulation. This stimulation can include temperature, touch, taste, smell, light, or sound.
• 2. Motor neurons stimulate action by conveying signals to muscles or glands and initiating a body’s response to stimuli.
• 3. Interneurons integrate the signals coming in from the sensory neurons and relay them to the motor neurons. These “middlemen” are located only in the brain and the spinal cord.

In the next section, we’ll look at how these three types of neurons interact and the general organization of the vertebrate nervous system.