3.1 Neurons: The Origin of Behavior

An estimated 1 billion people watch the final game of World Cup soccer every 4 years. That’s a whole lot of people, but to put it in perspective, it’s still only a little over 14% of the estimated 7 billion people currently living on Earth. A more impressive number might be the 30 billion viewers who tune in to watch any of the World Cup action over the course of the tournament. But a really, really big number is inside your skull right now, helping you make sense of these big numbers you’re reading about. There are approximately 100 billion nerve cells in your brain that perform a variety of tasks to allow you to function as a human being.

Humans have thoughts, feelings, and behaviors that are often accompanied by visible signals. Consider how you might feel on your way to meet a good friend. An observer might see a smile on your face or notice how fast you are walking; internally, you might mentally rehearse what you’ll say to your friend and feel a surge of happiness as you approach her. All those visible and experiential signs are coordinated by the activity of your brain cells. The anticipation you have, the happiness you feel, and the speed of your feet are the result of information processing in your brain. In a way, all of your thoughts, feelings, and behaviors spring from cells in the brain that take in information and produce some kind of output trillions of times a day. These cells are neurons, cells in the nervous system that communicate with one another to perform information-processing tasks.

Components of the Neuron

Figure 3.1: Golgi-Stained Neurons Santiago Ramón y Cajal used a Golgi stain (like the one shown here) to highlight the appearance of neurons. He was the first to see that each neuron is composed of a body with many threads extending outward toward other neurons. He also saw that, surprisingly, the threads of each neuron do not actually touch other neurons.

ED RESCHKE/GETTY IMAGES

During the 1800s, scientists began to turn their attention from studying the mechanics of limbs, lungs, and livers to studying the harder—to-observe workings of the brain. Philosophers wrote poetically about an “enchanted loom” that mysteriously wove a tapestry of behavior, and many scientists confirmed the metaphor (Corsi, 1991). To those scientists, the brain looked as though it were composed of a continuously connected lattice of fine threads, leading to the conclusion that it was one big woven web of material. However, in the late 1880s, Spanish physician Santiago Ramón y Cajal (1852–1934) learned about a new technique for staining neurons in the brain (DeFelipe & Jones, 1988). The stain highlighted the appearance of entire cells, revealing that they came in different shapes and sizes (see FIGURE 3.1).

Figure 3.2: Components of a Neuron A neuron is made up of three parts: a cell body that houses the chromosomes with the organism’s DNA and maintains the health of the cell; dendrites that receive information from other neurons; and an axon that transmits information to other neurons, muscles, and glands.

Cajal discovered that neurons are complex structures composed of three basic parts: the cell body, the dendrites, and the axon (see FIGURE 3.2). Like cells in all organs of the body, neurons have a cell body (also called the soma), the largest component of the neuron that coordinates the information-processing tasks and keeps the cell alive. Functions such as protein synthesis, energy production, and metabolism take place here. The cell body contains a nucleus, which houses chromosomes that contain your DNA, or the genetic blueprint of who you are. The cell body is surrounded by a porous cell membrane that allows some molecules to flow into and out of the cell.

Unlike other cells in the body, neurons have two types of specialized extensions of the cell membrane that allow them to communicate: dendrites and axons. Dendrites receive information from other neurons and relay it to the cell body. The term dendrite comes from the Greek word for “tree”; indeed, most neurons have many dendrites that look like tree branches. The axon carries information to other neurons, muscles, or glands. Axons can be very long, even stretching up to a meter from the base of the spinal cord down to the big toe.

In many neurons, the axon is covered by a myelin sheath, an insulating layer of fatty material. The myelin sheath is composed of glial cells (named for the Greek word for “glue”), which are support cells found in the nervous system. Although there are 100 billion neurons busily processing information in your brain, there are 10 to 50 times that many glial cells serving a variety of functions. Some glial cells digest parts of dead neurons, others provide physical and nutritional support for neurons, and others form myelin to help the axon carry information more efficiently. An axon insulated with myelin can more efficiently transmit signals to other neurons, organs, or muscles. In fact, with demyelinating diseases, such as multiple sclerosis, the myelin sheath deteriorates, slowing the transmission of information from one neuron to another (Schwartz & Westbrook, 2000). This leads to a variety of problems, including loss of feeling in the limbs, partial blindness, and difficulties in coordinated movement and cognition (Butler, Corboy, & Filley, 2009).

Cajal also observed that the dendrites and axons of neurons do not actually touch each other. There’s a small gap between the axon of one neuron and the dendrites or cell body of another. This gap is part of the synapse, the junction or region between the axon of one neuron and the dendrites or cell body of another (see FIGURE 3.3). Many of the 100 billion neurons in your brain have a few thousand synaptic junctions, so it should come as no shock that most adults have 100 to 500 trillion synapses. As you’ll read shortly, the transmission of information across the synapse is fundamental to communication between neurons, a process that allows us to think, feel, and behave.

Figure 3.3: The Synapse The synapse is the junction between the dendrites of one neuron and the axon or cell body of another. Notice that neurons do not actually touch one another: There is a small synaptic space between them across which information is transmitted.

Major Types of Neurons

There are three major types of neurons, each performing a distinct function: sensory neurons, motor neurons, and interneurons. Sensory neurons receive information from the external world and convey this information to the brain via the spinal cord. They have specialized endings on their dendrites that receive signals for light, sound, touch, taste, and smell. For example, sensory neurons’ endings in our eyes are sensitive to light. Motor neurons carry signals from the spinal cord to the muscles to produce movement. These neurons often have long axons that can stretch to muscles at our extremities. However, most of the nervous system is composed of the third type of neuron, interneurons, which connect sensory neurons, motor neurons, or other interneurons. Some interneurons carry information from sensory neurons into the nervous system, others carry information from the nervous system to motor neurons, and still others perform a variety of information-processing functions within the nervous system. Interneurons work together in small circuits to perform simple tasks, such as identifying the location of a sensory signal, and much more complicated ones, such as recognizing a familiar face.

Neurons Specialized by Location

Besides specialization for sensory, motor, or connective functions, neurons are also somewhat specialized depending on their location (see FIGURE 3.4). For example, Purkinje cells are a type of interneuron that carries information from the cerebellum to the rest of the brain and spinal cord. These neurons have dense, elaborate dendrites that resemble bushes. Pyramidal cells, found in the cerebral cortex, have a triangular cell body and a single, long dendrite among many smaller dendrites. Bipolar cells, a type of sensory neuron found in the retinas of the eye, have a single axon and a single dendrite. The brain processes different types of information, so a substantial amount of specialization at the cellular level has evolved to handle these tasks.

Figure 3.4: Types of Neurons Neurons have a cell body, an axon, and at least one dendrite. The size and shape of neurons vary considerably, however. The Purkinje cell has an elaborate treelike assemblage of dendrites (a). Pyramidal cells have a triangular cell body and a single, long dendrite with many smaller dendrites (b). Bipolar cells have only one dendrite and a single axon (c).