The brain is a fascinating organ. Like the rest of the body, it is composed of cells; but brain cells are different from other cells. Our discussion focuses on two types of brain cells: neurons and glial cells. Although the brain has many other types of cells, these are the ones most involved in learning.
The brain cell that gets much attention is the neuron. Neuron simply means “nerve cell.” Until recently, it was believed that the brain could not generate new neurons. Recent research shows that in one area, the hippocampus, there is evidence of new cells (Kinoshita, 1999). Before birth the brain produces about 250,000 neurons per minute. At birth, we have about 100 billion neurons, and although we maintain that number, the neurons may lose their connecting powers (Diamond & Hopson, 1998). If neurons are not used at appropriate times during brain development, their ability to make connections dies. Neuroscientists call this process “neural pruning.” So, yes, we are all losing our minds!
However, you don't need to panic about those lost connections. The ones that you have left can take care of anything you need to know or learn for the rest of your life. Some research implies that we use from 1 to 20 percent of our brain. However, we actually use all of our brain, but not all of its processing power (Chudler, 1998). The miracle of the brain is that it is built for continual learning.
What is learning, and how does it occur in the brain? Neuroscientists define learning as two neurons communicating with each other. They say that neurons have “learned” when one neuron sends a message to another neuron (Hannaford, 1995). Let's examine the process.
A neuron has three basic parts: the cell body, the dendrites, and the axon (see figure 1.1). Your hand and forearm are “handy” representations of a neuron. The cell body can be compared to the palm of your hand. Information enters the cell body through appendages called dendrites, represented by your fingers. Just as you wiggle your fingers, your dendrites are constantly moving as they seek information. If the neuron needs to send a message to another neuron, the message is sent out through the axon. Your wrist and forearm represent the axon. When a neuron sends information down its axon to communicate with another neuron, it never actually touches the other neuron. The message has to go from the axon of the sending neuron to the dendrite of the receiving neuron by “swimming” through a space called the synapse. As the neurons make connections, the brain is growing dendrites and strengthening the synapses. (See figure 1.2.)
Figure 1.1. A Neuron
Figure 1.2. How Neurons Communicate
If we have 100 billion neurons in our head, they must be very small. Imagine this: 30,000 neurons can fit on the head of a pin. That's impressive, but there's more. Each neuron may be linked with another 5,000 to 10,000 neurons. The brain has about one quadrillion neural connections (Wolfe, 1996). That's a lot of communication going on inside our heads! The process of neurons talking to each other is electro-chemical: the action within the neuron is electrical, but the message becomes chemical as it travels between neurons. The chemicals are called neurotransmitters. Chapter 2 provides more information about neurotransmitters.
Think about a small child's first experience when his mother points out a red bird and tells the child, “That's a red bird. It's called a cardinal.” The child attempts to repeat the word. “Cawdnal. Bood.” The child's brain has made a connection. A few neurons are now talking to each other about birds. If the child watched as the bird flew out of the tree, he may have the connecting neurons of bird-cardinal-fly. The next time he sees a cardinal, his brain will make those connections again. This time the neurons may connect faster, because when neurons learn or practice information, they become more efficient at connecting.
Neurons are stored in columns in the upper portion of the brain called the neocortex (Sylwester, 1995). The child might make other connections related to the cardinal. If he sees geese flying south, he might add that to the bird-cardinal-fly connection. From there, he might add a butterfly or an airplane.
This chain of neurons is called a neural network. The more often the brain accesses the network, the stronger the connections become. Those synapses, or spaces, become stronger as well. As these neurons are repeatedly “fired,” that is, talk to each other, the dendrites and axons become accustomed to the connections, and the connections are easier to make. Compare this to a path in the woods. The first time you create a path, it is rough and overgrown. The next time you use it, it is easier to travel because you have previously walked over the weeds and moved the obstacles. Each time thereafter, it gets smoother and smoother. In a similar fashion the neural networks get more and more efficient, and messages travel more swiftly.
Researchers are currently exploring an important theory called long-term potentiation (LTP). LTP suggests that every time a neuron fires information across a synapse, the memory of that information is encoded exponentially. That means the information is learned multiple times each time it is practiced. The signal has changed the potential of the receiving neuron, and it now has the potential to learn faster (Fitzpatrick, 1996).
During the first year of life, the brain makes neuronal connections at an enormous rate. Some scientists say that after the first two years, the brain never again learns as much or as quickly. What is happening during this time? The brain is first wiring the infant up to his body. It is making the connections for movement, sight, and sound (Begley, 1997). The baby is also making connections with his primary caretaker. Using his own sounds and movements, the infant communicates with those who are meeting his needs. He begins to recognize voices as well as the expression in those voices. The baby rapidly learns which sounds will get him the desired attention.
Because the brain is so immature at birth, it takes another 18 to 20 years to complete the wiring. We are a social culture, and each individual must “wire up” to a specific culture and society (Sylwester, 1997a). Specific brain areas develop at their own rates.
The second type of brain cell, the glial cell, is just beginning to get the attention it deserves. Glial cells are nurturing cells for the neurons. Glialmeans “glue,” and neuroscientists had good reasons for this name. Glial cells first assist in the migration of neurons during fetal brain development. Their fibers act like ropes for the neurons to hold onto as they make their way through the brain (Kunzig, 1998). The glial cells feed and do the housekeeping for the neurons, almost attaching themselves to the neurons to keep them nourished. The more often the brain uses neurons, the more glial cells it needs. Indeed, when researchers dissected Albert Einstein's brain, they found an extraordinary collection of glial cells in a specific area of his brain. They concluded that this area in Einstein's brain showed more possible use than the same area in any other brain ever studied (Diamond, 1996).
Unlike neurons in most areas of the brain, glial cells can reproduce, so we can have as many as our brain needs. Communication remains fast and easy because these glial cells work and nurture the neurons.
Another substance that neuroscientists are studying is myelin. This fatty substance coats the axons of neurons (see figure 1.3). The coating acts like insulation and allows messages to travel quickly without any loss of transmission. Currently two theories describe the production and release of myelin.
Figure 1.3. A Neuron with Myelin
One theory, supported by neurophysiologist Carla Hannaford (1995), says that myelin is added to the axon with use. In other words, as the neuron is called upon to fire, a coating of myelin is put down. If the neuron is part of a network of neurons fired often, the axon will be heavily myelinated. So, like the path in the woods that is constantly walked upon, the neuronal path becomes smoother and faster.
Other researchers, like Jane Healy (1994), theorize that the myelination of neurons is a developmental process that begins at birth. According to this theory, the brain releases myelin in stages, beginning with the lower brain areas. The final area of the brain to be myelinated is in the prefrontal cortex behind the forehead. This is where decision making, planning, and many higher-order thinking skills take place. This area is also associated with short-term memory.
What are the implications of these two theories? Could both be correct? In my study of the brain, I have read about both ideas and observed how the researchers have swung both ways on this pendulum. Let's look at some facts.
The development of the brain from birth through the end of adolescence parallels the child development stages identified by Jean Piaget. The researchers who believe in the developmental release of myelin state that the stages of myelin release coincide with Piaget's developmental stages (see figure 1.4). Piaget identifies four developmental stages:
- Sensorimotor stage (birth-2 years)—At this stage the child interacts physically with the environment. She builds a set of ideas about reality and how it works.
- Pre-operational stage (ages 2–7)—At this stage the child is not yet able to think abstractly. She needs concrete physical situations.
- Concrete operations (ages 7–11)—At this stage the child has accumulated enough experiences to begin to conceptualize and to do some abstract problem solving, though the child still learns best by doing.
- Formal operations (ages 11–15)—At this stage the child's thought processes are beginning to be like those of an adult.
Figure 1.4. Piaget's Stages and the Stages of Brain Development
Piaget' Four Stages and the Stages of Brain Development
Four Stages of Myelin Release & Brain Growth
Sensorimotor (birth-2 years)
Large Motor System and Visual System
Pre-operational (ages 2–7)
Concrete Operations (ages 7–11)
Manipulate Thoughts and Ideas
Formal Operations (ages 11–15)
Figure 1.4 suggests substantial support for this theory. Jane Healy (1994) states that the largest release of myelin may occur in the adolescent years. Once this dose is released, children have an easier time making decisions, planning for the future, and working out problems.
Although Piaget suggests that this stage occurs between the ages of 11 and 15, current research suggests that this stage varies with the individual. After spending some time teaching at the high school level, I have observed that many students appear to reach this final stage during their sophomore year, though some don't quite make it until senior year or afterward. Only 50 percent of the adult population reach this stage at all (Jensen, 1998).
Short-term memory does not reach capacity until approximately the age of 15. The capacity of short-term memory in a fully developed brain is seven chunks of information. At age 3, space exists for only one chunk. With the discovery by researchers like LeDoux (1996) that short-term memory is held in the frontal lobes, the last area myelinated, it makes sense that the frontal lobe's incomplete development due to the lack of myelin would influence short-term memory.
Many students today have difficulty with higher-order thinking skills. Although children of every age have some ability to synthesize, abstract, and evaluate, some children have more difficulty than others. Realizing that this difficulty may be due to the lack of myelin or its delayed release could lessen both children's frustration and that of the adults trying to help them.
Smooth transfer of information from neuron to neuron is greatly dependent on myelin. My two 4-year-old neighbors are a joy to watch. Their development and interests are very different. Joey loves to do acrobatics. He can do cartwheels better than I ever dreamed of doing them. He can almost do flips, and he loves any type of physical adventure. On the other hand, Mark is not very agile. He has difficulty doing somersaults. Instead of concentrating on the physical world, Mark is trying to read. He is constantly bugging his mother to tell him what written words say. Mark knows the alphabet and can spell some words.
Both boys are normal preschoolers. They are simply developing differently. Carla Hannaford (1995) believes that children benefit when neuronal connections are made through body movement. These connections will help them develop the neuronal systems for reading when they are ready. These boys obviously have different interests, which may have been inspired by their environments. Joey's sisters are acrobats, and perhaps he received recognition for mimicking their behavior. Because Mark is the older sibling in his family, he may be exhibiting behavior that he believes will win his parents' approval. Whatever the reasons, the firing of neurons is causing the learning.
The developmental differences among children are great. Whether these differences are caused by heredity or by the environment is a debate that continues. Whether myelin is released in stages or through use of the neurons, children still exhibit differences.
Myelin is a factor in brain growth and learning. I believe that both theories may be correct. It makes sense that as the brain continually uses its networks of neurons, transmission of information is swifter. It also makes sense that as their brains develop, children undergo vast changes.
Cartoonists often draw a lightbulb above the head to portray a character with an idea. This portrayal actually contains some element of truth. The brain has enough electrical power to light a 25-watt bulb. As mentioned previously, the process of neurons communicating is electro-chemical. The electrical part takes place within the neuron.
All matter has an electrical property. The electrical charges, called ions, are either positive or negative. The ions in the brain are sodium, potassium (each with one positive charge), calcium (with two positive charges), and chloride (with one negative charge). Some negatively charged protein molecules are also present. Neurons are surrounded by a cell membrane that may allow some ions to pass through and that block others. The openings in the cell membrane are called channels. While some channels remain open, others open only in response to chemical stimulation.
When a neuron is not sending a signal, the area inside the neuron has more negatively charged ions, and the area outside has more positively charged ions. This is called its resting potential (see figure 1.5). At this level potassium ions pass through channels easily, but chloride and sodium ions have very few channels to flow through, and protein ions have none. All of the ions want to move across the membrane, but because only the positively charged potassium does so readily, the outside of the neuron is positive and the inside is negative. This balance keeps the neuron at rest. During this time the electrical charge inside the neuron can be measured at about negative 70 millivolts and the outside at positive 70 millivolts (Dowling, 1998).
Figure 1.5. A Resting Neuron
When a chemical stimulus causes the opening of sodium channels, positively charged sodium ions rush into the negatively charged neuron, and the neuron becomes more positive (see figure 1.6). This state, called action potential, depolarizes the neuron. The millivolts within it increase, and at a voltage of about negative 55 millivolts the neuron fires. This firing is always of a fixed size. In other words, it is an all-or-nothing situation. This change in voltage causes an electrical energy output that sends the charge down the axon, across the synapse, and to the dendrites of the receiving neuron. Thus, a message is sent. When the potassium channels open again, potassium rushes out of the cell and the neuron goes back to resting potential.
Figure 1.6. An Active Neuron
Rats, Cats, Children, and Adults: How Do Their Dendrites Grow?
The brain's ability to grow and change is called plasticity. Neuronal activity, or the lack of it, causes these changes. The change process prompts questions such as these: How do we know it is happening? Where is the proof? Can it happen to anyone? Am I too old for brain growth? In other words, can you teach an old dog new tricks? The answers to these questions lie in years of research by some impressive neuroscientists. Let's examine the evidence.
Marian Diamond (1988) of the University of California at Berkeley has been studying the brain development of rats for more than 40 years, with impressive results. She and her colleagues and students conduct experiments in which they place rats in enriched environments. They use control groups to check for accuracy. In one of her tests, she placed a single rat in a regular rat cage—no fun toys for this one. The rat was given food and water as a normal lab rat would be. A larger cage housed one rat with toys. This rat also was tended to in a normal fashion. Then there was the fancy group—12 rats in a large cage containing rat toys, such as wheels to run on, trails to follow, and blocks to climb. The last cage housed 12 rats with no toys. Diamond called the cages with toys enriched environments and those without toys impoverished. The control group for this study consisted of three rats in a small cage with no toys.
The results of this study are exciting. Rats in the enriched environments (those with toys) had more dendritic connections than the rats in the impoverished environments; the dendritic branches were thicker as well (see figure 1.7). The study also showed that the control group with three rats learned more than either the rat left alone in the impoverished environment or the rat left alone in the enriched environment. Diamond concluded that the rats learned more by living together and even more by living together in an enriched environment.
Figure 1.7. The Effect of Environment on Neurons
Studies like this led to even more studies using rats. The rat brain is very similar in structure to the human brain, but because it has fewer “wrinkles,” it is easier to measure.
William Greenough of the University of Illinois discovered that rats in enriched environments had 25 percent more connections between neurons and performed much better in tests (Kotulak, 1996). He believes that synapses can be formed in seconds! (More dendrites create more synapses.) Researchers have found proof of changes in the brains of rats after only four days. In four days dendritic growth as a result of enrichment can occur, and in four more days dendritic death can occur as a result of lack of stimulation (Hooper & Teresi, 1986).
As an educator, I have a favorite rat story. In a 1985 study, Diamond placed baby rats and mature rats in the same enriched cage. She wanted to know if both the young rats and the older rats would grow more dendrites. The surprise came when the older rats refused to let the young rats play with the toys. The mature rats took over the cage and did not allow the baby rats to play. The result was that only the mature rats grew dendrites.
Why do I like this story? When I walk past classrooms with high-tech equipment such as computers, I like to watch what is happening. Often I see the teacher (the old rat) sitting at the computer showing the students how to do something. The students are sitting and watching. Who's growing dendrites here—the old rat or the babies?
We can conclude from Diamond's study that it isn't enough for students to be in an enriched environment. They need to help create that environment and be active in it.
Another rat study really intrigued me. During a visit to Japan to observe Japanese researchers' work with rats, Diamond learned that the Japanese rats were living to be 900 days old, which equals about 90 years for humans. Diamond's rats had been living only about 700 days, which is an expected life span for a laboratory rat. Intrigued, Diamond looked for differences between the two groups of rats. The food, temperature, and cages seemed to be similar for both groups. However, she did notice one difference. In Japan the lab assistants held the rats while the cages were being cleaned. In Diamond's studies, the rats were simply put into another cage. She concluded that this touching and holding may have increased the rats' life span. In addition, because the rats were not put into a “strange” cage while their own was being cleaned, they may have felt less stress. After Diamond returned to the United States, she instructed her lab assistants to hold the rats. The rats began living beyond their 700 days and had more dendritic connections than rats that were not held (Wolfe, 1996). We can conclude that gentle care can add to life span and contribute to brain growth.
Researchers have also conducted several studies with kittens. One study involved taking identical twin kittens at a critical time in their visual development and placing them in a large, circular container painted with black and white vertical stripes. These lines were the kittens' only visual stimulation. A balance beam with a basket on each end revolved in the center of the container. Each twin was placed in a basket. One of the baskets had holes for the kitten's legs, while the other did not. The kitten whose legs could go through the basket and touch the ground began walking around the container. His twin brother had a free ride. What the researchers discovered is truly amazing. The kitten who did the work and interacted with his environment developed great vision for vertical lines. The kitten who did not work could not see vertical lines at all (Healy, 1990). We can conclude that experiences cause brain growth, but one must actively participate in the experiences for growth to take place.
Now that we've talked about rats and cats, let's look at children and adults. After studying the results of such researchers as Greenough, Craig Ramey of the University of Alabama designed a study with children from an inner-city, impoverished environment (Kotulak, 1996). He took a group of children as young as 6 weeks old and exposed them to an enriched environment with playmates, good nutrition, and opportunities for learning and playing. Ramey followed this group and a control group for 12 years. Using intelligence tests and brain-imaging techniques, he found a significant difference in the way in which the children's brains had developed. The enriched children had significantly higher IQs, and brain imaging revealed that their brains were using energy much more efficiently, according to the scans. We can conclude that the brain is sensitive to its early environment and that enrichment can make a difference.
What can we do about growing dendrites? Researchers are addressing this question with a group of nuns in Mankato, Minnesota, who are participating in a study to examine the effects of remaining mentally and physically active in their work and daily lives. These women have lived well beyond the average life span, and researchers attribute their longevity to their active lifestyle. They constantly stimulate and challenge their brains (Golden, 1994).
Studies have compared the IQs of people in nursing homes with the IQs of those waiting to be admitted. People in the nursing homes have significantly lower IQs than those awaiting admission. In many cases, IQs go down measurably after just six months in a nursing home (Hooper & Teresi, 1986). Enriched environments can make a huge difference for everyone.
What Can We Learn from These Studies?
We can draw a number of conclusions from these studies. First, from the rat studies, a social environment is a form of enrichment. Rats do better when they interact with other rats and solve problems together. Humans are social creatures, and learning is a social activity. Gentle care was also a factor for the rats. We must take care when we work with others to help them in their quest for learning. Second, the studies with cats indicate that we need to interact with our environment. That means that both kittens must be able to walk around the container. We need to work together and all take part in the learning. Third, the studies of children tell us that the brain is very sensitive to its early environment, and enrichment affects its growth. Fourth, the study involving nuns indicates that brain stimulation at any age is important and helpful. Our lives must include some challenges. And the children, the rats, the cats, and the nuns tell us that play is important for learning.
Social interaction, care, challenge, and play are important for growing those dendrites. Whether it be in the classroom, in the home, at work, or in the community, all of these factors influence how much we learn.