One hundred billion neurones may seem like a lot of nerve cells, but is actually only about 20% of the number we originally start with. The number of neurones in our brain peaks prenatally and then they start to prune themselves out, one by one, through childhood. By the time we enter adolescence, our brain has chosen the final selective neurones it will keep throughout all our adult life. The decision is based on which cells we use and which we do not. The cells we do not use are pruned away leaving more room to add branches, or dendrites, to the neurones that we do use. New branches are added as the brain receives and processes any new information.
How does the brain actually "prune" the garden? The answer lies in a number of chemicals and their actions and reactions. The chief pruner is probably an enzyme named Calpain. Calpain has the ability to self-destruct a cell. Technically this is known as autolysis ("auto" meaning self, and "lysis" meaning to destroy).
Calpain is produced in the neurones when there is a heavy calcium ion concentration in their surrounding environment. Reduced blood flow can cause this high calcium ion build up between and within cells (for you biology enthusiasts, the calcium comes from the mitochondria and the ER as well as an influx from outside the cell). In other words, high activity in a brain region calls for heavy blood flow to service the cells, low activity requires little blood flow. Therefore, the less-used areas, with their limited blood flow activity, tend to build up calcium ions. This build-up triggers the secretion of the enzyme calpain, which causes the nerve cell to self -destruct.
New growth, on the other hand, comes in the dendrite development, or branching of well-used neurons. This branching is caused from chemicals known as neurotrophins. Neurotrophins are a group of proteins which are responsible for the growth and development of neurons. As you may suspect, we use a lot of neurotrophins during childhood as the brain has massive growth and development. But we continue to use neurotrophins all of our lives, especially in the hippocampus area, the brain region responsible for new learning and new memory formation.
There are many neurotrophins at work in the brain. The first one discovered is known as NGF (nerve growth factor). Others, discovered since, have equally self-explanatory names as brain derived neurotrophic factor (bFGF), and glial cell-line derived neurotropic factor (GDNF). These neurotrophins work by attaching themselves to receptor sites on neurones and causing the cell membrane to change shape, grow and branch.
Because most growth hormones throughout the body are especially active during sleep, it is thought that the majority of neurotrophic work is also done during sleep, especially the non-REM cycles of sleep. The work of Marcos Frank and Michael Stryker, at UC San Francisco, caught the education world by surprise in 2001 with their startling research showing the tremendous amount of branching and subsequent learning that took place during sleep. While most of the science community historically considered that the REM, or dreaming cycle of sleep was the time when most wiring took place, Stryker's work and the research following that study continue to show that it is actually the non-REM cycles that help hard wire in the information learned the previous day.
From a practical standpoint, sleep research continues to show the importance of sleep to the learning brain. Students MUST get sufficient sleep following the learning of new information if we want that information stored in a long-term, complex network of neuron branches.
The research on brain wiring and the biology behind it continues to be a fascinating topic. It gives hope to people with stroke damage, Alzheimer's and other neurological problems, as well as providing a better understanding for those of us who parent and teach young brains.
Remember to learn something new today. . . then sleep on it.
Kathie F Nunley is an educational psychologist in Salt Lake City, Utah. Founder of Brains.org and the developer of the Layered Curriculum (tm) method of instruction, Kathie can be reached via email at Kathie@brains.org.