The Adolescent Brain –

Some Basic Concepts

It is so exciting to have information that explains behaviors. The science can appear very complicated because there are so many parts to it. Most of us have little, if any, introduction to the details of these parts. So as we go through the process of learning about the adolescent brain, it's important to have some definitions. They'll be posted here as a reference page, and on a separate webpage that will allow for frequent updating as needed. Don’t feel intimated by the volume of information. It takes time to master this roadmap. The brain is the most complicated computer known. But unlike the computer, the brain has a genetic pre-disposition, and it grows, changes, and reacts to physical and psychological exposures. This makes it ‘plastic’, which means it responds to good and bad influences.

This is a quick, and hopefully easy, introduction to some of the basic science. (This will be a growing page!)

The brain is the center of the nervous system, with an estimated 120 billion neuronal (neurons) cells, and 120 billion non-neuronal (glia) cells. It's also estimated that there are 1000 trillion connections between the neurons; they connect via synapses. The brain is suspended in the cerebral spinal fluid, which softens how rapid movements that might damage it. It's also protected by a very zealous blood brain barrier that limits what can get into the central nervous system. But that blood brain barrier is not perfect, and they cannot keep out many drugs, alcohol, or other toxins.

The neurons are divided into two groups – the grey matter (with no myelin), and the white matter (with the white myelin coatings – these are similar to higher speed, larger computer cables). A neuron is a long cell that acts like a biochemical wire – when stimulated it transmits information. Each neuron can pick up signals from one or more other neurons, and each neuron can also send information out to one or more other neurons. A neuron is like an arm, and the reception and transmissions occur in areas of dendrites. Spread your fingers open and wide – they are like the dendrites. We have recently learned that there is a second burst of grey matter growth just before puberty.

Pruning is a natural process of removing neurons to further improve the networking capacity of a particular area of the brain, to make circuits less ambiguous, which is turn improves synaptic efficiency. There are appear to be two periods of grey matter synaptic growth followed by pruning, in early childhood and – which is new information -- in adolescence. A 3 year old will have many more synapses that an adult. Synaptic circuits most frequently activated will be preserved; ineffective, weak or unused synapses will be pruned, similar to a gardener shapes a tree. For humans, the gardener is a mixture of nature and nurture. We are rapidly learning the details of this second pruning in adolescence.

Glia, or glue cells, are the non-neuronal parts. They regulate the environmental outside the neurons. The role of glia cells has recently been much better understood, with consideration that they may have as important a role in many conditions, such as depression, as the neurons. The three types of glial cells produce a healthy, protective and clean environment so the neurons can operate as best possible.

Myelin is the product of a glial cell. It is an electrical insulation material that forms a sheath, around the axon. Myelin production of myelin starts in the 14^th week of fetal development, but is occurs most rapidly from birth to adolescence. Having myelin makes it possible for neurons to more rapidly transmit the information. Myelination reduces the loss of the electrical current from the axon.

Two broad classes of neurons exist, interneurons, which form local circuits, and projection neurons, which take information to other regions of the brain or body. Most neurons are interneurons. All of these make up the developing circuits and connections that we are interested in as the brain matures.

Cytoarchitecture is the term used to explain how Korbinian Brodmann organized the cortical neurons and glial cells into 47 areas. The organization of these areas will become important later on as we explore the developing adolescent brain. The highest area of computation is within the grey area of the cerebral cortex.

Neuroplasticity is the ability of the brain to change with learning and experiences.

Neuroimaging is the science of using x-rays, CT, MRI, PET and DTI to examine and explore the functioning or make-up of the brain. Some studies are static – they measure the size, location, or other structural and geographical aspects of the brain, such as looking for bleeds, tumor, atrophy, etc., that interfere with function. Some studies are functional – they look at blood flow, how the brain metabolizes glucose, etc. These produce pictures that ‘light up’ as the brain becomes active. Diseases are suspected when the pictures are unusual. DTI, or diffusion tensor imaging, enables us to see the location, orientation, and health of the brain’s white matter tracts. This is becoming increasingly valuable in understanding brain development and disease or injury. DTI gives information known as the anisotropy of the white matter.

Anisotropy. When things happen all in one direction, it is known as anisotropy. When things happen equally in all directions, it is isotropy. Light going through a polarizing lens is changed from isotropy into anisotropy, and that is why the glare is reduced – less interference and confusion. Information traveling through the white matter is best when it is focused and on target. Damaged neurons in the white matter tracts will lessen the strength and purity of the desired signals, so abnormal or less anisotropy means abnormal white matter tracts. Think of it as a leaky electrical cable. DTI, using anisotropy, measures the degree of such leaking. This is important information when understanding brain damage, brain growth and development, etc. Anisotropy is usually associated with health and proper functioning; isotropy is not.