2. Building the brain's architecture

The brain’s basic structure is formed early in prenatal development as 100 billion neurons begin to join up to form neural pathways and networks. Neurons communicate with each other to form circuits and share information using both electrical and chemical signals to carry information across the brain and body’s nervous systems. Electrical impulses are transmitted along the neuron’s axon and chemicals carry the electrical signals across the synapse to the dendrites of another neuron. The receiving neuron then fires another electrical signal, and the signal is relayed to the next neuron in the neural chain.

Figure 2.2

In 1949, Canadian psychologist Donald Hebb postulated that when one cell excites another repeatedly, a change occurs in one or both cells that contributes to a stable link between them.¹² In other words, “neurons that fire together, wire together.” Hebb’s work pushed the frontier of scientists’ reluctant recognition of the inextricable role of our biology in how we think, learn, socialize and behave.¹³ Hebb and the next generation of scientists emphasized the importance of networks of neural circuits. Experiences carried to the brain influence how the neurons join up with each other to construct neural networks that make up the brain’s architecture.

All perceptions, thoughts and behaviours result from combinations of signals among neurons. Proper nervous system function involves coordinated action of neurons in many brain regions. The nervous system influences and is influenced by all other body systems (e.g., cardiovascular, endocrine, gastrointestinal and immune systems).

Figure 2.3

However, this is not the whole story. The basic building blocks of neural pathways are brain cells: neurons and glial cells. Neurons make up about 15 percent of our brain cells and glial cells make up the rest. Neuron and glial cells are intimately connected with each other. “Neurons are elegant cells, the brain’s information specialists. But the workhorses? Those are the glia,”¹⁴ says Douglas Fields of the U.S. National Institute of Child Health and Human Development. Scientists now report that glial cells are significant players in how the brain and the body’s nervous system function.

Glial cells can control communication across the synapses and are therefore implicated in our learning, behaviour and health.¹⁵ Neurons “speak” across synapses by generating electrical impulses that trigger chemical communication between neurons and prompt more impulses in other neurons. Glia have receptors (receiving docks) for many of the same chemical messages used by neurons. They are able then to eavesdrop on the neurons and respond in ways that help strengthen the messages. Without glial cells, neurons and their synapses fail to function properly. Some varieties of glia wrap around axons, the “wires” that connect neurons, forming insulation called myelin and contribute to more efficient learning. Others work in concert with the immune system to prune out inefficient neural connections.

Gene–environment interactions shape the quality of the architecture of the brain. As we come to better understand the processes that regulate gene function, we are gaining a better understanding of how experiences at different stages of life affect gene functions in neurons and glia cells.

The plasticity of the brain refers to its ability to learn, remember, forget, reorganize and recover from injury. The brain is more receptive to stimuli during earlier stages of development. For example, children who are dyslexic have difficulty with language and expression that handicaps their learning and work. They tend to have sound sensing and speech functioning distributed more on the right side of the brain instead of on the left. Intensive stimulation with phonemes by 6 years of age can lead to reformation of the neural pathways to left side of the brain, indicating that neural plasticity, including neurons and neural pathways, is sufficiently malleable at this age that normal function can be restored.¹⁶

Next: 3. Sensory Pathways