North American Ed - Brain development

1.1 Brain architecture

Basics of genetics

Brain development begins soon after conception and continues well beyond birth. Genes, the basic physical unit of heredity, are key to brain development. Genes set the parameters for the basic structures of the developing brain. Genes and experiences work together, and we can think of them as being inextricably linked.  

The next reading provides an overview of some key terms that are important for understanding that both genetic and environmental factors influence brain development. On the pages that follow, we will look more closely at these concepts.

Now check your understanding of some of these terms.

In the next video, Dr. Marla Sokolowski, professor of Ecology and Evolutionary Biology at the University of Toronto, describes what a gene is.

Listen now as Sokolowski describes cell differentiation during the early prenatal period. She discusses differential gene expression that leads to the production of different types of cells.

For some background on gene expression, listen to this explanation by Dr. Charles Nelson, professor of Pediatrics and Neuroscience at Harvard Medical School.

Nelson explains that gene expression is influenced by the environment; in other words, one’s experiences. In the next video, Dr. Meaghan Jones, assistant professor, Department of Biochemistry and Medical Genetics at the University of Manitoba, uses the analogy of a dimmer switch to explain epigenetics.

The next video is from KidCareCanada. Dr. Thomas Boyce adds to our understanding of epigenetics and explains that experiences – both positive and negative – affect how genes are expressed without changes to the genes themselves.

In the next video, Dr. Michael Skinner, professor in the School of Biological Sciences at Washington State University, discusses the concept of genetic determinism and explains that as we learn more about how epigenetics regulates gene function, we have a better understanding of how certain genes get turned on or off – and it is not all about genetic sequence.

Listen now as Skinner reviews the difference between genetics and epigenetics and explains that epigenetics is the “molecular factors around the DNA that regulate how the DNA functions, completely independent of the DNA sequence”.

…imagine a computer in which DNA or the genome (the complete set of 23 chromosomes) is the hardware. Epigenetics is the software that tells the genome how and when to work, as well as how hard to work” (Clinton, 2020, p. 84-85).

The field of epigenetics is relatively new and research is cutting edge. Neurobiological and genetic studies confirm that a range of early human experiences have an effect on brain architecture (National Scientific Council on the Developing Child, 2007a). Negative experiences, such as poor nutrition, environmental pollutants, drugs, and chronic stress, all can have long-lasting, possibly even multi-generational deleterious effects on health, learning, and behaviour. On the other hand, positive experiences such as nurturing and stimulation also affect the genome.

See the next reading from the Center on the Developing Child for an infographic that concisely explains epigenetics as well as details about common misconceptions related to genes and early development.

We will explore epigenetics research on p. 2.1 of this module.

Neurons

Our brains are made up of special cells called neurons,  which are the basic building blocks of the brain and spinal cord. Unlike other areas of the body, in which new cells grow throughout our lives, most of the approximately 100 billion neurons are formed before birth. This happens through a process called neurogenesis. This process continues in some brain structures until about age two. There is controversy as to whether any regions of the brain make new cells as we age. Neurons collect signals from several sources, integrate and transform information, and distribute information to other cells.

Synapses are the connections between neurons, while neurotransmitters are chemicals that transfer across the synapse from one neuron to the next. Connected neurons make up millions of neural pathways in the central nervous system. These pathways form the brain’s communication system within the brain and with the rest of the body. Glial cells are other cell types that support and insulate the neurons.

More neural connections develop in the first few years of life than at any other time. Over 1 million neural connections form every second during these years! (Center on the Developing Child at Harvard University, Brain Architecture, n.d., para. 2). These connections are vital in building a healthy brain.

Click on the Interact that follows to learn about the main components of a synapse and how neurons form connections with each other across the synapses.

Genes and environment interact throughout brain development. Genetic programming organizes neurons and neuronal development and lays the groundwork for basic networks and connections among major brain regions. Environment and experience refines these connections, enhancing some connections while eliminating others. Every experience excites some neural circuits and leaves others alone. While neural circuits used over and over become stronger, those that are not used are eliminated, resulting in synaptic pruning.

Even during pregnancy, experiences and genes interact to establish neural circuits and shape the brain’s architecture. We know from research that many experiences during pregnancy have the potential for detrimental life-long consequences on fetal brain development, for example, exposure to tobacco, drugs and alcohol; stress, infection, violence during pregnancy; certain medications; limited access to prenatal care and screening; and environmental hazards.

The following video, from the National Scientific Council on the Developing Child at Harvard University, provides a visual summary of brain cells forming connections and pathways in the early years.

Building neural connections and pathways is not all that happens in the first months and years of life. Neurons, connections and even whole neural pathways are discarded, while others are strengthened. The brain increases its efficiency by eliminating little-used pathways and reinforcing useful ones. This process of connecting (“wiring”) and refining (“pruning”) continues through infancy, childhood and adolescence. Synaptic connections are overproduced then pruned over time so that by age six, a child’s brain has many more synapses than the adolescent or adult brain. Click on the Synaptic density image to have a closer look at this.

In the following clip, neuroscientist Dr. Bryan Kolb, professor at the Canadian Centre for Behavioural Neuroscience, University of Lethbridge, describes the early brain processes of wiring and pruning using the metaphor of a piece of marble that is sculpted and changes from a stone to a beautiful object of art.

In the next video, the late Sir Michael Rutter, professor of Developmental Psychopathology at the Institute of Psychiatry, King’s College London for many years, explains that biology is not deterministic. While genetic programming organizes neuronal development, there are individual differences in those that are strengthened and those that are eliminated.

Brain structures

The brain is divided into three regions, from the most primitive to the most evolved brain area: the hindbrain, the midbrain and the forebrain. These three regions comprise the main structures of the brain. The hindbrain at the back of the brain includes the cerebellum. The brain stem is in the midbrain at the base of the brain. The forebrain includes the corpus callosum, limbic system and cerebral cortex.

In humans, the cerebral cortex is highly specialized and significantly larger than the cortex found in other mammals, including monkeys and chimpanzees. The cerebral cortex is comprised of two hemispheres (right and left) each of which is divided into four lobes: frontal, temporal, parietal and occipital. Play the following game to learn where the brain structures are located.

The following labelled diagram provides more information on some brain structures, including the four lobes within the cerebral cortex. Play the game that follows to test your understanding of the function of each brain structure.