Introduction

The brain is the most complex organ in the human body, and the only organ to study itself. How does the brain develop and what is necessary to maintain its health? The answers to these questions are important because they inform and impact everything we do in our lives, especially when working with children. As we will learn, the brain develops quickly in the early childhood years and continues to change throughout our lives.

When we understand the brain, we understand the power and impact of positive early childhood experiences. We also come to understand the impact on young brains from toxic stress and trauma and how we can prevent this. Building healthy brains from the start benefits everyone.

Brain Anatomy

At the cellular level, the brain is made up of 86 billion nerve cells called neurons. There are at least 10 times more support cells, called glial cells. Neurons communicate with each other through billions of connections in an electrochemical process. There are about 500 trillion connections in the adult human brain.

Although there has been a long-standing debate about whether we are more impacted by nurture (our environment) or nature (our individual biology), we now understand that it is actually a unique combination of both. Neither nature nor nurture fully explains what makes us human; it is a complex relationship between the two. Biology and genetics provide the potential, but our social environment shapes our ability to access that potential.

Neurons

At the most basic level, a neuron is made up of a dendrite, cell body, axon, and myelin. Between neurons there is a small gap called the synaptic gap.

A dendrite and its spines receive information from other neurons. The number of dendrites on a neuron varies from a few hundred to thousands. Dendrites are covered with spines (varicosities) that are neurotransmitter receptor sites.

The cell body and its DNA genetic system use the nutrients that the blood brings to maintain the cell and to synthesize neurotransmitter molecules (messengers between cells).

The axon sends information from the neuron to other neurons. Each neuron generally has one axon branching out into many terminals. Axons vary in length from 1 millimeter to about 3 feet! Mature axons are covered in an insulated coating, which looks like sausage links, called myelin.

The synaptic gapis the tiny space between neurons; the neurons don’t actually touch. Neurotransmitters are released into the gap and act as chemical messengers to the receiving neuron.

Neurons transmit information to each other through axons and dendrites by using the synaptic gap to exchange neurotransmitters. The axon sends a message through a series of electrical impulses called the action potential. When the impulse reaches the end of the axon the electrical activity ceases. A chemical process takes place in the form of neurotransmission. If the message is “transmit information” an electrical charge is triggered in the next neuron. That neuron’s dendrite receives the message and electrically sends it through the axon to the next neuron. The process repeats until the message has reached its destination. If the message is “don’t transmit information,” the message is not passed on.

Neurotransmission

When the electrical impulse that carries information reaches the end of a neuron’s axon, it is stopped at the tiny synaptic gap that separates them from the receiving neuron. The circuit is broken. Neurotransmitters are chemical messengers secreted at the synapse that have the potential to continue the circuit and transmit information between neurons.

Without neurotransmitters the brain could not process information or send out instructions to run the rest of the body. They affect the formation, maintenance, activity, and longevity of synapses and neurons. Neurotransmitter molecules are produced within a specific type of neuron (different neurons are specialized in different neurotransmitters) and stored in tiny sacs known as vesicles. When an electrical signal reaches the vesicles, they release their neurotransmitters into the synaptic gap.

Each type of neurotransmitter has a unique shape that acts like a key. Released neurotransmitters attempt to attach to receptor sites (usually on the receiving neuron’s dendrites). Each receptor site is shaped like a lock that will fit only certain types of neurotransmitters. If the “key” fits, the neurotransmitter will send a message to turn on a receiving neuron (excitatory message) or off (inhibitory message).

When a neurotransmitter’s job is done, the receptors release the molecules, which are either broken down or recycled. Each neurotransmitter has a very specialized function. Some neurotransmitters carry emotional information that impacts our mood, outlook on life, and behavior. For example, cortisol has an impact on our stress response system, and dopamine has an impact on our motivation, satisfaction, and pleasure. Serotonin plays a role in our mood management.

Growth and Development of the Brain

Brains begin development in the womb, starting as a neural tube and rapidly developing from the bottom up to form the lower, midbrain, and outer brain. Development is impacted by the mother’s nutrition, stress, environment, and other factors such as their mental health. During this time neurons migrate along the glial cells and move into place. The specific time table for migration is unknown and can be negatively impacted by the mother ingesting drugs or alcohol during pregnancy.

Babies are born with an estimated 86 billion brain cells. They create new connections, in the form of neural pathways, in response to active engagement in stimulating experiences. In the first few years of life more than 1 million new neural connections form every second (Center on the Developing Child, n.d.-a). Most neural pathways are created after birth as a result of stimuli coming from the environment that the child interacts with through the senses.

Each time the brain responds to a similar stimulus there is an increased propensity for the neurons to reconnect along the same pathway. Connections grow in the brain when experiences are repeated over and over or when an experience triggers a strong emotional reaction. The brain becomes hard wired to respond along established pathways. Think back to the opening story of the baby in the high chair dropping their spoon. That repeated experience with the caring adult is building pathways in the brain as they learn about cause and effect.

Neurons physically change as a result of this activation. Neurons grow new dendrite branches and receptor sites allowing the brain to process information more effectively and efficiently in more areas of the brain. The brain changes in response to experience by making connections between new input and what is already known and in place. The brain learns by recognizing patterns to make sense of new experiences. For example, when a baby tracks a toy with their eyes while grasping at it with their hand, their visual and motor pathways are connecting and growing stronger. Experience literally sculpts the brain!

The most active period for creating connections is in the early years of life, but new connections can form throughout life. After this rapid proliferation early on, unused brain cells and connections wither away in a process called pruning. Pruning is necessary in order to make room for the pathways the child needs most to survive in their world. Creating room also has the function of making the remaining pathways more efficient. Think of how pruning a fruit tree is essential to make room for new growth and fruit to mature. Pruning too many neurons that are important will decrease the brain’s efficiency. Pruning happens most rapidly between ages 2 and 10, but is happening in some form throughout life, starting at about 8 months and ending in the late 20s (Cafasso, 2018). The intensity of the pruning is dependent on which area of the brain is being affected at the time.

Plasticity of the Brain

Plasticity describes the ease with which the brain can change itself. Our genes provide the blueprint, and our experiences are the architect. Which genes get turned on or off is determined by our experiences and environment. The brain’s pathways strengthen as they are used. As stated above, the neurons that are not used are subject to pruning. In other words, we need to use our neurons or risk losing them.

There is a remarkable increase in synapses during the first year of life. In the beginning of life the rate of connections is about 1 million per second (Center on the Developing Child, n.d.-a). The brain is most plastic early in life, and it is easier to influence a baby’s brain than try to rewire parts of it in the later years.

Windows of Opportunity

There are some stages of brain growth where parts of the brain become more active in response to what the senses absorb. This is called  windows of opportunity in the brain . It is when  optimal growth occurs.ii These parts of the brain grow and learn faster than at any other time in life.

Children need the right experiences at the right time for their brains to fully develop in these areas. Some areas in the brain have windows that are merely an easier time to learn a task (like a second language) and others are more critical. Sight is one of these critical windows of opportunity. If the eyes are deprived of sensory input early in life the neurons poised to connect for visual pathways reassign to areas in the brain where there is more experiential input happening (Eagleman, 2020). Most windows of opportunity are only optimal times and not absolutes. Every child is on their own timetable and so the age they reach the window will vary.

When developmental stages are interrupted or skipped, or an injury of any degree is experienced, some sensory-motor and cognitive functions may be impaired or missing. For most functions it is never too late to grow new neurons and pathways, but it gets increasingly harder to do as the brain ages. The human brain has a remarkable ability to heal. Windows don’t slam shut but slowly close as we age, never really permanently shutting.

Enriched Environments

Children need active involvement in a stimulating, challenging, and loving environment for the brain to grow and flourish. This is called an enriched environment for the brain to develop. Passive involvement, isolation, and an impoverished environment diminish the brain.

What is included in an enriched environment for the brain? Sleep, nutrition, water, a safe environment, positive role models, and more. It is very important that babies, children, and adult brains have adequate sleep (see Table 1). Sleep is when the brain renews itself and cements learning.

Brains need proper nutrition with the right types of fat, protein, fruit, and vegetables. We are what we eat, and our brain can only function as well as the fuel we give it. Foods high in refined sugar are toxic for a growing brain. The American Association of Pediatrics recommends limiting the amount of sugar children consume each day to no more than six teaspoons for ages two and older; a typical child consumes more than triple that on average (Jenco, 2016). A great resource for a balanced diet for children is MyPlate by the U.S. Department of Agriculture.

Water is essential for the brain and body to stay hydrated. Encouraging children to drink water instead of juice reduces the amount of sugar they are consuming while hydrating their brain. WebMD suggests the following: Toddlers need 2-4 cups, children 4 to 8 years old need 5 cups, 9-13 year-olds need 7-8 cups and children over 14 years need 8-11 cups (Wheeler, 2016).

In addition to sleep, nutrition, and water, children need a safe environment with appropriate boundaries. Giving kids the freedom to explore while making sure that the environment is free from toxins and hazards helps young brains grow. They need the chance to interact with interesting materials and be given clear guidance about what is safe and not safe. We can think of boundaries as a fence we provide that surrounds the child and enlarges as they mature. The fence keeps them safe but within it they are free to explore and push against the boundary, so they know they are safe. Emotional warmth and safety is key!

An enriched environment also includes positive role models and guidance. Adults should model the lifestyle and behavior they want from children. Healthy eating, drinking water, getting adequate sleep and exercise, and modeling emotional intelligence and growth mind set skills are all part of this. If the adults around children strive to keep their brains healthy, then chances are that kids will follow suit. Positive guidance lets the child know they are safe. Because behavior is a learned skill, children learn how to behave by watching the adults in their lives. Learned skills like tying their shoes or following positive guidance activate neurons to build strong pathways.

Young brains do best when media is limited, and when they have daily exercise with time in nature. Movement of bodies creates an increase in the oxygen and blood flow to the brain, helping to keep it healthy at any age. Movement is important not only for keeping the brain healthy but also for improved mental health and school success. “For example, researchers found that children who had an opportunity to run for 15 to 45 minutes before class were less distracted and more attentive to schoolwork” (Wilson & Conyers, 2014, para. 5).

Nature provides the brain with a complex bath of sensory input that will strengthen pathways and connections in a way that can’t be replicated indoors, while helping kids build confidence, creativity, and responsibility. “Many researchers agree that kids who play outside are happier, better at paying attention and less anxious than kids who spend more time indoors” (Cohen, 2023, para. 1).

Our brains need down time and unstructured play. Down time for brains allows children to follow their own interests and develop mastery over skills they are learning. It is through unstructured play time that children feel free to learn about their world and strengthen their abilities. Young brains need practice repeating positive developmentally appropriate experiences with caring adults supporting them.

It is important not to stress the child by pushing them to do things they are not ready for. In addition, try to avoid providing an overstimulating environment. The best approach is to follow the lead of the child and focus on their interests and unique timetables.

The child’s brain is not a smaller version of an adult brain. Neurons are still moving into position. As the brain develops, neurons migrate from the inner surface of the brain to form the outer layers. Immature neurons use fibers from cells called glia as highways to carry them to their destinations.

Note: Data from the Centers for Disease Control and Prevention (2022).

Myelination

Mature neurons have axons that are coated by a fatty layer called myelin, the protective sheath that covers communicating neurons. Myelin acts in two ways: it provides substance for the brain and insulates the cells. The myelination of axons speeds up the conduction of nerve impulses through an ingenious mechanism that does not require large amounts of additional space or energy. Areas of the brain do not function efficiently until they are fully myelinated. Babies are born without much myelin.

According to Harvard Health, “how the brain begins is how it stays” for the rest of life (McCarthy, 2018, para. 3), so it is important to make sure nerves grow and connect and get covered with myelin. The essential nutrients for brain growth include:

• Protein. Protein can be found in meat, poultry, seafood, beans and peas, eggs, soy products, nuts and seeds, as well as dairy.
• Zinc. The food that has the most zinc, interestingly, is oysters — but it’s also found in many meats, fish, dairy products, and nuts.
• Iron. Meats, beans and lentils, fortified cereals and breads, dark leafy vegetables, and baked potatoes are among the best sources of iron.
• Choline. Meat, dairy, and eggs have lots of choline, but so do many vegetables and other foods.
• Folate. This nutrient, which is especially important for pregnant mothers, can be found in liver, spinach, fortified cereals and breads, as well as other foods.
• Iodine. Seaweed is a great source of iodine, but we also get it from iodized salt, seafood, dairy products, and enriched grains.
• Vitamin A. Liver, as well as carrots, sweet potato, and spinach are good sources of this vitamin.
• Vitamin D. This is the “sunshine vitamin,” and the best way to get it is to get outside. The flesh of fatty fishes such as salmon have Vitamin D, as does fish liver oil, and products fortified with it, such as fortified milk. (McCarthy, 2018, para. 6)

Breast milk contains a fat almost identical to the fat in myelin, so if possible, mothers should nurse during the first year or more of life. Recent research has shown positive neurodevelopment and longer term cognitive outcomes for babies that are exclusively breastfed at least the first three months of life (Deoni et al., 2018). If formula is necessary, it is important to make sure it includes ingredients as close to the composition of breastmilk as possible.

In order to protect a babies’ unmyelinated neurons, never shake a baby. Although there may be no outside sign of damage, the neurons get whipped around and have no myelin to protect them from the impact to the skull.

Boundaries and Readiness

The brain has boundaries around how quickly it can develop that are established by myelination timetables. Myelination can be stimulated when the brain is ready, but it cannot be rushed. Pushing a child to do something before they are ready can result in learning problems later on. Follow the child’s cues: their interest and frustration level will tell you when their brain is ready (or not) to learn a new skill.

Myelination continues to develop slowly all during childhood and adolescence in a gradual progression from lower to higher level systems. Early childhood is spent primarily on the brain stem, cerebellum, and sensory cortex. Puberty is when the limbic system is primarily being myelinated and late adolescence is when the prefrontal cortex finishes myelination.

Layers of the Brain

The brain develops sequentially from the brainstem up, with the cortex developing last and continuously throughout life.

The Brain stem and midbrain are the first to develop and are mostly concerned with survival. The autonomic nervous system is regulated by the brain stem. It is the first part to mature. Babies are born with autonomic nervous system neurons fully myelinated. These neurons control survival needs such as heartbeat, breathing, and sucking. The brainstem and midbrain monitor the outer world through sensory input and activate the body to respond in ways that ensure self-preservation. The brain stem processes information at a subconscious level; it is quick and reactive. Some of its functions include autonomic nervous system, fight/flight/freeze/fawn response, defense mechanisms, territoriality, reflexes, rote responses, routine, and habits. It is the least plastic layer of the brain and the most highly resistant to change. The reason habits are so hard to break is because they reside in this region of the brain.

The cerebellum is mostly in charge of coordination. It controls automatic movements and the coordination of movement and thought or balance. The cerebellum is where procedural memory is stored like our motor skills. It does not involve conscious thought except when we are first learning something (like riding a bike). This area of the brain matures in early childhood and works in coordination with the brain stem.

Thelimbic system is where emotions are processed. The limbic system is made up of many structures in the middle of the brain including the amygdala, hippocampus, thalamus, and olfactory bulbs. This area receives, interprets, and responds to emotional signals sent from the body. It processes information at the subconscious level and forms emotional patterns. This area is also associated with long-term memory and matures during puberty.

The cortex is where higher level thinking at the conscious level occurs. This includes, making sense of the world, decision making, creativity, reason, logic, imagination, self-awareness, and self-control. Everything that makes us uniquely human is the result of the interplay between the cortex working in harmony with the lower brain structures. The cortex loves change, novelty, fresh input, and variety. It is the most plastic layer of the brain. The cortex is divided into specialized areas called lobes that are determined by their function. It matures over a long period of time, from the back to the front of the brain.

Lobes of the Brain

The cortex is split up into areas that are responsible for different functions. The back and side lobes are mostly related to sensory functions.

The occipital lobe is mainly responsible for vision and develops very early. The temporal lobe processes hearing, speech, language, and memory. The parietal lobeprocesses incoming sensory information like touch, pressure, pain, cold, heat, taste, and proprioception. Thefrontal lobe is responsible for gross and fine motor movements.

The prefrontal lobe, the very front section of the frontal lobe, is responsible for critical thinking, creative thinking, and problem solving. It is the part of the brain that allows us to imagine, plan and rehearse future actions. This area connects to the limbic system to regulate emotions. It is this integration of emotions with thought that is essential to the decision-making process. This area of the brain starts to develop around eight months and continues to develop late into adolescence (around age 26).

Executive function and self-regulation are also associated with this area of the brain. A child who develops the ability to self-regulate has better impulse control, mental flexibility, and emotional intelligence. These functions are critical for learning. Although children do not have executive function from birth, it can be strengthened through practice with games and activities specifically aimed at reinforcing these skills.

Attributions

1. Figure 3.2: Beta-Amyloid Plaques and Tau in the Brain by NIH Image Gallery in the Public Domain; The NIH Image Gallery on Flickr provides images that are free to use with credit. Materials produced by federal agencies are in the public domain.
2. Figure 3.3: Untitled by OpenStax is released under CC BY 4.0
3. Figure 3.4: Action of SSRIs and NRIs by Arran Lewis for the Wellcome Collection is released under CC BY 4.0
4. Figure 3.5: Synaptic Density by Harry T. Chugani is released under CC BY 4.0
5. Figure 3.6: Untitled © Center on the Developing Child, Harvard University Used with permission from the author.
6. Figure 3.7: Human Brain Development © Center on the Developing Child, Harvard University Used with permission from the author.
7. Figure 3.8: image by NIH Image Gallery in the Public Domain; The NIH Image Gallery on Flickr provides images that are free to use with credit. Materials produced by federal agencies are in the public domain.
8. Figure 3.9: by Nicholas Malara, for WA Open ProfTech, © SBCTC, CC BY 4.0
9. Figure 3.10: Diagram of the brain for a phrenological textbook by Bernard Hollander, Wellcome Collection in the Public Domain; Granted use for any purpose without restriction.