Human Development--Brain


John Jaccard, M. D.


The human consists of only two cells at the time of conception, the ovum and sperm. The DNA code contained therein leads to the production of the entire human over the months to follow to birth and beyond.


The brain develops upwards of 50,000 cells or neurons per second at times and these neurons are guided to their appropriate place in the brain. "Once the neurons reach their final location, they must make the proper connections for a particular function, such as vision or hearing to occur. They do this through axons. These stalk-like appendages can stretch out a thousand times longer than the cell body from which they arise. The journey of most axons ends when they meet the branching areas, called dendrites, on other neurons. These targets can be located at a considerable distance, sometimes at opposite sides of the brain. In the case of a motor neuron, the axon may travel from the spinal cord all the way down to a foot muscle. The linkup sites, called synapses, are where messages are transferred from one neuron in the circuit to the next." 1 (figure 1).


The brain has more cells or neurons at birth than as a fully grown average adult's 100,000,000 neurons. Only about one half of the neurons survive to function in the adult brain. As we grow, the decision to keep or not keep these neurons is based on activity or use. There are times in development, called "critical periods", when certain groups of neurons will be activated or they will be disposed of based on whether the nervous system has "obtained certain critical experiences, such as sensory, movement or emotional input, to develop properly" 2. This could be called a "use it or lose it" approach to brain development.

 The adult brain has on average 100,000,000 neurons which average 10,000 connections or synapses each.




There are primary and secondary messengers which make the communication from the axon of the sending neuron to the dendrite of the receiving neuron. The primary messengers are called neurotransmitters and operate in the synapse or space between the axon and dendrite. The second messengers operate within the receiving neuron and start a series of biochemical events leading to the desired effect, muscle movement, emotion, thought, etc.


Neuron Types


Glutamate accounts for about 30 percent of the neurons and is the largest collection. It is excitatory in nature and could be compared to stepping on the gas. It can lead to great damage when over stimulated, like a speeding car. The brake is provided by the GABA system which is involved in about 40 percent of the various connections between neurons where it modulates or controls activity.


If one thinks of the brain as a television screen, the glutamate, GABA and other cell types produce the picture, while other cells tune the picture. One could say that these tuning cells control the contrast, brightness and sharpness of the picture. These equate to the things like mood, and mental sharpness of the person.


These cells are primarily the Norepinephrine, Serotonin and Dopamine cells. There are only about 250,000 or less of these cell types, compared to the billions of other cell types. The cell bodies are buried deep in the lower part of the brain, and traveling to the highest parts of the brain, have the longest distance to go to their targets and therefore, the longest axons.


The growth of the Myelin Sheath covering the axons strengthens the axons and helps signals pass more quickly. This covering grows at different rates in different parts of the brain. These tuning cell axons don't have fully developed sheaths until humans are about 14 years old. That means that these very long axons are more susceptible to damage, particularly head injuries, before 14 years old.


Neural Structure


As noted above, sensory input or experience shapes the brain. Initially, the lower, more primitive and more reactive areas of the brain are more developed. The infant lacks the ability to regulate itself. This can easily be seen when stimuli like loud noises cause the infant's whole body to jump or react in the startle response or reflex. This reflex reaction and some others can be seen to slowly diminish as the individual grows and the brain matures. Experience leads to the activation of specific neural pathways, or collections of connected neurons, which leads to the expression or triggers genes that cause chemical events leading to stronger connections within those neural pathways. The strengthened pathway then becomes stronger through repetition. This is how we learn or create habits.


The most significant early stimulator is the mother. The infant is visually sensitive to the human face within hours after birth, and within days can distinguish its mother's voice, and within weeks prefers to look at its mother face. "The child operates within a structure provided by the mind of the adult." (3) Through stable, consistent care giving, the appropriate genes are activated, and the neural pathways created for the child to:


* Regulate emotions
* Feel connected to other people
* Establish memories of their life story and have a sense of who they are
* Move into the world with motivation and confidence
"The expression of motives and emotions between young children and their caregivers and companions regulates the acquisition in the human world." (4)


Brain Maturation


Stage of Growth
Brain region




Implicit memory- remembers emotionally but can't recall the event. May be responsible for gut feelings or intuition. Stays throughout life.


12 - 18 months


Medial temporal lobe
Semantic memory - able to remember or recall Hippocampus events and sequence of events. Able to encode or store and then recall specific events.


18 - 36 months


Prefrontal regions, esp. Orbitofrontal Cortex
Autobiographical memory - able to recall self and the experience of self in the events




Continued growth of Frontal areas
Ongoing development of a sense of self with increased skill in autobiographical memory remembering self in the experience. Better use of words after 4 years old. Cooperative play.


6 - 11 years


Continued growth of Frontal areas
Increasing capacity for more complex thought Increasing motor coordination, and language skills. "Essentially a period of consolidation of all earlier developments." (5) Moral judgment progresses from mainly that which is forbidden to more concern with inequality.




Pruning, and continued development of higher brain areas esp. the frontal areas
Continued development of reasoning, integration of sensory input and other executive functions, such as the ability to hypothesize.




1 Brain Facts, A Primer on the Brain and Nervous System. The Society for Neuroscience. P 10
2 " " P 11
3 Wexler, B Early sensory input shapes brain's neural structure. Cli Psych News, May 2005.
4 Trevarthen. From Siegel,DJ. The Developing Mind. The Guilford Press, 1999.
5 Lewis,M Clinical Aspects of Child development. 2 nd Ed. Lea and Feabiger. 1982
Siegel,DJ. The Developing Mind, Guilford Press, 1999.
Schore,AN. Affect Regulation and the Origin of the Self. Lawrence Erlbaum Associates, Publishers. 1994.