DEVELOPMENT OF CENTRAL NERVOUS SYSTEM

Following fertilisation, the nervous system begins to form in the 3^rd week of development. It continues after birth and for many years into the future.

Structurally, the nervous system is divided into two parts:

Central nervous system – consists of the brain and the spinal cord.

Peripheral nervous system – consists of cranial and spinal nerves, ganglia, plexuses, and sensory receptors.

From birth to age 5, a child’s brain develops more than at any other time in life. And early brain development has a lasting impact on a child’s ability to learn and succeed in school and life. The quality of a child’s experiences in the first few years of life – positive or negative – helps shape how their brain develops.

At birth, the average baby’s brain is about a quarter of the size of the average adult brain. Incredibly, it doubles in size in the first year. It keeps growing to about 80% of adult size by age 3 and 90% – nearly full grown – by age 5.

The brain is the command centre of the human body. A new born baby has all of the brain cells (neurons) they’ll have for the rest of their life, but it’s the connections between these cells that really make the brain work. Brain connections enable us to move, think, communicate and do just about everything. The early childhood years are crucial for making these connections. At least one million new neural connections (synapses) are made every second, more than at any other time in life.

Development of the brain from 25 days to 9 months during pregnancy:

FIRST TRIMESTER

The development of the brain begins in the first few weeks after conception. Most of the structural features of the brain appear during the embryonic period (about the first 8 weeks after fertilization); these structures then continue to grow and develop during the fetal period (the remainder of gestation).

The first key event of brain development is the formation of the neural tube. About two weeks after conception, the neural plate, a layer of specialized cells in the embryo, begins to slowly fold over onto itself, eventually forming a tube-shaped structure. The tube gradually closes as the edges of the plate fuse together; this process is usually complete by four weeks after conception. The neural tube continues to change, eventually becoming the brain and spinal cord.

About seven weeks after conception the first neurons and synapses begin to develop in the spinal cord. These early neural connections allow the fetus to make its first movements, which can be detected by ultrasound and MRI even though in most cases the mother cannot feel them. These movements, in turn, provide the brain with sensory input that spurs on its development. More coordinated movements develop over the next several weeks.

SECOND TRIMESTER

Early in the second trimester, gyri and sulci begin to appear on the brain’s surface; by the end of this trimester, this process is almost complete. The cerebral cortex is growing in thickness and complexity and synapse formation in this area is beginning.

Myelin begins to appear on the axons of some neurons during the second trimester. This process – called myelination – continues through adolescence. Myelination allows for faster processing of information: for the brain to achieve the same level of efficiency without myelination, the spinal cord would have to be three yards in diameter.

THIRD TRIMESTER

The early weeks of the third trimester are a transitional period during which the cerebral cortex begins to assume many duties formerly carried out by the more primitive brainstem. For example, reflexes such as fetal breathing and responses to external stimuli become more regular. The cerebral cortex also supports early learning which develops around this time.

YEAR ONE

The remarkable abilities of newborn babies highlight the extent of prenatal brain development. Newborns can recognize human faces, which they prefer over other objects, and can even discriminate between happy and sad expressions. At birth, a baby knows her mother’s voice and may be able to recognize the sounds of stories her mother read to her while she was still in the womb.

The brain continues to develop at an amazing rate throughout the first year. The cerebellum triples in size, which appears to be related to the rapid development of motor skills that occurs during this period. As the visual areas of the cortex grow, the infant’s initially dim and limited sight develops into full binocular vision.

At about three months, an infant’s power of recognition improves dramatically; this coincides with significant growth in the hippocampus, the limbic structure related to recognition memory. Language circuits in the frontal and temporal lobes become consolidated in the first year, influenced strongly by the language an infant hears. For the first few months, a baby in an English-speaking home can distinguish between the sounds of a foreign language. She loses this ability by the end of her first year: the language she hears at home has wired her brain for English.

YEAR TWO

This year’s most dramatic changes involve the brain’s language areas, which are developing more synapses and becoming more interconnected. These changes correspond to the sudden spike in children’s language abilities – sometimes called the vocabulary explosion – that typically occurs during this period. Often a child’s vocabulary will quadruple between his first and second birthday.

During the second year, there is a major increase in the rate of myelination, which helps the brain perform more complex tasks. Higher-order cognitive abilities like self-awareness are developing: an infant is now more aware of his own emotions and intentions. When he sees his reflection in a mirror, he now fully recognizes that it is his own. Soon he will begin using his own name as well as personal pronouns like “I” and “me.”

YEAR THREE

Synaptic density in the prefrontal cortex probably reaches its peak during the third year, up to 200 percent of its adult level. This region also continues to create and strengthen networks with other areas. As a result, complex cognitive abilities are being improved and consolidated. At this stage, for example, children are better able to use the past to interpret present events. They also have more cognitive flexibility and a better understanding of cause and effect.

Information Processing System
The brain’s reticular activating system filters incoming information to focus on what’s important, excluding anything trivial which leads to meaningful perception (Wolfe, 2001 as cited by Schunk, 2012). This process is adaptive and perceived importance is based on factors such as novelty, intensity, and movement. In terms of its application in the classroom, these aspects can be used to maintain student’s attention and educators can find ways to build these factors into their lesson and student activities.

Specifically, sensory inputs are processed in the sensory memories area of the brain and the ones that are retained move to the working memory where it primarily resides in the prefrontal cortex of the frontal lobe. The cortex and medial temporal lobe are involved in memory and information processing. The LTM is located in the frontal and temporal cortex. The parts of the brain responsible for long term memory differ based on the type of information whether it is declarative memory (facts, definitions, events), or procedural memory (procedures, strategies).

Table 1: Illustrates how long term memory works in the brain

Type of information	How LTM occurs in the brain
1.Declarative	Sensory registers in the cerebral cortex (visual, auditory) receives input and transfers it to the hippocampus and nearby medial temporal lobe. Inputs are registered in the same format as they appear (auditory, visual stimulus). The hippocampus acts as a processor and conveyor of inputs. Stronger neural connections develop based on multiple activations of memories in the neural networks.
2.Procedural	Becomes automated as certain procedures can be done without conscious awareness (i.e. typing). Initial procedural learning occurs from the prefrontal cortex, the parietal lobe, and cerebellum which ensure that we consciously attend to the steps in the correct sequence. Practice helps these areas to become less involved and other areas such as the motor cortex become more involved in their place.

According to Bandura (1986), cognitive neuroscience supports the idea that learning occurs through observation. Nonmotor procedures (i.e. decoding words) involve the use of the visual cortex. Yet, repetition can change the neural structure of the visual cortex. These changes allow us to quickly recognize visual stimuli (i.e. words, numbers) without consciously having to process their meaning. Conversely, conscious processing is involved when it requires extended activity (i.e. pausing to understand a passage).

Neural Connections in Memory Networks
From a cognitive neuroscience perspective, learning involves forming and strengthening neural connections and networks. Hebb’s theory elaborates on this by explaining the role of two cortical structures: cell assemblies and phase sequences. Cell assemblies are structures with cells in the cortex and subcortical centers. It is the neural counterpart of a simple association that is developed through repeated stimulations. Phase sequence is a series of cell assemblies which form an organized pattern. Hebb believed that when a cell assembly was activated that it would in turn activate the neural and motor responses. For example, if we repeatedly see a friend’s face then multiple cell assemblies are activated enabling us to meaningfully perceive who it is. Thus, Hebb’s theory can be summarized best by this statement “cells that fire together, wire together” as it describes associative learning. Hebb’s theory is still relevant today as it is linked with recent research on how learning occurs and memories are formed. For instance, several studies have shown that enriched environments are key to leading to improved learning outcomes (van Praag et al., 2000).

Generally, people are born with a large quantity of neural connections and our experiences work on this system so that our connections are selected or ignored, strengthened or lost. According to the National Research Council (2000), the process of forming and strengthening synaptic connections, essentially learning, changes the physical structure of the brain and alters its organization. Although we tend to think that the brain determines learning, it is a reciprocal relationship since the “neuroplasticity” of the brain or its capacity to change its structure and function is a result of an individual’s experience.

Recent research indicates that memory is not formed at the time initial learning occurs. Memory forms as a continuous process called consolidation where stabilizing and strengthening of neural connections occur in the hippocampus. The brain plays an active role in storing and retrieving information. Instruction also plays an important role in helping impose a desired structure on learning.

Table 2: Illustrates instructional applications to help improve memory

Factors that improve consolidation (memory)	Instructional applications
1. Organization	Creating timelines, labelling, use of visual organizers
2. Rehearsal	Practicing for a play, repeating vocabulary words, using lists, essays
3. Elaboration	Surveying students to gauge their interests, relating concepts, encouraging students to elaborate on their questions/answers

Language Learning
The brain’s cerebral cortex is responsible for reading and the posterior cortical association areas of the left hemisphere are important for understanding language and normal reading. There are several brain structures involved in the following aspects of reading. For educators it is important to know how the brain develops since developmental changes need to be considered in planning instruction to help students learn best.

Table 3: Illustrates the brain structures associated with reading

Brain Structures	Aspects of Reading
Primary visual area	Orthographic- refers to letters or characters processing
Superior temporal lobe	Phonological- phonemes or syllables
Broca’s area in the frontal lobe and medial temporal lobe	Semantic- meanings
Broca’s area in the frontal lobe	Syntactic- sentence structure processing

Brain development
Educational implications for teaching and learning vary depending on the level of brain development. Factors influencing brain development are illustrated below.

Table 4: Illustrates how factors influence brain development

Factors influencing brain development	How it occurs
Genetics	Human brains have a similar genetic structure but they differ in size and structure. Genetic instructions determine the size, structure, and neural connectivity of the brain.
Environmental stimulation	Brain development requires experiences from the environment. These experiences develop neural circuitry that can receive and process stimuli and experiences.
Nutrition	Lack of good nutrition can have major effects on brain development. Crucial period is between the 4^th and 7^th months of gestation when most brain cells are produced.
Steroids	Hormones can affect brain development. Excess stress hormones can led to the death of neurons.
Teratogens	Foreign substances (i.e. alcohol, viruses) can cause abnormalities in the developing embryo or fetus. Teratogens can have effects on the development and interconnections of neurons and glial cells and can cause birth defects.

Critical Developmental Phases
At birth, the human brain has over a million connections which represent about 60% of the peak number of synapses that will develop over a lifetime. Brain connections that are not used or needed disappear.

Key highlights include:

· 2 years old- a child will have as many synapses as an adult

· 3 years old- a child will have billions more than an adult

· 5 years old- a child’s brain has acquired a language and developed sensory motor skills and other competencies.

· Teenage years- major structural changes occur in the brain. The frontal lobes which handle reasoning and problem solving mature and the parietal lobes increase in size. The prefrontal cortex which controls judgments and impulses matures slowly. There are also changes in neurotransmitters especially dopamine that can leave the brain more sensitive to the effects of drugs and alcohol. There is a thickening of the brain cells and massive reorganization of synapses which makes it a key time for learning.

Table 5: Four aspects of crucial brain development

Type of Brain Development	Why it is important
Sensory motor development	Systems associated with vision, hearing and motor movements develop extensively through experiences during the first 2 years of life.
Auditory development	By age 6 months children can distinguish between most sounds in their environment. In the first two years children’s auditory systems mature in terms of range of sounds heard and ability to discriminate among sounds. Problems in auditory development can lead to problems learning language as it is dependent on hearing the speech of others.
Vision	Develops in the first year of life especially after 4 months. Proper visual development requires a visually rich environment where infants can explore objects and movements.
Language	Children who are normally developing show extensive bilateral and anterior cortical activation and left-sided activation in language and speech areas. Reading development is based on anterior activation on both sides of the brain. A critical period of language development is from birth to age 5. During this time, a child’s brain develops most of their language capabilities. There is a rapid increase in vocabulary between age 19-31 months. Development of language is enhanced when children are in language-rich environments where parents and others talk with children.

Instructional Applications
Instruction can help facilitate language development by stressing perceptual, motor, and language functions. Teachers can work with students of all ages to help develop their language skills by coordinating the components of language- seeing, hearing, thinking and speaking. Some examples include:

Helping students learn phonemes using cards and practicing writing/reading it in sentences

Using picture and word cards for names and spellings

Using magnetic number lines students put the magnetic bar on the appropriate number line. Next converting the numbers to a common denominator to place in the correct order.

For learning historical events, have students engage in role-playing activities where they read documents as historical characters

Using case studies

Conclusion
Overall, brain development is a lifelong process, and requires ongoing stimulation especially after the age of 2 years. The brain is continuing to add, delete and reorganize synaptic connections and changing structurally. Individuals of all ages can benefit by engaging in stimulating learning environments.