Neurodevelopment: Nature and Nuture
At birth, babies have approximately the same number of neurons as an adult, about 100 billion, but approximately 10 times fewer connections or synapses.
Each neuron consists of a cell body which are connected to dendrites and an axon. The presynaptic terminals at the end of the axon make contact with the dendrites of other neurons allowing connections to form between neurons. In this way, complex neural networks can be created.
Development of the neural tube begins during the first weeks of gestation. Cells that will compose the brain, spinal cord, and gut arise from the tube. Prior to birth, these cells migrate to the different regions where they will inhabit in the adult brain. By 7 months gestation, almost all of the neurons that will comprise the mature brain have been created and will live until death. A neuron can live 100 years or more.
After birth, brain development or learning is shaped by experience. It consists almost exclusively of the growth of connective elements; axons, synapses and dendrites in a process called synaptogenesis. Synaptogenesis occurs rapidly in the first years of life. A process of ‘pruning’ selectively eliminates those connections and neurons that are not used. The process of pruning helps to structure the brain’s architecture into organized networks. As a result each child’s brain is unique, shaped by that child’s individual experiences.
Another important brain tissue is glia. Glial cells are cells which provide nourishment, support, and insulation or myelin for neuron axons. Myelination makes connections more efficient and the transmission of neural signals about 10 times faster, but reduces neuronal plasticity because axon growth is more limited.
The brain is made up of many different areas, each having a particular structure and function. Brains develop from bottom up. First developed is the lowest and evolutionary oldest part, the hindbrain, including the brain stem and cerebellum. The brain stem controls most basic functions of life such as breathing, heart rate, blood pressure, body temperature, and fear. The cerebellum is involved in motor learning, coordination, equilibrium, posture, precision, and many cognitive processes that require careful timing such as language, music and movement. The hind brain must be functional by birth for the baby to survive. This is followed by development of the higher parts – the midbrain, which handles sensory motor integration, the limbic, where emotional information is stored, and the cortex, which control abstract thought, cognitive memory, and many "higher-order" functions like language and information processing.
The cerebral cortex is divided into two hemispheres, left and right, joined together by a thick band of nerve fibers called the corpus callosum. Language centers are usually found only in the left cerebral hemisphere. For more details on the hemispheres go here.
Each hemisphere is further divided into the frontal, parietal, temporal, occipital lobes. Each lobe is specialized to function. In 1909, Dr. Korbinian Brodmann further divided the lobes into 52 areas, based on the types and organization of neurons or cytoarchitecture. Within a Brodmann area, similar processing is preformed.
Within the cortex are many types of memory systems that follow the same basic pattern. There are three main steps in the formation and retrieval of memory: encode, store, and recall.
Encoding is receiving, processing, and combining of sensed information. Storage is creation of a permanent record of the encoded information. Recall is retrieving stored information in response to some signal for use. All three are used in learning require attention or control circuits for learning to happen. Only recall from long term memory is necessary for certain practiced tasks. People with strong right or left hemisphere dominance can have smaller short term memory buffers or loops on the opposite side, if they do not have special training. A complex behavior such as reading will make use of multiple memory systems, therefore determining the exact source of comprehension challenges can be difficult without computer-based assessment.