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Chapter 4: Growth & Health
THE DEVELOPING NERVOUS SYSTEM
Organization of the Mature Brain
• Neuron: The basic unit of the brain and nervous system, specialized in receiving and transmitting
information. The cell body contains the biological machinery; the highly-branched dendrite allows one
neuron to receive input from thousands of other neurons; the slender elongated axon sends information
to other neurons.
• The axon is wrapped in myelin, a fatty sheath and increases conduction speed from 2m/s to 15m/s. At
the end of the axon, terminal buttons release neurotrasmitters that carry information to nearby neurons
across the synapse.
• An adult brain contains 50-100 billion neurons, weighing around 1.5kg.
• The cerebral cortex regulates many functions, and consists of two hemispheres linked by millions of
axons in the corpus callosum.
• The frontal cortex in particular regulates higher level functions such as planning, reasoning, goal-
directed activity, and decision-making. Production and understanding of language is in the left
hemisphere, while spatial, artistic, and music abilities are in the right hemisphere.
The Developing Brain
Emerging Brain Structures
• At 3 weeks, a group of cells form the neural plate (neuralation). At 4 weeks, the neural plate folds to
form a tube which will become the brain and spinal cord.
• Neurogenesis occurs through cell division, born from neural stem cells, until week 18-20, by which
point the developing brain has virtually all the neurons it will ever have. During this time, neurons form at
• Postnatal neurogenesis occurs in select areas of hippocampus (memory formation) and olfactory bulb
• Neural migration moves neurons to final adult positions in stages, beginning with those in the
innermost layers being positioned first. This continues until all 6 layers are placed in the 7 month.
Migration occurs by wrapping around supporting glial cells.
• In the 4 month, axons begins to myelinate; this continues through infancy, childhood, and
adolescence. Neurons carrying sensory information myelinate first, and the cortex last; myelin plays a
role in improved coordination and reaction times as the child ages.
• Rapid synaptogenesis leads to peak number of synapses at 1 year, overabundant; soon after,
synaptic pruning reduces number of synapses to weed out unnecessary connections and increase
efficiency of useful connections. This pruning depends on the activity of the neural circuits, with active
synapses preserved. Synaptic density then remains relatively stable until the later years of life.
• Pruning in order: sensory and motor functions (visual cortex experiences huge growth after birth, with
intense visual stimulation after none