Neuron Development: in human brains, approximately 10 billion cells are needed to form just the “cerebral cortex” that blankets a single hemisphere. To produce such a large number of cells, about 250,000 neurons must be born per minute at the peak of “prenatal” brain development. (Kolb, 195)

As neurons are born, they migrate to their proper locations in the brain and connections between them and other neurons begin to form. (Goldberg, 39)  Recently, neuroscientists have discovered that the brain does change throughout life. (Best of the Brain-Fred Gage, 121) “Neurogenesis” continues into old age, though at a slower rate than in earlier decades. And even that slowdown may not be inevitable, but rather a side effect of 'monotony.' Adding 'complexity' to a person’s social environment primes new “learning,” enhancing the rate at which the brain adds new cells. (Goleman, 239)


Arborization: a process of early growth of “dendrites.” (Goldberg, 39) Dendrites begin as individual "processes" protruding from the "cell body." Later, they develop increasingly complex extensions that look much like the branches of trees visible in winter. (Kolb, 198) Verb - ‘arborize.’

Cell Migration: (process of) newly formed cells traveling to their correct location. Begins about 8 weeks after “conception” and is largely complete by about 29 weeks. (Kolb, 195-196) During “embryonic development” in animals, cells migrate to their appropriate positions within the body. (Brooker, 203) Only 50% on average migrate successfully, the others perish. Newborn stem cells need to move away from their “precursors” before (the precursors) can “differentiate.” (Best of the Brain-Fred Gage, 123)

Myelination: the process by which “glial” cells wrap around long axons, forming a fatty protective coating called “myelin.” The dramatic increase in brain weight during the first years of life is largely due to myelination. The brain structures are not fully functional until the axons connecting them are insulated with myelin, and the time course of myelination varies vastly from structure to structure. (Goldberg, 40-41) Begins at the “axon hillock” and stops at the “axon terminal.” (Characterized by) discontinuous segments. (Patestas, 30) Adjective - ‘myelinated.’

Internode: each myelinated segment (of an axon). (Patestas, 30)

Myelin Sheath: multilayered wrapping of cell membrane around an axon. The electrical insulation on axons that is formed by “oligodendrocytes” in the “central nervous system” and “Schwann cells” in the “peripheral nervous system.” (Fields, 317) Allows faster and more energetically efficient conduction of impulses. (GHR) Acts as an electrical insulator that allows nerve impulses to travel faster by increasing the ‘resistance’ and decreasing the ‘capacitance’ over that found in unmyelinated nerve fibers. (NCIt) Myelin is white, which gave rise to the term “white matter” as opposed to the term “gray matter” which includes all the neuron (neuron bodies and dendrites) and short local non-myelinated “pathways.” Facilitates signal transmission along the axon, greatly enhancing and improving transmission of information within large coordinated neuronal ensembles. (Goldberg, 40) Also referred to as ‘myelin.’

Nodes of Ranvier: the ‘discontinuities’ of myelin between adjacent internodes. (Patestas, 30) Regularly spaced gaps in the myelin sheaths of peripheral axons. Allow ‘saltatory conduction,’ that is,  jumping of impulses from node to node, which is faster and more energetically favorable than continuous conduction. (MeSH) Richly endowed with "voltage" sensitive "channels." Tiny gaps in the myelin sheath. Sufficiently close to one another that an "action potential" occurring at one node can trigger the opening of voltage sensitive gates at an adjacent node. In this way, a relatively slow action potential jumps at the speed of light from node to node. (Kolb, 128) 

Neurogenesis: the process of forming neurons. Begins about 7 weeks after conception and is largely complete by 20 weeks. (Kolb, 196) The process of “stem cells” dividing and developing into functional new brain cells in the brain. That this happens in the adult human was firmly established in 1998. (Ratey, 282) The brain’s daily manufacture of new neurons. Neurogenesis is regulated by a variety of naturally occurring molecules called “growth factors.” (Best of the Brain-Fred Gage, 125) The important element ‘FGF-2’ that helps tissue grow, is necessary for neurogenesis and is increased during exercise. As we age, production of FGF-2, “BDNF” and other ‘growth factors’ naturally tails off, bringing down neurogenesis with it. (Ratey, 53) Editor's note - for “glial cells,” referred to as 'gliogenesis.’ Also referred to as ‘cell birth.’

Neuron Differentiation: process of “precursor” cells (changing) into the right type of neuron or “glial” cell. Begins about 8 weeks (after conception) and is largely complete by about 29 weeks. (Kolb, 195-196) The emergence of distinct types of cells in the brain does not result (only) from the unfolding of a specific genetic program. Instead, it is due to the interaction of genetic instructions, timing, and signals from other cells in the local environment. (Kolb, 198)

Pruning: brain process of getting rid of unneeded neurons. Occurs after birth and also unfolds at different time courses for different parts of the brain, the “frontal cortex” being the last. Pruning is akin to ‘sculpting,’ a process that the great sculptor Augusta Rodin described as ‘eliminating everything that does not belong.’ Pruning is not random, but rather is a consequence of reinforcing heavily used neural structures and letting go of those under-used or not used at all. (Goldberg, 40) During the final months of prenatal development, there is fierce competition among neurons to make connections and survive. Neurons that don’t make connections are eliminated. (Hockenbury, 377) Also referred to ‘synaptic pruning’ and ‘neuronal pruning.’