Brain Development: Synaptic Plasticity, Sensitive Periods, and Epigenetics

Synaptic Development and Plasticity

Early Brain Development: Elaboration of Neurons and Synapse Formation

After birth, the increase in brain weight is primarily attributed not to the creation of new neurons, but to the elaboration of existing ones. This includes axons becoming longer, an increase in dendritic branches, and the growth of more connections, with each dendrite becoming densely packed with synapses. Similar to the early overproduction of cells and neurons, the brain initially forms an excess of synapses, following the principle that it is better to have more than needed than to lack necessary connections. This excess is part of a developmental strategy.

Synapse Rearrangement and its Significance

Synapse rearrangement is a crucial stage of development involving the remodeling of these connections. It means the brain essentially 'rewires' how neurons communicate, leading to the loss of some synapses and the development of others. This process is continuous throughout life, as new synaptic connections are essential for obtaining and learning new information. However, retaining too many synapses that are not being used or strengthened can be problematic.

Fragile X Syndrome: An Example of Defective Synapse Elimination

Fragile X syndrome is an inherited intellectual disability caused by a fragile or damaged site on the X chromosome, where the DNA is unstable and prone to breaking. This instability results in cortical neurons possessing an abundance of small, immature dendritic spines, leading to many immature synaptic connections. These numerous unrefined connections disrupt the efficient transfer of information throughout the cerebral cortex, impairing intellectual development. This condition exemplifies how blocking the normal elimination (pruning) of unneeded synapses after birth can severely impact mental development. The strengthening of synaptic connections occurs when a presynaptic neuron repeatedly sends chemical messengers to a postsynaptic neuron. If these connections aren't strengthened, they become like unused