but some developmental processes, such as axon myelinization, extend into adulthood and continue until 40 years of age (Sowell, Thompson, & Toga, 2004). Then after middle adult age, multiple aging-related changes occur in the brain, which are clearly detectable by the age of 50-60: the weight and the volume of the brain decreases (Dekaban & Sadowsky, 1978); the number of neurons diminishes and their morphology changes (Masliah, Mallory, Hansen, DeTeresa, & Terry, 1993); the number of synapses decreases (Peters, Sethares, & Luebke, 2008); the synthesis of neurotransmitters and the density of their receptors decrease (Amenta, Zaccheo, & Collier, 1991; Ota, et al., 2006; Wong, et al., 1984). As a result of those aging-related changes the brain loses some of its plasticity (Burke & Barnes, 2006), which is clearly observed in the decline of certain cognitive functions and a rise in neurological disorders (Salthouse, 2009).
