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The concept of adulthood has legal and socio-cultural definitions. The legal definition [4] of an adult is a person who is fully grown or developed. This is referred to as the age of majority, which is age 18 in most cultures, although there is a variation from 15 to 21. The typical perception of adulthood is that it starts at age 20 or 21.
In response to the signals, the gonads produce hormones that stimulate libido and the growth, function, and transformation of the brain, bones, muscle, blood, skin, hair, breasts, and sex organs. Physical growth —height and weight—accelerates in the first half of puberty and is completed when an adult body has been developed.
Several motorneurons compete for each neuromuscular junction, but only one survives until adulthood. [36] Competition in vitro has been shown to involve a limited neurotrophic substance that is released, or that neural activity infers advantage to strong post-synaptic connections by giving resistance to a toxin also released upon nerve stimulation.
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The infant brain will increase in size by a factor of up to 5 by adulthood, reaching a final size of approximately 86 (± 8) billion neurons. [4] Two factors contribute to this growth: the growth of synaptic connections between neurons and the myelination of nerve fibers; the total number of neurons, however, remains the same.
The axolotl is less commonly used than other vertebrates, but is still a classical model for examining regeneration and neurogenesis. Though the axolotl has made its place in biomedical research in terms of limb regeneration, [19] [20] the model organism has displayed a robust ability to generate new neurons following damage.
Emerging adulthood is characterized by a reevaluation of the parent-child relationship, primarily in regard to autonomy. As a child switches from the role of a dependent to the role of a fellow adult, the family dynamic changes significantly.
Synaptogenesis is particularly important during an individual's critical period, during which there is a certain degree of synaptic pruning due to competition for neural growth factors by neurons and synapses. Processes that are not used, or inhibited during their critical period will fail to develop normally later on in life.