CHAPTER Brain plasticity and brain damage Chapter 4 4 Brain plasticity and brain damage Brain development in infancy and adolescence Impact of injury to the cerebral cortex and adaptive plasticity KEY KNOWLEDGE ■ the use of animal studies and neuroimaging techniques to Parkinson’s disease develop understanding of human neurological disorders ■ infancy and adolescence as periods of rapid development including Parkinson’s disease. Chapter summary and changes in brain structure and function, including Chapter 4 test development of myelin, synaptic pruning and frontal lobe Brain development in infancy and adolescence 147 development ■ the impact of injury to the cerebral cortex on a person’s Impact of injury to the cerebral cortex biological, psychological and social functioning and the and adaptive plasticity 151 ability of the brain to undergo adaptive plasticity, illustrated Parkinson’s disease 163 by rehabilitation of people with brain injuries Before neuroimaging techniques became available, In this chapter we examine the onset and it was widely believed that the brain stopped process of brain development in infancy through developing at around the age of 12. One reason to late adolescence, by which time most areas have for this belief is that the brain’s overall structure reached maturity. We then examine the adaptability of is almost complete at birth and it grows very little the brain in response to experience. In particular, we in size during childhood. consider how experience can change the brain’s Although the sizes and shapes of our brains structure, especially when damage through injury look very alike, no two human brains are actually interferes with its normal functioning. identical. Genetic information directs the course of our brain’s development and the experiences we have throughout life actively shape its structure. In BRAIN DEVELOPMENT IN this sense, our brain never reaches a point where we can describe it as ‘fully developed’. INFANCY AND ADOLESCENCE From birth through to the end of life, neurons and Infancy and adolescence are periods of rapid the connections between them change in response to development and change in brain structure and our experiences. They change to represent and store function. These occur in a genetically programmed, this information so that we can learn and remember. orderly way. At birth, the infant’s brain has just Sometimes neurons die and connections are about all the neurons it will ever have despite eliminated, especially if not needed or used. But the being only about one quarter the size of an adult’s more we repeat a thought, feeling or action, the more brain. By six months of age the brain will reach connections that are dedicated to it and the stronger about half its adult size, almost three-quarters those connections become. In some cases, the brain the size by two years of age and about 90–95% can repair itself and a healthy part can take over of its adult size by the age of 6. By the mid-20s the function of a damaged area. However, despite its or so the brain will have reached adult size by remarkable adaptability and capacity for change, the most estimates, but some parts are still maturing, brain cannot be transplanted or replaced. particularly cortical areas. 2 months 5 months Newborn 2 years 6 years 12 years 25 years (fetal) (fetal) Weblink FIGURE 4.1 Changes in brain and body proportions as we grow older. The brain reaches Video on brain full adult size by the mid-20s but some areas are still developing. development 8m 7s CHAPTER 4 Brain plasticity and brain damage 147 Brain growth and development are orderly processes, Myelination does not occur in a uniform way but this does not mean that they occur at the one pace across the cortex. Sensory and motor areas are within each individual. There are bursts and spurts, myelinated by age 3 or 4, whereas association areas most notably in early childhood and adolescence. in the frontal and temporal lobes responsible for Areas deep within the brain that are responsible for more complex functions are myelinated last (see vital survival functions develop rst. It is essential that gure 4.2). breathing, heartbeat, circulation, sleeping, sucking and swallowing are possible when the infant leaves the Synaptogenesis and synaptic uterus (Epstein, 1986; Kolb & Wishaw, 2014). Although the brain quadruples in size from birth pruning to adulthood, it is not due to an increase in the Synaptogenesis accounts for most of the brain’s number of neurons. A substantial amount of its growth in size. Synaptogenesis involves the formation growth is due to two processes — the development of new synapses between the brain’s neurons. A of myelin and the growth of new neural connections synapse is the place where neighbouring neurons through synaptogenesis and the branching of connect and communicate — where messages are dendrites. passed from axon terminals to dendrites. After birth, the neurons continue to develop new dendrites, the Development of myelin dendrites can grow more branches and each branch The growth and development of white, fatty myelin can grow spines, making the dendrites extremely around many axons through myelination contributes ‘bushy’ in appearance (see gure 4.3). The end of to the increase in brain size. This important process each of these branch-like extensions provides a site allows neurons to be more ef cient in sending where a neuron can connect with an adjacent neuron messages to other neurons (although not all axons are and collect information. ever myelinated). Generally, synaptogenesis occurs more quickly in Myelination starts before birth during fetal sensory (and motor) areas than in association areas. development and continues through childhood, It is believed that this allows the brain to have the adolescence and into adulthood. The most intense capability to respond to the constant stream of new period of myelination occurs shortly after birth. By environmental input; for example, to deal with touch this stage, axons have also grown in size. They are sensations and all the new sights, sounds, smells and longer, with denser branching at their ends because so on that bombard the sense organs. there are more axon terminals. There is also a burst Synapses in the brain begin to form long before of myelination in adolescence. birth. After birth, synaptogenesis occurs so Myelination typically emerges in the hindbrain rapidly within the rst 15 months that the total then spreads over time into the midbrain and number of synapses increases tenfold. The infant forebrain. Within the cerebral cortex, sensory areas brain forms far more neural connections through are myelinated before motor areas. This progression synaptogenesis than it will ever use. So, weak or of myelination through the brain is consistent with unused connections are ‘pruned’. This process of the overall course of brain growth and development. eliminating synaptic connections is called synaptic pruning. Synaptic pruning is considered to be the means by which the brain ‘ ne tunes’ its neural connections. It is a long-term process, continuing for many years, but in different areas of the brain at different times. Synaptic pruning also tends to occur rst in sensory areas and last in association areas. It is complete in the visual cortex by about age ten, but the elimination of dendrites can continue in association areas of the frontal lobe until well beyond age 20, eventually stabilising in adulthood around age 30. There is a burst of synaptic pruning during early adolescence, with almost half the synaptic connections eliminated at this time (Kolb & Wishaw, 2014; Spear, 2010). What is constant across different areas of the brain, FIGURE 4.2 Progress of Weblink however, is that the number of synapses in an adult is myelination in the cerebral Explanation of synaptic cortex. Light shaded areas about 40% less than the number in a three-year-old. pruning and myelination Which connections will be kept and which will be myelinate last. in adolescence 3m 37s pruned is based on experience. The connections that 148 UNIT 1 How are behaviour and mental processes shaped? At birth 1 month 3 months 6 months 15 months 24 months FIGURE 4.3 The infant brain forms far more neural connections than it will ever use. the brain then ‘prunes’ any synaptic connections that are not used in a process based on a ‘use it or lose it’ rule. have proven to be valuable and have strengthened The progression of myelination, synaptogenesis and through repeated use are retained. Those that have synaptic pruning through the frontal lobe (and other become weak or obsolete decay and disappear. The lobes) follows this overall pattern. Within the frontal entire process occurs as if the rule ‘use it or lose it’ lobe, synaptogenesis occurs more quickly in the is being followed. It is also a process that closely ties motor cortex and other areas towards the back of the experience to brain development (Gazzaniga, Ivry & lobe than it does in the prefrontal cortex at the front. Mungun, 2014; Kolb & Wishaw, 2014; Spear, 2010). Synaptic pruning and myelination also occur last in the prefrontal cortex. Frontal lobe development During the early childhood years, between the Brain development continues into adulthood until ages of 3–6 years, there is a signi cant increase in about the mid-twenties or so, with the frontal lobes last the number of neural connections established in to fully mature. The prefrontal cortex — the association the frontal lobes through synaptogenesis. As this area just behind the forehead — is the very last part of occurs, children become increasingly sophisticated the brain to mature. This is consistent with the general, in cognitive abilities, especially when compared to overall pattern of brain growth and development. Areas capabilities at birth.