VOL: 99, ISSUE: 17, PAGE NO: 28
William T. Blows, BSc, PhD, RMN, RGN, RNT, OStJ, is senior lecturer (applied biology), City University, London.
The human brain is formed before birth but continues to develop for at least another 20 years. During that time, specific skills are attained, although there are times when development does not take place as normal. An understanding of normal development will help alert nurses and carers to potential problems in a child’s cognitive development and to understand the reasons behind specific behaviour patterns.
The brain is entirely enclosed within the skull, and relies on the sensory nervous system to bring it information about the surrounding world. This information is then used by the brain to shape its connections and neuronal pathways. It is thus true to say that environmental experiences mould the mind.
A serious loss of any sensory system, or the exposure of the senses to severe adverse stimuli, can have profound and often permanent effects on the way the brain functions and perceives the world. Sensory deprivation and environmental insults such as drugs or alcohol at key stages of development can have a permanent detrimental influence on brain development and function.
Before birth: a time of neuronal migration
This is the time when the brain is under construction, that is, when the original neural tube becomes a recognisable organ system: the head end of the neural tube expands to form the brain; the tail end becomes the spinal cord, while the tube’s lumen becomes the ventricular system filled with cerebrospinal fluid.
The neural tube forms by the end of week three of gestation. Cells destined to become neurons replicate by mitosis to create 250,000 new cells per minute. At birth, the brain has 200,000,000,000 (200 billion) neurons, after which the creation of new neurons drops dramatically, and is restricted to specific brain areas only.
In the fetus, new neurons are formed in the deeper layers of the tube, near the lumen, and then migrate from there to a point closer to the outer surface. Three waves of migration take place over a three-month period during the second trimester of pregnancy. This creates three main cell layers within the cortex (outer shell) of the brain. The position of each neuron is critical and irreversible (Blows, 2003:168). Once in place, the neurons begin the complex task of making synaptic connections. Incorrect routing of migrating neurons causes parts of the brain to be in the wrong place at birth, with no possible means of correction, and this leads to inadequate or wrong synaptic connections. It is now recognised that wrong synaptic connections may be involved in some mental health disorders such as schizophrenia. Already some ‘symptoms’ of pre-schizophrenia are recognised (Yakeley and Murray, 1996; Blows, 2003), therefore, if nurses, midwives and health visitors were to make routine observations of babies from birth onwards for signs of such problems, those children at risk of developing schizophrenia later in life could be closely monitored and treated early. Unfortunately, nothing can be done to correct the problem once the neuronal migrations have happened, but it may be possible in the future to identify families where the genetic risk of schizophrenia is high, so making schizophrenia preventable.
Brain sexual orientation
The sexual orientation of the brain depends on two factors: genetic heredity patterns and perinatal exposure to hormones - androgens in males and oestrogens in females. Perinatal exposure of the brain to androgens from the adrenal cortex in males causes the development of a larger pre-optic (anterior) region of the hypothalamus than in females. Because the anterior hypothalamus shows significant sexual dimorphism (variation between the sexes) in the pattern of synaptic connections found there, this part of the brain has been called the sexually dimorphic nucleus. This is significant given that the hypothalamus is involved in sexual behaviour from puberty.
Male brains are physically larger than those of females, but women have a greater concentration of grey matter cells (or neurons) in the areas concerned with communication - the dorso-lateral pre-frontal cortex (involved in memory and initiative, 23 per cent more cells in females) and the superior temporal gyrus (involved in listening, 13 per cent more cells in females). Table 1 indicates those skills with which the male and female brain is more competent (Kimura, 2002).
In female brains there is more overlap between the functions of the hemispheres in verbal skills than in males. Women, therefore, do better in these skills than men. There are also other physical differences in the brain between the sexes, the significance of which is not always understood. For example, females have longer temporal lobes than males; and in females the posterior corpus callosum (connecting the left hemisphere with the right hemisphere) is bulbous and wide compared to that in the male which is cylindrical and uniform in width throughout its length.
Occasionally, the sexual orientation of the brain is not in consort with the sexuality of the body, in which case a crisis of gender can manifest as the child grows older. Parents who are not conversant with the problem may suppress the child’s preferred mental orientation in favour of his/her physical state, and thus may worsen problems during childhood. This could affect schooling, interpersonal relationships and family life. Careful counselling by health care professionals and detailed medical investigations, coupled with sensitive handling of the problems by the parents and professionals alike, mark the way forward.
Birth trauma caused by obstetric complications may starve the infant brain of oxygen or glucose long enough to cause neuronal malfunction and damage. Neurons are very sensitive to oxygen and glucose levels, and require a rapid changeover from placental oxygenation to pulmonary ventilation in order to maintain adequate oxygenation, and early feeding after birth so as to maintain blood glucose levels. Anything that causes prolonged absence of oxygen or glucose may be detrimental to further brain development. Prevention of birth trauma, therefore, whenever possible, is fundamental to normal brain development.
Birth to five years: growth and exploration
Rapid physical development in this period indicates the great changes taking place in the brain. The maturity of the brain is most advanced in its lower segments (brain stem) and least advanced in its upper segments (cerebral cortex), a situation that is sustained until adulthood is reached. Brain stem maturity at birth is vital, since it is this that allows the newborn to sustain its own life. The main life-supporting reflexes present at birth are the conscious state; breathing; feeding and digestion; elimination and sleep, plus the major sensory inputs, for example, vision, hearing and body (somatic) sensations.
From birth onwards three major factors guide brain development: myelination, synaptogenesis and plasticity.
Myelination of neuronal axons is well under way at birth, more so with the sensory system than with the motor system. The cerebral cortex cells are largely unmyelinated and remain so for many years.
Myelination of the nervous system proceeds gradually during the first year and beyond in a head-downward sequence. Increasing myelination of the motor system allows the infant to ‘do’ more; for example, lift the head, reach out, roll over and crawl, and eventually walk and run (Shaffer, 2002).
Motor skills are dependent on practice, which allows sensory feedback to the brain on performance. Sensory ‘feedback’ eventually gives way to ‘feed-forward’, where motor skills have been achieved and are carried out without conscious effort. Nurses and parents will monitor for key stages in motor development, such as turning, sitting, crawling and standing so that late development of these skills can be investigated.
Synaptogenesis, the formation of synaptic connections, continues from the second trimester of pregnancy throughout childhood and into adult life. Synaptic pruning, the process of removing unwanted (or unused) synapses is also happening at this time. The choice of synapses for destruction depends on the environmental stimulation; thus those not stimulated or used will be lost forever. Environmental stimulation reinforces and strengthens those synapses involved, and these are the synapses that will last for years, forming the basis of memory.
Plasticity is the brain’s ability to change, adapt and cope with environmental changes, and to be able to compensate for losses. Synaptic pruning retains enough unused synapses to allow the brain to evolve, adapt and to switch or develop functions as the need arises (Shaffer, 2002).
Synaptic pruning makes it clear that brain development is dependent on all forms of mental stimulation from the outside world. In the introduction to this article, the point is made about the isolation of the brain, except through the sensory nervous system. Early experiences in visual, aural and tactile stimulation allow for the preservation of more synapses, which in turn improves brain function and allows a greater degree of plasticity. This highlights the need for play and parental bonding, and communication and mental stimulation during these early years. Music, particularly classical music, is rapidly becoming recognised as a vital stimulant for early brain development. Making the most of the sensory nervous system in babies promotes synaptogenesis. This and the plasticity of the brain make the recovery from brain injury quicker and easier in children than in adults.
The cerebellum gradually learns its functions of balance and equilibrium, smoothing muscle activity, synergy (co-ordination of eye and hand movement), and muscle tone regulation. These activities, essential as they are for locomotion, rely heavily on sensory feedback from the semi-circular canals (vestibular sensations), and from muscles and joints (proprioception sensations), to the cerebellum.
Balance is a major achievement of the cerebellum as the infant attains the unaided erect posture necessary for bipedal locomotion. Identification by parents and health care professionals at what point these skills are demonstrated for the first time by the child keeps a check on cerebellar development.
A brain growth spurt occurs over the first two years of life. At birth the brain is about 25 per cent of adult weight, but is 75 per cent of adult weight by two years of age. During the first year of life the brain gains 1.7g a day in weight. This growth increase is due to replication of glial cells, especially astrocytes, and developments in myelination and synaptogenesis (including synaptic pruning), especially of the cortex (Shaffer, 2002).
Lateralisation, the processing of assigning functions to either of the cerebral hemispheres, is well under way by birth and continues into childhood (Table 2).
Speech development is based on maturation of both the Wernicke’s (language) and Broca’s (motor speech) areas of the left cerebral hemisphere. As these mature, both the vocabulary and the use of language increases, but this does depend on sensory input (that is, the vocabulary and language use to which the child is exposed) (Table 3). Infancy is often defined as the period from birth to the development of a language.
A wide range of genetic and chromosomal disorders can result in varying degrees of mental retardation, or even arrest of further mental development, especially rare gene defects on the X chromosome. Disorders including autism (Aylott, 2000), Rett syndrome, attention-deficit hyperactive disorder, and Tourette syndrome are now widely believed to have a genetic basis (Folstein et al, 1998; Blows, 2003:ch 5).
Genetic screening and counselling of affected families by health care professionals will become increasingly more important as genes are discovered and genetic testing is made more available. Management of familial genetic disorders requires sensitivity on behalf of the health care professional in understanding the dilemmas facing the family. This applies particularly when families must answer questions about whether or not to have children. In order to facilitate the choices open to the family, knowledge of the science of genetics is as important to the nurse or counsellor as an understanding of the management options.
Five to 10 years: the early education
This period is critical for the moulding of the brain, and education is the key factor in this process. Most of the brain’s synaptic connections are by now in place, but they need reinforcing to establish permanent neuro-logical pathways. This is achieved by further myelination, strengthening synapses and by utilising plasticity.
Education plays a vital role in the process of moulding the brain, as those synaptic pathways that are used regularly will be reinforced, and this forms the basis of learning. Rehearsing knowledge (such as learning spelling) creates synaptic connections that will last as long-term memories for life. Synaptic pruning ensures that established connections become efficient, without the clutter of unwanted synapses. These are the best years for learning; for example, music skills are best learnt up to the age of nine years. Music training in pre- and junior school children is the best way to develop the type of neural connections needed for abstract reasoning, science and mathematics later in life (Anon, 2002).
A lack of education during these years results in deficits in learning that are likely to be a problem for life. For this reason, children in hospital for long periods should be provided with teaching in the wards. Changing schools can also be disruptive to learning, especially if this happens regularly - children of travelling parents, for example. A long period of stability at the same school is desirable for the best brain development. Other factors affecting learning, such as persistent bullying at school or an unhappy home life, will take a toll on both the brain and intellectual development.
Fine-tuning of motor skills continues throughout childhood, with the development of motor skills programmes (rather like computer programs), which are stored as long-term memory in the motor association cortex. These programmes allow skills such as riding a bicycle to become second nature. Developing motor programmes involves repetitive practice of the motor skill, with feedback on performance, and usually forms part of the education curriculum at school, for example, hand-writing, gymnastics, football or learning musical instruments.
Child abuse, be it physical or psychological, has very severe implications on brain development. Detrimental changes in the pattern of synaptogenesis and plasticity occur as a result of the stress from abuse, and these changes have life-long consequences. Child abuse is a menace that threatens the mental capabilities of our next generations (Teicher, 2002; Blows, 2003:142).
Ten to 20 years: adolescence
Significant changes continue to take place in the adolescent’s brain until maturity at about 20 years. Most of the changes are made to improve the functional efficiency of the frontal lobes and to shift the emotional emphasis away from the more primitive limbic system towards the sophisticated frontal lobes as these come ‘on line’.
Myelination of the neurons of the frontal lobes is incomplete until the mid to late teens, or even into early adulthood (the frontal lobe is the last part of the brain to mature). As myelination progresses, more of the frontal lobes contribute to brain function, and this gradually increases the individual’s attention spans and improves the speed of processing information, both of which then improve with age.
As the frontal lobe’s role in emotions increases, there is a shift in emphasis away from the amygdala for emotional responses to the frontal lobes. This means that the raw, unmodified emotional responses generated by the amygdala gradually give way to the more reasoned and sophisticated responses of the frontal lobes. But this switchover in function is not without consequences: there now occurs a dramatic slump in the adolescent’s ability to recognise other people’s emotions: at about 11 years of age, an individual is 20 per cent slower in recognising anger or happiness than someone younger. It can make the teenager grumpy, ill-tempered and petulant, with confused emotions. The problem improves annually until, by the time an individual is 18 years old, his/her emotional abilities are fully restored.
Two brain growth spurts occur in relation to puberty. The first (13-15 years of age) increases the size and function of the brain, especially of motor areas and spatial perception activity. The second (17 years onwards) increases frontal lobe size and its connections with the rest of the brain. The final adult brain weight of between 1300g and 1400g is achieved in the late teens.
A major change in memory occurs between 13 and 25 years of age, when memory is at the best it will ever be. This memory peak is called the ‘Reminiscence bump’. The hippocampus (the site of short-term memory) shows considerable plasticity during this time (Carlson, 2001). Cognitive improvement begins about 12 years, and continues into adulthood. This involves using wider cognitive strategies, increased reasoning, hypothetical thinking and systematic problem-solving. After 25 years of age the memory starts to decline, but it can still remain good throughout life.
In boys, testosterone levels during puberty influence behaviour patterns, particularly with regard to sexual behaviour, mood and aggression (Carlson, 2001). In girls, the influence of oestrogen on brain function is less well understood but it appears to have a calming effect on the brain, reducing aggression. Low oestrogen (or higher than average testosterone) in girls is more likely to cause aggression, and females affected by this are sometimes called ‘tomboys’ (that is, girls born to high testosterone mothers). The implications of these hormonal effects are that during puberty, when hormones are both high and possibly unstable, and the frontal lobes are less than mature, obscure and irrational (or even criminal) behaviour may be demonstrated. It is unreasonable to say that these changes excuse these individuals for their behaviour, but it is certainly a driving factor, and cannot be ignored.
Alcohol and drugs
Alcohol and illicit drugs taken during adolescence are likely to disturb brain (particularly frontal lobe and hippocampal) remodelling, and any change will be permanent. These changes may seriously affect mood, emotional responses, thinking, reasoning, judgement, interpersonal skills, memory and attention (Carlson, 2001). The effects worsen with every additional drug or alcohol exposure, but even a single drug exposure (for example, to pethidine or Ecstasy) is shown to cause significant life-long changes or damage (Brown et al, 2000).
Researchers in this field suggest that large numbers of current and future teenagers taking drugs or excessive alcohol will result in an entire cohort of adults with impaired mental skills. As always, prevention is the better course of action. Parents, educationists and health care professionals should take the lead in teaching older children about the hazards of even single bouts of drinking or drug exposure.
The human brain has a long period of development - up to at least 20 years. During this time, specific skills are attained which allow parents and health care professionals to note the milestone in a child’s development. Identifying deviations from normal patterns of development will alert nurses and carers to the possibility of potential problems in the child’s cognitive development, and help them to understand the reasons for specific behaviour patterns that may occur.