What to learn about (interactions between) body, brain and health? - Chapter 5

What are the building blocks of growth and health? 

As said before, genetics and environmental factors interact to create development. Growth for instance depends partly on genetics, but a genetic predisposition to be tall cannot be realized if the environment is not nurturing enough - through malnutrition for instance, maybe through diseases like celiac disease (allergy for gluten). Treatment/bettering the environment can result in catch-up growth, thus getting back on the genetic path of growth.

Looking at the workings of systems in the body can help understanding growth:

• The endocrine (hormonal) system.  This system consists of endocrine glands that put hormones into the blood. Examples of those glands are the ovaries/testes, the hypothalamus, pancreas and adrenal glands. The pituitary gland in the brain, controlled by the hypothalamus, is critical since it triggers the release of hormones from all other glands. It also produces growth hormone. The thyroid gland is also important for physical growth and development and the development of the nervous system. Thyroid problems can lead to intellectual problems and slow growth. The endocrine glands in the sex organs are important as well. The testes give out testosterone and androgens, which stimulate growth hormone and development of sex organs and sexual motivation. The ovaries produce estrogen and progesterone, also for growth hormone, development of breasts and other female sex characteristics and menstruation. Adrenal glands give out hormones that contribute to maturation of the bones and muscles and also create sexual attraction/orientation. The endocrine system stays important through life, for example through the adrenal glands helping the body cope with stress.
• The brain and nervous system. The nervous system contains the brain, the spinal cord (central nervous system) and the neural tissue that extends into all parts of the body (periphal nervous system). They feature neurons. Neurons have dendrites that receive signals from other neurons, and the axon of a neuron transmits signals to other neurons or to a muscle cell. Neurons can make connections called synapses. Releasing neurotransmitters, neurons can stimulate or inhibit actions of other neurons. Myelination is the process of neurons becoming encased in myelin, which speeds up transmission. Myelineation causes developmental changes, like a vocabulary spurt when there is rapid myelination in the areas of the brain involved in language development.

There are three principles of growth:

• cephalocaudal principle, which states that growth occurs in a head-to-tail direction (a prenatal's head is very big compared to its body, during life a human "grows" in proportion, first the trunk and then the legs). 
• proximodistal principle, which states that muscles are developed from the center outward to the extremities (e.g. chest and organs form before hands and fingers). 
• orthogenetic principle, which states that development starts globally and undifferentiated, and moves toward differentiation and hierarchial integration (e.g. one cell becomes billions of highly specialized cells that become organized and integrated into systems). 

The life span developmental model can be applied to health, since it is a lifelong process influenced by lifelong choices, it is influenced by both genetics and environmental influences, it's multidimensional (changes in one area of the self influence other areas of the self), both gains and losses occur, and there's a sociohistorical context around all of this. 

What to learn about the infant developing in a healthy and good way?

Most brain development in infancy and childhood is about neurons making connections with each other. There is a process of synaptogenesis (growth of synapses between neurons) and synaptic pruning (removal of unnecessary synapses). The brain development is influenced both by genetics as the environment. The brain has plasticity, it responds to experiences and can develop because of them. It is also very vulnerable to damage, but can sometimes bounce back from damage by adapting (neurons taking over damaged neurons). Brain plasticity is greatest in early development, but cognitive catch-up growth is still possible when the environment improves. However the "sensitive period" for brain development is during late prenatal period and early infancy. 

A full-term newborn already has a set of reflexes (unlearned, involuntary responses to stimuli) which can be divided into two categories. Survival reflexes are for instance the breathing reflex, eye-blink reflex, pupillary reflex (pupils changing as a response to degree of light), sucking reflex (so the child can take in nutrients) and swallowing reflex. Primitive reflexes are not clearly useful then and are for instance the Babinski reflex (fanning, then curling toes when bottom of foot is stroked), grasping reflex (putting fingers around for instance mother's finger when it touches their palm), swimming reflex (moving arms and legs in water, holding breath) and stepping reflex (stepping when they are held upright and their feet touch a surface). All these reflexes let us know that the nervous system is working, and some disappear after a while and get replaced with more developed ones. 

Settling into a good sleep-wake pattern is another way to see if the nervous system is developing well and is integrating a myriad of external signals with internal states. The sleep-wake cycle gets stable around 6 months. Newborns spend half their sleeping hours in REM sleep (active sleep), and this decreases as they get older. This probably has to do with the role of REM sleep in brain maturation. 

Infant health has improved hugely since administering vaccinations. In all cases, if possible, infants should be taken to the doctor for a control visit regularly. 

What to learn about the child developing in a healthy and good way?

Growth in childhood can be characterized as slow and steady. During middle childhood (6-11 years) it seems like there's little growth, probably since the gains are small in proportion to the child's size. The brain is still developing as well. An important feature is lateralization (or assymetry and specialization of functions) of the two hemispheres of the cerebral cortex. The functions do not develop identically but diverge. The left cerebral hemisphere controls the right side of the body and is adept at the sequential processing needed for analytic reasoning, logic and language processing. The right hemisphere typically controls the left side of the body and is skilled in simultaneously processing the information needed to understand spatial information and visual-motor information. It also processes emotional content and creativity. The hemispheres communicate through the corpus callosum and they work together in all tasks. If one hemisphere is damaged the other one can take over the lost functions. The signs of brain lateralization are already visible at birth, thus it seems genetic.

Children learn to adjust to a changing environment (like catching a ball), whereas toddlers can only control their movements in a stationary world, and they refine motor skills. 

Accidents are the leading cause of death throughout childhood years. Non-fatal injuries can have a lasting influence. Also children's nutrition and eating habits should be looked at to keep them healthy, since children long for unhealthy stuff. A stressful environment can also cause "comfort eating". For instance the mother is stressed because of work, this can make children eat more (an example of Bronfenbrenner's exosystem: events in the mother's life still influence the child). Children also need enough physical activity, a challenge with the amount of "screen time" that is now usual. The environment can stimulate this physical activity, but children can also be predisposed to be "couch potatoes". 

What to learn about the adolescent developing in a healthy and good way?

The grey matter in the brain (primarly cell bodies and dendrites) increases, peaks and then decreases through the teen years. This might be because sypnatogenesis increases just before puberty, and then synapse pruning follows for a period. The white matter (clusters of axons) increases in a linear way, with myelination of axons steadily progressing. 

Adolescence is known for puberty (biological change resulting in sexual maturity and becoming capable of producing a child) and for the typical adolescent's risky behavior. This last phenomenon could come from the fact that the part of the brain involved in self-control has not yet matured. Furthermore, the reward system of the brain (e.g. in the nucleus accumbens) is very responsive in adolescence, which could lead to more reward-seeking behaviors. The frontal lobes are not fully developed yet as well, and these are essential for planning and decision making, so this could cause adolescents to not fully think through the consequences of actions. Not all teens are similar in levels of risk-taking though, depending for instance on the relationship between teen and parent. Adolescence is also known for the adolescent growth spurt which is triggered by an increase in circulating growth hormones. 

The sexual maturation process contains adrenarche, which is the increased circulation of adrenal androgens that contributes partly to sex characteristics like pubic hair. The more obvious signs emerge with increased levels of gonadal hormones (androgens produced by the testes/progesterone by the ovaries). These are primarily responsible for developing sex characteristics, and even sex feelings. The first menstruation cycle is called menarche. For boys the sexual maturation process begins with enlargement of the testes and scrotum. The first ejucatulation of a boy is called semenarche. Physical and sexual maturation are processes initated by the genes and executed by hormones, but environment also plays its part: for instance, illustrated by the secular trend (a historical trend in industrialized societies toward earlier maturation). This trend is probably explained through better nutrition, better medical care, higher obesity levels and exposure to chemicals that can alter hormone production. The role of environment is also seen in the fact that family situations can affect the timing of puberty for girls, probably since it can cause stress. 

All those adolescent changes and maturation processes can cause self-consciousness and insecurities in teens. It also influences teen-parent relationships, with more distance growing between them and conflicts arising more often.This also depends on culture. When the transition is complete, adolescents get closer to their parents gain. Parents can help adolescents through puberty by maintaining closeness and helping them accept themselves. Once again, biological changes interact with psychological characteristics and the environment to influence the experience of adolescence. 

Early-developing boys generally have a positive experience, late-developing boys find more difficulties. In contrast, early-developing girls face more difficulties and late-maturing girls seem to benefit academically. It seems pre-existing childhood differences in mental health-related factors influence whether an individual is early-maturing,  so earlier behavior problems are related to early puberty. Early-maturing girls have the greatest likelihood of experiencing long-term issues. But beliefs and attitudes play a part in how all of this is experienced, just like peer and family-member reactions.

Today's teens are unhealthier than ever, due to the "sedentary society" and eating/drinking unhealthy. The obesity rate is increasing, especially among certain ethnic minority groups, and the diabetes rate is rising too. Obesity can also influence brain function through metabolic syndrome (MeTS), a combination of risk factors associated with obesity and including high blood pressure and unhealthy cholesterol levels, since research showed adolescents with MeTS to function less good cognitively. Also, adolescents typically suffer from sleep problems, for instance through melatonin (sleep hormone) rising later at night for adolescents. However a consistent sleep-wake cycle and enough sleep time is essential for optimal cognitive skills and physical health, so it should be everyone's mission to have a good sleeping schedule.

What to learn about the adult developing in a healthy and good way?

Brain development is never really complete, since it responds to experience and is capable of neurogenesis which is the process of generating new neurons, for instance after physical or mental exercise. Even culture can alter the brain: different cultural experiences are associated with different patterns of brain activity. But though the adult brain can make new neurons, it does so at a much lower rate than a young brain.

So what happens when you age? Normal aging is associated with gradual, mild degeneration within the nervous system. This means losing neurons, other neurons functioning less well, and potentially harmful changes in the tissues surrounding neurons. Brain volume decreases over the adult years and neuron loss is greater in the areas of the brain that concern sensory and motor activities than in the association areas (linked to thought) or the brain stem and lower brain (linked to basic life functions like breathing). As an adult ages the levels of neurotransmitters decline as well, and so-called "senile plaques" are formed (hard areas in the tissue surrounding neurons, that may interfere with neuronal functioning). Also, reduced blood flow to the brain may starve neurons of the oyxgen and nutrients they need. An implication of this degeneration of the brain is that older people generally process information slower. However! Middle age comes with greater integration of the left and right hemispheres. This can increase creativity and cognitive functioning. And physical and mental exercise still means a lot for the brain. In some people the degeneration may win, but others can improve with their age through brain plasticity. There is also the scaffolding theory of aging and compensation (STAC) which says the brain may adapt to losses by compensating in other brain areas. 

Through adult life, menstrual cycles keep going. Premenstrual dysphoric disorder (PDD) is a severe form of "normal" PMS with severe complaints. There is much variability in how women experience their menstruation and how they experience the end of menstrual cycles, namely the menopause, in which levels of estrogen and other female hormones decline. This process is gradual, taking 5-10 years and can feature symptoms like hot flashes (sudden warmth) and vaginal irritation. Earlier, hormone replacement therapy (HRT) was viewed a cure for menopause symptoms. However it was found that HRT increased chances of breast cancer and heart attacks, so for many women the risks outweigh the benefits. Menopause does not seem to affect women's level of psychological problems. The presence of symptoms of menopause depends on having a history of menstrual problems, attitude and culture.

Men seem to experience andropause (called age-associated hypogonadism as well) which is linked to slowly decreasing testosterone and symptoms like low libido, lack of energy, erection problems and memory problems. An andropause is more gradual, more variable and less complete than a menopause.

Again, in adulthood, obesity is an increasing problem. As we all know with older age more health problems arise, like diseases or other chronic impairments. An example is osteoporosis, a disease in which loss of minerals leaves the bones very fragile. An elder person that has fallen and broken something will probably be more anxious and careful, but this can make them more vulnerable since muscle and bone then decrease further. Preventing osteoporisis can happen through dietary habits (having more calcium), weight-bearing exercises like walking, and HRT. Another thing that comes with aging is the gradual decline in the efficiency of most bodily systems so almost every physical function gets more troubling. And there's a decline in the reserve capacity of many organ systems (thus, their ability to respond to demands for extra output, like in emergencies). Through looking at things like the strength of a handgrip or walking speed in aging adults, their wellness as elderly can be predicted. Wellness of eldery varies much, depending on their fitness and activeness. Regular exercise by older adults can delay the onset of physical problems by up to 7 years. Centenarians are people who live to be 100 or older.

To answer why we age and die, two main categories of theories have emerged:

• Programmed theories of aging suggest that aging follows a predictable, genetic and species-specific timetable with a maximum life span. A promising theory of this kind discovered that cells from human embryos could only divide a certain number of times, namely 50, plus or minus 10 (this is the Hayflick limit), and cells from adults even less. And the maximum life span seems related to the Hayflick limit. The mechanism behind this is telomeres (DNA that forms the tips of chromosomes and that shortens with every cell division, so that eventually a cell can not divide anymore). Chronic/post-traumatic stress can relate to shorter telomeres, just like lack of exercise, smoking, obesity and low SES. Manipulating genetics can lengthen life according to this theory. This could for instance be done by administering the enzyme telomerase which prevents the telomeres from shortening and thus causes cells to keep replicating, however this could also cause cancerous cells to multiply more rapidly. The role of individual genetic makeup in longevity also supports these theories.
• Damage/error theories of aging suggest that an accumulation of random damage to cells and organs over the years ultimately results in death. After "use and abuse" of a certain number of years, we might just be worn out, like cars. One leading error theory states that damage to cells that compromises their functioning is done by free radicals, which are toxic and chemically unstable by-products of metabolism or the everyday chemical reactions in cells like those involved in the breakdown of food. Free radicals are created when oxygen reacts with molecules in cells. They have an unpaired electron (so can steal electrons from other cells) which can be damaging to other molecules in the body, like DNA, and the genetic code is damaged. Eventually cells lose their ability to function and the organism dies. The damage of free radicals is visibly shown in age spots on the skin, they also seem to relate to diseases like Alzheimer's and cancer and they feature in the aging of the brain. So the damage of free radical should be prevented. Antioxidants like vitamins E and C can unstable free radicals by safely pairing them with their electron. Resveratrol is an antioxidant of interest, which effects can be similar to the effects that come from caloric restriction (nutritious but restricted diet with a very big cut in caloric intake). Caloric restriction can extend length of life and slow the progression of age-related diseases, by reducing the number of free radicals and other toxic products of metabolism. A restricted diet seems to alter gene activity and trigger the release of hormones that slow down metabolism and protect cells against oxidative damage. A lot of extra research has to be done on this subject.

 Again, it seems many factors interact and again, nature and nurture both influence aging and dying. And we can't avoid the biological reality of aging, but we can make choices to better and lengthen our life, through physical and mental activity for instance, and by having a positive attiutude.