Some basics of human biology

Authored by Henry Strick van Linschoten

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While the human mind cannot be reduced to physiological properties, psychology is grounded in biology. Psychological understanding and psychotherapeutic practice are thus significantly enriched by a good knowledge of biology.

Modern biology, the “science of life”, is informed by a number of principles that give a strong unity to the subject:

  • Life has a cellular basis
  • The diversity of life forms can be explained by evolution
  • Living organisms preserve homeostasis, i.e. relatively constant internal conditions that are different from the environment
  • Life is best understood by a systems view at many levels of complexity, e.g., the molecular level, elements that jointly have one function, organ systems, ecosystems.
  • Living organisms respond to their environment and need energy and raw materials to continue living
  • For a type of organism (species) to survive over time, it needs adaptive processes of growth, development, reproduction and evolution
  • In order to adapt to its environment, an organism needs to obtain information about its environment, process and integrate all the information it has and take actions in response, acting on the environment in turn

Living things are complex, and can be described and studied at many levels:

  • molecular
  • cell
  • tissue
  • organ
  • organ system
  • organism
  • population and community (ecology)
  • ecosystem
  • the biosphere

Living organisms are also characterised as containing substantial numbers of organic molecules (molecules containing carbon and using covalent bonds). Some of the most used and essential molecules are:

  • fats and sugars for energy storage
  • phospholipids for cell membranes
  • nucleic acids (RNA and DNA) for instructions about growth processes in cells
  • proteins for transport, catalysis and a range of other functions

Proteins are long and complex chains of smaller building blocks called amino acids. Relatively short chains of amino acids are called polypeptides.

Organ systems are groups of organs which are classified together on the basis of the broad functions they serve, ultimately either for survival of the organism or the species. This paper will describe several organ systems, but not give much detail about the skin, skeletal, muscular, respiratory and urinary systems, important as they all are.

The cellular basis of life

The smallest unit of life, seen as perhaps the most characteristic property of life, is the cell. In the modern view of biology all life is seen as consisting of cells. The human body is estimated to have between 10 and 100 trillion cells. There are many specialised types of cells – the human body has around 210 types (Raven et al., 2008).

This complexity is typical for animals, even small ones. A thoroughly studied roundworm (nematode), the about 1 mm long C. elegans, has 959 somatic cells in the adult hermaphrodite form, and 302 neurons in its nervous system. These have been completely mapped, as has been its genome, which has six chromosomes and a mitochondrial genome. It is an example of one of a few “model organisms” that have been exhaustively studied and which help in the understanding of larger and more complicated animals.

Cells are the smallest units of life that can replicate (reproduce) independently. Two fundamental distinctions about organisms are whether they are unicellular or multicellular, and whether they are prokaryotes or eukaryotes. All eukaryote cells have a nucleus. In a eukaryote cell most of the DNA is contained in the nucleus. All animals, plants, fungi and algae are eukaryotes. The first (prokaryote) cells emerged on earth at least 3.5 billion years ago; eukaryote cells at least 1.5 billion years ago.

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Evolution offers an explanation for the phenomena of differences and change amongst living organisms, especially diversity amongst members of a species at one time, and variation through the generations. Differences and change are ubiquitous, between species, amongst members of a species, over time, over shorter, longer and very long time periods. The widest concept of evolution can be defined as any heritable changes in the frequency or distribution of traits in a population, no matter what mechanism makes those traits heritable. (Pigliucci & Kaplan, 2006) This is wider than the traditional gene-centred reference to changes in the allele (alternative version or state of a gene) frequencies of a population. Change may be smaller or larger, quick or slow. Evolution is a matter of historical results, the observable outcome over time of many processes. One of those processes can be natural selection. Even when natural selection takes place, it can only have an evolutionary impact if the traits selected are heritable, genetically (via its impact on DNA sequences transmitted from parent(s) to child) or otherwise.

Evolution assumes by definition that there are mechanisms of heredity, but does not depend on knowing what they are. Indeed, in the 19th century most concepts of heredity were wrong – Lamarck was wrong (the theory that traits or behaviours acquired during adult life could be passed to one’s children via heredity); Darwin’s pangenesis theory (the whole of the parental organism would participate in heredity, through tiny heredity particles called gemmules) was similarly flawed. Only Mendel developed a better concept (mid 19th century) proposing that traits are inherited, resulting in fixed proportions in the next generation, but he was unread for 50 years.

In the course of the 20th century much progress has been made in understanding the properties of genetic inheritance (the rediscovery of Mendel), and the material of genes – DNA, its structure, function and potential modification. However, there remains a barrier in understanding how proteins influence disease processes or behaviour, in contrast to the very detailed understanding of how DNA instructions lead to the manufacture of the proteins (Rutter, 2006).

The most studied and probably most generally operative process leading to evolutionary change is natural selection. It works as follows:

  • There is a population consisting of organisms which differ from one another in a particular trait
  • The trait can be inherited from one generation to the next (the way in which it is inherited does not matter); inherited means that offspring are more like one or both of their parents in the trait than the average of the population
  • The differences in the trait make organisms with certain traits more likely to (survive and) reproduce than others
  • Natural selection takes place: over a number of generations the presence (frequency) in the population of the variant of the trait that is more successful, and their genetic or otherwise heritable basis, will increase

Of these different aspects of the process the origin of the first step, the cause for variation or innovation, is the most controversial (e.g. Müller, 2010). Once there are differences, especially when these are significant differences that materially affect the probability of survival and reproduction, the other steps will tend to work as expected. But a number of the major controversies in thinking about evolution are linked to the question how the initial difference in DNA has come about; whether the difference is big or small (satltationism, punctuated equilibrium or phyletic gradualism); and whether the more important changes are likely to be or have been random mutations in genetic material (DNA), were initiated by environmental events, or by the developmental processes in which the genes interacted with the environment in producing proteins. It may be useful to regard this overall question unsettled scientifically, and be open to at least some of the different possibilities.

In any real-life population, it is highly unlikely that the frequency of traits will remain stable over the generations. Reasons for change in frequency other than natural selection may be:

  • Mutations
  • Non-random mating (e.g., phenotypically similar individuals may be more likely to mate with each other)
  • Migration in and out of the population (gene flow)
  • Genetic drift, an effect occurring by chance, especially in smaller populations
  • Artificial selection, e.g. selective breeding

Even when natural selection is the main engine behind a particular case of evolutionary change, it may be in response to environmental change, e.g., the need to avoid newly introduced predators, the introduction of toxic substances, or to deal with climate change.

There is a lot of unsettled controversy about evolution, and the major mechanisms that lead especially to long-term evolutionary significant change – which is of course the question evolution theory is supposed to answer. As these questions keep coming back with practical consequences about the inheritance of characteristics and behaviour of human beings, it may be useful for psychotherapists to take an interest. The following are a number of high-level books that describe some of the main controversies that are important at the moment: Pigliucci & Kaplan (2006)Oyama et al. (2001)Neumann-Held & Rehmann-Sutter (2006)Jablonka & Lamb (1995)Jablonka & Raz (2009).

To summarise some major conclusions:

  • There are no scientists who deny the influence of the environment on the nature and characteristics of organisms. The questions are how and how much, not if.
  • Preformationism, the old idea that organisms grow from miniature versions of their fully grown state, is not supported (but see Oyama, 2000).
  • There are very few serious defenders of an absolute determinism, genetic, environmental, “biological” or otherwise. Any currently existing theory accepts that outcomes will be probabilistic, that there are random elements in whatever processes are described as explanations, and that new scientific knowledge about processes usually leads to ways of influencing those processes.
  • Excluding supernatural factors as explanation is a question of choice. The general biological discourse excludes these. But if someone wants to subordinate or incorporate science (in)to a religious or spiritual system, they have the right to do so.
  • There are very few scientists who would disagree with a central role for DNA in inheritance and genetics, and a central role for natural selection as the explanation of a great deal of evolutionary phenomena. There continue to be arguments about alternatives, about relative importance, about the most useful ways of defining key concepts, etc., but they all tend to leave this major place to natural selection in evolution, and to DNA in inheritance systems.
  • Evolution can be studied at all biological levels, at those of genes, organisms, species, and groups.

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Please also refer to our module paper Controversies: Genetics

Genetics is the science of heredity. It studies the variation in the traits (characteristics) of living organisms, and how variation relates to the reproduction of organisms (with changes) from one generation to the next. A gene is the smallest physical unit of heredity. It is generally identified with a sequence of DNA that is enough to code for a single polypeptide or protein.

The words genotype and phenotype are much used in genetics. The genotype is the genetic constitution of an organism or the allelic composition of one or a few genes under investigation relative to a trait. The phenotype is the observable properties (physical appearance or functional expression) of an organism or a trait. It is important to note that a phenotype undergoes development – always has its own ontogeny; any phenotype has its own developmental history.

Trait is used for the characteristics of an organism, either (physical) structures or behaviours. Many traits are not inherited, e.g. dyed hair, a surgically removed kidney or a scar.

Genetics is about the mechanism for change passing down through generations of reproducing organisms, not at the species-level, which is studied by evolution theory, but at the level of the individual organism. If a phenotype changes through mutilation, or through learning something, this is not in general of interest to geneticists, as these are not changes that will be passed on. This is the question that Lamarckism tried to address, as well described in Jablonka & Lamb (1995).

Classical (“Mendelian”) genetics believed that heredity could only be transmitted by nuclear genes located on the chromosomes of both parents (meiosis) or one parent (mitosis). This belief was already held by some geneticists before the mechanism or even the identity of DNA had been discovered. It is now well-known that there are many other methods (“developmental systems” or “developmental mechanisms”) through which traits can be passed on through the generations. One other ingenious possibility is that the balance between DNA-genetic and “other” processes may have been different at earlier stages of evolution than it is now, as in Newman & Müller (2006). The epigenetic details which widen the purely DNA-centred position are now described in complete textbooks (e.g. Allis et al., 2007), and take a major place in up to date general textbooks about genetics (e.g. Klug et al., 2013). An article discussing non-DNA-based epigenetic inheritance is Jablonka & Raz (2009).

Genetics is often seen as controversial. Few people would doubt the clear role that heredity plays in the inheritance of traits of plants, bacteria, fruit flies and other insects. Without the increase in yield of major cereals and rice during the Green Revolution between 1960 and 1980, hundreds of millions of people would have died. This was the result of selective breeding and crossing, and preceded the current capabilities of genetic engineering. Equally most people recognise the long record of horses, dogs and cats being bred artificially to achieve a special appearance or qualities such as speed or endurance, but also temperament. Dogs have been bred for a long time specifically for certain kinds of behaviour (Plomin et al., 2013). For all these activities genetics, and the enormous progress achieved by genetics between 1900 and the present day, has increased their effectiveness. The non-biologist reader is urged to read Bürglin (2006), which gives an introduction to how genes function in a tiny animal, the nematode or roundworm C. elegans, which has about 20,500 protein-coding genes (Bürglin, 2006), and of which the cell structure and the nervous system are known in great detail. Whilst this animal has about the same number of genes as humans (Pennisi, 2012), it has far fewer base pairs of DNA (about 100 million vs 3 billion for humans).

Turning now to human beings, many people accept that eye colour, blood type and height of humans have a large heritable component. For a number of diseases there is a good deal of knowledge about their genetic origin, e.g., phenylketonuria; Huntington’s disease; Down’s syndrome; type-1 diabetes; sickle cell anaemia. There are specific less common forms of early onset Alzheimer’s disease and of breast cancer which are strongly heritable. For all these the cause lies in genetical defects or abnormalities, but for standard trisomy 21 Down’s syndrome the abnormality in the DNA of the child is not found in that of the parents. A rare variant of Down’s syndrome is however heritable. There are also diseases such as leukaemia, which is rarely considered as heritable.

Some of the above diseases involve a single gene, others several or many. There are far more heritable traits that are polygenic (i.e., involve multiple genes) than are monogenic, although that still leaves many thousands of human diseases that are held to be monogenic.

Information similar to the one given in the above examples exists about a number of mental disorders. The special field within genetics studying these is called behavioural genetics. Two source books about this are Plomin et al. (2013) and Rutter (2006).

There is considerable controversy about behavioural genetics. Some critical assertions made by its opponents are as follows:

  • Behavioural traits, including mental disorders, can never be inherited
  • Even if there would be some heritability, it is bound to be very small
  • It is not possible to determine whether behavioural traits can be inherited or not, and certainly the degree or strength of heritability cannot be measured, as the methods used are incoherent
  • Even if heritability could be determined, this is it not useful
  • It is impossible to distinguish between genetic and environmental influences
  • Mental disorders do not exist, or cannot be coherently defined, or at least so far it is not known how to do so

In practice, a majority of biologists, medical practitioners and of course geneticists, and probably many psychiatrists, agree that there are considerable practical and methodological problems, but that they are not sufficiently different from those for any genetic research. In addition, it is widely believed that over the years the genetics field will make substantial progress in what it can do. There is a particular interest in research that has been done and that continues in genetic influences on autism spectrum disorder, drug and alcohol abuse, bipolar disorder and schizophrenia, which, partly based on twin and adoption studies, are widely held to have a substantial (but very complex, polygenic and mixed with environmental and epigenetic influences) degrees of heritability.

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Embryology and the prenatal period

Animal embryology is a fascinating discipline, which, starting on modern principles in the first half of the 19th century, studies the early development where one zygote (a fertilised ovum or egg cell) develops up to its fetal stage. As part of the multiplication of the cells, they also become differentiated, allowing the formation of patterns that include positioning of the multicellular embryo so that specialised cells continue growing in specific locations.

There is a sequence of significant stages in the prenatal development of mammals, with the new animal referred to first as embryo, then as fetus (weeks are from fertilisation):

  • the blastula (blastocyst for mammals) stage (characteristic of and limited to animals)
  • the gastrulation period
  • organogenesis, for vertebrates starting with neurulation (for humans starts around days 18-20, and shows a clear separation between forebrain, midbrain, hindbrain and spinal cord at the end of week 4)
  • the embryonic stage (until 8 weeks for humans)
  • the fetal stage (for humans from 8 weeks until birth)

The period of gestation (counted from the last menstruation) varies for mammals from about 20 days in mice to 21-23 months for elephants.

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Phylogeny and ontogeny

These words were coined by Ernst Haeckel in 1866 and continue to be much used by biologists. Ontogeny (sometimes ontogenesis) is somewhat equivalent to the more general word development, but is intended to start at conception, and is used to emphasise that it includes the development as influenced by all conceivable biological, cultural and social factors. It is only applied to living beings. Although less often stated, it only ends with the death of the organism.

Phylogeny (sometimes phylogenetics) is the development, studied in all its aspects, of a whole species, through generations, – including the major element that genetics plays in development, together with environmental influences and events.

The contrast is important; for both there are elements of genetic and environmental influence, but they interact and combine very differently whether the species is studied long-term, or a single individual over its lifetime.

Haeckel is known for the idea that “ontogeny recapitulates phylogeny”, also known as recapitulation theory. He thought that it would be possible by studying the development, especially the early embryonic development, of organisms to derive valid conclusions as to how the species had developed. As a general theory this has been thoroughly disproven, but studying early development of embryonic development remains worth while, as it brings out species similarities (homology and homoplasy) and their place in evolution. Sigmund Freud was fond of these ideas, and used them repeatedly in his writings.

Gould (1977) describes the development of ideas about ontogeny and phylogeny.

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Epidemiology is a fundamental discipline for public health, prevention, and the understanding of society-wide causes and wider connections about mental health. This field has enormously grown in the last 30 years, and is of importance for psychotherapists, as it gives answers and approaches for a number of issues of direct relevance, including the usage of drugs, views of causation, ideas about what evidence means and where it can be found. It would be a good complement to these papers to read at least one short introduction to epidemiology such as Rothman (2012).

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A system or organism is in homeostasis when certain variables of its internal environment are maintained at a stable and relatively constant level. It is a fundamental quality of living organisms. Typically an iron atom will not be structurally effected, even under extreme physical circumstances: it may react with and form molecules with other atoms, it may vaporise, but it does not cease to exist. Only under extreme conditions of radiation could it be split. But any living organism, even simple unicellular ones, needs to maintain certain internal parameters, usually at least connected with its fluid balance, to maintain its integrity. If the homeostasis breaks down, the organism dies.

For multicellular organisms especially, and even more for the large multicellular organisms represented by animals, homeostasis of a range of internal properties within narrow limits is essential for survival. Mammals have a long list of variables that need to be regulated and stay in homeostasis, such as body temperature, the concentration of a long list of substances in the blood and in other body fluids, blood pressure and the number of hours slept per day. For humans one of many examples is that of body temperature. A human person can survive in a range of external temperatures, but the internal temperature that is an indication of health lies in a range of less than 1°C around perhaps 36.8°C, and internal temperature must be within a range of less than 15°C perhaps for the human body to survive. Human regulation depends on a number of well-understood detailed mechanisms that will not be described here.

This example and most other examples depend on there being a “no action” range of a variable, inside of which no attempt will be made to adjust, with a maximum and a minimum which are trigger points outside of which considerable changes will be made inside the organism in order to move the variable back inside the “proper” range. Many psychotherapists are familiar with the picture of general arousal, above which there is hyperarousal and below which there is hypo-arousal, both of which are considered to be states of dysregulation (Rothschild, 2000Porges, 2011). This picture follows the general form of a homeostatic system.

Another example of regulation is that of emotions and affects, which are often described as requiring regulation so much that this can be a major target of psychotherapy (Schore, 2012). As it is not possible in general therapy conditions to measure affect and emotion in quantitative terms, this usage of homeostasis and regulation is more metaphorical, but that does not take away its importance or effectiveness.

Many diseases and disorders can be understood, at least partly, as disturbances in homeostasis, or as unsuccessful attempts to restore homeostasis. Homeostasis is not always positive, however. A behaviour pattern, an interpersonal exploitative dynamic or an immovable set of roles in a family system, can all maintain their stability and lack of change through homeostatic mechanisms: homeostasis can also be the maintenance of rigidity.

The technical term for the mechanism whereby moving beyond a particular level of a variable leads to activity to bring the variable back to where it came from is “negative feedback”. Despite homeostasis being a very typical mark of living organisms, the concepts of homeostasis and negative feedback are used of systems in general, including mechanical and non-living physical systems, and can help to understand the dynamic “behaviour” of the system.

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The heart and cardiovascular system

The heart and the circulatory system are vital for the life of most animals with a coelom, a body cavity, to the extent that death follows rapidly when the circulatory system ceases to function. Larger animals – most reptiles, mammals and birds – have two circulatory systems, one for the lungs and one for the whole body, The circulatory system is key for distributing blood and other substances throughout the body, regulating the body by transporting hormones, and protecting the body through the blood clotting mechanism and its part in fighting viruses and bacteria.

Blood is regulated by a number of homeostatic mechanisms:

  • Blood pressure must be kept within a narrow range, avoiding hypotension as well as hypertension
  • The blood’s acidity must be regulated in narrow boundaries
  • The blood glucose level must be regulated
  • If oxygen and carbon dioxide levels are not right, the heart beat will be adjusted, and signals go to the respiratory system for correction
  • Water and ion concentration must be kept at the right level – if there is too much, this will be secreted by the kidneys – if too little, thirst feeling will suggest intake of fluids

Anything that threatens the functioning and regularity of the circulatory system tends to have a survival meaning, as it could so easily develop into a life-or-death problem. Regulating the heart rhythm is a vital function. The heart rate is controlled by the autonomic nervous system, coordinated by the medulla in the brainstem. Inhibitory signals come from the parasympathetic fibres in the vagus nerve. The heart can malfunction in a number of ways, gradually or suddenly, with a whole range of causes. Heart problems increase as a part of normal ageing, but heart problems can also be congenital and become apparent in the womb, during or shortly after birth. Peripheral arterial disease can be problematic. An important class of problems is cerebrovascular disease, involving the blood vessels supplying the brain. The latter includes stroke, but there are also less easily detectable and slower-developing problems that lead to or exacerbate the neurocognitive disorders (dementias).

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The immune system

The immune system has the function of protecting the body against disease and the consequences of accidents, in particular against the invasion of the body by foreign elements usually called pathogens. These can be bacteria, viruses, prions (infectious proteins) and others. To play this role effectively, the immune system must have the capacity to distinguish disease and health, and to determine the status of cells and substances.

The immune system needs to work when other primary defences (e.g. the skin, or the respiratory or digestive systems) have failed. The human immune system is complex and advanced, and apart from its different protective functions has the ability to learn about diseases, i.e., to acquire immunity based on its experience of disease (e.g., vaccination, whether artificial or naturally achieved through having suffered a disease). Many of the cells produced by the immune system need to be transported to the place needed, via the blood, or the lymph – the fluid that gives its name to the lymphatic system, which is a circulatory system linked to the blood circulation but accessing every part of the body. Specific organs supporting the immune system are the bone marrow, the thymus, the spleen and the mucosal-associated lymphoid tissue.

Because of its sensitivity and complexity, there are a number of things that can go wrong with the immune system:

  • It can fail, and not function as intended
  • There are diseases such as cancer, which are body cells that multiply out of control; as they are cells of the own body, they will not normally be detected as “foreign” bodies by the immune system
  • There are in the order of 40 known or suspected autoimmune diseases, jointly affecting about 5-7% of the human population. They are based on the body’s immune system somehow treating parts of human tissue as the antigens of a foreign invader, thus triggering a specific immune response against them, which causes damage. This can only be corrected by suppressing some of the action of the immune system, which of course has the downside that it is then also less available to fight other real diseases or invasions.
  • Hypersensitivity means an overreaction by the immune system, which overproduces antibodies against what is not really a dangerous substance. The reaction can be so strong as to completely overpower the body in what is called systemic anaphylaxis or anaphylactic shock.

When the body is threatened, in many cases there will be an “inflammatory response”, mobilising the immune system. The immune system has a number of mechanisms, including:

  • The use of leukocytes (white blood cells), of which macrophages, neutrophils, and NK (natural killer) cells are three major examples
  • A number of proteins can help deal with foreign attackers, including about 30 proteins belonging to the “complement system” and interferons, another class of proteins.
  • The specific immune system organises a tailor-made defence against attacking pathogens. This uses specialised cell types to identify antigen molecules which are part of the invading pathogen, which are a special kind of leukocyte called lymphocyte, developing into B cells and T cells. These cells further play a major role in putting together the antibodies that will grapple with specific antigens. The antibodies are immunoglobulins, of which there are five classes with specific characteristics.
  • When a particular attack has triggered the specific immune system, a “memory” will be left in the form of (lymphocyte) “memory” cells that are able to “recognize” the attacking cells if they return, so that a secondary immunity builds up which will act more quickly and effectively.
  • The same types of molecules that could be attacked as invaders are also part of the human body. It is essential that the lymphocytes recognise what are “friendly” or self cells, and differentiate them from foreign bodies. The self cells become marked by the immune system as part of the major histocompatibility complex (MHC)

It is not only the human immune system that is complex, versatile, and has ways of developing defences and learning from each one of them. The attacking pathogens have ways to circumvent or “outwit” the immune system. This includes making changes to their identifying antigens, and in the case of bacteria their ways of changing can be analysed as forms of evolution. Viruses keep changing and evolving, too. A more sophisticated way of successfully attacking is not to attack the body directly, but undermine the immune system, as is done by human immunodeficiency virus (HIV) infection. AIDS is a disease of the human immune system.

There is a major overlap between the endocrine system and the immune system in that changes in one influence the other.

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The endocrine system and hormones

One of the main challenges for multicellular organisms is ensuring internal integration, especially when the cells have differentiated and specialised into organ systems. This requires a means of internal communication, where coherence is maintained through ways of passing “signals” throughout the body. In this internal communication system the nervous system and endocrine system play key roles. Nerve cells (neurons) receive information from the sensory organs, communicate with each other, and send output to the motor (muscle) system; it passes this information via synapses. The endocrine system moves chemicals through the body, mainly by using the circulatory system, i.e. the blood, and the lymphatic system. Some chemicals move locally from cell to cell, bypassing the blood, and are called paracrine regulators.

The chemicals used to send signals throughout the body via the blood are called hormones. The chemicals called hormones are mainly classified in three groups:

  • Peptides and proteins. These are shorter (peptides) and longer (protein) chains of amino acids. Examples are insulin, oxytocin, growth hormone and corticotrophin (ACTH).
  • Biogenic amines, derived from amino acids. Examples are the catecholamines noradrenaline and dopamine, and other amines such as serotonin and melatonin.
  • Steroid hormones and eicosanoids. These are all derived from cholesterol. Examples of steroids are cortisol and testosterone, as are the sex hormones or sex steroids in general (oestrogens; other androgens; progesterone). Prostaglandins are a main class of eicosanoids.

Hormones are produced in many parts of the body, by the hypothalamus (part of the brain), the pineal, pituitary (in two parts), (para-) thyroid and adrenal (in two layers) glands, by the thymus, pancreas, testes or ovaries, and also in substantial quantities by the kidneys, and by the whole gastrointestinal system. Hormones are usually distributed throughout the body (only not reaching the brain, as this is protected by the blood-brain barrier), but they only affect specific types of cells, called “target cells”.

There is no space to enumerate all the different glands and organs and the hormones they produce. In making sense of the multitude of hormones, a number of systems have been identified which link several hormones and activities under one functional heading. Some of the major systems are:

  • The hypothalamic-pituitary-adrenal axis
  • The hypothalamic-pituitary-thyroid axis, regulating metabolism, using various thyroid hormones
  • The renin-angiotensin system regulating blood pressure and the body’s fluid balance
  • The hypothalamic-pituitary-gonadal axis, controlling growth / development, the sexuality-reproduction system and (part of) ageing.

The overlap between the nervous system and the endocrine system is substantial. Though the hypothalamus is part of the brain, its function is central for the endocrine system. Many of the chemicals identified as hormones are called neurotransmitters when functioning in the nervous system. The secretory activity of many glands is controlled by the nervous system – though they are also self-regulating through feedback loops. A number of the major systems described as endocrine are better called neuroendocrine, as they mobilise activity in the nervous system as well as in the rest of the body. A major difference is that endocrine action tends to be slower and longer lasting, compared with the rapid action associated with control by the nervous system.

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Sexuality and reproduction

This system has a dual name. Especially for human beings, sexuality, although closely linked to reproduction, needs to be regarded as a separate (behavioural) system. Amongst the non-egg-bearing placental mammals, humans and apes are unusual in having a menstrual cycle. Most other mammals have an oestrous cycle, in which periods of fertility alternate with non-fertility, and the animals mostly mate only during the fertile period. Another system is that of cats and rabbits, who are induced ovulators, which means that the females only ovulate immediately after copulation, in a reflex behaviour. Humans and apes mate throughout the menstrual cycle.

It is relatively difficult to study the human sexual and reproductive system as there is great variety amongst animal, vertebrate, mammalian and even primate species in this system. This means that it is less possible than usual to use the conclusions from animal research for the functioning of sexuality and reproduction.

The menstrual cycle, ovulation, the changes during pregnancy, the initiation and process of childbirth, sexual arousal and sexual intercourse are all regulated by (sex) hormones. The important ones that have been identified are:

  • Follicle-stimulating hormone, a glycoprotein
  • Luteinising hormone, a glycoprotein
  • Oxytocin, a peptide
  • Vasopressin, a peptide
  • Prolactin, a protein
  • Testosterone and its variants; in general, the group of androgens; these are precursors to

oestrogens and mostly steroid hormones

  • The oestrogens (not one, but several), steroid hormones
  • Progesterone, one of the progestogens, steroid hormones

The regulation of the sexual / reproductive system is referred to as the hypothalamic-pituitary-gonadal axis.

The complete physiological cycle of what takes place in humans during sexual intercourse has been studied to a considerable extent. It can be reasonably well described, and at least largely explained in terms of the mechanisms regulating the process. The complete process is particularly complex, as at various stages both the sympathetic and the parasympathetic branches of the autonomic nervous system need to be activated and deactivated. To co-ordinate these both simultaneously is more complex than when only one of the autonomic divisions is involved.

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Nutrition and the digestive system

Nutrition plays an essential role in well-being and health; unlike some other simple organisms, animals are not self-sufficient, but are dependent on taking in other organisms to survive. In aetiological research or in client assessment, in addition to the many other factors that need to be considered (e.g. early family environment, attachment, infections, genes, trauma) recent past and current nutrition should never be ignored as another possible influence, risk factor or protection factor (Rutter, 2006).

Nutrition includes a balanced diet, the presence of certain essential nutrients, toxicity of substances consumed, allergies, toxic factors in the environment (radiation; air pollution).

The digestive system is composed of the mouth, oesophagus, stomach, small intestine and large intestine, supported by accessory organs and glands, and the endocrine and nervous system. The digestive system has a substantial part of the nervous system built into its walls, the enteric nervous system. Although the enteric nervous system and the gastrointestinal muscles are linked with other parts of the nervous system, too, they continue to function when these links are interrupted, or when parts of the rest of the body have ceased functioning.

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Complex behaviour

More complex forms of animal and human behaviour, when they can clearly grouped together because of functional commonality and because they engage a similar well-defined network of parts of the brain, a brain system, are studied by a number of disciplines. Apart from neuroscience, this is in ethology (the science of animal behaviour), neuroethology, comparative psychology, evolutionary biology, developmental biology and attachment theory. Bowlby (1982) had coined the term “behavioural system”, but this has not been significantly adopted outside attachment theory.

As regards the behaviours central to the attachment behavioural system, a good summary of to what is known, and how important it is to study attachment in a developmental context, can be found in Marvin & Britner (2008)Coan (2008) gives a good summary of findings about the neurobiological basis of attachment.

There are other groupings of complex behaviour into systems which would benefit from a similar systematic treatment and research programmes, such as sexual behaviour, caregiving behaviour, social behaviour and exploration behaviour, but they have not had as much attention or systematic treatment as attachment behaviour using the combined strengths of systems theory, developmental theory, learning theory, ethology and neuroscience.

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Ageing is a natural process, as opposed to the deterioration of the major neurocognitive disorders, which are caused by defined disease processes. As the whole organism changes throughout the life-span it is difficult to define a real difference between ageing and development. There is no completed and fully convincing understanding of why ageing takes place or what is causing it. It may well be the result of a combination of three phenomena:

  • Most known cells in nature stop reproducing after a certain number of times. It is not understood how or why this happens. It is possible that some part of the cell system is programmed to stop after a certain number, and this may also apply to some or all of human cells.
  • Although a lot of the body gets replaced and regenerated over time, it is possible that replacing and repairing cells leads to deterioration over time and ultimately to accumulated small changes and defects that are beyond renewed repair – sometimes called the accumulation of misrepair.
  • Different body systems and body parts age (change) at different rates. It is possible that every time that a specific part of the whole organism fails or deteriorates, all the other parts and organ systems also suffer and are damaged as a consequence. This would emphasise that in order to survive much longer than humans in general do, it would be necessary to keep all body parts and systems at a top level of health, which can be seen to be a difficult result to achieve. This would be even more plausible if it would be caused by certain forms of tissue damage, say, being caused by protein damage that when it took place would affect most other tissues too.

It is well documented and observable that the major organ systems usually age at different rates. This is in itself a problem to manage. The muscle, bone and skin systems all age, and start ageing early in life. The heart muscle over time becomes slowly stiffer. Some glands needed for the immune system decrease in size. Neurons die, and few if any are replaced (however, the remaining neurons often create more synaptic connections). Finally, the reproductive system functions optimally earlier in life, and with advanced age fertility almost always declines for men, and always for women.

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Biology is a huge topic and this paper only brings out highlights, mainly referring to issues that may concern psychotherapeutic practice and mental health issues. The great majority of information in this particular paper is standard and can be found in any biology textbook.

As biology is the basis of Interpersonal Neurobiology, and the latter offers a significant elaboration of psychotherapy, the reader may want to review the W.W. Norton series on this subject. Module owners have an automatic 30% discount on these books (apply the discount code WNPRO )

It may be useful to read parts of a biology university textbook or to review online courses in biology and / or videos. Some of the many books worth considering are: Raven et al. (2013) and Johnson (2013).

An additional possibility is to take up one of the online biology courses that are available. The Biology I course of the CK-12 Foundation (2010) is free and comes with a considerable amount of online support. It covers cell biology, genetics, evolution, ecology, and physiology.

It is recommended to look up some of the unexplained terms in this paper in Wikipedia, which in the field of biology has a number of excellent write-ups which can help to deepen one’s understanding of the subject.

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