Robin Balbernie
Transcription
Robin Balbernie
Circuits and Circumstances. robin.balbernie@gmail.com Nature v. Nurture. 15th January, 2015. © R. Balbernie. The Importance of Early Relationships: A View from Interpersonal Neurobiology. “Human relationships, and the effect of relationships on relationships, are the building blocks of healthy development. From the moment of our conception to the finality of death, intimate and caring relationships are the fundamental mediators of successful human adaptation.” (p. 27) National Research Council and Institute of Medicine (2000) From Neurons to Neighbourhoods: The Science of Early Childhood Development. Committee on Integrating the Science of Early Childhood Development. Jack P. Shonkoff and Deborah A. Phillips, eds. Washington D. C. :National Academy Press. The brain is the only ‘computer’ that can: adapt to any input by changing both its own software and hardware; automatically scrap unnecessary, unused components; dictate the interests and skills of the operator; So eat your heart out ! and, crucially, it never exists on its own. This is simply how humans have evolved. There may be exceptions. Evolution and brain development. Human babies are helpless and less developed compared with most mammals. – The primate with the least developed brain at birth. The pelvis changed to aid walking as our ancestors became more gracile, forcing the bones surrounding birth canal to be closer. But the enlarged skull made birth risky. Homo sapiens. Solution. Evolution of bipedal locomotion lead to wider pelvic girdle in females. This allowed bigger babies with bigger brains. Expanded brain size enabled tool use and social complexity. Big brains then selected for. More processing power. Homo habilis. And so Smaller babies needed! Neuroplasticity increases adaptability. Attachment, common to all Early caregiving now alters mammals, is co-opted by the baby’s brain, allowing this We never look back. evolution. closeness to import culture. “The evolution of human beings has consisted largely of adaptation to one another.” (p. 27) (Wright, R. (1996) The Moral Animal London: Abacus. • “Our brains coevolved with culture and are specifically adapted for living in culture – that is, for assimilating the algorithms and knowledge networks of culture.” (p.11) • “The human brain is the only brain in the biosphere whose potential cannot be realised on its own. It needs to become part of a network before its design features can be expressed.” (p.324) Merlin Donald, (2001) A Mind So Rare. W.W. Norton & Co. Culture is the mechanism for the survival of ideas. Humans have evolved the capacity to use language in order to teach and learn cultural information (e.g. tool use, child rearing customs). The evolutionary significance of ‘plasticity’. As brains evolved and became more complicated their formation became more patterned by the surroundings in which they must function – the ‘knowledge networks of culture’– so that specialised circuits are formed in response to the demands of the local environment. The structural organisation of the brain reflects it’s history. “Most of human knowledge cannot be anticipated in a species-typical genome … and thus brain development depends on genetically based avenues for incorporating experiences into the developing brain.” (p.53) Neurons to Neighborhoods. Your brain is what you do with it. It functions as an anticipatory machine; using past experiences to create a neurological filter. Thus preparing us for the next. Flexibility enhances survival. “Rather than slowly adapting specialised structures in the brain to environmental demands over the course of evolutionary time, selection has designed a brain whose adaptation is its ability to adapt to local environmental demands throughout the lifetime of an individual, and sometimes within a period of days, by forming specialised structures to deal with these demands.” (p 142) Buller, D. J. (2005) Adapting Minds. The MIT Press. So what do we adapt to? “For the developing infant the mother essentially is the environment.” (p. 78) (Schore, A. N. (1994) Affect Regulation and the Development of the Self: The Neurobiology of Emotional Development. New Jersey: Erlbaum.) And how do we adapt? “By initially overproducing connections that have been spread to a variety of targets, and then selecting from among these on the basis of their different functional characteristics, highly predictable and functionally adaptive patterns of connectivity can be generated with minimal prespecification of the details.” (p. 202) Deacon, T. (1997) The Symbolic Species. London: Penguin. There is plenty of choice – A piece of brain tissue the size of a grain of sand contains 100,000 neurons, and has 100,000,000 connections, all communicating with each other. “Every physical feature of the human nervous system – the brain cells, or neurons, that transmit information; their axons and dendrites that reach great distances to connect with one another; the tiny synapses that are the actual sites of connection; and the supporting cells, or glia, that keep it all going metabolically – responds to life experiences and is continually remodeled to adapt to them. The brain changes when you learn to walk and talk; the brain changes when you store a new memory; the brain changes when you figure out if you’re a boy or girl; the brain changes when you fall in love or plunge into depression; the brain changes when you become a parent.” (p.6) Lise Eliot. (2012) Pink Brain Blue Brain. London: Penguin. Nurture and nature are inextricable. “Genetic susceptibilities are activated and displayed in the context of environmental influences. Brain development is exquisitely attuned to environmental inputs that, in turn, shape its emerging architecture. The environment provided by the child’s first caregivers has profound effects on virtually every facet of early development, ranging from the health and integrity of the baby at birth to the child’s readiness to start school at age 5.” Neurons to Neighborhoods. Genes are not destiny. • The epigenome is the operating system that controls the structural genome by switching genes on and off. • These chemical signals are written on top of the gene without altering the genetic code. • Such changes can be temporary or enduring, the latter playing key roles in brain and behavioural development. • Early experiences cause epigenetic adaptations in the brain that influence whether, when and how genes build the capacity for future skills to develop; e.g. how well or poorly we respond to stress. “Structural genes … are codes for resources needed for development. They are not the codes for the course and end-points of development itself.” (p.30) Richardson, K. (2008) How dynamic systems have changed our minds. pp. 25-40 in: Fogel, King & Shanker (Eds.) Human Development in the Twenty-First Century. Cambridge: Cambridge University Press. Within the developing mind neural signaling sets off the production of gene regulatory proteins, which then attract or repel the enzymes that in turn add or remove epigenetic markers. “For the growing brain of a young child, the social world supplies the most important experiences influencing the expression and regulation of genes” (p.32). Siegel, D. J. (2012) The Developing Mind (Second Edition). New York: The Guilford Press. The new science of Epigenetics. There is now a “growing body of work from the developmental neuroscience field that supports the view that epigenetic changes underlie the long-term impacts of early-life experiences.” (p.404) Roth, T. L. & Sweatt, J.D. (2011) Annual Research review: Epigenetic mechanisms and environmental shaping of the brain during sensitive periods of development. The Journal of Child Psychology and Psychiatry, 52 (4), 398-408. Epigenetics refers to the mechanisms that alter a gene’s form, structure and function without changing its sequence. (Only the sequence is inherited.) It is as if certain characteristics and behaviours are authorized while others are prohibited, or switched off. Early experiences alter gene expression. (1) Early experiences spark signals between neurons. (3) Gene regulatory proteins attract or repel enzymes that add or remove epigenetic markers. Gene – a specific segment of a DNA strand. (2) Neural signals launch production of gene regulatory proteins inside cell. Chromosome. DNA strands encircle histones (protein spools) that determine whether or not the gene is ‘readable’ by the cell. (4) Epigenetic ‘markers’ control where and how much protein is made by a gene, effectively turning a gene on or off, thus shaping how brains and bodies develop. • Epigenetic changes allow a response to the environment through changes in gene expression. For the baby this environment is largely defined by close relationships and family setting. • Epigenetic modifications are inherited during mitosis (and sometimes in meiosis) and can be transmitted to the next generation. • Such intergenerational transmission may persist into the fourth generation despite a lack of continual exposure to the original environment. • Thus, in a negative environment, mental health difficulties might be transmitted from parent to child (or even from a grandparent) via a genetic mechanism. • The emotional environment, especially during pregnancy and infancy, activates and silences good and bad genes that are crucial for mental well-being and social and emotional development. In the same way, more positively, the relationship with the mother actively builds the baby’s brain. The heightened states of excitation and elation that the baby feels during interactions with the mother correlate with biological changes involving neurochemicals centrally involved in the regulation of brain metabolic energy level and the maturation of the cortex and limbic systems. This also triggers the birth of new neurons, protein synthesis and neural growth. Thus caregiving activates the growth of the brain through emotional availability and reciprocal interactions. The brain is comprised of four areas. diencephalon limbic system cortex stem Growth of the brain occurs from the inside out and the bottom up. You are born with 100 billion brain cells, neurons, but these are largely unconnected. There can be about 10,000 synaptic connections for each cell. They can wire up at the rate of 1 million per second! You have more than 2 million miles of neuronal fibres! Growth from the bottom up and inside out. Neocortex Cerebral cortex. Cognitive and executive functioning. Abstract thought, metacognition. Conscious awareness. Strategic thinking, planning, interpretation, setting priorities, suppressing impulses, weighing consequences of actions. Limbic system Attachment programming. Sexual behaviour. Emotional processing and regulation. Emotional reactivity. Hippocampus. Formation & recall of memory. Developmental Amygdala. insult may have a Awareness Monitor stimuli for threat. . cascade effect Fight, flight or freeze responses. on the growth of all ‘downstream’ later maturing brain areas that will receive input from the affected neural system. Diencephalon Hypothalamus. Thalamus. Cerebellum. Brain stem. Plasticity. Spinal cord. Complexity. Reflexes. Motor regulation. Procedural motor patterns. Arousal / aggression. Appetite/satiety. (All autonomic functions) Sleep cycles. Instincts. Co-ordination of movement. Cardiovascular functions. Body temperature. As the maturing brain becomes more specialized in order to assume more complex functions, it is then less capable of reorganizing and adapting. But this does not imply it is impossible. See: Schwartz, J. M. & Begley, S. (2002) The Mind and the Brain. New York: Harper Collins. Neurons. Many intrauterine and perinatal insults can alter migration of neurons and have a profound impact on functioning. e.g. infection, lack of oxygen, malnutrition, psychotropic drugs, lead poisoning, ionising radiation, maternal stress, smoking and alcohol. Principal internal structure of a Multipolar Neuron. Neurons have 3 sequential levels of information exchange, or messenger systems: 1) The communication across the synapse, that - 2) changes the internal biochemistry of the cell, which - 3) activates mRNA (messenger ribonucleic acid) & protein synthesis to change brain structure. View of a synapse. Axon Neurotransmitters Dendrite It is the process of synaptic transmission that stimulates each neuron to survive, grow and be sculpted by experience. Experience-expectant brain growth. • This take place when the brain is primed to receive particular classes of information from the environment in order to build basic skills. • Since the brain over-produces synapses they are ‘forced’ to compete. This over-abundance of synapses occurs during sensitive periods • Neurons that fire together wire together. • The ‘fittest’, or most used and useful, synapses are selected; and in neural development this is defined by the level of electrical activity. Initial growth of synapses. Birth. 15 months. 2 years. And then it is all downhill! 6 year old. 14 year old. Highly active synapses – receiving more electrical impulses and releasing a greater amount of neurotransmitters – stimulate their post-synaptic targets more efficiently. This heightened electrical activity also triggers molecular changes that stabilise the synapse. The axons of stabilised neurons become coated in myelin, an electrical insulator. It plays a role in the transfer of energy to neurons and may also support neuronal functioning. This myelin sheath is essential for the proper functioning of the nervous system. This shows by an increase in white matter and a decrease in grey matter. Less active synapses fail to stabilise, and so eventually regress. It is a matter of: “Use it or lose it!” right from the start. Synaptic pruning fine-tunes the functional networks of the brain. For the first 8 months after birth the rate of creating new synapses far outstrips that of pruning. By age 1, and through early childhood, the rate of reabsorbing redundant connections gains on the rate of creating new synapses. By adolescence, in most cortical areas, this process again reaches equilibrium. Fewer but faster connections. A second wave of synaptic proliferation and pruning within the cerebral cortex occurs in late childhood; and the final, critical part of this, affecting the higher mental functions, occurs in the late teens. This overproduction, or exuberance, occurs in the parietal lobes – maintaining sense of self, logic and spatial reasoning, connects senses with motor abilities and creates the experience of a sense of our body in space; and the temporal regions – linked to auditory processing, language and memory. The maturation of grey matter in the cortex. The regions that mature last – not until early adulthood – are associated with higher-order functions such as planning, reasoning and impulse control. A decrease in grey matter shows increased myelination. Decreases in grey matter reflect selection of neurons, organisation of neural networks and enhanced processing efficiency. This occurs in a wave that starts at the back of the brain and progresses to the front. The prefrontal cortex (the area of ‘sober second thought’, or the ‘chief executive’) is the last part of the brain to mature. Adults depend on this area of the brain to process emotional information, resist impulses and exert voluntary control. Unexpected stress may exhaust the prefrontal cortex resources of the adolescent, undermining executive functioning. Teenagers still rely heavily on the amygdala to process emotions, and frequently read the cues wrongly so the emotion, in self or other, gets wrongly labelled.