Brain Development PDF - Prenatal Abilities & Behaviors

brain development (Stages of prenatal) During the embryonic period, the embryonic disc develops into a three- layered sandwich of cells comprising 1. an outer layer called the ectoderm 2. a middle layer called the mesoderm 3. an inner layer called the endoderm By weeks 3–4, the d...

brain development (Stages of prenatal) During the embryonic period, the embryonic disc develops into a three- layered sandwich of cells comprising 1. an outer layer called the ectoderm 2. a middle layer called the mesoderm 3. an inner layer called the endoderm By weeks 3–4, the development of the nervous system begins in earnest. By day 18, chemical signals are sent to the ectoderm from the layer below, causing the growth of a thick neural plate. day 20, the plate has a groove all along its midline with folds of tissue either side (like a river bed with canyon walls) week 4 (about day 22), these folds arch over the groove, wrapping around to form a hose-shaped structure, the neural tube (hose-shaped structure forming the basis of the brain and spinal cord in the embryo the precursor to the brain and spinal cord) This folding starts at the middle of the groove, moving towards each end in a ‘zipping’ manner until the tube is sealed except at the ends By about 3½ weeks, there is a defined swelling at the top end of the tube and by week 4, this shows three clear divisions corresponding to the future forebrain, midbrain and hindbrain  After the closure of the tube in week 4, neural development proceeds frantically in several phases which can overlap in different brain areas Neurogenesis: the production of neurons in the embryo The first phase -Neurogenesis, the rapid and teeming production of neurons within the neural tube. Neurons cannot copy themselves but are made from other cells in the neural tube that rapidly divide to form more cells, which are in turn changed under genetic control into neurons. - Neurogenesis begins in weeks 3–4 and is especially active up to week 16, with hundreds of thousands of neurons being made each minute and most of the brain neurons formed by about week 24. This frantic proliferation of neurons is reflected in the corresponding growth in the size of the infant brain, with the head enlarging to one-half of the body size by week 8. Neuron production generally ceases at birth, although some new neurons may be made in the adult brain (possibly in the hippocampus, the part of the brain important in making new Outer layer (ectoderm) The second phase Neuronal migration: the movement of neurons to appropriate locations in the brain and body. neuronal migration, in which the neurons move to different locations in the emerging brain and nervous system. Some only go short distances, but others must travel far. By a pushing or pulling process under the control of genetic and biochemical factors, they move along fine filaments sometimes swinging like monkeys from one filament to another This exodus of neurons establishes different neural groups or populations that will eventually contribute to different structures and systems in the brain and nervous system THE THIRD PHASE neuronal differentiation – begins when genes further modify the newly arrived cells to make them specialised for their future roles in the brain regions and systems. During the process of embryonic cell differentiation, two other ‘phases’ also occur at the same time. synaptogenesis: the development of potential connections or synapses amongst the emerging myelinisation or myelination – the coating of the neurons due to the growth of extensions or axons in fatty insulating material to improve branches at the top of the neuron called their efficiency, although this process increases dendrites and at the bottom called terminals. markedly after birth with use of the brain The final phase In the prenatal brain there is considerable over-supply of synapses. This leads to the final phase neural development, involving ‘pruning’ of connections and rivalry amongst neurons for target neurons The basic brain structures are in place early in the foetal period by weeks 9–10, with the main aspect of development thereafter being the growth in brain mass. During the last few months of prenatal life, the cerebral cortex of the brain shows increasing folding due to neuron production and growth of synapses and myelin. Not all regions of the brain develop at the same pace and, even within the cortex, there are different timetables for different regions with these discrepancies continuing after birth. As a result of this rapid development of the brain and nervous system, even the embryo soon displays basic behaviours and responses reflecting organised neural activity. prenatal states: foetal sleep and waking cycles By the end of the first trimester, the foetus shows daily (24-hour or Circadian) rhythms of heart and lung activity, and hormone Prenatal development and infancy are critical release. Sleep cycles with rapid eye to the early development of the brain, movements (REM sleep) appear at about 28 especially the organisation of the visual, weeks gestational age, with REM accounting auditory and touch systems and the ability to for most sleep, declining to about 50% by form long-term memories. birth.  Prenatal abilities and behaviours One of the most fundamental human capacities is the ability to sense and respond to a stimulus external to the body. The five main sensory systems of vision, touch, hearing, taste, and smell all begin to develop in the embryo. Touch- The earliest sensory channel to develop During intrauterine diagnostic procedures, it has been established that by 8 or 9 weeks, the foetus moves its head if touched in the mouth region and by 12 weeks will grasp at anything touching the fingers. Taste and smell- formed in the foetus by about 18 weeks a(taste) and 15 weeks for (smell) The foetus becomes familiar with the smell and taste of the amniotic fluid environment, as shown by preference for own mother’s amniotic fluid at birth Auditory/hearing- functional to some extent by about 20 weeks. The foetus will respond to loud sounds with movements, perhaps showing a startle reflex By 22 weeks, the foetus will show a more sophisticated reaction by orienting or attending to sounds Furthermore, if the tone or sound is repeatedly presented, the foetus will eventually show habituation or a decline in responsiveness, evidence of simple learning or memory Vision- The least developed of the sensory channels in prenatal life. by about the fourth month, the main parts of the eyes have developed and the next month sees the specialisation of cells along the visual pathways for colour and spatial detail. the 7month, the visual region of the brain (the visual cortex) has developed. Brain development (The postnatal) Many of the brain systems for sensation and perception are functioning well at birth, as is the basic ability to learn and store information in memory. The neonatal brain is therefore sufficiently developed to allow the infant to perceive and sufficiently pliant to allow adaptation to the context for development. The neonatal brain resembles the adult brain apart from being only about 25% of its weight. Connecting and pruning the brain after birth New-born brain only has about one-sixth of the connections or synapses of the adult brain. In the months after birth, the synaptogenesis that began in prenatal increases in earnest. There is now a massive increase in the number and length of dendrites and terminal branches, leading to such a rapid increase in number of synapses that by about 12 months of age, the infant brain has twice as many as the adult. Synaptogenesis and pruning do not occur uniformly across brain areas and are linked to the usage of the brain region. The earliest regions to show these processes are the brain areas for sensory processing Lateralisation LEFT BRAIN GROWS AFTER BIRTH RIGHT BRAIN GROWS AFTER BIRTH left hemisphere bias for positive emotions such e right hemisphere shows early dominance for as happiness in 4-month-olds and for the processing the relational aspects of facial categorical perception of orientation in 5- patterns, colour categorisation and negative month-olds emotional arousal such as fear or sadness Mapping activity in the infant brain: developmental neuroscience A most promising future direction for studying the infant brain and its abilities is that provided by research approaches in developmental neuroscience (studying brain regions and systems) and developmental cognitive neuroscience (studying the brain during different states and activities related to perceptual–cognitive processing) a longitudinal fMRI study of the infant brain from 4 to 9 months of age revealed increasing long- range connections from the frontal cortex to visual and sensorimotor areas. Patterns of EEG activity in the resting brain also undergo development in the first 12 months, becoming adult-like during that time and studies with infants around 7–8 months have found infant EEG patterns to be indicative of infant cognition. EEG (electroencephalogram): record of brain activity recorded by means of electrodes placed around the scalp is being increasingly used to map infant brain activity during different states and cognitive tasks. In infants, as for adults, brain activity in the form of ‘waves’ from the scalp can be detected by EEG technology and ‘dense-array’ sensor-nets have made it easier to apply this technique to young infants. A very recent EEG approach is to search for bursts of activity in particular frequencies in the EEG waves that have been linked to different types of cognitive processing. EEG frequencies are conventionally grouped in bands related to how frequently the waves occur (like fast or slow ripples on a pond), with these bands denoted by Greek letters: d Definition EEG (electroencephalogram): record of brain activity recorded by means of electrodes placed around the scalp. In adults, for example, the brain shows theta waves when holding items in working memory, beta activity when processing information from the environment and alpha activity when awake but in relaxed mode (as with eyes closed). The superfast gamma may arise when there is a need to synchronise processing across different brain regions – for example, when different visual features of an object such as colour and shape need to be bound together. There is growing evidence of similar types of EEG activity in the infant