This lecture focuses on brain development from in utero stages through the first few years of life.
Key topics include neurons, glia, neurogenesis, synaptogenesis, myelination, apoptosis, synaptic pruning, and the impact of experience on brain growth.
The primary reading material is from the textbook "How Children Develop" by Siegler et al., available on the LMS.
Why Study Brain Development?
Childhood and adolescence are marked by significant behavioral, emotional, hormonal, and cognitive changes.
Brain function is closely related to brain structure, with different areas developing at varying rates.
Longitudinal studies offer insights into age-related changes within individuals.
Many psychiatric disorders emerge during late adolescence, potentially linked to abnormal brain development, as well as genetics, epigenetics and environmental factors.
The Adult Human Brain
The human brain is larger and more complex than that of other mammals, weighing about 1.3 kg.
It comprises roughly 60% fat and 40% water, protein, carbohydrates, and salt.
The brain contains blood vessels, neurons, and glial cells.
Grey matter is the darker outer portion, while white matter is the lighter inner section (this order is reversed in the spinal cord).
The First and Second Brains
The "first brain" refers to the brain in the skull, while the "second brain" is the enteric nervous system (ENS) in the gastrointestinal tract.
The ENS regulates digestion, gastric acids, local blood flow, gut hormone release, and interacts with the immune system.
It contains 400-600 million neurons of about 20 different types and develops during late gestation and after birth.
Over 90% of the body's serotonin is found in the GI tract, along with a considerable amount of dopamine.
The vagus nerve facilitates communication between the central nervous system and the ENS.
The ENS can function independently of the brain.
Neurons
Neurons are the basic units of the brain, transmitting signals for movement, sensation, thought, and memory.
There are over 100 billion neurons in the grey matter, categorized into three main types:
Sensory neurons: Transmit signals from sensory receptors to the central nervous system (physical inputs include heat, light and touch; chemical inputs include taste and smell).
Motor neurons: Carry signals from the central nervous system to muscles and glands (lower motor neurons travel from the spinal cord to muscles; upper motor neurons travel between the brain and spinal cord).
Interneurons: Connect sensory and motor neurons.
Neurons consist of a cell body (soma), dendrites (receiving input), and an axon (conducting electrical signals away from the cell body).
Neurons communicate via electrical and chemical signals across synapses, microscopic junctions between axon terminals and dendritic branches.
Some neurons have as many as 15,000 synaptic connections with other neurons.
Neurons take different forms and have different structures.
There are brain neurons involved in sensory processing, like those in the visual and auditory cortex, and others involved in motor processing like those in the cerebellum and motor cortex.
Glial Cells
Glial cells are essential brain components that perform critical functions.
They form the myelin sheath around axons, increasing information transmission speed and efficiency.
They act as neural stem and progenitor cells during prenatal development and continue into adulthood.
Glial cells react to brain injuries by increasing in number, protecting the brain and aiding in regeneration.
Different types of glial cells:
Astrocytes: Star-shaped cells that maintain a neuron's working environment by controlling neurotransmitter levels, supplying nutrients, and regulating synapse formation.
Oligodendrocytes: Provide support to axons by creating myelin sheets that speed up electrical activity; multiple sclerosis (MS) is caused by a loss of myelin sheath around the neurons.
Schwann cells: Form a layer around the axon helping to conduct electrical impulses, which are the main glial cell found in the peripheral nervous system; they act a lot like the oligodendrocytes
Microglia: Act as the immune system in the brain, initiating responses to remove toxic agents and clear dead cells; they may become hyperactive in Alzheimer's disease, promoting neuroinflammation.
NG2+ cells (polydendrocytes): Precursors to oligodendrocytes, astrocytes, and neurons distributed throughout the grey and white matter.
Brain Growth
Brain volume is about 35% of adult volume by two to three weeks after birth, doubling in the first year and increasing by 15% in the second year (80% of adult volume by the end of the second year).
After age two, brain growth increases gradually.
Grey matter volume develops rapidly compared to white matter volume soon after birth, but white matter volume increases much more gradually.
From birth to one year, grey matter volume increases 108% to 149%, versus 11% for white matter.
From age one to two, grey matter increases by 14% to 19%, whereas white matter increases by 19%.
After age two, total grey matter volume shows minimal increases throughout childhood and decreases in adolescence, while white matter volume steadily increases throughout early adulthood, peaking at around 30.
Cortical thickness peaks at one to two years and decreases afterwards. The surface area develops rapidly and continues to expand thereafter.
Brain Development Schedule
Major events include neurulation, neural proliferation, neural migration, apoptosis, synaptogenesis, and myelination.
Neurulation: Forming the neural tube (precursor to the brain and spinal cord) during weeks two to three of gestation; failure to close can result in spina bifida and Arnold Chiari malformation.
Neurogenesis: Generating new neurons from neural stem cells via cell division (mitosis), beginning at 42 days after conception and nearly complete by mid-gestation.
Neurons migrate outwards from the center of the brain toward the developing neocortex.
The diversity of neurons results from regulated neurogenesis during embryonic development.
Neurogenesis in Adulthood
In 1998, scientists established that the formation of new neurons seems to occur in the hippocampus, which is the area of the brain responsible for learning and memory, and that this continues in adulthood.
In this 1998 study, the postmortem brains of cancer patients were injected with dye and they were examined.
There was evidence of polydendrocytes, the glial cells, which are the precursor to neurons.
These glial cells actually have also been found more recently in the amygdala.
Adult neurogenesis is currently controversial, with conflicting study results on the existence of robust adult neurogenesis in the hippocampus. Some suggest it continues until the eightieth decade of life in healthy individuals, while others find no evidence to support the notion.
Discrepancies may stem from methodological issues, such as time elapsed from tissue death to brain preparation, techniques used, and sample variations.
Other Processes
Arborisation: Increase in the size and complexity of the dendrite tree from around the twenty eighth week onwards which increases the dendrite's capacity to form connections with other neurons.
Myelination: Formation of the myelin sheath around axons, beginning prenatally and continuing into early adulthood; it progresses from deep in the brain upwards and outwards to the cortex, enhancing complex movement; the sensory areas mature faster than the executive function areas.
Synaptogenesis: Growth of axonal and dendritic fibers, resulting in a wildly exuberant generation of neuronal connections. In early development, synapses occur at the rate of approximately 40,000 per seconds during the 34th week of gestation.
Apoptosis and Synaptic Pruning: The brain produces an excess of neurons and synapses, with about half the neurons dying early in life via apoptosis (programmed cell death) and about 40% of synapses are eliminated through synaptic pruning; this is a normal and necessary process.
Early Experiences and Plasticity
Early experiences play a central role in "use it or lose it" scenarios.
The more often a synapse is activated, the stronger the connection between the neurons involved, and conversely, when a synapse is rarely active, it is likely to disappear.
Neural connections are constantly being created and reorganised by experiences in a process referred to experience dependent plasticity.
Research in this area has focused largely on non human animals because we can control their environments and manipulate them a bit more readily.
Animals raised in complex, stimulating environments develop more dendritic spines, synapses, and a thicker cortex compared to those in bare laboratory cages.