These are my notes from reading Lise Elliot’s What’s Going On in There?.
“The Basic Biology of Brain Development”.
This chapter covers prenatal development of the brain and the nervous system, and an overview of the structure of nerve cells.
A few interesting facts I learned:
- The nervous system starts out flat as a pancake, then curls up into a groove, and then a tube.
- The brain cortex is made up of units perpendicular to its surface. More intelligent species have more grooves in their brains, and thus a larger surface.
- Initially too many connections are created between neurons, leading to noisy connections. This excess is pruned during early childhood.
Pre-natal brain development
Cell differentiation in the embryo starts with the development two tissue types: the endoderm (which later becomes the internal organs) and the ectoderm (which becomes the skin and the nervous system). Then a middle layer, the mesoderm (which becomes the bones and muscles) forms between them, along a track down the centre of the embryo. As this thread of mesoderm threads its way between the other two layers, the ectodermal cells it touches start turning into the nervous system. The embryo is now 19 days old.
The embryo is flat at this point, and so is the nervous system, so it’s called the neural plate. Over the next week this plate folds up into a groove, and then becomes a tube (called the neural tube). The top end is enlarged and will become the brain; the rest becomes the spinal cord.
So skin cells and nerve cells start out the same. The ones that come in contact with the mesoderm are folded inside the embryo and become nerves; the ones that stay outside become the skin.
By day 28, when the embryo is 3 mm long, the top of the tube separates into three lumps – the forebrain, midbrain and hindbrain. These then split further, so by 6 weeks all the major parts of the brain exist. The fluid-filled spaces between them are called ventricles.
The simpler, more basic structures form first. The spinal cord starts to function already at the beginning of week 9, causing the first fetal movements. The brain, on the other hand, doesn’t even have its final shape yet. After 3 months the mid- and hindbrain are mostly done, but the forebrain is not. Over the next few weeks the hemispheres grow, covering deeper parts of the brain, and connect to each other.
By 24 weeks the fetus is capable of surviving outside the womb: the lungs can breathe air, and the brain stem can direct breathing movements. But the cortex, the outer layer of the brain, is still far from done: it is smooth, without the characteristic “brainy” look.
The cortex is made up of lots of small processing units perpendicular to its surface. So the larger the surface, the more units the cortex can hold. More highly evolved species have larger brains, but also more and deeper grooves in their brains. These grooves grow during the end of pregnancy and the first year of life.
The brain continues to grow after birth. During the first year the brain triples in size, from 1/4 to nearly 3/4 of its final size.
Neurons and synapses
The brain is made up of two kinds of cells: neurons (nerve cells), and between them glia which provide structure and energy to the brain.
Each neuron is shaped like a tree: dendrites / roots that receive signals, a cell body, and an axon / trunk that splits into many branches and sends signals. A signal is transmitted electrically within a neuron, and chemically between neurons. The gap that the signal has to cross, from the axon of one neuron to the dendrite of the next neuron, is called the synapse.
Nerve cells are generated on the walls of the ventricles, the spaces within the neural tube. This process (neurogenesis) begins as soon as the neural tube forms (3 weeks), reaches a peak at 7 weeks, and is pretty much done by 18 weeks. 100 billion neurons are generated during the first half of pregnancy, which means half a million per minute! Only a few neurons are produced later, a very few even after birth. Glia on the other hand are produced throughout life.
All neurons start out at the surface of a ventricle. From there they migrate outward, along long glia. At the beginning the neurons only have a tiny axon, very few dendritic branches, and hardly any synaptic connections, so they cannot do much. They start growing new dendrites and forming synapses as soon as they are in place. This takes a lot longer than neurogenesis – it continues throughout pregnancy and most of the first year, in some regions even a part of the 2nd year.
How does a nerve cell find the right neurons to connect to? How does a path develop from a section of the eye to the right part of the visual processing centres in the brain? That’s not entirely clear yet. What is known is that neurons initially form too many synapses, with a lot of overlap and noisy signals. Synapses are then pruned – the ones that are used and useful get stronger, while the others regress. This pruning leads to more streamlined and coherent brain processes. The more stimulating the environment, the more synapses survive.
Adult axons are coated with a fatty substance called myelin. This keeps nerve fibres from touching each other and interfering with each others’ signals. Before the myelin layer is in place, many nerve fibres are so leaky that they cannot transmit a signal all the way to the synapse. So even though the neurons are in place and connected, they don’t work properly.
Myelination starts at 5 months of gestation in the spinal chord, and in the 9th month in the brain. The process is slow, and like synapse formation it starts at different times in different parts of the brain – the more primitive parts first. It is not known if the child’s experience can affect myelination.
Myelin is 80% fat, and is produced by a special kind of glia. The number of these glia cells is sensitive to nutrition in early life, which is why malnutrition can affect myelination negatively. Myelin production is the main reason why children’s diet should have a high level of fat – children should drink full-fat milk during the first 2 years, for example. An extremely high-fat diet is even used to treat some forms of epilepsy in children.
The order of events, and critical periods
The nervous system matures from tail to head. The spinal cord and the brain stem are almost fully formed and myelinated by birth. The midbrain begins myelination just after birth. The inner parts of the forebrain follow later in the first year. The cerebral cortex is slowest. Sensory and motor areas mature relatvely soon, but the higher-order parts responsible for language, judgement, reasoning and emotion are still pruning synapses during in the teenage years.
The activity levels in the brain follow the same sequence: the primitive parts are most active first. Activity levels are estimated by measuring glucose use. Glucose use increases through childhood, reaches a peak between 4 and 7 years (depending on the part of the brain) and then decline. The peak is about twice the adult level. This overshoot-and-decline is similar to what happens with synapses. So it seems that the brain uses the most energy when the critical decisions are made about which synapses to keep.
Each part of the brain has a critical period, during synapse pruning, when it is permanently shaped by the child’s experience. Once a given brain region has passed the pruning stage, the opportunities for rewiring are very limited. Because of the order in which the brain regions mature, critical periods for basic abilities end earlier than those for more advanced skills.