Genesis of neurons and connections
Any decision, movement, or thought is due to the detailed and precise connections between the 85 billion neurons in the human body. I recently read a book called “The Brain: The Story of You” by David Eagleman, which went into depth into how different kinds of neurons and their vast connections can result in the complexity of the human experience. In chapter 23 and in the lecture this week we discussed how neurons and their connections are formed.
Neurons develop in three stages: proliferation, migration, and differentiation. Proliferation starts in the ventricular zone. From there, a cell performs a “cellular ballet,” moving to different positions, and ending with the division of the cell. The kind of cell formed depends on different factors of the precursor cell, such as its age, position in the ventricular zone, and its environment.
Cell migration happens as daughter cells crawl across fibers formed by radial glial cells, that connect the ventricular zone and the pia. Roughly one third move horizontally instead of vertically. The destiny of the cell, in other words what kind of neuron the precursor cell will eventually become, is dependent on the order of which the precursor cells leave the ventricular zone and form the cortex. For example, the first to leave the zone are destined to become subplate cells. The next layer of neurons pass the previous layer and form a new cell layer above. In this way, the cortex is formed “inside out. ”Eventually, when all cells have found their final place, six layers or neurons are formed.
Connection generation also happens in three stages: pathway selection, target selection, and address selection. During the critical period axons grow to meet their targets. Axons going to the same target can grow in unison due to cell adhesion molecules which connect the surface of axons together. Axons are able to complete the long intricate paths by meeting intermediate targets along the way. One of the main ways axons know where to grow is through the use of guidance cues. Chemoattractants like netrin are secreted by target neurons. The axon cone has receptors to those chemoattractants that attract them to the target. There also exist chemorepellent, like slit, to direct the growing axon to the next intermediate target. The ability for axons to find their exact targets in a large complex space is remarkable.
The wiring of the brain and connections all of these axons make between neurons is vast. Connectomes are wiring maps of the brain. Some companies are mapping the connections of the brain of a rat by analyzing electron microscope slices. The human brain is significantly larger and more intricate. Even if we know all the connections in the brain, there would still be a lot of missing information, including synaptic strength and contributions from glial cells. correct pathway.
Plasticity in the brain
Synaptic plasticity is the property of synapses to strengthen or weaken over time according to Hebb’s hypothesis. When the presynaptic axon is activated and the postsynaptic axon is strongly activated, the connection strengthens, i.e. neurons that wire together, fire together. On the contrary, when the presynaptic axon is activated but the postsynaptic neuron is weakly activated, the connection is weakened, i.e. neurons that fire out of sync, lose their link. This ensures that only synapses that are able to participate in the firing, remain in the neural circuits.
As we already know, learning and neural plasticity are greatest in early childhood. For example, it is a lot easier for children to learn new languages, behavioral patterns, etc. and even recover from brain trauma. However, it was interesting to read about the actual processes behind the plasticity. In early childhood, developmental plasticity is considered to be a result of three mechanisms: synaptic plasticity, homeostatic plasticity and learning which all are interconnected. But why does developmental plasticity (critical period) diminish? We don’t have a clear answer but different hypotheses have been proposed. It can be because of (1) axon growth ceases, (2) synaptic plasticity diminishes, (3) cortical activity is constrained, or some other explanation we are yet to discover. Luckily, brain plasticity doesn’t diminish completely over the years and the environment will still keep modifying our brain to learn and create more memories.