Blog #7: ch-ch-ch-changes
Hi friends! Welcome back to another week of samsplaining science!
This week, I finished my second week at my new job! Though there’s some overlap between what my new lab and what my previous one, there’s a decent amount of on-the-job training and learning I’ve done so far and will continue to do in the upcoming weeks. This got me thinking about and appreciating how the brain can deal with changes, how it learns new things, and how it adapts to new environments. That’s why, this week, we’re talking about how our brains change with new experiences – a process called neuroplasticity.
To talk about this, first I should give some background into the human brain and how it works. Your brain has billions of nerve cells, called neurons. These neurons work to transmit nerve signals across the brain and spinal cord. I took the cartoon of the neuron below from Szymik, B.(link) to help explain this a bit more. This drawing does a great job of defining critical parts of the neuron (and is also very helpful because I lack the skill and tools to properly draw a neuron electronically myself lol).
The nerve signal that a neuron will propagate or pass starts at the dendrites. Here, a stimulus, such as the presence of a neurochemical glutamate, dopamine, etc. or a sensory stimulus will start a chain reaction, which will spread throughout the cell. This reaction (called an action potential) is essentially an electric current that will pass through the cell body, which is where the cell nucleus is located, and down an axon. The axon is a long projection of the cell body. The projection helps brain cells from different parts of the brain physically connect to one another. At the end of the axon, we have the axon terminals. At this point, depending on the neuronal cell and where it is projecting, it will often release another signal onto another neuron’s dendrites by releasing neurotransmitters into the space between the “sending” cell (the axon terminal) and the “receiving” cell (the following cell’s dendrites). This space or connection between cells is called a synapse, and it is crucial for proper brain function. Without synapses, our brain cells wouldn’t be able to talk to each other, leaving the neuron’s function of propagating signals useless.
Img from Szymik, B. (link)
Right now, you’re reading the words on this blog. The visual receptors in your eyes will send signals to your visual cortex, the part of your brain which processes visual stimuli. Then, the nerve signals will be projected to areas of the brain that help us recognize the letter shapes, and then to the area of the brain that is responsible for language comprehension. This all happens in the span of milliseconds. And, again, all possible because of neurons (brain cells) and synapses (the connections between those cells).
Most of our neurons were actually generated during gestation. The number of synapses or connections between neurons most drastically increases as the brain is developing (Ref: Chapters 2, 6, and 7 of my thesis lol), ranging from mid-gestation until adolescence in humans. BUT, that doesn’t mean that your brain stops changing after this period! Our brains are plastic throughout our lives, meaning that they are malleable and can change with time. Did you know that our brains grow new neurons into adulthood?! Well, sort of… this process of neurogenesis has been shown in the adult hippocampus– a region of the brain that is responsible for learning and memory – of rodents and humans! Additionally, studies in rodents and nonhuman primates have shown that synapses across the brain can “re-organize” throughout life, depending on functional use and experience.
I found this interesting review article that summarizes a few studies that have explored the mechanisms that make the brain malleable. Usually, the easiest way to model neuroplasticity is in animal studies, where the environment can be controlled and often times brain samples can be acquired and evaluated after the study is completed. One article that this review mentioned was one written by Barnes and Finnerty in 2010 (source) suggest that brain circuits in the mature rodent cortex can be physically “rewired” by axon remodeling, growing new dendrites, and synapse turnover/relocation. Axon remodeling consists of changes in interactions with other neurons along a cell’s axon. Sometimes, an axon can have bumps (also known as varicosities) along them which are often sites of connections with other cells. These varicosities are sensitive to sensory experiences, such that they may increase or decrease in size/density, depending on the type of connection (excitatory or inhibitory) and the degree of change in input. Dendrites, which are located post-synaptically as the starting point of a nerve signal in a neuron, can also be formed or lost as brain networks change. Like axons, the formation of these dendrites is sensitive to sensory experiences and are believed to play a role in rewiring brain circuits in response to these experiences. Lastly, synaptic relocation is another method the brain uses to change it’s microstructure. This method requires pre-synaptic changes to the synapse, in which the synapse “moves” and becomes the input of one dendrite instead another. This process keeps the synapse number constant while still adapting to changes in the brain.
So, now we know how our neurons can change. But, what can make them change? In Barnes and Finnerty’s article (2010, source), many of these studies looked specifically at sensory changes, modeled by altering sensory exposures by changing the length of the whiskers or by housing the animals in an “enriched” environment, which has extra toys and treats for the animals that standard research housing does not include. In enriched housing, for example, the density of dendrites increases in sensory regions of the brain. In cases of sensory exposures, studies have shown that after periods of sensory-loss, neurons tend to remodel their axons and dendrites to compensate for the loss of input, but there is still a lot that is not well understood about this process. This ability to change and reorganize neural circuits that are deprived of sensory inputs is one main focus for spinal cord injury research, in which it is believed connections may be reorganized to help restore function after injury (source). Even though there is still a long way to go to totally understand the mechanisms that our brains use to re-wire, and how we can potentially use these mechanisms to help people, it’s pretty remarkable what our brains can do.
If you’re starting something new – whether it’s a new job (me too!), a new skill, a new language – just know, you will adapt! You (and your brain) got this!
Talk to you next week!
-s
