The purpose of this page is to investigate the possible functional roles of neurogenesis (the birth of new neurons). For more information about the location and function of the areas involved in neurogenesis check out our where page.
A Brain of Metal and Clay
In order to do this, we must first understand the brain and its components. The circles in the brain represent neurons, the fundamental units of the brain. The lines represent connections between neurons, the pathways through which neurons communicate. There are billions of neurons communicating through trillions of connections. These neurons and connections allow you to do everything from brushing your teeth to thinking about the universe.
Plasticity is the brains ability to change itself in response to maturation, learning, and changes in environment. It is the reason we can learn everything from languages to the rules of Monopoly. Plasticity in the brain allows us to adapt to our environment at many different levels.
Plasticity can refer to a few processes: the rearrangement of connections between neurons, changing the strength of connections, or even adding new neurons – the process known as neurogenesis (Lledo, Alonso, and Grubb, 2006).
There is a trade-off between stability and adaptability (plasticity). Our brains must be stable enough to maintain our identities while staying flexible enough to learn new things and accommodate growth. Historically, we believed the human brain must be very stable to support our complexity (Rakic, 1985).
However, this was questioned when vast amounts of neurogenesis was uncovered in birds (Neurogenesis in Birds).
Further investigation showed neurogenesis in many other species including: crustaceans, rats, mice, monkeys, and humans. (Gould, 2007). These findings shifted our view of the human the brain towards plasticity.
Effects of Neurogenesis
The effects of neurogenesis can be seen on three levels – the level of a cell, the level of multiple cells (in a network), and the level of a whole person (in behavior) (Kempermann, Wiskott, and Gage, 2004).
1) Cell Level
In searching for function at the cellular level, its important to look at both neurogenesis and its compliment process, cell death (the death of neurons). Together, these processes constitute neural turnover as seen in the figures above. The neurogenesis figure shows the birth of new neurons and their integration into an existing network. Integration refers to neurons making connections to other neurons. These freshly minted neurons may have a higher potential for plasticity compared to older, more rigid cells. The cell death figure shows an existing network of neurons in which one of the cells dies. By trimming unnecessary cells, the brain is able to optimize resource use in order to become more efficient (Abrous, Koehl, and Le Moal, 2005).
2) Network Level
Networks are comprised of connected cells. In the animation on the right, we have a network of three interconnected neurons. In a simplified world made up of shapes, these three neurons can conform to any kind of triangle through rearrangement of connections – one of the forms of plasticity.
In the figure to the lower left, this world of shapes changes to not only include triangles, but also rectangles. No matter how they rearrange, they can never conform to a rectangle. This situation would represent the human brain without neurogenesis; it has the potential to alter connections, but it cannot add new neurons.
In the figure to the lower right, we have allowed multiple forms of plasticity: neurogenesis, neural death, and rearrangement of connections. Now, our initial three-node network can handle both triangle and rectangles. In fact in can handle any shape. Together, neurogenesis and cell death allow for our network to conform to any number of sides, while rearrangement of these cells allow for them to handle variations within shape (Cecchia and Magnasc, 2005).
3) World Level
A world of changing shapes is akin to the ever-changing real world. Neurons in our brains constantly adapt to new environments, which allows for the adoption of new behaviors and new ways of thinking. How do our neurons accomplish this?
Rearranging and altering the strength of connections only get our brain so far. Computational modeling is leading us to the conclusion that neural turnover (the processes of neurogenesis and neural death) is a necessary component of plasticity.
Cecchia and Magnasc (2005) modeled hippocampal neurogenesis. They used simplified versions of neurons and connections in computer software to investigate the effects of adding new neurons to a network. They found that neurogenesis was necessary when the complexity of stimuli is not constant (ie. when the world changes), or when you want to organize information in your head with minimal interference (eg. remembering that today’s events and yesterday’s events are separate). Four other groups performed similar computational modeling of neural turnover. They all converged on the conclusion that neural turnover helped networks form distinct memories and adapt to new environments. (Zhoa, Deng, and Gage, 2008; Lledo, Alonso, and Grubb, 2006)
Kempermann, Wiskott, and Gage (2004) posited that the function of the hippocampus is to consolidate memories. They proposed that new neurons acted as gatekeepers to new memories, controlling the amount of new information that is allowed into an existing network.
From our readings, neural death and neurogenesis are both vital for keeping the brain organized. Neurogenesis gives our brain the ability to adapt and handle new information. The role of cell death is often downplayed or discussed in a negative light, but it is essential for consolidation or trimming of information. Our brains only have a finite amount of space, so cell death appears to keep our brain out of information overload. Death enforces stability and compels us to only hold onto important information (use it or lose it).
By Matt McGranaghan and Mike Hadley