Brain Basics

What cells are in the brain? Can we make new ones? How do we learn?

Jul 01, 2024

Hey Brainiacs,

Welcome to this week’s newsletter! We’re going to get back to the basics and learn more about neuroplasticity, how we can create new synapses, and if we can make new brain cells.

The Brain

Our brains are made of two main types of cells: the neuron, which is responsible for sending signals throughout the brain, and glial cells, which are responsible for supporting neurons and maintaining brain health. Neurons are what people usually refer to when they say ‘brain cell’, and the diagram below nicely depicts their structure.

Diagram depicting the structure of a neuron. Created with BioRender.com

Neurons generally contain a cell body, where they receive signals from other cells, an axon, which they use to transport signals electrically, and the axon terminals, where they release chemicals to send the signal to another cell. Myelin is a substance covering the axon which acts as an electrical insulator, allowing signals to run from the cell body to the axon terminals quickly.

You’ve probably heard me talk about synapses (especially in the context of plasticity), and these are the areas where one neuron's axon terminals meet another neuron's cell body to release chemical signals and allow for communication. Check out the below diagram¹⁴.

Diagram depicting the synapse between two neurons. Created with BioRender.com

Babies are born with 100 billion neurons, on average¹. Until they reach the age of 2, they continue growing new neurons and creating new synapses². However, by age 2, they have more neurons and synapses than they need². Thus, the process of removing unnecessary neurons and synapses begins. This is known as synaptic pruning, and continues until age 10². Synaptic pruning is necessary to allow permanent neurons the space and energy they need to function. It is also a critical process for learning and memory³.

The human brain can form new synapses and modulate existing synapses throughout adulthood. These processes are known as synaptogenesis and synaptic plasticity, respectively. However, whether neurogenesis can occur in the adult brain has largely remained in debate.

Neurogenesis

The first evidence of adult human neurogenesis was published in 1998⁴. They used human post-mortem brain tissue and markers of cell division to determine that new neurons are produced in the hippocampus, the brain region responsible for learning and memory. However, in 2018, a paper was published that swung the pendulum of the debate toward neurogenesis does not exist in this region⁵. They could not find any young neurons in the hippocampus of adults. Then, in 2019, a new paper was published which swung the pendulum of the debate towards neurogenesis does exist⁶. They found young neurons in the hippocampus!

Part of the reason for this controversy is that the technology necessary to answer this question is still developing. It is also a tricky area of research because of the technical limitations of working on the human brain (namely, the need to use post-mortem brain samples). Although it remains unclear if, and to what extent, new neurons can be created in the human brain, it is clear that the connections between neurons can be altered.

Synaptic Plasticity

Synaptic plasticity is, as the name suggests, at the level of the synapse. Synapses can become stronger in response to repeated use (otherwise known as long-term potentiation; LTP)⁷. A ‘stronger’ synapse means that the pre-synaptic neuron is releasing more chemicals and the post-synaptic neuron is producing more receptors to accept those chemicals. Alternatively, when synapses are not used as frequently, they can be weakened (otherwise known as long-term depression; LTD)⁸. Both strengthening and weakening of synapses contribute to learning and memory⁹. The extent to which they contribute is debated, but experiments have clearly shown that blocking these processes also blocks learning¹⁰.

Neuroplasticity

Neuroplasticity may be confusing because it seems to imply that our neurons can come and go, and I just said we don’t know if neurogenesis exists. However, neuroplasticity does not refer to one single neuron. It refers to the ability of large networks of neurons within our brain to adapt, reorganise, and change¹¹. This adaptation and change may be through the growth of the network, which is not caused by the creation of new neurons, but by including neurons into the network that were not previously part of it¹¹. And the way this inclusion may be done is through synaptic plasticity! Or it could be that the network reorganizes, so new connections are being made with different brain areas. This type of reorganization is commonly seen after a brain injury, and explains how some people can recover different abilities, such as language and movement¹¹.

This is why in a lot of my content I emphasize the ability of the brain to change, without necessarily meaning the brain can make new neurons. Even if the brain can make new neurons, neuroplasticity and synaptic plasticity are still two major processes through which we learn and adapt.

Glial Cells

I will only mention these guys briefly, although they do deserve more attention. Glial cells include astrocytes, microglia, and oligodendrocytes¹². Astrocytes are important in clearing excess neurotransmitters, or the chemicals that neurons release at synapses. They also help with synapse formation, and regulating the blood-brain barrier. Microglia are the immune cell of the brain, and they are important in clearing damaged cells and protecting against potential attacks. Finally, oligodendrocytes are the cells that produce the myelin which covers the axon.

Until Next Week,

Nicole x

P.S. Please leave a comment with any topics you want covered in future newsletters!

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