I bet you have already heard that humans only use 10% of their brain. This has even been theme for many movies and series in which super humans or drugs are able to unlock the full potential of the human brain, granting amazing abilities (yes, I am referring to the movies Lucy and Limitless). I’m sorry to disappoint you but the idea that we only use 10% of our brain is a complete myth; we do fully use the potential of the human brain.
So, if that is not true, where did this “10% myth” came from? Well, it is many times difficult to trace back the origin of a myth, and this one seems to be product of studies from psychologists in the late 19th and early 20th century. But another reason may be because, actually, our central nervous system (CNS) was believed to be mostly made out of non-neuronal cells, called glial cells. For half a century it was thought that glial cells were 10 times more abundant in the CNS than neurons, therefore our brain would be 90% made of cells that were regarded as having little importance. However, recent research has shown that, even though the ratio between glial and neuronal cells vary depending on the brain region, the total numbers of neurons and glia cells in the brain are very similar: around 85 billion cells each, making it a ratio of around 1:1, way different than 10:1. This alone already discredits one of the explanations for the 10% myth. But don’t even try to start a new 50% myth based on the 1:1 ratio, because the expanding research field on glial cells is revealing that these once neglected cells are crucial for the development and functionality of the CNS, and play important roles in several neurological diseases.
Curiosity: the name of the glial cells comes from the Greek word “glue” and “to stick”, and as such these cells were thought to be non-functional glue that keeps neurons together.
So what are glial cells? They are divided into macroglia, the group of several cell types that have a similar origin as neurons (ectoderm), and microglia, a single cell type that has a hematopoietic origin, similar as blood cells (mesoderm). The most abundant and well studied glial cells of the CNS are oligodendrocytes, astrocytes, and microglia. Of the glial cells, the oligodendrocytes are the most abundant (45–75%), followed by astrocytes (19–40%), while microglia contribute to 10%.
Ok, so you know what glial cells are and I gave an idea of their numbers, but what do these cells REALLY do in our brain if they are not neurons?
These cells are responsible for producing the myelin sheaths that protects the neuronal extensions and most importantly, enable the fast transmission of the electrical signal. Without myelin, the signal is conducted at speeds of about 0.5 to 10 meter per second (think this is fast?). Myelinated neurons can conduct velocity up to 150 m/s (equals to 540 km/h, faster than a formula 1 car!). Imagine the first is like having a normal internet cable connection and the other one is an optic fibre internet capable of 10x more speed! Whenever oligodendrocytes malfunction or die, the neuronal signal becomes slow and neurons are susceptible to cell death, which leads to neurological deficits. Examples of demyelinating diseases are multiple sclerosis and neuromyelitis optica.
With their name coming from their star-like shape, these cells responsible for the brain scaffold, are also the ones who provide a barrier between our central nervous system and the rest of our body (called blood-brain-barrier for obvious reasons). Astrocytes make contact with our blood vessels, like a hug, and are like gate keepers, or the bouncers from that expensive night club, that only allow VIP to go through from the blood to the brain. They have many other functions and give support to neurons by, for example, recycling neurotransmitters and secreting growth factors. Whenever the BBB is broken, complications can occur including infections, such as meningitis, and neuroinflammation, such as the inflammatory reaction from the infiltrating peripheral immune cells into the brain (which is one of the characteristics of multiple sclerosis).
The smallest of the glial cells (that’s why they are called micro), microglia are the resident cells of the brain and they do not originate there. These cells come from the yolk-sac to the brain even before neurons are mature during embryogenesis. After they are established in the brain, microglia self-renew without contribution of peripheral cells from the blood (don’t even try to replace them or kick them out, they belong where they belong!). Microglia are the immune cells of the brain (specifically, the brain macrophages), and are very versatile cells. As sentinels and warriors, they continuously sense their surroundings and can become activated and migrate to sites of injury or to fight infections. Moreover, as cleaners, microglia are responsible for removing debris (rest of dead cells and other harmful particles such as protein aggregates). Also, as barbers/hairdressers, they trim neurons to ensure that they correctly meet each other at the synapses (who wouldn’t like a nice hair cut for a successful date?). Finally, microglia also play the role of nurses, since they can promote neuroprotection and remyelination by secreting growth factors. With such a diverse role, it is not so surprising that microglia participate in several diseases, ranging from Alzheimer’s and Parkinson’s, to prion disease, multiple sclerosis and stroke. There is a lot to talk about microglia, but that’s enough for now. These wonderful cells (my passion) deserve a post of their own.
Besides oligodendrocytes, astrocytes, and microglia, other glial cells in the central nervous system include ependymal cells, responsible for producing the cerebrospinal fluid (the liquid in which our brain and spinal cord “float”) and radial glia, progenitor cells that produce neurons in the cerebral cortex and also other glial cells such as astrocytes and oligodendrocytes. (see image below)
For the peripheral nervous system (the nerves and other neuronal structures other than the brain and spinal cord), the glial cells are different, and they are: Schwann cells, which have similar myelinating function as the oligodendrocytes; satellite cells, which help regulate the external chemical environment, similar in function to astrocytes; finally, there are also the enteric glial cells, which are related to the digestive system function. (see image below)
Now that you know what glial cells are, how abundant and how important they are, just be happy knowing that if you ever feel lonely, remember your neurons are never alone in your brain. 🙂
Some references/further reading:
Neuron vs Glial cells numbers