The Building Blocks of the Brain
The brain contains some 86 billion neurons. While these cells are miniscule in size, they are vastly complex. An easy way to wrap your head around how neurons work is to look at their anatomy.
Basics of neuron anatomy. Daniela Gamba, Seung Lab.
Information flowing from one neuron to the next to the next! Daniela Gamba, Seung Lab.
The basics of neuron anatomy include a few main structures. These main structures are synapses, which allow for neurons to communicate with each other, dendrites, which receive incoming signals from other neurons, somas (cell bodies), which contain DNA and crucial information for the cell’s survival, and axons, which send signals to surrounding neurons.
Based on our understanding of basic neuron anatomy, we can zoom into real data acquired by the Allen Institute and view images of neurons reconstructed by Princeton University's Seung Lab!!
Images above: reconstructions by Princeton University's Seung Lab from data acquired by The Allen Institute. Funded by IARPA MICrONS.
Neurons are connected by synapses, and if all these connections are mapped, we have a connectome. Synapses are dynamic, and they constantly evolve as we experience and learn new things. Little is known about which neurons are connected and why, but researchers, even in our lab, are working to expand our knowledge in this field.
Among the known cells types in the brain, there are neurons that signal fast, and neurons that signal slow. There are neurons that fire in pulses and neurons that just turn their voltage up and down to communicate. There are neurons that connect vastly different areas of the brain, and there are neurons that only connect in their local neighborhood.
Pyramidal neurons from a cortical column. Amy Sterling, Seung Lab, using data from The Allen Institute funded by IARPA MICrONS
Cells in the Brain
Similar to the various species that exist on planet earth, there are many different kinds of neurons and cells in the brain.
In fact, scientists don’t even know exactly how many types of neurons there are in the human brain! There are at least broad categories of cells that most neurons fall into.
The primary cell of the brain is a pyramidal neuron. They get their name from a slightly triangular-shaped cell body. Pyramidals cause other neurons to fire more when they send an action potential, so we call them excitatory.
There are many types of inhibitory cells. They dampen the firing rate of cells they synapse to. Some of the main types are shown in diagrams below.
Glial Cells (Glia)
Glia are non-neuronal cells that assist existing neurons. While glial cells themselves do not send electrical signals to other cells, they are extremely important because they ensure that the environment is ready to go for neurons to do their thing. In fact, glia are so important that they outnumber neurons by a ratio of about 3 : 1! There are different types of glial cells including astrocytes, oligodendrocytes, and microglial cells. And no, these are not the names of dinosaurs that once walked on earth!
Astrocytes mainly function to provide and maintain a good chemical environment for neurons. Oligodendrocytes lay down myelin for some but not all neuronal cells, contributing to how quickly neurons can communicate with each other. Microglial cells are the resident immune cells in the brain. They remove cellular debris from areas of injury and secrete cell signaling molecules like cytokines which play a role in the regulation of inflammation and cell survival or death.
Astrocytes create "force fields" around neurons. They ensure that the chemical environment surrounding neurons is good for signaling. Image: Daniela Gamba, Seung Lab.
Oligodendrocytes are like car mechanics. They provide "accessories" for neurons that make neuronal signaling more efficient. Image: Daniela Gamba, Seung Lab.
Microglia are like medics arriving to the scene of an accident. The more severe the injury, the more microglia arrive to help regulate inflammation and save the day. Image: Daniela Gamba, Seung Lab.
How Neurons Communicate
Neurons talk by sending electrical and chemical signals to one another. When a neuron is sending or receiving a signal, we say that the neuron is active. You could think of the neuron's signal relay as similar to how many people in a city might be talking, texting, or tweeting with each other. Someone makes a popular tweet that goes viral and suddenly many others are relaying that message far and wide.
Neurons can communicate across long distances, similar to how we can reach our friends across country through cell phone calls! Daniela Gamba, Seung Lab.
Our brains are constantly active - even when we are asleep! Neural activity refers to the spontaneous activity in the brain that allows us to function on a daily basis.
On a relatively macroscopic level, sophisticated behaviors important for human survival require the coordinated neural activity of many different brain regions: the communication of different brain regions. To learn more about how the brain integrates different regions in order to initiate functional behaviors, visit our page on Systems Neuroscience!
On a more microscopic level, the entirety of neural activity can be traced to the activity between brain cells, most specifically the signals sent between neurons: action potentials. To learn more about the impressive communication system in the brain, check out our Action Potential deep dive page!
Modeling Neuron Behavior
While it might seem silly to model the behavior of neurons because they are so small, models of neuron behavior can have pretty significant implications. For example, researchers studying epilepsy have found great value in computational models of neuronal activity. These models have provided important applications for therapeutic purposes, and implantable devices have been developed that detect seizures and respond appropriately so that seizures are suppressed.