The Building Blocks of the Brain
The brain contains some 86 billion neurons. While these cells are miniscule in size, they are vastly complex. Let's start to think about how neurons work first by looking at their anatomy.
A neuron has a soma as its center, where its nucleus and most organelles are found. It's branches are called neurites and come in a couple types. Dendrites, shown as the spiky branches at the top, receive action potentials from other cells through synapses. Each cell has one axon that can fork hundreds of times through which it sends its own impulses to other cells.
Daniela Gamba & Amy Sterling
Information flows through synapses from one neuron to the next to the next!
Daniela Gamba & Amy Sterling
Take a closer look at these 3D renders of those structures. These reconstructions are shown at nanoscale resolution and were mapped by AI at Princeton University's Seung Lab using electron microscope images acquired by The Allen Institute.
Images above: Amy Sterling, Seung Lab
Neurons are connected by synapses, and if all these connections are known, we would 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 are working to expand knowledge in this field.
Inside a synapse. The yellow dots are vesicles that are full of neurotransmitters like dopamine and serotonin. They are released when an action potential passes through the axon (purple) and are absorbed by the dendrite (blue). Image: Amy Sterling
Pyramidal neurons from a cortical column. Each spine sticking off these neurons is a synapse. Image: Amy Sterling
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. 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.
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. Their somas look a bit triangular under a microscope. Pyramidal neurons 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 above.
Image pair: Daniela Gamba & Amy Sterling
Blue: pyramidal neuron; yellow: 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 sometimes outnumber neurons! 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.
Above: glia (teal) next to a pyramidal neuron (pink).
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.