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Have you ever wondered what your skin is made of or what your eyes are built from? The answer is the building block of all living matter— the cell. Cells form your skin. They form your eyes. And perhaps, most astoundingly, they form your mind.

These cells found in your brain and throughout the body are neurons. Like batteries, neurons generate electrical activity from chemical events. About 100 billion neurons, each with sometimes up to 10,000 connections, make up the human brain.

Well, the brain is complex. And if it weren't, we wouldn't be very complex. So it's a good thing.

The neuron is the basic unit in the brain. Electrical and chemical activity of neurons is the basis of all emotions, thoughts, and behaviors.

To think about it simply, it basically has a part that receives information from other neurons. It has a part that helps process that information. And then it has a part that sends that information— its decision— on to other, different neurons.

So you have a cell body, where the nucleus is. Then you have information coming in from the dendrites— this big arbor. It's like tree roots, just like that, branching out everywhere, taking in information to the cell body. And then there's one primary outflow tract. So we have dendrites, cell body, and the axon.

Many things come in. So what the neuron does is it sums all this stuff. And then when you get to a certain threshold, it fires. If you don't get to the threshold, it doesn't fire. So there's many, many ways to modulate, to control, this by all kinds of chemistry that comes in.

You can think of the neuron as a boss, sitting in her office waiting as her workers, the dendrites, gather chemical inputs and bring them to the cell body. As it is summed together in the cell body, more positive input brings the neuron closer to making her all or nothing response. And finally, it's time for a response.

Fire.

The cell fires.

So as a signal goes from the cell body, down the axon, to the end of the axon, it then has to communicate to the next neuron by releasing a neurotransmitter. And when it does that, the neurotransmitter leaves the end of the axon, crosses a very small, tiny space, lines to the dendrite on another neuron, and triggers a whole process in that other neuron.

So again, a cell is— a neuron is a cell. It has a cell body and a nucleus. It has dendrites that receive inputs from other neurons. And it has axons that send signals to other neurons.

The neuron's basic function is to conduct electrical and chemical information. It does this by a very elegant mechanism involving charged ions. When the cell is at rest, pumps and channels in the cell's membrane make sure that enough positively charged sodium ions are kept outside of the cell to maintain a slightly negative charge inside.

It's like a coil that's bound up waiting to spring. And what that does for neurons and for our brain is it allows neurons to fire really quickly. There are these networks that take considerable energy that keeps the charges different inside and outside of the neuron. And that allows it to be ready to fire.

This primes the cell to fire an action potential. When chemical input from other neurons enters the cell body via the dendrites, their total charge is summed together and can lift the cell out of its resting potential and above threshold. Once a cell hits threshold, it fires a burst of electricity known as an action potential. And the action potential is the same every time. It is an all or nothing response.

You reach a threshold and the neuron fires because you have enough gates open that the sodium rushes into the neuron.

The neuron fires, and the action potential leaves the cell body and travels down the axon. Along the way, it is sped up by a coating called a myelin sheath. Like the insulation around electrical wires, this cellular wrapping keeps the positive charge from leaking out of the axon as it travels to the next synapse. The myelin sheath is crucial for neuro-communication.

The sheath is formed by glia, a family of cells in the brain that account for most of the matter between our ears. They do everything from cleaning up toxic waste, to preventing infection, and transporting nutrients to neurons. Some glial cells also communicate electrically with one another and with neurons. It is clear that this family of cells plays a central role in our nervous system.

Now the point where an axon and a dendrite meet is really a space. And that space is called the synaptic space. And the connection between the two neurons is called a synapse.

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So when information comes down an axon and releases a transmitter, and it binds to the dendrite of the next neuron, it binds to specific structures on those neurons called receptors. And receptors are groups of proteins that work in a lock and key fashion. So the receptor is normally locked.

And when the neurotransmitter binds to the receptor, it has to go in just the right way, just as a key would fit into a lock. And it opens the lock and allows ions to pass between the neurons. When the key has opened the lock, ions in the extracellular space can cross into the other neuron, and influence the electrical properties of that neuron, and cause it to have action potentials, and so forth.

Neuron A can increase the chance that neuron B will fire, and that's called an excitatory. They're a transmitter. On the other hand, neuron A may— when this fires— play a role in inhibiting, or at least lowering the chance that neuron B will fire. And that happens via inhibitory connections.

There are drugs, or chemicals, that can either facilitate the transmission of information from one neuron to the other. And those are called agonists. Or there are also drugs that can inhibit the transmission of these neurotransmitters from one neuron to the other. And those are called antagonists. A lot of the drugs that are used to cure a whole variety of clinical disorders, such as depression, play a role in directly, at the synaptic level, helping to govern how well neurotransmitters will travel from one neuron to another. And that, in turn, governs cognitive function and mood.

The neurotransmitter serotonin is the target of many drug therapies. Selective serotonin reuptake inhibitors, or SSRIs, are used to treat disorders like depression. Reuptake refers to the process where the sending neuron reabsorbs the excess neurotransmitters in the synapse.

There's a serotonin theory of depression where it's believed that there's too little serotonin stimulating receptor cells. And so SSRIs like Prozac and Alexa Pro, and those sorts of things, what they do is they're reuptake inhibitors. So they keep the serotonin molecules in the synapse. It blocks it from coming back into the neuron that's firing so that there's more activation. And it raises that level of activation in the receptor cell.

Drugs for other conditions act in different ways. Sometimes the problem is too much transmitter, not too little. So instead of stopping neurons from reabsorbing chemicals, some drugs might stop them from getting into receptors.

The brain is anything but a static organ. Contrary to long held beliefs, we now know that the brain demonstrates plasticity. New neurons are born well into adulthood. And existing neurons are constantly adapting, shifting, shrinking, and growing new branches as we interact with the world.

So neurons continue to grow dendrites, or processes, connecting with other neurons in a way that allow them to organize. We're learning. And also there's the traditional model where neurons that fire together, wire together. And those pathways become more efficient and more likely to occur.

So it really is an issue of what we're born with— our genetics— understanding how our genetics sets the parameters, one extreme to another. But understanding how much the environment influences within that range that's determined by genetics. So neuroscience is not one of those things, it's all of those things. We're learning things on a subcellular, molecular level.

But we can now, by using imaging and other techniques, understand how those differences on the molecular level actually translate into function. And so it's that integration from the molecular to the macro which is the most exciting part.

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Fire.

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