Why neurons are awesome
How do you see? How do you feel? How do you hear? Our ability of perception is possible because of our nervous system. Not only does it let you detect your surroundings, but actually recognize them. It is responsible for the fact that you can remember you’ve been in a specific place before, and why you’ve been there before. Your ability to read this article is dependent on your nervous system, and so is your ability to not walk in front of a bus when it’s speeding across the street.
Neurons, which transport information throughout the body, are often referred to as the building blocks of this nervous system. There are more than 85 billion neurons in your brain alone, each connected to as many as 10,000 other neurons. Together, they form an unimaginably complex but incredible network of communication that is essential for, well, pretty much everything. So, in essence:
Neurons are awesome.
As opposed to your stereotypical avocado-shaped cell, a neuron has a really funky shape, as seen above. Though it costs a lot to form and maintain, the shape is crucial for its function. Neurons can be broken into 3 main parts:
The Cell body or Soma, which receives and synthesizes incoming information. It also contains the genetic details of the cell.
The Dendrite, which takes inputs from other neurons and brings it into the soma.
The Axon, which is a long, tail-looking, extension that sends output information to other neurons. At the end, it branches off as it “communicates” with other cells, in a place we call the axon terminal.
How neurons send information
If inputs, captured by its dendrites, stimulates the neuron enough, a neuron will send an electrical impulse down its axon which sends another input to other cells. This impulse is called an action potential and is the quick rise and fall of the cell membrane’s voltage. This rapid back and forth voltage shift is caused by the movement of charged atoms, or ions, across the axons membrane. To really understand how they work, we have to first look at…
The set up of a neuron
All animal cells, including neurons, are normally more negatively charged than the salty fluid around them. The difference in electrical charge between the inside of the cell and outside of a cell is called membrane potential. When the cell is not stimulated, neurons have a membrane potential of -70 millivolts (mV), which means the inside of neurons are 70 millivolts more negatively charged then it’s surroundings. This “resting” membrane potential is called a neurons resting potential.
When the voltage outside of a cell is higher than the inside, we call it polarized. Being depolarized means the inside voltage is rising, making the cell less polar. On the other hand, being repolarized means the inside voltage is dropping, making the cell more polar. Additionally, when the inside voltage drops a lot more, its called hyperpolarization. It's basically the same thing as repolarization, just more extreme.
When cell body is stimulated enough to cause the membrane potential to rise above the threshold, the neuron “fires”.
In the section where the cell body is connected to the axon or the axon hillock, the voltage inside the neuron (otherwise known as the membrane potential) rapidly rises and falls, leading the depolarization to spread down the length of the axon. This fluctuation of membrane potential is our action potential, and it all happens in about one millisecond! For perspective, that is 100 times faster than your quickest blink.
The increase in membrane potential causes the next part of the axon membrane to also rise in charge. This continues until the action potential has traveled down the entire length of the axon.
This fluctuation of membrane potential is caused by the movement of certain ions (charged atoms), mostly Potassium/K+ (the thing in your bananas) and Na+ (the thing in your salt). For the ions to move across the cell membrane, they go through voltage-gated ion channels. These are small passageways that let a specific ion through when the membrane potential around them reaches a certain charge. (Example: The voltage-gated sodium channel opens when the cell membrane reaches -55 mV)
Action potential timeline:
An initial stimulus and/or strong enough input, sent from other neurons, triggers the membrane potential to rise from -70 mV to the threshold, -55 mV. If the stimulus is not strong enough to pass the threshold, we call it a graded potential.
Once the membrane potential is -55 mV, the Na+ channels open. Many positively charged Na+ ions rush into the cell, drastically increasing the voltage inside the cell. As the charge increases, the Na+ spreads out, creating a chain reaction as it opens all the other sodium channels. This increase in membrane potential depolarizes the cell, making it less polar.
Once the cell reaches a positive 30 mV, the K+ channels open and Na+ channels close. The Na+ stops coming in and positively charged K+ rushed out of the cell. The now positive membrane potential dives. This decrease in membrane potential repolarizes the cell, making it more polar.
Once the membrane potential becomes more and more negative again, the K+ channels close. The channels close a little bit too slowly causing the to dip a bit below it’s resting potential of -70 mV. This extra decrease in membrane potential hyperpolarizes the cell, which makes it even more polar.
The work of Sodium-Potassium pump restores the distribution of K+ and Na+ ions to get ready for the next action potential. The cell then goes back into its resting state.
When a neuron passes the threshold, it opens up all of the sodium channels, no more, no less. Therefore, all the impulses are always the same size and power. No sodium channels will open until you pass the threshold. Because you either only get all of the channels or none of them, its called the all or nothing principle. For neurons to show intensity, like the difference between a punch and a puff of air, instead of using the size of impulses, it fires more action potentials in a shorter amount of time.
A note on myelin
Once the action potential is fired, the change in membrane potential travels down the axon which is wrapped by myelin, an insulating sheath that significantly increases the speed that the impulses travel. Between the myelin sheets are the nodes of ranvier, small spaces around 70 times thinner than a strand of your hair (or 1 micrometer thick). These gaps are filled with ion channels, which helps repeat/reignite the action potential. When the nerve impulse travels down the axon, it jumps between nodes through a process called saltatory conduction.
Neuron communication - Synapses
When the signal reaches the axon terminal at the end, it branches off to relay the information to other neurons. The section where two neurons meet is called a synapse, and is formed by the meeting of two things: the presynaptic (sending) neurons' axon terminal; and the postsynaptic (receiving) neurons' dendrite. Most of the time, neurons do not touch, and are separated by a small gap called the synaptic cleft.
The action potential reaches the end of the presynaptic cells' axon terminal, increasing the membrane potential around it.
The rise in membrane potential opens voltage-gated Calcium/Ca+ channels, causing Ca+ to rush in.
The Ca+ bonds to synaptic vesicles, little balls filled with chemicals. It releases chemicals, called neurotransmitters, into the synaptic cleft.
These neurotransmitters interact with receptors on the postsynaptic cells' dendrite, and triggers the opening or closing of ion channels. Different neurotransmitters open or close different channels, letting in or blocking out certain ions from entering the cell.
The different distribution of ions, or charges, either raises or lowers the membrane potential of the postsynaptic neuron, causing a stimulus. If enough positive ions are let in, and/or enough negative ions are flushed out, then the stimulus can trigger an action potential, passing along the impulse. The stimulus can also inhibit the next neuron by either letting in enough negative ions or sucking away enough positive ions, making it harder to fire.
I like this video because not only does it have some really cool sound effects, it also visualizes the entire process, from action potential to synapse transmission:
Note: It is a simplified animation, so some details such as myelin and the nodes of ranvier are omitted
0:00 - 0:03: Epic brain zoom in
0:04 - 0:10: Action potentials as they spread out through the neuron system
0:11 - 0:17: Na+ ions crossing the membrane into the cell, raising the voltage, and causing an action potential (which is the white band).
0:18 - 0:27: Cell depolarization opens Na+ channels (not shown), sucking in Na+. Repolarization and the work of the Sodium-Potassium pump, or ATPase, pushes the Na+ back out, getting ready for another action potential.
0:28 - 0:34: Impulse triggers the release of neurotransmitters (orange balls) into the synaptic cleft
0:35 - 0:38: Neurotransmitters open ion channels
0:39 - 0:42: Ions (blue balls) flow into the postsynaptic cell, in this case, stimulating another action potential.
0:43 - 0:59: Same thing, but slower.
How neurons make decisions:
When you make the decision to, for example, eat a cookie, your brain weighs the advantages and disadvantages. Advantages such as, "It tastes good" or "My brother is eating them too" could be used, while disadvantages like, "They are not healthy" or "They are old" could be considered. If the advantages outweigh the disadvantages, you would make the decision to eat the cookie, and vice versa.
How does your brain decide do that? Synapses.
Due to the high density of sodium ion channels, the axon hillock is when an action potential originates. If the membrane potential reaches the threshold there, it’ll spread down the axon and signal other cells that you will “eat a cookie”. Therefore, the decision to "eat a cookie" is made if a strong enough stimulus hits the axon hillock.
Most stimuli are inputs from other neurons. Advantage and disadvantage neurons connect to our “eat a cookie” neuron at synapses, and excites or inhibits an action potential.
Excitatory, like the “It tastes good” synapses will temporarily increase our membrane potential by letting in positively charged ions such as Na+, in an action we call excitatory postsynaptic potential (EPSP). Inhibitory, like the “They are not healthy” synapses will temporarily decrease our membrane potential by either letting in negatively charged ions, such as Cl+, or letting out positively charged ions like K+. We call this inhibitory postsynaptic potential (IPSP). If enough EPSPs occur simultaneously, the membrane potential will be enough for an action potential to be fired. The more IPSPs that occur, the harder it will be to start an action potential in the postsynaptic neuron.
Not all synapses, advantageous or disadvantageous, effect your decision equally. The synapse is stronger if the size of the receiving dendrite, how close it is to the axon hillock, and/or, there is an increase of released neurotransmitters. This way, more positively or negatively charged ions are let through. For example: the closer “the cookies are old” synapse is to the axon hillock in my brain, the more I would think that “the cookies are old” is important.
These inputs centralize in the cell body, and the net charge, or summation, reaches the axon hillock. If the summation of charges is above -55 mV, then the signal is passed on to the next neuron as an input, and the process repeats.
To learn is to grow
Learning changes the brain. Learning causes neuroplasticity, which creates new synapses that connect different neurons. It also causes synapses to be strengthened in a process called Long-Term Potentiation (LTP). This increases the efficiency of the synapses, and can last for hours, weeks, or even forever, and is essential for memory and long-term learning. The more we use them, the more neurons myelinate, making the myelin sheaths thicker, increasing the speed at which action potentials are sent along the axon. This explains why when you learn about something more, the more it makes sense, and the more easy it becomes. But also explains why you begin to forget things right after you learn them, which helps emphasize the importance of practice and repetition.
Neurons are one of the most perplexing, amazing, and important cells in our body. Their dance of ions allows us to perceive our surroundings, remember it, and question it. The inconceivably complex neural system that they form, and the one that we take for granted, defines not only who you are, but who we all are as a species. Somehow, the billions of teeny tiny blobs between your ears can muse about the meaning of life. Somehow they can retract your finger quickly, after touching a hot pan, faster than you can even realize that you did it. Or just somehow can decide if you want to eat a cookie or not.
It is the miracle of the neuron.
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