Now that we got the right mindset, and understand how the brain processes stimuli from the environment, we can safely continue with the study of our physiology and how the nerves work.
Let's talk a bit about the chemical path. The action potential (our current) runs along the tail of the nerve (called its Axon) and meets at the end of the next nerve (called his dendrite), in their meeting point the current allows the release of chemical compounds called neurotransmitters (NT from now on).
These NT's are released and attached to the dendrite of the following nerve.
And when they do, they create a chemical reaction that leads either to an excitation of that nerve or its inhibition. In order to activate the next nerve we need to get a certain threshold of excitation.
Understand that a single nerve can receive input from many other nerves, and the summation of their excitation or inhibition signals is what determines whether or not the current will continue forward.
There is a delicate balance between the "on" and "off" switches that allows a fine control over which messages get through and which not, and in which cases. If it does get through, then the current continues to run forward to the next nerve in line and can lead to a certain action (such as the contraction of a muscle).
There isn't really a “final stop” for this current - just like in an electric circuit. We have feedback loops that get regulated by the brain, and they lead to a change in the pattern of the firing signals (the action potentials). The intensity and intervals of such signals are really what’s changing, but there isn’t some “off switch”, not even when you sleep.
Here's an excellent video that sums what we discussed so far. It also adds some extra info on how exactly the action potential works, so if that confuses you – don't worry, it's not critical to the understanding of the rest of the series.
Besides the "normal" NT which simply send "activate" or "deactivate" signals, we have some special ones, which I'm sure you heard about. These "special" NT transmit signals that affect your mood and trigger special states. Let's see some examples:
Norepinephrine and Epinephrine (or adrenaline) -
"I just love the adrenaline rush"
Well, your body probably less so - since it's used mostly in a state of emergency by your brain and the autonomous nervous system (ANS) to enhance your vigilance and readiness for action. I'll elaborate more when we get to the ANS.
Both types are also secreted by the adrenal gland (which sits on top of your kidney) and then they serve as a hormone in your body in times of stress. (On the difference between Neurotransmitters and Hormones - read on)
Dopamine - the satisfaction NT, although it has some other jobs as well. When you have cravings - you can thank this stuff, since it's released in your brain when you experience pleasure or satisfaction. What for? To encourage you to do more of whatever you're doing at that moment. If you taste something and it feels good - this is your brain's way to remember it as a good experienc, which he'll encourage you to repeat.
Now, obviously not all the stuff that make you feel better is really good for you, and it's no accident that many drugs stimulate its release and absorption.
Drugs that encourage Dopamine release or increase its duration in your brain (like cocaine) stimulate us and can make us feel awesome and energized, but also make us psychologically addicted to that superficial feeling of hype.
Serotonin - This NT is involved in many many processes and can affect our mood, digestion, aggressiveness and sleep.
We know that drugs that increase serotonin levels in the brain help fight depression since they help alter your mood, and many antidepressants are based on this Seratonin.
Also, many psychedelic drugs bind to specific serotonin receptors in your brain and cause chemical manipulation which can make you see hallucinations and alter your state of mind.
Now let's talk about hormones - hormones are quite similar to NT, but where NT are used in communication between nerves, hormones are secreted by specific glands (not the brain!) located in different place in your body (for example the adrenal gland is located above your kidney). Once they're released, these specific hormones travel through your blood system to target other tissues.
Note: It’s not really critical info, but you can remember that NT are chemical messengers that work between nerves, and hormones are chemical messengers that work through your bloodstream. Some chemicals can be both - such as the adrenaline I mentioned.
They usually facilitate long term process such as in growth and maturation, or as a tool to facilitate certain states in your whole body. For example, when you experience fear, Cortisol and Adrenaline are immediately released to maintain your body in a state of awareness, to "keep the engines running" even some time after the danger is over. It makes sense, since you want to stay alert until you're absolutely sure it's safe to rest.
Let's see some other examples:
Oxytocin - AKA the love hormone. Up until recently this hormone was only related to the facilitation of post-birth processes such as lactation and the bonding of the mother with her child. But today it's also seen related to intimate feelings and sex in couples and in social behavior in general.
Androgens and especially testosterone - are the family of male sex hormones that determines the male gender and later its maturation. Besides that, there is a correlation between testosterone levels and aggression, assertiveness, dominance and risk taking behavior.
Note: although it's a "male" hormone, women have it too, only in much smaller quantities.
Estradiol and progresterone - 2 female sex hormones. They also facilitate the menstrual cycle, each is released in different stages to prepare the female egg for fertilization.
Cortisol - a stress hormone, which released in times of... stress, or when you have low blood sugar. Its job is to signal your body to release the reserves of energy stores in your cells and turn them into glucose. Glucose is like the basic unit of fuel in your body, which the muscles burn in order to operate. So naturally, when you sense there's a lot of action that's going to happen - you want those reserves up in the front, ready to be used.
One last introductory concept in neuroscience that I want to present you is the Autonomic Nervous System (ANS): The ANS is part of the central nervous system, but like its name suggests it autonomous, not in our direct control.
The ANS controls many things we usually don't need to think about but are critical to our survival - like breathing, keeping the heart pumping and digesting. Imagine if we had to actively think every moment about such things, when we barely remember where we put our keys... better leave it on auto-pilot.
This system has 2 parts, which are opposite to each other in function and therefore keep one another in check - the Sympathetic system and the Parasympathetic system.
Both innervate the same organs but send entirely opposite signals - where one makes your heart beat faster, the other slows it down.
The sympathetic system is responsible mostly for the stimulation and activation of your body, and as I mentioned above, it also uses epinephrine. It's mostly activated in times of stress or strenuous activity and does the following:
The parasympathetic system does exactly the opposite and is activated most when you relax and rest. After all, you need time to recover and can't be in turbo mode all the time.
I present these 2 systems because they play a critical role in our behavior and emotions, which we'll talk about later in the series.