In every course I teach, I wind up giving a lecture I call “A Crash Course in the Brain”. I like to think that any college student will have some basic literacy in the organ at the root of their thoughts, but I also suspect that this literacy comes from cop shows and assorted Internet sources of questionable repute. The Crash Course tries to get all students on the same page about how the brain works, before we explore topics of child development or belief or language in more detail.
The brain is made up of neurons, about 80-90 billion of them. A neuron is a special kind of cell (and I do assume my students have enough high school biology to remember what a cell is), usually drawn like a sideways tree. The “branches” of the tree are the dendrites, gathering information from other neurons; if enough information is received, the neuron sends a signal down the “trunk”, or axon, which ends in synapses with the dendrites of other neurons.
Individual neurons respond to very specific things, and each neuron ends up representing one specific thing. Neurons that process what you see can be very picky; there are many, many neurons that respond to lines, but some respond only to straight up-and-down lines, some to lines at a 10-degree angle, some to a 20-degree angle, and so on. Other neurons respond to faces. Again, many neurons have something to do with “faces”, but some respond to a face that’s head-on, or in profile; others respond to the expression on your face, but only for one particular emotion. You may even have one particular neuron in your brain that responds only to Jennifer Aniston. Neurons are not just limited to pictures or simple shapes, either; some very important neurons, called “mirror neurons“, respond to goals, like grasping a peanut or executing a plie, and seem to be critical to our ability to understand other people.
Neurons that respond to similar things tend to be found near each other, following the old saw “birds of a feather flock together”. The neurons that help make sense of vision tend to be found in the “visual cortex”, near the back of the head, while neurons that respond to touch are found in the “somatosensory cortex“, arranged by body part (with more space dedicated to fingers and other regions of the brain that are more sensitive to touch). This is useful, since neurons that respond to similar things probably need to talk to each other, and it would be awkward if those axons had to spread all through the brain like spaghetti; it’s also a potential problem, since an injury to one even a small region of the brain has the potential to wipe out an entire skill, like our ability to recognize other people, or to speak. Despite these sensible, arrangements, though…
Beware anyone who says they’ve found the X part of the brain, because any given region of the brain rarely does just one thing. There is a region of the brain called the “fusiform face area”, because it seems to be specialized for faces. But despite the name, it doesn’t just do faces; if you are a judge in a dog show, this region will respond to dogs, and if you are a car fanatic, it will probably respond to cars; researchers suggest the real fundamental role might be helping us make fine distinctions in complex images. Or take the prefrontal cortex, the region of the brain right behind your forehead, which is often held up as the difference between humans and “lesser” animals; it helps us hold information in mind, delay gratification, organize long-term memories, and make moral decisions, to name just a few functions. And not only does one region of the brain play multiple roles, but one skill can be spread across several regions of the brain, like the half-dozen different areas of the brain involved in understanding language.
Take care of your neurons, because if you lose any you aren’t likely to get them back. This is why paralysis is usually permanent; the axons connecting the brain to specific muscles in the body were destroyed, and the neurons can’t grow new axons or get replaced by new neurons. It was groundbreaking and paradigm-shifting when researchers discovered that humans can grown new neurons, but this ability still seems limited to certain regions of the brain (like the hippocampus, which is key in our storage of long-term memories), and we’re a long way from promoting regeneration without a lot of heavy mental lifting (such as the work London cabbies have to go through to learn the streets and get licensed, which helps grow the hippocampus). You might think that this lack of neuron growth is a problem: how can we learn if we only have so many neurons to work with? Well….
Connections between neurons are just as critical as the neurons themselves. Connections between neurons are how we learn; my entire purpose as an educator is to make use of the existing neurons, refining what they respond to and forging connections between this neuron/idea and that neuron/idea. The number of neurons might not have seemed so impressive, but there are an estimated 10 thousand trillion connections between neurons in the human brain (the only number I work with that is bigger than the national debt). It is these connections or synapses that may be the key to our “superior intellect” over animals, the reason behind the slight link between brain size and intelligence, and the foundation of creativity. This is something that we should keep in mind when we remember that…
The “chemistry” of the brain is in how neurons communicate at the synapse. At the end of the axon, one neuron releases chemicals called “neurotransmitters”, such as serotonin and dopamine. The dendrites of a receiving neuron respond to the presence of these neurotransmitters; if there are enough of them, the receiving neuron becomes a transmitting neuron that sends signals on to still other neurons. The popular phrase “chemical imbalance” (usually associated with depression, anxiety, or other psychological disorders) refers to the availability and effectiveness of these neurotransmitters. This is what most psychotropic medication tries to change; for example, the anti-depressant “selective serotonin reuptake inhibitors” work to keep the neurotransmitter serotonin in the synapse for a longer period of time, so the receiving neuron has more of a chance to recognize it and react.
And there you have your brain in a nutshell (although speaking from my neuroscience lab experience, only rabbit brains could possibly be mistaken for walnuts), the basic literacy I try to make sure all my students have before we delve into how the brain gets wired into this complex structure to begin with, how we can use those mirror neurons to “read minds”, or what goes wrong in the brain to contribute to psychological disorders.