These posts, tagged “Primer,” are posted for two reasons: 1). to help me get better at teaching non-scientists about science-related topics; and 2). to help non-scientists learn more about things they otherwise would not. So, while I realize most people won’t read these, I’m going to write them anyway, partially for my own benefit, but mostly for yours.
I can’t say I’ve been excited about writing this one, as brain anatomy is, quite possibly, the most boring thing I can think of to write about. I did a rotation at SLU in a lab that focuses on anatomy and how individual brain structures interact with one another, and that 6 week period was more than enough for me. As that professor told me, it’s very important work that someone needs to do, even if it may not seem all that interesting. This kind of work is how researchers have figured out which brain component “talks” to which other one(s), and how intertwined all these connections really are throughout the brain.
For the sake of this posting, I’ll simply point out that brain mapping has been carried out in a variety of ways. Quite a bit of it has been done over decades when people would hit their heads. If they would lose their memory, or their sense of smell, clinicians could localize the injury to a specific area of the head, then look at the brain post-mortem and see what happened. Ultimately, they would find a lesion of dead tissue in that region that lead to the deficiency. Similarly, the study of stroke victims also provided clues to the function of certain brain locations, as a stroke occurs when blood flow is cut off to an area of the brain, typically leading to damage. Alternatively, modern science uses a series of stereotactic injections of traceable materials in mice, rats and primates that can be visualized in brain slices, showing that a series of neurons in one area are connected with neurons in a separate region of the brain.
It is through this work that certain pathways were elucidated, including the reward pathway (very important for drug addiction, gambling addiction, etc.); the movement pathway (mostly for Parkinson’s disease, but important for voluntary movement, in general); the sensory systems (how the visual cortex signals, the auditory cortex, etc.); the amygdala (figuring out what this structure did and where it went led to quite a few labotomies back in the day); and memory (signals transfered between the hippocampus, the reward system, and the cortex…very complicated network…). It is through brain mapping like this that helped determine where everything connects together, and which areas are important.
While the human brain is a difficult nut to crack, it can be divided up into different portions. For the sake of this little blurb, we’ll focus on the three primary divisions of the brain: the prosencephalon (forebrain), the mesencephalon (midbrain) and the rhombencephalon (hindbrain).
The prosencephalon, or forebrain, is further divided into the telencephalon and the diencephalon. The telencephalon consists, primarily, of the cerebrum, which includes the cerebral cortex (voluntary action and sensory systems), the limbic system (emotion) and the basal ganglia (movement). As you can see from that list, for the most part, the telencephalon is what constitutes what “you” are: your thoughts, your feelings, and your interaction with the world around you. It’s where a lot of your processing happens. The telencephalon in humans is quite a bit more developed than in other species, which is really what separates their brain from other, lesser developed species (i.e. the human telencephalon is what really separates them from a chimpanzee). The diencephalon, on the other hand, consists of the thalamus, hypothalamus and a few other structures. The thalamus and hypothalamus are very important for various regulatory functions, including interpretation of sensory inputs, regulation of sleep, and release of hormones to control eating, drinking, and body temperature.
The mesencephalon is comprised of the tectum and the cerebral peduncle. The tectum is important for auditory and visual reflexes and tends to be more important in non-vertebrates, as they don’t have the developed cerebral cortex that humans do (more on that later). The cerebral peduncle, on the other hand, is a mixed bag of “everything in the midbrain except the tectum.” It includes the substantia nigra, which ties into the movement system and reward system. I think it’s fair to say that, aside from these things, the function of the midbrain, overall, has yet to be fully determined.
The rhombencephalon is quite important, even though it’s probably the oldest part of the brain, from an evolutionary standpoint. It includes the myelencephalon (medulla oblongata) and the metencephalon (pons and cerebellum). The medulla oblongata is important for autonomic functions like breathing and heart function. The pons acts primarily as a relay with functions that tie into breathing, heart rate/blood pressure, vomiting, eye movement, taste, bladder control and more. Finally, the cerebellum is important for a feeling of “equilibrium,” allowing for coordination of movement and action, timing and precision.
As you may have noticed, if you go from back-to-front, you’ll get increasing complexity in brain function. For example, the hindbrain is important for very basic things like breathing, heart rate, and coordinated movement. These are functions that are important in nearly all organisms, but especially so all the way down to the smallest worm and insect. Further up, the mesencephalon starts to work in further control of reward and initiation of voluntary movement, giving the organism voluntary control rather than solely reflexive control. Then, the diencephalon starts acting like a primitive brain, working in regulatory functions and more complicated reflex action to help maintain the more complex organism that has been assembled. And finally, the telencephalon yields the ultimate control over the organism, with things like memory, emotion, and greater interpretation of sensory inputs. As the image above dictates, the hindbrain (to the right-hand side) remains a large portion of the brain in the rat and the cat, but the human forebrain (the top/left-most portion) gets much larger, relative to the hindbrain. With that size comes greater development of brain structure and function.
So yeah, the brain is kinda complicated. Actually, it’s really complicated and, for the most part, I do my best to ignore all of the complex wiring networks that occur within. However, it is important work that needs to be done in order for surgeons to do what they do, and for neuropharmacologists to develop drugs that target some brain areas and not others. For the most part, I’ll leave this research to more interested people…