Neuroanatomy for the Rest of Us
The basics of brain anatomy and fundamental neuroscience in plain English.
I hated high school biology. I was always a math and physics type person and biology never made any sense with all its rote memorization and complicated vocabulary. And like many things in my life, I wish I had paid better attention and learned more then. So a few decades later, I am attempting to learn basic high school biology. I’ve read some books and articles, watched several videos, and done lots of googling and wikipedia research to try to get an understanding of the basics. Better late than never…here we go!
The Obvious
The brain is the 3 pounds of ridged gray matter stuffed between our ears that controls all the functions and activities of the body, and interprets the information from the outside world. The brain doesn’t actually interact with the world, its locked away in our skull but takes input via various sensors (sight, hearing, smell, taste, and touch) and fundamentally acts to process that information and then drives action consciously (e.g. move left pinky, sing the high note now) or subconsciously (raising heart rate, sweating, etc.).
The following article will share at a high-level:
High-Level Brain Anatomy
As shown in the image above, fundamentally the brain is made up of the three pieces: the cerebrum, the cerebellum, and the brainstem. The brain and spinal cord make up the central nervous system and the spinal nerves and cranial nerves emanating from the spine and brain make up the peripheral nervous system.
Brain anatomy is often described in evolutionary terms: the oldest part of the brain is the Reptilian Brain, essentially the brainstem and cerebellum, responsible for the body’s basic vital functions such as heart rate, breathing, etc; the next oldest part of the brain is the Limbic System, first seen in mammals and straddling the brainstem and the cerebral cortex, responsible for emotions and value judgments; and the newest part of the mammalian brain, unique to primates and humans, is known as the Neocortex and drives all the fancy higher-level thinking associated with humans — language, abstract thought, imagination, consciousness, and learning.
This is a nice site where you can visually move around an image of the brain, zoom in and out, and read about the various anatomy.
Cerebral Cortex and the Main Lobes
The surface of the cerebrum is called the cortex. The folds increase the brain’s surface area so that more neurons can fit inside the skull. Interestingly the nerve cell body give the cortex its gray color — hence the name gray matter. Buried deep within the brain the nerve fibers (axons) are white in color and known as white matter.
The cerebrum is divided up physically into sections or lobes as can be seen in the image above. The main lobes of the Cerebral Cortex are the frontal lobe, occipital lobe, parietal lobe, and the temporal lobe. This image shares the key functions of each:
The much discussed prefrontal cortex (PFC) is part of the frontal lobe and responsible for the higher level cognitive functions and what we think of as consciousness. Although the diagram above shows one area responsible for one or more functions, the brain is highly interconnected. Take something like vision — the occipital lobe and the visual cortex do much of the raw data processing of the visual information passed in from the eyes, but interpretation of that data is done in conjunction with parietal lobs and perhaps temporal lobes. Facial recognition, for example, has evolved specially and has its own fully dedicated apparatus knows as the fusiform face area (FFA) in the temporal love.
“Rest” of the Brain
For brevity (and because I don’t fully understand all of it), I’m not going to detail all the subsections of the brain and their numerous details. The brain is a complex structure with many overlapping sections and interconnected layers between lobes, hemispheres, and sections. The limbic system, for example, sits physically underneath the cerebral cortex in the inner brain and borders the cerebrum and the brainstem. I have attempted to capture some high-level aspects of the anatomy and their key functions. For convenience I attempted to organize and summarize them in a mind-map. Mind map. See what I did there? :) Bad jokes aside, click on the image to expand.
Neuron Anatomy
Beyond the high-level structures of the brain, its super interesting to understand more about the internal working of the brain and the underlying neuroscience.
The fundamental building block of the brain is a cell known as the neuron or nerve cell. Neurons are interconnected to each other via synapses and communicate information via electrical and chemical signals. As seen in the picture above the neuron cell body, or soma, is the primary information processing center. Neurons pass messages along via axon and receive messages via dendrites. The connection between one neuron’s axon and another neuron’s dendrite is a small gap known as the synapse. Once a message is received by a neuron’s dendrite, its processed in the soma and potentially passed along through the axon and then communicated to one or many other neurons and picked up by their dendrites. A single neuron can have thousands of synapses effectively connecting to thousands of other neurons. While the neuron body and dendrites are quite small (microns) in order to carry nerve information from all over the body, the axon can be quite long — as long as 1 meter!
What’s also remarkable is that there are ~100 billion neurons in the brain and ~100 trillion synapses or connections. Yes trillion with a T. The brain is highly interconnected and neuroscientists are just starting to understand the complexity of the interactions and intercommunications.
Brain Circuity: Neurotransmitters and Hormones
The method of passing information between neurons is electro-chemical. At the end of the axon, the axon terminal, neurotransmitter molecules are released, cross the synapse, and plug into special receptors on the receiving neuron’s dendrite.
Fundamentally there are two types of neurotransmitter — excitatory and inhibitory. Excitatory neurotransmitters increase the likelihood that a neuron will fire; inhibitors decrease the likelihood. In this way secretion of hormones can enhance or diminish certain neurons from firing. This maps with what we know about something like adrenaline for example: while our brain is receiving environmental information as normal, the addition of adrenaline causes an excitatory response and kicks off the fight or flight systems.
On the other-hand certain neurotransmitters like serotonin, that are involved in emotions and moods, are inhibitors. This is why in certain mental health cases, like depression, physicians may prescribe SSRI’s (selective serotonin reuptake inhibitors) like prozac or zoloft. SSRI’s increase the level of serotonin in the brain by blocking the reabsorption of the chemical. Having additional active serotonin in the brain can help regulate those brain pathways associated with emotion and moods.
A final clarification: neurotransmitters are proteins or amino acids and released by the axon terminals into the synapse; hormones are produced in the endocrine glands and secreted into the blood stream.
Certain chemicals (e.g. serotonin, dopamine) are both neurotransmitters and hormones. There are MANY hormones, this table list just a few common ones:
That’s it for the very basic tour of neuroanatomy and neuroscience!
This article was the start of a series of posts about the brain. Feel free to check out the start of the series here which includes links to the other articles on behavior, learning, AI, and free will.
