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Subcortical Brain Structures, Stress, Emotions, and Mental Illness

Brain midsagittal view

We will only discuss the major subcortical structures of our mammalian animal brain. Our ancestral brain is not nearly as simple as the following short discussions might imply. In Affective Neuroscience: The Foundations of Human and Animal Emotions (1998), referring to the instinctual emotional processing that goes on in our subcortical brain structures, Jaak Panksepp notes the likelihood that, "in a deep evolutionary sense, many of the complex information-processing potentials of the cortex are servants (often unconscious, automatized servants) to the dictates of the affective forces that ruled behavior prior to cortical evolution."

In An Odyssey with Animals: A Veterinarian's Reflections on the Animal Rights & Welfare Debate (2009), Adrian R. Morrison provides a great description of just how mammalian we humans are. We humans share common subcortical brain structures with all other mammals. Morrison writes:

My cat, Buster, flinches and yowls or curses at a sudden painful stimulus, and our legs both jerk in response to a tap on the patellar tendon of the knee. The spinal organization of the neurons responsible for these activities is the same in cats as it is in humans.

Moving forward into the lowest part of the brain, in both Buster and me the same neurons control basic bodily functions, such as regulation of breathing, heart rate, and vomiting. Farther forward reside the nerve cells that regulate the behaviors of sleep and wakefulness, which are identical in humans and other mammals, and where dysfunction results in similar problems, such as narcolepsy … and REM sleep behavior disorder. In this brain region in all mammals are found the neurons containing the neurotransmitter dopamine, which degenerate in Parkinson's disease.

At the base of the cerebral hemispheres is the almond-shaped amygdala, where mechanisms leading to fear and anxiety in people and animals operate. Monkeys and rats have contributed much to our understanding of the amygdala. The overlying cerebral cortex is where all of us mammals analyze the sensations coming from the skin, muscles and joints via the spinal cord, or eyes and ears in the cases of vision and hearing.

Where we depart from our animal brethren is in the great development of the front part of our cerebral cortex, the frontal lobes, and the greater proportion of cerebral tissue, called association areas, which integrate the information obtained from the regions that directly receive sensory information. These latter regions are called the primary sensory and motor areas because they receive simple, pure sensations and direct the movement of the body. It is within the frontal lobes that we humans mull over the past, prepare for the future, and reflect on its implications. Animals do not have this last capability in particular, as far as we can discern. Animals prepare for the future in a limited, instinct-driven way: Think of squirrels gathering and burying nuts for the winter. …

Our animal brain:

Temple Grandin and Catherine Johnson, in Animals in Translation: Using the Mysteries of Autism to Decode Animal Behavior (2005), recount Grandin's first encounter with real brain tissue. "The pig brain was a big shock for me because when the lower-level structures like the amygdale are compared to the same structures in the human brain we couldn't see any difference at all. The pig brain and the human brain looked exactly alike. But when we looked at the neocortex the difference was huge. The human neocortex is visibly bigger and more folded-up than the animal's, and anyone can see it. You don't need a microscope."

HypoThalamus Thalamus Hippocampus

The illustration above links to an NIMH fact sheet on autism spectrum disorders. In the illustration, you can see the amygdala, hippocampus, and corpus striatum. Remember that both the left and right hemispheres contain each of these structures, which are mirror images of each other. The pair of structures called the corpus striata has, in the past, been referred to as the basal ganglia. The corpus striata play a key role in generating obsessions and compulsions. We will discuss the corpus striata structures in more detail in Parts 2 and 3 of

Cerebral cortex and basal ganglia

n the picture to the right, all of the strangely shaped subcortical nuclei are nestled within the much larger and more consistently formed neocortex. John A. Beal, Department of Cellular Biology and Anatomy, Louisiana State University Health Sciences Center, provides this image. What has traditionally been called the basal ganglia, and what is called the corpus striatum, is labeled 1 and 2. The thalamus is labeled 5 and the hypothalamus is labeled 7. In my view, a very powerful force—whether you see it as God, evolution, or a concerted effort—stitched the subcortical nuclei together as needed to ensure the survival and continued development of animal life.

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