What is the anatomy? How does it all connect? How does this change with depression?
Franz Joseph Gall, a German neuroanatomist, was the first to propose that specific parts of the brain control specific parts of our behavior. It turns out that he was correct; the brain is not a uniform ball of cells; rather, it is more like a DJ’s mixing table: press one button and the song changes; turn one dial and the beat changes. As psychologists like to say, “all that is psychological is biological”: our thoughts, feelings, and behaviors are all rooted in our living, active bodily tissue. All of the functions that make us human - including what we see, what we say, whom we recognize, what we remember - come from specific groups of neurons (cells) within the brain tissue. Furthermore, since our neurons are electrically excitable, if you were to electrically stimulate specific groups of neurons in a human brain, you could change what a person thinks, feels, or does. Stimulate one part of the brain, and the person’s leg will move; stimulate another part and they will see something that isn’t there; stimulate another part and they will recall a vivid memory from their childhood. As all of these aspects of the brain come together, we form what we know as the mind. The mind is what the brain creates. Or, as the saying goes, “the mind is what the brain does.”
The nervous system is composed of two parts: the central nervous system (CNS) and the peripheral nervous system (PNS). The central nervous system is composed of the brain and spinal cord and is mostly responsible for the creation of our thoughts, feelings, and behaviors: our minds. In contrast, the peripheral nervous system is the system of neurons which aid in gathering our sensations and reporting back to the brain. In depression, we are primarily concerned with the central nervous system, and specifically, the brain.
The brain has layers - like an onion. Its core is the oldest part of the brain. This core, sometimes referred to as the ‘Old Brain’, has similar structures to that of simple animals like reptiles and is therefore associated with our vital, automatic tasks. It includes the brain stem, which is where the spinal cord connects to the brain, and the medulla, which controls automatic functions like the breathing of our lungs and beating of our heart. It also includes the pons, which coordinates our sleeping and waking. The Old Brain also includes the cerebellum, which coordinates our ‘muscle memory’, balance, and the perception of time, as well as the thalamus, which receives all of our senses about what we see, smell, taste, hear, and feel. These Old Brain structures compose most of the brain for many reptiles. Humans do not have completely distinct brains from other animals, rather, we have developed additional brain layers on top of this Old Brain core. One of these additional layers is the limbic system. The limbic system includes the hypothalamus, which regulates hormones, pleasure and rewards, hunger, circadian rhythms, etc. It also includes the amygdala, which is involved in fear and emotions as well as memory formation. Also in the limbic system is the hippocampus, which is involved in learning and memory. Finally, on top of these two layers is perhaps the most complex layer: the cerebral cortex. This layer makes up about 85% of the brain’s weight and produces our ability to think, talk, and feel.
The cerebral cortex - or outer layer - of the brain has four main areas: the frontal lobe, parietal lobe, occipital lobe, and temporal lobe. Each lobe has unique functions and responsibilities. The frontal lobe is behind our forehead and is generally involved in our ability to plan, judge, think abstractly, speak, and is responsible for many features of our personality. The parietal lobe is at the top of the head and is involved in our sense of touch and body position. The occipital lobe is at the back of the head and processes information related to sight. The temporal lobe is at the side of the head (around our ears) and processes sounds, conversations, and our ability to communicate.
These are very simple descriptions of the brain’s structure and functions, and there are many sub-regions to these lobes and circuits which allow them to be interconnected. Despite the complexity of the brain, Franz Joseph Gall was certainly correct: different parts of the brain are responsible for different functions. Damage to specific regions of the frontal lobe can make us more impulsive, aggressive, or sexual, while changes to the hypothalamus can lead us to eat uncontrollably. Our thoughts, feelings, and behaviors are all the creation of these cells’ activities. All that is psychological is biological. Depression is biological.
Now that we’ve reached a simple understanding of the brain’s structures, let’s zoom in closer to understand the brain’s chemistry and its effect on how we think, feel, and behave. Let’s begin with a general discussion of how neurotransmitters and synapses work.
The synapse is where our neurons meet. It consists of the sending end of one neuron and the receiving end on the next neuron. The space between is known as the synaptic cleft. During a communication between brain cells, known as a “synapse”, electrical activity in one neuron leads to the pushing out of neurotransmitters, which are our natural brain chemicals. These neurotransmitters leave from the neuron’s terminal end and travel across the synaptic cleft to the receptors on the end of the next neuron, causing electrical activity in this next neuron. This is how information is relayed between neurons in the brain.
There are various neurotransmitters that produce different effects. We can better understand the disease of MDD and its treatments by becoming familiar with some of the most prominent neurotransmitters. To begin, norepinephrine is involved in creating our sense of wakefulness and alertness, as well as honing our attention and encouraging feeding behavior. Histamine similarly contributes to our level of alertness and attention. Acetylcholine is involved in learning and memory. Serotonin is involved in regulating our sleep and sense of wakefulness, and its receptors have been implicated in our regulation of emotional states and our sleep-wake cycles. GABA can block the firing of neurons and decrease worried or anxious feelings. Dopamine is involved in producing our sense of motivation, responding to rewards, learning, and also plays a critical role in our coordination of bodily movements. Each of these neurotransmitters is involved in the development of different neurological or psychiatric diseases such as major depressive disorder, schizophrenia, and addictions.
In addition to the neurotransmitters, the other major group of natural chemicals which affect our brain tissue are our hormones. These are chemical messengers which are released by our body’s glands, travel through our blood, and exert an effect on our brain and other tissues. These are more long-lasting, slowly acting messengers than neurotransmitters. Some of the body’s hormone-secreting glands include the adrenal glands which produce adrenaline and cortisol, as well as the pancreas which produces insulin. In addition, there is the thyroid and parathyroid glands which change our metabolism and calcium levels, the ovaries and testicles which produce our sex hormones, and the pituitary gland which communicates directly with the brain’s hypothalamus to coordinate many of our body’s hormones. All of these hormones affect our brains, and our brains can also affect these hormones. It’s a dynamic relationship. This brain-gland-hormone-brain loop creates the complex interplay between the brain and the body that we see in mental illness. Between the physical brain, its neurotransmitters, and our body’s hormones, there is a complex biochemical environment involved in the development of major depressive disorder.
Now that we have a general understanding of how the brain is composed, the function of its various lobes, and the effect of neurotransmitters and hormones, we can better conceptualize the many ways that the brain can become diseased. As someone with MDD or a loved one supporting someone with MDD, our ability to defeat the mental health stigma begins with recognizing the organ-ness of the brain. By recognizing the brain as an organ that relies on various biological systems for its proper function, we can appreciate how someone’s thoughts, feelings, and behaviors may change when the organ is diseased. Only by reaching this biological understanding can we begin to assign MDD the same importance and respect as disease of any other bodily tissue, like the heart or lungs.
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