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Neuroscience
Do Some Thoughts and Feelings Live Outside our Brain?
It turns out our nervous system is much more than neurons
Posted May 16, 2015
Comedian Will Rogers observed “It isn't what we don't know that gives us trouble, it's what we know that ain't so.”
What we know, or used to know, is:
- Our nervous system is made up exclusively of neurons and glia
- Immunity to disease derives only from our immune system, (e.g. many varieties of immune cells, antibodies, signaling proteins such as cytokines, tissues such as thymus, bone marrow and spleen that make immune cells, etc.)
- Our brain does all of our thinking and feeling for us.
We now know that all of these things ain’t so.
Recent research has shown that we must broaden our definition of the nervous system to include both our immune systems and the trillions of bacteria that inhabit our bodies inside and out.
Our immune system communicates directly with our nervous system, acting as a sensor in the same way that neuronal sensors communicate to the brain. Immune cells secrete cytokines, which act as a neurotransmitter, alerting our brains that all is not well. As a result Neuroscientists are coming to view the immune system as just another neuro-sensor alongside rods and cones in our eyes, hair cells in our inner ears, tactile receptors on skin, proprioceptors (position sensors) in our muscles and chemical sensors in our tongues and noses. For example, when our immune system attacks foreign invaders, it releases cytokines that communicate to the nervous system, inducing “sickness behaviors” (taking it easy) and even depression.
Conversely, our immune system receives commands from the brain to change its behavior. Neurotransmitters such as catecholamines, 5-hydroxytryptamine, acetylcholine, histamine and neuropeptides, have all been shown to alter immune response. Activation of the vagus nerve, which innervates the spleen, can slow down the production of cytokines that promote release of Tumor Necrosis Factor (TNF), a major contributor to autoimmune diseases such as rheumatoid arthritis. Doctors have already begun to relieve symptoms of this autoimmune disease by electrically stimulating the vagus nerve, which instructs the spleen to decrease TNF production.
And the link between brain activity and other diseases has been well established. Chromic depression, for example, suppresses the immune system leading to increased infections, cardiovascular disease and other maladies. Conversely, happy people are usually also healthy people who live longer lives, in part due to strong immune systems.
The trillions of "biome" bacteria that inhabit our body (there are ten times more bacterial cells in and on our body than our own cells) also exert influence on our nervous system and behavior. Gut microbes, for example, send signals to our brains by releasing neurotransmitters that stimulate the vagus nerve, one of the brain’s cranial nerves. When certain microbes get “hungry” for nutrients such as sugars or fat, they tell our brain to eat sweet, fatty foods and our brain obeys. Overweight people often have a different mix of gut microbes than thin people, suggesting that an imbalance of gut microbes contributes to obesity. Secretions of gut bacteria also can produce anxiety and even depression, leading to new treatments of these mood disorders with probiotics (as in yoghurt) that restore healthy bacterial balance to the gut.
"Good’ microbes in our gut also play a crucial role in helping our immune system. Dr Sarkis Mazmanian and colleagues at Cal Tech found that gastro-intestinal bacteria promote the growth of immune cells, such as neutrophils, macrophages and monocytes, that fight invading pathogens both inside and outside our digestive systems. Since our immune systems “talk" to our nervous systems, gut bacteria may communicate indirectly to the nervous system through the immune system, in addition to the direct bacteria-to-neural communication paths.
Finally, our nervous system communicates with bacterial colonies on our skin as well as our gut. When our autonomic nervous system instructs glands in our skin to release sweat, some of this sweat contains neuropeptides that change the activity of skin microbes, which in turn can promote both health (skin microbes can help ward off disease) and illness (skin microbes can also cause infections such as MRSA).
From all of this discussion, it’s clear that the lines between our brain, immune system and the bacterial colonies that we host are rapidly blurring, and that some of our thoughts and feelings probably originate outside of our brains. The figure below shows just how complex the relationships among our immune system, bacterial colonies (biome) and brain can be.
But, as the figure suggests, there’s more.
Our very, very distant, invertebrate ancestors were basically simple “tubes” that took food in at one end, and secreted waste out the other. The nervous systems of these primitive creatures consisted of networks of neurons and ganglia that promoted the ingestion, movement, digestion and excretion of food.
As time progressed and our ancestors grew increasingly complex, this "tube”, and the nerves that operated it, evolved into a gastro-intestinal (GI) system that was distinct from other parts of the body such as musculoskeletal system and central nervous system (brain and spinal cord).
Today, the separation of “enteric” nervous system that controls our GI system from our brain persists. For example, during development of human embryos, the enteric nervous system grows out of different tissue than the central nervous system. In fully grown humans, the enteric nervous system contains about 500 million neurons, comprising a “brain” with almost as many nerve cells as the brain of a cat.
A growing body of evidence suggests that this sophisticated “second brain” not only influences the thoughts and feelings of our “primary brain”, but can even think on its own.
Feelings of excitement, stress and fear (e.g. butterflies) can originate in the gut and travel to the brain through signals transmitted from the enteric nervous system to the brain through the vagus nerve.
And the enteric nervous system appears to be capable of forming its own memories, even when its link to the brain is severely damaged. Dr Michael Gershon, author of The Second Brain : The Scientific Basis of Gut Instinct and a Groundbreaking New Understanding of Nervous Disorders of the Stomach and Intestines, tells the story of an Army medical ward where a nurse administered enemas to constipated paraplegics (patients with severed spinal cords) at 10 AM every day. When another nurse replaced the original and gave enemas on a different schedule, the patients continued to have bowel movements at 10 AM, as if the enteric nervous systems of the paraplegics had remembered when they were supposed move bowels.
One of the more intriguing questions that arises from the new findings about the complex relationships among our nervous systems, immune systems and bacterial colonies, is “what other truths about the origins of our thoughts, feelings, memories and behaviors ain’t so?”
Do other organs, such as our livers, kidneys and pancreases exert influence on cognitions and feelings? And what about organisms other than bacteria, such as mites, that inhabit our hair follicles? Do these microscopic arachnoids, like gut bacteria, tell our brains to engage in behaviors that promote their own selfish agendas?
Although we don’t yet know the answers to such questions, the truth is, it’s almost certain that we are not finished dispelling old truths and dramatically altering our beliefs about where our thoughts, feelings and behaviors come from.
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