In an online lecture, neuroscientist Robert Sapolsky asked his Stanford students what behavior looks like. If we look back a half a second, we are talking of the world of neurons and circuitry. This world gets most of our attention.
But, says Dr Sapolsky:
Just as we are about to get happily settled into that bucket, we push back a bit and say what smell, what sound, what sensory stimulation in the environment caused those neurons to get activated and produce that behavior?
Then we need to push back one step further, to hormone levels in the blood in the last few hours that changed how sensitive we are to those sounds and smells.
We can work our way back even further, through early development, fetal life, genes, and natural selection. Human behavior is a complex of interconnections between all these worlds. But let’s focus on one, for now: the interconnection between neurotransmitters and hormones.
Both are chemical messenger packets. Neurotransmitters work in the synapse between brain cells, hormones in the blood. Some packets, such as norepinephrine, operate in both locales, and are considered both neurotransmitters and hormones.
The HPA Axis
Where do we start? Where is the start? Let’s start with the brain’s limbic system as it senses danger. Bear The limbic system sends a signal to the hypothalamus, just above the brainstem. This sets in motion a hormonal cascade known as the HPA axis.
The hypothalamus is a gland whose main function is to link the nervous system and the endocrine system. In the context of the stress response, the hypothalamus releases the hormone CRF through a short network of blood vessels to the adjacent pituitary, known as the master gland. In response to CRF, the pituitary secrets another hormone, ACTH, into the blood stream, which prompts the adrenal glands above the kidneys to release adrenalin and glucocorticoids, which primes the body for running like a deer or fighting like a tiger.
Way back in 1884, the psychologist and philosopher William James raised this intriguing question: When running from a bear, do we run because we are afraid or are we afraid because we are running?
Another way to phrase it: Are our hackles already rising before we are even aware of the bear? Are we actually running before we know we are running from a bear?
Ah, the mysteries of the neuroendocrine system.
**More Neuroendocrine Mysteries **
Nervous system or endocrine system - which is acting upon which? For instance, the pineal gland in the brain produces the hormone melatonin in reaction to light. Melatonin is involved in regulating sleep and our circadian rhythms.
Melatonin is the end product of serotonin. So, are we depressed because we can’t sleep? Or are we not sleeping because we are depressed?
Then there is the thyroid. Insufficient thyroid function may mimic depression. A thyroid pill may resolve the depression. But what caused the thyroid to shut down in the first place? Stress?
Then there is the pancreas, which regulates blood sugar. Junk food plays havoc with our blood sugar, which can result in mood crashes and mood spikes. To make matters worse, our depressions may set us up for carb and sugar cravings. It’s not the sugar we are addicted to - it’s the serotonin that sugar helps produce. So are we getting depressed because we eat bad food, or do we eat bad food because we are depressed?
Finally, let’s not forget testes in men and ovaries in women. All great works of art, literature, and music owe their existence to these glands. Does estrogen account for the higher rates of depression in women? Or is testosterone masking men’s depression in anger and antisocial behavior? Is love the answer? Or does that make us even more crazy?
Women get a bad rap for their fluctuating estrogen and progesterone levels, but surging testosterone in young men is the source of just about all violent crime, and declining testosterone results in some very grumpy old men.
Two Feel-Good Hormones
Oxytocin, which also acts as a neurotransmitter, is produced by the hypothalamus and secreted through the pituitary. Fall in love and watch what happens. The hormone is involved in orgasm, child-birth, breast-feeding, attachment, and feelings of trust and empathy.
If falling in love is too extreme for you, you may stimulate the feel-good effects of oxytocin through yoga, exercise, or just being nice to people.
Vasopressin, tag-team partner of oxytocin, is mainly involved in water retention and blood-constriction, but the difference between monogamy and promiscuity may lie in just a small concentration of these receptors in the right place.
The Moral to this Story
So which influences which? Maybe it’s best to think of our nervous system and endocrine system as one system, both firmly embedded in the other, each, in turn, highly responsive to our environment. In many ways, we are the sum total of all three, which doesn’t leave much room for free will. "I think, therefore I am." Or is my own thinking merely an illusion?
Something to think about.
Also check out: Our Neurotransmitters: It’s All About Signaling and Circuits