Our new MSc in Psychology and Neuroscience of Mind-Body Interface offered by the Institute of Psychiatry, Psychology, and Neuroscience at King’s College London aims to answer this question.
The blood has many functions, including, most notably, the spreading of oxygen to all our organs through red blood cells, and the distribution of immune cells (white blood cells) to the site of an injury of an infection. However, its most important function might well be the communication between the mind and the body by carrying hormones, that is, chemicals released into the blood that transmit signals between the brain and the body.
As part of our newly established MSc in Psychology and Neuroscience of Mind-Body Interface, we are producing a series of ITM pieces dedicated to educate and inspire students on the psychology and the neuroscience underlying brain/mind processes, and their connection with physical symptoms. We already have two pieces by the MSc lead, Dr Alessandra Borsini, on how this knowledge can project the clinical and academic career of students, and on the many mechanisms connecting the brain, the mind and the body. Today, we will talk about one such mechanism: hormones, and the blood that carries them around.
Today, we read hormones’ names routinely in our blood tests: cortisol, T4, oestradiol, testosterone... But this name was only created at the beginning of the 20th century, by two physiologists at University College London, William Bayliss and Ernest Starling.
Until then, the prevailing understanding of bodily functions was that nerves – i.e., direct connections originated in the brain through the sympathetic and parasympathetic nervous systems – are needed to regulate how organs, like the stomach, heart, and intestine, work.
However, in their 1902 experiments, Bayliss and Starling found that the duodenum (a part of the intestine) releases a substance in the blood that reaches the pancreas (the organ that controls glucose levels) and stimulates the section of the digestive substances. This was an incredibly original and creative discovery at that time.
Three years later, Starling created the term “hormone”: from the Greek participle ὁρμῶν (hormôn), the present participle of ὁρμάω (hormáō), that is, "setting in motion, to excite or arouse”.
The discipline that specifically investigates the bidirectional communication between hormones and the brain is called “psycho-neuro-endocrinology”. This discipline studies how some brain cells are part of both the nervous system (i.e., receive direct communication from other brain cells) and the endocrine system, as they secrete hormones in the blood to reach other parts of the brain or other organs. These psycho-neuro-endocrine mechanisms are very important for emotions and behaviours.
More recently, this concept has been expanded to include the communication between the brain, the endocrine, and the immune system, as often discussed on ITM. Immune cells are both moving targets of hormones and moving sources of hormones (called “cytokines” or “interleukins”) which not only interact with other immune cells but also with other organs, like the brain, the muscles, and the liver. The effects of cytokines on the brain are one of the most exciting recent developments in our understanding of emotions, behaviours, and mental health, and a central tenet of mind-body interface.
There are more than 50 hormones in the body, and so a summary of their names and function is necessarily quite limited. Here is my attempt to do it.
Adrenaline and cortisol are the two life-saving hormones that give us the energy and the concentration to deal with stressful situations. High levels of these hormones are good in the short term (minutes or hours) but bad in the long term (days or weeks), and they are involved in fear, arousal, anxiety, and sadness, and thus contribute to a range of mental health problems, from depression to panic attack and post-traumatic stress disorder.
FSH, LH, and progesterone regulate the physiology of the menstrual cycle and the preparation of the body for pregnancy. They are also particularly important for the instability of mood – the mix of sadness, agitation, and irritability – experienced during the days preceding the arrival of the menstrual period or the days following childbirth.
Interleukin-1 and interleukin-6 are examples of hormones secreted by the cells of the immune system. These hormones not only regulate the immune response, facilitating the elimination of viruses and bacteria during an infection, but also reach the brain and shut down our interest in social activities, as well as increase our fear of the external environment, so that we naturally go into “social isolation”, and thus the infection is contained as much as possible. These cytokines also affect the brain in situations of psychological stress unrelated to infections, contributing to mental health problems such as depression and anxiety, and can be targeted by anti-inflammatories as treatment for mental health problems.
Leptin and incretin decrease our appetite while ghrelin increases it. They are produced in different parts of the body (Leptin in the adipose tissue, ghrelin in the stomach, incretin in the intestine), reach the brain and regulate appetite. They are relevant to the cause, and potentially the treatment, of problematic eating behaviours and of metabolic disorders, such as anorexia, bulimia and obesity. Other hormones that regulate blood sugar levels, like insulin, also regulate hunger, while vasopressin regulates thirst.
Oestradiol and testosterone are the two types of sex hormones, present predominantly in, respectively, women and men, while prolactin is the breastmilk-producing hormone. These hormones have an important role in our sexual development as well as our sexual desire – and in the fact that different mental health problems are more prevalent in males or females. These hormones also regulate brain areas relevant to sexual desire, and the anticipation, preparation, and experience of sexual activity. They also have a role in abnormal emotions and behaviours leading to excesses of depression (in females) and rage (in males), or in sexual problems such as low libido or erectile dysfunction.
Oxytocin and vasopressin tell the brain when we are in love, or prepare the brain for having a baby. Vasopressin explains why some animals and humans, but not others, choose to be monogamous in long-term relationships. They are relevant to mental health difficulties related to social interaction, such as autism.
Pregnenolone and allopregnanolone belong to the wider class of neurosteroids, which protect the brain from injury, help brain cells survive physical trauma or lack of oxygen, and stimulate the growth of myelin, the protective sheet surrounding peripheral nerves. They have a role in dementia and stroke.
Triiodothyronine (T3) and thyroxine (T4) regulate virtually all aspects of our body and give us the energy required to live. Their abnormalities can manifest as both overtly low (hypothyroidism) and overly high (hyperthyroidism), and both can make us feel very tired, although the former makes us feel tired and cold, and the latter makes us feel tired and hot. Thyroid hormones are very important in depression and bipolar disorder, where are sometimes prescribed as medications.
As we conclude this piece, it is important to emphasise that the notion that substances circulating in the blood can affect our emotions and behaviour has permeated our culture for centuries, starting from the “humours theory” of the Greek philosophers. Now we finally know how hormones are essential for mind-brain-body communication and can contribute a strong, holistic, destigmatising message about the reality and physicality of our mental process.