Brain chemistry: how our emotions, thoughts, and behaviors take shape

The brain functions through a constant interplay of electrical and chemical signals. Neurons produce electrical impulses that travel along their extensions, then release molecules called neurotransmitters when they reach a synapse, the tiny gap separating two cells. These substances bind to receptors on the neighboring cell and can stimulate, slow down, or modify its activity. Once they have done their job, they are broken down by enzymes or taken up by the neuron that released them.

More than 200 neurotransmitters have been identified since the first one was discovered in 1921, according to the U.S. National Institute of General Medical Sciences. Each can perform multiple functions depending on the brain region, the type of receptor, and the circuit in which it is involved. Brain chemistry, therefore, is not like a control panel where each molecule corresponds to a specific emotion. Rather, it is a dynamic network in which billions of signals are constantly modifying each other.

Dopamine, serotonin: roles often oversimplified

Dopamine is often referred to as the "pleasure molecule," but this definition is too restrictive. It plays a role in motivation, learning, reward anticipation, and motor control. In Parkinson's disease, the progressive loss of dopamine-producing neurons in certain brain regions leads to tremors, slowness of movement, and balance problems. Therefore, a high or low concentration cannot be simply labeled as good or bad without considering the specific brain circuit involved.

Serotonin influences mood, sleep, appetite, and digestion, but it doesn't solely control happiness or sadness. Glutamate is one of the brain's main excitatory neurotransmitters, while GABA generally reduces neuronal activity and helps maintain balance. Norepinephrine is involved in attention, wakefulness, and the stress response. Acetylcholine, for its part, plays a role in muscle contractions, attention, and certain memory mechanisms. These molecules act simultaneously, and their effects are closely linked to their receptors.

The brain also communicates with the rest of the body

Brain chemistry isn't solely based on neurotransmitters. Hormones like cortisol, adrenaline, melatonin, and oxytocin circulate in the blood and also influence brain function. During a stressful situation, the body releases cortisol and adrenaline, among other hormones, to increase alertness and prepare the body to react. This response is helpful in the short term, but prolonged stress can disrupt sleep, attention, memory, and emotional regulation.

Sleep, physical activity, diet, light, social interactions, and daily experiences all influence these mechanisms. The brain is plastic: its connections can strengthen, weaken, or reorganize themselves with learning and the environment. Glial cells, long considered mere support structures, also participate in the functioning of neurons, their nutrition, and the regulation of their chemical environment. Thought, therefore, never results from a single isolated substance, but from the coordinated activity of cells, circuits, and the rest of the organism.

"Chemical imbalance": an overly simplistic explanation

The idea that depression is simply caused by a lack of serotonin is now considered inadequate. Mental health disorders can involve brain chemistry, but also genetics, the structure and function of neural circuits, hormones, trauma, environment, and living conditions. The National Institute of Mental Health emphasizes that depression results from a combination of biological, psychological, genetic, and environmental factors, with no single cause applicable to all patients.

Drugs that act on the brain remain essential in the treatment of many conditions. Antidepressants alter the availability of certain neurotransmitters, Parkinson's treatments partially compensate for the loss of dopamine, and anxiolytics reinforce certain inhibitory mechanisms. Their effectiveness does not mean that a disease corresponds to a simple deficiency of a molecule, as their effects can also gradually lead to a reorganization of brain circuits. Brain chemistry thus offers a major key to understanding human beings, but it is never sufficient, on its own, to fully explain a personality, an emotion, or a disease.