How neurons talk to each other, and how the brain controls hormones
Neurons pass on a message by releasing molecules into a tiny gap, the synapse: 20 to 40 millionths of a millimeter. The same brain also controls most of the body's glands, through the hypothalamus and the pituitary gland. Choose a molecule, send an impulse, and watch.
A synapse in motion
In slow motion: in reality, it all happens in a few thousandths of a second.
The brain, conductor of the hormones
The hypothalamus, at the base of the brain, sends orders to the pituitary gland, which passes them on to the glands. The glands' hormones then come back to slow down the brain: this is feedback, like a thermostat.
Hormones that talk to the brain
The path also works the other way: many of the body's hormones act on the brain.
Cortisol
It acts on the hippocampus, a memory area. A brief peak helps you remember a striking event; a prolonged excess might instead damage it.
Brief peak: shown in humans. Effect of prolonged excess: mostly seen in animals.
Melatonin
Made at night by the pineal gland, it tells the brain and body that it is night. It prepares you for sleep, it does not force it.
Shown in humans.
Ghrelin and leptin
Ghrelin from the stomach stirs up appetite; leptin from fat calms it. Both act on the hypothalamus.
Shown in humans.
Estradiol and progesterone
They change body temperature, sleep and, in some people, mood over the cycle.
Shown in humans.
Oxytocin
Both a hormone and a brain messenger: contractions, milk, and a role in bonding.
Contractions and milk: shown. Bonding: mostly seen in animals.
Frequently asked questions
What is the difference between a neurotransmitter and a hormone?
Mainly distance. A neurotransmitter crosses a synapse, a fraction of a micrometer. A hormone travels in the blood and acts far from where it is made. Noradrenaline plays both roles: a messenger in the brain, a hormone when it leaves the adrenal glands.
What does an SSRI antidepressant do?
It slows the reuptake of serotonin: it stays longer in the synaptic gap. Check "Block reuptake" with serotonin to see it. The effect on mood, however, takes several weeks to appear, which this blocking alone does not explain.
How many neurons are there in the brain?
About 86 billion in adults, according to a count published in 2009 (Azevedo et al.). Each one can have thousands of synapses.
Why do hormones slow themselves down?
To stay in a useful range. When cortisol or thyroxine rise, they slow down the hypothalamus and pituitary, which then order less of them. That is why a hormone taken as a medicine can put the gland to rest.
Sources: physiology textbooks (Purves, Neuroscience; Kandel, Principles of Neural Science; Guyton and Hall, Textbook of Medical Physiology); Yano et al., "Indigenous bacteria from the gut microbiota regulate host serotonin biosynthesis", Cell, 2015 (PubMed); Stamatiades and Kaiser, review on GnRH pulse frequency (PMC); Azevedo et al., "Equal numbers of neuronal and nonneuronal cells make the human brain an isometrically scaled-up primate brain", J Comp Neurol, 2009 (PubMed). Molecule drawings made by us, based on PubChem.