How many brains do we have? Depending on your brain counting methodology, the answer could be as high as a dozen.

You have probably heard of most of these brains. The main one, the one that “thinks,” is the neocortex. It’s the big one and is clearly a brain. The others are quite small and tend to have more proscribed functions. So what is a brain? Here we are using the word very broadly. Any organ fortunate enough to be identifiable and having mostly neurons—let’s put the cutoff at a million—we will call a brain. The neuron is the basic brain cell. It receives inputs—electrical stimuli actually—and produces an output, which it typically sends on to more neurons. Techies would call this a “threshold logic device.”

Our brain definition may give short shrift to the bugs and crustaceans, as they won’t make the cut here. But they really don’t seem to be able to learn any new tricks. These small-brained creatures, though sometimes equipped with several hundred thousand neurons, operate mainly in reflex arcs. If this, eat it. It that, reproduce, if the other, run away.

Starting with the vertebrates, everything changed. Suddenly the number of neurons soared and special purpose brains developed. We went from lobsters, 100,000 neurons, to 10,000,000 for the smaller fish—a hundred-fold leap. Running a cluster of neurons takes a lot of energy, but apparently this is well worth it because as soon as the vertebrates appeared, the neuron count took off. Mice are around 100 million. Cats 700 million, and lesser monkeys, 3 billion. We come in at over 100 billion, but, interestingly, we are not the winner. That title goes to elephants with 250 billion. However in the grey matter department, we have more that the elephants. But, even here, we can’t claim first prize: some dolphins have more neocortex neurons than us. A bit humbling. (American billion used here: 1,000,000,000.)

For humans, the neocortex—the main brain—the ol’ grey matter—gets most of the publicity. There are actually two of these. The right and left brain, and they do different things, as is well known. These are connected by a huge cluster of nerves, so each knows what the other is doing. Unless of course, that cluster of nerve is severed, as frequently occurs with professional boxers. Interestingly, women have a greater cluster, which would mean their two halves are more connected. But this post is about the other brains. So two down and ten to go.

Here is the list:

• Hypothalamus
• Hippocampus —two of these
• Amygdala—two of these
• Thalamus
• Cerebellum
• Medulla oblongata
• Pons

Oops, only eleven—close enough. Here’s a drawing. The hippocampus and amygdala pairs are missing, but basically they’re crammed in next to the hypothalamus.

The pons, medulla oblongata, thalamus, and cerebellum

These are the boring brains. They send and receive signals to and from the rest of the body, rather like a switchboard or relay station. Not too exciting, but somebody has to do it. They can do some sorting of the signals and can do some reflex arcs, but, as far as is known, there isn’t much going on in the way of what we might call thinking. Certainly coordination and many other rather tricky processes are bound up in these brains. No mean feat.

Remember that 100 billion neurons we are blessed with? One would suppose that most of these are in the neocortex—the big thinking brain. However, the cerebellum wins this one hands down, having around 80 billion. The neocortex has a mere 25 billion or so. However, not all neurons are created equal, and the ones in the neocortex are far more elaborate.

This leaves the hypothalamus, the hippocampus, and the amygdala.

The interesting brains

The amygdala comes in pairs, right and left, seems to be involved in emotional memory and decision making where a strong emotion might be involved.. They weigh in at 12 million neurons.

The hippocampus pair has about 50 million neurons each and is involved in spatial memory and memory consolidation. The hippocampus can grow new material if needed. Taxi drivers have larger ones. Deterioration of the hippocampus is frequently observed in Alzheimer’s patients.

Last, but certainly not least, is the hypothalamus. This tiny brain has a billion neurons and has a starring role in the book Quantitative Medicine, where it is depicted as a six-armed curmudgeon, exasperated at our ubiquitous, unhealthy behavior. It is indeed, the key to good health. It usually is controlling many, if not most, of the various hormonal and circulating biochemicals that largely determine it.

The hypothalamus is in charge of metabolism and energy management. It very directly decides what to do with the food you eat: whether to burn it or store it as fat. It decides whether or not to burn the fat you already have. It controls your sleep, overall energy, whether cells repair or degrade, whether muscle or bone is built or torn down, and so on. It decides the levels for many of the hormones and other circulating biochemicals, and tightly regulates several of them. It has numerous timers: some daily ones that manage sleep cycles, one for the menstrual cycle. It even decides when to start puberty.

The hypothalamus does not control your conscious self, though it can certainly affect your mood, alertness, pep, etc., but these are consequences of its actions, not an attempted takeover. You do have free will. You can do that which pleases the hypothalamus, or that which thwarts it. In this way, you affect some control over it. The hypothalamus is strong-willed, but quite predictable. The exercise and dietary regimes found throughout this blog are designed to please the all-important hypothalamus.

We don’t know a whole lot more than we do know

We might close with this: the function of all these assorted brains is only roughly known. In most cases, knowledge comes from observation of the effects of damage or removal to the various areas. This is a very crude way to figure out thing, analogous to figuring out how your laptop works by drilling holes in it and seeing what breaks. The neocortex is the biggest mystery of all. Therein is the seat of consciousness. 25 billion neurons, but with a quadrillion total connections. That’s one with 15 zeros after it. None of our largest computers and networks have anything remotely close to this.