Lulled in the countless chambers of the brain,
Our thoughts are linked by many a hidden chain.
—ALEXANDER POPE
UNLIKE THE HEART AND LUNGS, THE HUMAN BRAIN doesn’t look from the outside as if it’s for anything in particular. It doesn’t appear to have moving parts. It looks very like an inert lump, silent and motionless. Ancient peoples tended to underestimate its importance. From our brain-savvy, brain-centric world, it’s easy to scoff at this, and at why it was that the heart, so obviously merely a blood pump, should once have been thought the seat of the moral self. Plato was considered radical in his idea that the brain might be its real location, as was Hippocrates, who declared, in the manner of a man who expected to be contradicted, that “from the brain and the brain alone comes pleasure, happiness, laughter, as well as sorrow and pain.” The ancient Egyptians discarded the brains of those they were mummifying, removing them in bits through the nose with long hooks and binning them, though they preserved the hearts of their kings for the kings’ later use.
The brain is a big thing and heavy. It can weigh three pounds. The outer layer, the cortex (from the Latin for tree bark), is wrinkled so as to cram more surface area into the limited space. The cauliflower shape is actually a twinned half-cauliflower pair, two halves, left and right, with functions allocated, divided, and shared. People who’ve had the connection between the two halves severed medically experience a troubling dual consciousness in which one hand really doesn’t know what the other one’s doing.
Brain use is tiring work. Whether purposely trying to think or not, conscious or unconscious, the system takes a lot of energy. Up to a fifth of food energy is dedicated to fueling brain functions. Glucose is the brain’s gasoline, and brain glucose levels plummet in Alzheimer’s; one of the newer diagnostic tests measures these levels in living subjects. Aside from the 100,000 million neurons, there are ten times as many neuroglia (from the Greek for glue), cells that form the support network, feeding and repairing the lead actors. This support network also suffers devastating cell loss in the Alzheimer’s forest fire.
They’re gray, these neurons, Hercule Poirot’s leetle gray cells—gray with white axons. And so many of them: 100,000 million is a big number. There are fewer than 7,000 million people in the world. I read somewhere that the phone system covering the whole planet, with all its connections and interconnections, parts of it at rest and parts of it firing with calls, is nowhere nearly as complicated as the interior of one human brain.
While neurons are different shapes according to function, those illustrated in neurology texts are generally star shaped. Neurons are microscopic, but an axon can be as much as half an inch long, directing its communications network in particular sequences, though most stay very local, passing information along like firemen passing water in buckets (albeit huge numbers of firemen and buckets), passing water too fast for the naked eye to see.
The neurons are packed tight in the cortex, and the cortex divides into four main areas, or lobes. The frontal lobe, the front third of the brain, in and behind the forehead, is where we think in the most obvious, self-conscious sense, plan, imagine, debate, decide. It’s the area that develops last in the growing child. It’s the area that best distinguishes us from the rest of the animal kingdom. It’s our executive center, the seat of the executive I. It has vital secondary roles in all kinds of brain function and is crucial in the retrieval of memory.
The temporal lobes, worn like earmuffs at the sides of the head, are memory banks and instrumental in language and the comprehension of language. They analyze sensory input and, with the auditory cortex, interpret sound. The temporal lobes work in emotion as well as memory. The so-called God spot is here, the mysterious brain area that may give us our sense of the divine. In an experiment done with nuns, it was the same small location in the right temporal lobe that lit up within each, shown on a scanner, when they were asked to focus on communication with the Almighty. Richard Dawkins, the biological theorist who wrote The God Delusion, thinks that this God spot, in evolutionary terms, has to do with belonging to a tribe and the socially unifying effects of tribal genuflection. The bishop of Oxford thinks it’s provided by the Lord as an interface.
The parietal lobe, at the upper part of the back of the head, helps orient us, giving us spatial awareness, our three-dimensional sense of the world, our own detailed body map, and our orientation to left and right. Number recognition, and the ability to manipulate numbers, is worked on here also.
The occipital lobe, at the lower rear of the head, is responsible for vision. Vision takes up a lot of space and energy. Other centers in the brain collaborate to process visual information. Among its visual tasks, the occipital lobe helps interpret writing.
Across the top of the head like a stretchy headband runs the motor cortex, and behind it lies a second headband-type strip, known as the somatosensory cortex, where messages from the nerve endings in the body arrive for processing and analysis from the spine.
Deep beneath the cortex, the limbic system, folded away in its own compartment, includes the hippocampus and amygdala and our sense of smell. A dulled sense of smell (like Nancy’s) may be a predictor of Alzheimer’s and contributes to problems with appetite. The amygdala has been described as the fear zone, the seat of primitive emotions, instinctive, fearful, and aggressive. The egglike thalamus, at the center of the system, acts as mediator between the limbic system and the cortex, between instinct and abstract thought, and may be the brain area that most specifically corresponds to the experience of consciousness. The hippocampus processes short-term memory, which may or may not then be laid down into long-term memory. It’s called hippocampus because it’s supposed to look like a sea horse.
The brain stem is in evolutionary terms the original organ and resembles the whole brain of simpler animals like lizards. It handles all the basic regulatory functions, the heart rate, hormones, sleep, breathing, blinking, blood pressure. It’s a bulbous small area at the top of the spine.
The cerebellum, at the base and back of the skull, is an onion-shaped organ that’s thought to be a minibrain in itself, a minicomputer, and may be a sort of backup generator for the rest. Traditionally, its main responsibilities are thought to be for movement, coordination, posture, balance. It’s also the seat of our most secure, most deeply embedded memories. How to walk, for instance. Automatic actions, the kind we don’t need to think about anymore—cleaning our teeth, riding a bike—are handled from here. The cortex learns things and then delegates, once we have the thing mastered. Forty million fibers connect the cerebellum to the cortex.
The romantic view of the brain as an interior landscape predated the Romantic movement by over two thousand years, in its using cave and weather and smoke metaphors. “Caverns there were in my mind,” Wordsworth writes, “which sun could never penetrate.” Coleridge’s “intellectual breeze, / At once the soul of each and god of all.” Erasistratus, born three hundred years before Christ, talked about “vital spirit,” the pneuma, a liquid life force flowing around our bodies like blood. The second-century doctor-scientist Galen thought the cerebrospinal fluid to be the pneuma and discounted the hard-boiled-egg-consistency, gray-and-white matter that surrounded it as merely protective.
J. K. Rowling uses this antique idea of selfhood as something vaporous, silvery, swirling through the caverns of the mind like mist. In the Harry Potter books, memories can be decanted, studied, held in a pensieve. The dying Snape’s memories emanate with his final breath and are caught by Harry in a flask, to be reviewed later. Nurses on intensive care wards open windows to let the souls of the just-deceased escape the walls of the hospital. Absurd though the idea of memory as a silver mist might be, it’s in truth far closer to our own idea of the workings of our thoughts than the actual mechanism is. The actual mechanism has all to do with electricity made by the body. How can a body make an electrical impulse? Chemistry provides the answer, down at the cellular level—the fact that chemical molecules carry electrical charges that react with others. Your body may be a temple but it’s also, far more intriguingly, a laboratory within which chemical reactions are ongoing.
The resting potential of a cell is created by potassium leaking out of it. There’s a high concentration of potassium inside cells, and a weak solution of it outside, where there’s a high concentration of sodium. The potassium flowing out of a cell creates a negative charge (–70 millivolts). That’s the resting potential of a cell. Along comes electrical information—from a pain in your leg, say, or something seen, or something learned, or a memory—and astoundingly, it seems that the information in every case is of the same order; it’s just the question of where it comes from and where it’s directed in the brain that translates it into pain, vision, knowledge, recall. What happens is that the sodium outside the cell flows in through a hinged gate, creating a wave of positive charge, which happens to be 110 millivolts, so that the balance from the original ‒70 is +40 millivolts. Sodium flows in, potassium flows out: It takes about a thousandth of a second. The electrical charge passes to the next excited cell, and onward in waves, at fantastic speeds. After the sodium/potassium exchange has occurred, a protein inside the cell is responsible for ushering out the excess sodium, chaperoning back the potassium, so that the cell is reduced to its usual state, ready for the next impulse (which is called an action potential).
All this was confirmed, incidentally, by Alan Hodgkin and Andrew Huxley’s research using the squid, which has a giant axon, a millimeter thick and visible to the naked eye. They were awarded the Nobel Prize for their work in 1963. Hodgkin commented that the prize should have gone to the squid.