Chapter Nineteen

BRAIN

The Enchanted Loom

WHAT FORCE RACES THROUGH THE BODY to connect its many parts? Could it possibly be electricity? To former generations, the very concept of electricity was as mysterious, and terrifying, as nuclear energy is to ours. Benjamin Franklin risked his life by launching a kite into the teeth of that fiery power. What relevance could the feared juice of the heavens have to nerve cells buried in the body’s soft tissue?

Before Luigi Galvani, an Italian who lived thirty years after Franklin, scientists and doctors had accepted the ideas of the Greek physician Galen, who described the body’s communication system as a flow of pneuma, or spirit, through a network of hollow tubes. Then one humid day, Galvani brought a few frogs home for dinner and hung them on his porch.

Following one of those farfetched hunches that have formed the history of science, he beheaded the frogs, skinned them, and ran a wire from a lightning rod to the frogs’ exposed spinal cords. He recorded what happened next as a summer thunderstorm swept across Bologna: “As the lightning broke out, at the same moment all the muscles fell into violent and multiple contractions, so that, just as does the splendor and flash of the lightning, so too did the muscular motions and contractions . . . precede the thunders and, as it were, warn of them.”

Galvani did not bother to describe the expressions on the faces of his guests, who watched headless frogs jerk and twitch as though kicking across a pond. He stuck to the science, concluding that electricity, not pneuma, had surged through the nerves of the frogs and stimulated movement in dead animals. Entranced, Galvani performed many other experiments. One bright day he hung several beheaded frogs on copper hooks just above the iron railing of his porch. Whenever one of the frog legs drifted toward the railing and made contact, it jerked violently. Reflexes during a lightning storm are one thing, but dead frogs high-kicking on a sunny day—that’s the kind of discovery to set the scientific community on its ears. And so it did.

Galvani’s rival, Alessandro Volta, decided that the electric current had nothing to do with the frogs and everything to do with two dissimilar metals joined by an organic conductor. He went on to invent the battery, and we have him to thank for flashlights, laptop computers, and cars that start on below-zero mornings. Galvani insisted the reaction came from “animal electricity,” and we have him to thank for EKG monitors, biofeedback machines, and electric shock treatment.

Wires Within Us

The neuron plays the key role in carrying out orders from the head. Inside each of us, twelve billion neurons, so fine that a hair-width bundle of them contains one hundred thousand separate “wires,” lie poised for action. Medical specialists view them as the most significant and interesting cells in the entire body.

The neuron begins with a maze of minute, lacy extensions called dendrites, which, like the root-hairs of a tree, ascend to a single shaft. These dendrites wrap around every square millimeter of skin, every muscle, every blood vessel, and every bone, interweaving so intricately that even through a microscope it is nearly impossible to discern where one ends and another begins. I liken the sight to standing on the edge of a forest on a winter day. Before me marches a line of several hundred trees, each thrusting dark lengths of snow-laced branches up and out. If all those trees were somehow compressed into a few square yards, with their branchlets filling in the spaces without touching each other, the resulting image would resemble a nerve bunch in the body.

A debate raged in neurophysiology for decades: Do the dendrites actually touch? In the electrical wiring of a home or the circuit board of a computer, every live wire connects with every other wire, resulting in a closed loop. Eventually, it became clear that in the human body each of the twelve billion neurons stops just short of its neighbors, forming a precise gap called a synapse.

The synapse allows for staggering complexity. Take just one motor neuron controlling one muscle fiber in the hand. Along the length of that single nerve cell, knobs from other neurons form synapses at many junctions. A large motor nerve may have ten thousand points of contact, and a brain neuron may have as many as eighty thousand. If an impulse prods one motor nerve into action, instantly thousands of other nerve cells in the vicinity go on alert.

I want to move my index finger to type a letter, so my brain sends a command to a motor nerve. That nerve relies on surrounding neurons to help it compute how many muscle fibers should be mobilized for action, as well as which opposing muscles to inhibit. Neurons carry these electrical messages, as many as a thousand per second, with an appropriate pause between each. Every impulse gets monitored and influenced by all ten thousand synaptic connections along the path. A stupendous crackling wildness streams through all of us at every moment.

Master of Delegation

The brain does not consciously order every decision in the body, for that would defy the management principle of delegation. Instead, a dependable reflex system handles many ordinary situations.

Earlier I referred to the knee-jerk reflex, which doctors often test. Even when I tell a patient to stifle the reflex, the leg recoils anyway. I smile with approval, for I value the reflex as a sign of protection, not insubordination. Normally—in fact, almost always except in the case of a reflex test—abrupt tension in that tendon means a person’s knee has just absorbed a sudden stress, and the reflex straightens the leg to avert a fall. The brain delegates such safeguards to the reflex arc in the spinal cord, which explains why my patient cannot easily overrule the reflex kick.

It shows good management, this delegation to sneeze, cough, swallow, salivate, and blink. Blink: we do it without thinking, some twenty-eight thousand times per day. I yearn for such a reflex in my leprosy patients, many of whom go blind because their deadened pain cells do not inform them when the dry cornea needs a lubricating blink. We can forestall blindness in many patients by teaching them to blink with regularity, and I naively assumed that patients with eyesight at stake would make eager learners. I soon discovered that conscious movements are not nearly as reliable as reflexes.

We have to train our leprosy patients to blink, using placards and stopwatches—drilling, scolding, praising, and cajoling them. The higher brain resists giving priority to something as elementary as a reflex (who would force a supercomputer to count to ten every thirty seconds?). Many patients do not learn, and their eyes eventually dry out.

Some functions need more direction than the robot-like response of reflex. The brain stem itself coordinates the next level of guidance, the subconscious regulators of breathing, digestion, temperature, and circulation. These need more attention than reflexes: when I race up the stairs, my heart and lungs must shift into another gear, and the act of breathing alone must coordinate ninety different chest muscles.

Highest of all in the hierarchy of the nervous system are the cerebral hemispheres of the brain, the body’s CPU, most shielded by bone and most vulnerable to injury if that barrier is ever breached. There, some eighty billion nerve cells and many more glia cells (the biological batteries for brain activity) float in a jellied mass, sifting through information, storing memories, creating consciousness. Anatomists estimate the human brain contains more than a hundred trillion synapses. From all this buzzing activity, we make conscious choices.

I marvel at the nervous system’s sophisticated design. When a sudden danger—touching a hot stove, protecting eyes in a dust storm—requires a quick response, the brain delegates it to a reflex loop that functions below the level of consciousness. Yet the body reserves the right to overrule this reflex loop under unusual circumstances. An expert rock climber clinging to a precipice will not straighten his leg when a falling stone hits the patellar tendon; a society matron will not drop a too-hot cup of tea served in Wedgewood china; a mother does not reach out to break her fall when holding a baby.

The Final Common Path

Although the hierarchy seems neatly ordered, one anomaly keeps showing up. The final decision, the localized “will” that controls muscles and movement, resides not in the magnificent crevasses of the brain but in the humble neuron that controls the muscle fibers. Sir Charles Sherrington discovered this discomfiting feature and grandly labeled it “the final common path.”

Along its length, each neuron receives a spray of impulses from surrounding nerves. It stays alert to muscle tension, the presence of pain, the action of opposing muscles, the degree of strength required for any given activity, the frequency of stimuli, the oxygen available, the fatigue factor. An order from the brain arrives: lift a heavy box. Accomplishing that act will involve a whole army of motor units. After sifting through all the synaptic signals, the motor neuron itself decides whether to contract or relax its particular muscle fiber. After all, it is best equipped for such a decision, being in intimate contact with so many local synapses.

Only the “final common path” can decide between incompatible commands and reflexes, and we should be grateful. I stand on a cliff on one of the sheer granite hulks in the Rocky Mountains. Ahead of me, just beyond my reach, I see a delightful wildflower I cannot readily identity. I plant my feet, squat down, and lean forward, adjusting my camera under instructions from my brain. My close-up lens approaches within inches of the wildflower when suddenly a string is jerked and, like a marionette, I tip backward away from the flower. My heart is pounding, and I look around to see who interrupted my photography. No one is there, save a raucous, scolding jay.

Ever since I peered over the edge of the cliff to the ravine two thousand feet below, my cells have been chemically flooded with a heightened awareness of the potential danger. My conscious brain wanted a picture of the flower, but my subconscious reflexes received alarming reports from the balance organs of my inner ear. Short-circuiting the conscious brain, an “Emergency!” message convinced the nerve cells that control my muscles to yank me backward.

Sometimes my brain overrules, and sometimes it delegates. The response to its commands ultimately depends on the local, autonomous cell—the final common path. The microscopic computer in each nerve cell gauges my intentions, consults other muscles, calculates available energy, and monitors any signs of pain; only then does it fire a yes or no order to its assigned muscle group.

The Brain Exposed

Medicine knows no more daunting procedure than brain surgery. No one who opens a human skull can avoid a grim sense of defilement, almost like sacrilege.

I have observed a living human brain on maybe a half-dozen occasions. Each time I feel humble and inadequate, an intruder. Who am I to invade the hallowed place where a person resides? Solzhenitsyn once referred to a man’s eyes as “sky-blue circles with black holes in the center and behind them the whole astounding world of an individual human being.”

During my medical training, I chose as my senior project the task of exposing the major nerves in the head, tracking them all the way into the brain. Two years of medical school had not steeled me sufficiently for the experience of getting my own cadaver head, whole and perfectly preserved though shrunken slightly by the chemicals. It had belonged to a middle-aged man with plentiful hair and bushy eyebrows.

Over the next few weeks I spent most of my waking hours with the head of my anonymous friend. “That skull had a tongue in it, and could sing once,” Shakespeare wrote, and I had to push from my mind images of this hunk of wrinkled tissue on the table singing, talking, winking, smiling. I felt almost grateful for the pungent odor of formaldehyde that seeped through my skin and affected the taste of food, for it reminded me I was carving away not on a man but on a specimen of preserved tissue.

The skull, I learned, is a nearly impregnable orb. In my training I had watched as brain surgeons huffed and puffed, leaning in heavily to force a whirring drill bit through its quarter-inch armor. The bone barrier had sealed off my cadaver’s brain from all direct encounter with the outside world. Paradoxically, that secluded brain had stored all its owner’s knowledge of the world, thanks to the frail, white nerves leading into it. One nerve had controlled all the subtle movements of his lips that made possible speech and eating and kissing. Another had brought in every nuance of color and light to form his visual construct of the world.

I began my exploration with the familiar shapes of eye and ear. Then I proceeded inward, like an explorer searching for the source of the Nile, following a small strand of white into the penetralia of the brain itself. I chiseled the facial skeleton in thin layers, coaxing out slivers of bone while taking care not to cut too deep and sever the nerve. The orbit of the eye, for instance, consists of seven bones fused together in a socket. Fortunately, I had worked as a stone mason for a full year, and before long, the process of chiseling away layers of bone the thickness of tissue paper seemed natural and even artistic.

I remember being most impressed by the range of textures. I picked up a scalpel to make a slice through satiny muscle and fat, holding my breath and keeping the blunt edge toward the nerve—one quiver of my finger would sever it. Next, I laid down the scalpel, picked up a mallet and chisel, and attacked the bone with all my force.

After several arduous weeks of dissecting, fine white fibers led from the cadaver’s ear, eye, tongue, nose, larynx, and facial muscles to disappear into the cavity that contained the brain. Finally, I was ready to enter the brain itself. After vigorously sawing through bone, I reached the three membranes, or meninges, that sheathed the brain. I slit each one, remembering with a smile the arcane Latin names I had learned in anatomy class: dura mater (hard mother), arachnoid (cobweb), and pia mater (tender mother).

The innermost membrane fit like Saran wrap over the convolutions of the brain, and when I punctured it, a small piece of the brain bulged through the opening like a tiny fist. I stared at it a full five minutes before continuing. The organ weighed barely three pounds, yet that fragile, grayish jelly once contained a person’s entire life experience.

I touched it: gray matter had the consistency of cream cheese, softer than any bodily tissue I had yet encountered. Its landscape dipped and rose and turned in on itself—a topographical map of all the mountains on earth compressed into a small space. Red and blue lines crisscrossed the topography, and I breathed a prayer of thanks that I was practicing on a dead brain. A surgeon operating on a living patient spends much time avoiding those vital channels of blood and stanching the vessels cut by his scalpel.

Although I had hoped to trace sensory nerves to their origin, the brain does not easily yield to map-making. Nerves there, ensconced in the ample armor of skull, have a doughy consistency and will break at the slightest tug. And even the most experienced surgeon has difficulty with orientation in the brain, for everything appears soft and white, like an Arctic landscape.

My professor took tutorial delight in my project, displaying the pickled result in a medical museum. I had schoolboy fantasies of becoming a brain surgeon. Years later, when of necessity I attempted a few hazardous ventures into neurosurgery, I felt immensely grateful that I had not pursued that enormously challenging field.

Forces Within Forces

Brain surgery on a living patient is a very different experience. Sometimes, the patient stays awake in order to cooperate with the exploring surgeon, and the fact of the patient’s consciousness serves to dampen the normal prattle of surgery. Standing on the sidelines of such procedures, I notice the sounds: the faint electronic beeps of monitoring machines, the sighs of the respirator, the shrill whine of the drill, slight pops from the electric cautery, the clinking of instruments being passed around like dinnerware. The object of all this attention glistens in the bright lights, and if I look closely I can see it heaving gently. The brain is alive.

Brain surgery remained in a primitive state until one remarkable discovery. When a surgeon inserts a needle-like electrode into a portion of the brain and switches on the current, the brain responds, indicating what function that area controls. The patient will say something like “I feel a tingling sensation in my left leg” if the surgeon lightly stimulates a particular spot.

Wilder Penfield, a neurosurgeon in Montreal, recorded remarkable results from such exploration. While trying to locate the source of epileptic seizures, he found that his electrical stimulations could evoke specific memories in sharp detail. One young South African patient began laughing, reliving second by second an incident on a farm in his native land. A woman recalled every note in a symphony concert she had heard long before. The memories revived in such detail for one patient that she recollected a scene at a train crossing years before, describing each train car as it went by. Another counted aloud the number of teeth of a comb used in childhood.

Thanks to these discoveries and more recent results from functional MRIs, we now have a fairly reliable map of the brain. Most brain research centers on the cerebral cortex, which is far more advanced in humans than in any animal. The thickness of the sole of a shoe, the cortex directs the higher activities of thought and memory while also processing all the information received from the sensory organs. The majority of neurons live in that layer of gray matter, the fertile topsoil of the brain.

Nerve cells divide into two groups: “the way in,” or afferent cells that carry impulses from the body into the brain, and “the way out,” efferent cells that carry instructions from the brain out to the extremities. All visual images, all sounds, all touch and pain sensations, all smells, the sensations of hunger, thirst, and sex drives, muscular tension—all the noise from the entire body—occupy only one in a thousand of the brain’s cells, the afferent cells. Similarly, the efferent cells make up a fraction of one percent of brain cells, enough to control motor activities: playing a musical instrument, speaking a language, dancing a ballet, typing a letter, playing a video game.

The remaining, vast majority of brain neurons join together in a network of intercommunication to allow the processes we know as thought and free will. One brain biologist likens this network to billions of bureaucrats constantly phoning each other about plans and instructions for keeping a country running. More poetically, Sir Charles Sherrington rhapsodized about an “enchanted loom” with lights that flash on and off as messages weave their way through the brain—the very image later captured by functional MRIs.

The entire mental process comes down to the brain’s cells spitting chemicals at each other across synapses. Its complexity defies description, with the total number of connections far exceeding the number of galaxies in the universe. A mere gram of brain tissue may contain as many as four hundred billion synaptic junctions. As a result, each cell can communicate with every other cell at lightning speed—as if a population far larger than earth’s were linked together so that all inhabitants could talk at once.

Mercifully, we are hardly aware of the process. I decide to write the next sentence; in a flash my brain computes first the thoughts and then the words I will use, then the elaborate coordination of muscles, tendons, and bones required to type the words. Before I finish typing, my brain begins composing the sentence to follow.

Steven Levy records what happened when, on a visit to Princeton, he came across a jar containing Albert Einstein’s brain:

I had risen up to look into the jar, but now I was sunk in my chair, speechless. My eyes were fixed upon that jar as I tried to comprehend that these pieces of gunk bobbing up and down had caused a revolution in physics and quite possibly changed the course of civilization. There it was.

In the human head, concludes Nobel laureate Roger Sperry, “there are forces within forces within forces, as in no other cubic half-foot of the universe that we know.” Perhaps if I worked on brains daily I would grow more callous. I doubt it—the neurosurgeons I know still speak of the brain in hushed, almost worshipful tones.

And yet nothing on earth is so fragile. One dosage of a powerful drug can permanently upset the delicate balance inside a brain. One bullet may destroy it or one spill from a motorcycle. Deprived of oxygen for a mere five minutes, the brain will die, and with it the whole body.