Inside Your Brain
The much-maligned bird brain has remarkably sophisticated cerebral organization and an amazing flight control system. The shark may have puny cerebral hemispheres, but its brain is an olfactory-processing marvel that lets Jaws live in a perceptual world of smells, and respond like a machine to minute changes in the environment. We can’t do that.
But we can think great thoughts, which are important if you’re small, naked, and vulnerable in a dangerous world. It’s our skill, and our brain is best built to do it. Let’s look under the human hood and see how – by touring the big, anatomical brain structures we can see with the naked eye. Bear in mind that your mind is really much more complicated than this. That’s why medical school takes four years, and neuroanatomy is a least favorite subject.
The Cerebral Hemispheres –
a.k.a The Neocortex
Open the skull and you can look right at the cerebral hemispheres, or neocortex – the “new” part that accounts for most of big mammalian brains. Alive, the hemispheres are a rosy ecru color, soft – like a well-set pudding. They pulsate in quiet rhythm with your heart.
Each hemisphere is a mass of soft, fat folds called gyri. The valleys are called sulci. The deepest valleys are fissures, and the deepest of all is the interhemispheric fissure – the split down the middle between the left and right halves of the brain. Another one, the Rolandic fissure, divides the front half from the back, and a third one, the Sylvian fissure, divides the top half from each side.
The Frontal Lobes
Fissures are the landmarks that separate the lobes, which are worthy of individual names because they do different jobs. Most of our knowledge about which parts do what comes from studying people with brain injury or illness – people like Phineas Gage, whose face and forehead were impaled by an iron rod. His case helped define the role of the big frontal lobes, which are part chief executive officers, part directors of voluntary movement. They are responsible for ambition and drive, strategic planning, and control of emotional expression.
The Temporal Lobes
On the sides, the temporal lobes are the first waystation for sound processing and memory formation. They gather ideas, and integrate massive amounts of sensory data. The left temporal lobe (in most people) transforms ideas into words for the left frontal lobe to express. In diseased states, the temporal lobes can produce remarkable hallucinations of smells, sounds, and sights, and peculiar states of depersonalization, or absence of the self.
The Occipital Lobes
The occipital lobes are at the back of the brain, mostly on the inside of the two hemispheres, tucked away in the interhemispheric fissure. On the outside, they merge into the back part of the temporal lobes. This part of the brain takes in visual data and processes it, sending it forward to other parts of the brain to be fashioned into recognizable images. When troubled by disease or drugs, the occipital lobes produce hallucinatory misperceptions and illusions.
The Parietal Lobes
Between the frontal lobes and the occipital lobes, and above the temporal lobes, lie the parietal lobes, which map out bodily sensations and do the herculean work of integrating input from all other areas of the neocortex and from the more primitive brain below. They reveal their complexity in the variety of sensory and cognitive symptoms produced by disease. When Alice went down the rabbit hole, she was romping around in her parietal lobes.
The Limbic System
The cerebral hemispheres look like mirror images, but one, usually the left, is in charge of speech and handedness, while the other is more or less in charge of visual-spatial thinking. Split the brain down the middle, along the interhemispheric fissure, and you’ll see a big bridge of nerve fibers – the corpus callosum – connecting the two sides. The gyri surrounding it and the deep parts of the brain are part of the “limbic system,” one of the least understood brain regions.
Here lie your emotions, tangled up with memory formation, your sense of smell, the regulation of fight-or-flight reactions, hunger, thirst, and temperature. Here are laughter, rage, crying, and pleasure. Here is the border – and the connection – between our brand-new big brain and the little old brain we share with all the other animals.
The Brain Stem
Put your finger at the back of your head, at the notch at the base of your skull, and you’ll be just outside the lower end of a jam-packed, finger-sized structure called the brain stem. It is an exquisitely organized, multiple-lane highway for information speeding between the brain above and spinal cord below. Interspersed among the lanes are brain stem nuclei – little traffic control centers for parts of the body. A little damage here can kill, paralyze, or even lock people in, leaving them with no way to communicate that they are still inside.
The top part of the brainstem is the midbrain, the middle part is the pons, and the lower part, which merges into the spinal cord, is the medulla oblongata. In the midbrain, we swallow, hiccup, yawn, and move our eyes – sometimes deliberately, sometimes reflexively. The pons houses the on-off switch for consciousness. And the medulla controls the automatic, life-sustaining rhythm of breathing and influences other crucial, “vegetative” functions like heart rate and digestion.
The Basal Ganglia and Cerebellum
The brain stem swells at its top into big grey globular pieces that connect it to the neocortex. These are the basal ganglia, part of the control system for motor activities. They work in concert with the cerebellum, a delicately convoluted little accessory brain that first appeared in a big way in birds. It’s a motor computer that controls the rate, force, and timing of all movements. When you start up a staircase, your cerebellum tells you by the second step exactly how high you must lift your foot to clear this flight’s riser height.
The cerebellum is visible at the back of the brain, nestled under the occipital lobes and connected by nerve fibers that wrap around the pons. The basal ganglia and the cerebellum are continuous feedback systems that control the body while the neocortex above tells it what to do.
Your Vulnerable Self
The brain may be the boss of the body, but like all dictators, it’s vulnerable to its vassals, should they fail to serve. The big neocortex demands much energy and will fail quickly if oxygen or glucose doesn’t arrive, leaving behind a body driven by an automatic pilot in the tougher old brain – and wiping out much of your self.
So, where in this incredible organ is the self? No one knows. To the naked eye, the monotonous surface of the brain yields no clue to its phenomenal specialization and organization, or to the self that lies inside. Einstein’s brain looks like the one in the guy next door.
