The Human Brain Book: An Illustrated Guide to its Structure, Function, and Disorders
NO ORDINARY ORGAN
The human brain is like nothing else. As organs go, it is not especially prepossessing—3lb (1.4kg) or so of rounded, corrugated ﬂesh with a consistency somewhere between jelly and cold butter. It doesn’t expand and shrink like the lungs, pump like the heart, or secrete visible material like the bladder. If you sliced off the top of someone’s head and peered inside, you wouldn’t see much happening at all.
SEAT OF CONSCIOUSNESS
Given this, it is perhaps not surprising that for centuries the contents of our skulls were regarded as relatively unimportant. When they mummiﬁed their dead, the ancient Egyptians scooped out the brains and threw them away, yet carefully preserved the heart. The Ancient Greek philospher, Aristotle, thought the brain was a radiator for cooling the blood. René Descartes, the French scientist, gave it a little more respect, concluding that it was a sort of antenna by which the spirit might commune with the body. It is only now that the full wonder of the brain is being realized.
The most basic function of the brain is to keep the rest of the body alive. Among your brain’s 100 billion neurons, some regulate your breathing, heartbeat, and blood pressure and others control hunger, thirst, sex drive, and sleep cycle. In addition to this, the brain generates the emotions, perceptions, and thoughts that guide your behavior. Then it directs and executes your actions. Finally, it is responsible for the conscious awareness of the mind itself.
THE DYNAMIC BRAIN
Until about 100 years ago, the only evidence that brain and mind were connected was obtained from “natural experiments”—accidents in which head injuries created aberrations in their victims’ behavior. Dedicated physicians mapped out areas of the cerebral landscape by observing the subjects of such experiments while they were alive— then matching their deﬁcits to the damaged areas of their brains. It was slow work because the scientists had to wait for their subjects to die before they could look at the physiological evidence. As a result, until the early 20th century, all that was known about the physical basis of the mind could have been contained in a single volume.
Since then, scientiﬁc and technological advances have fueled a neuroscientiﬁc revolution. Powerful microscopes made it possible to look in detail at the brain’s intricate anatomy. A growing understanding of electricity allowed the dynamics of the brain to be recognized and then, with the advent of electroencephalography (EEG), to be observed and measured. Finally, the arrival of functional brain imaging machines allowed scientists to look inside the living brain and see its mechanisms at work. In the last 20 years, positron emission tomography (PET), functional magnetic resonance imaging (fMRI), and, most recently, magnetic encephalography
(MEG) have among them produced an ever more detailed map of the brain’s functions.
Today we can point to the circuitry that keeps our vital processes going, the cells that produce our neurotransmitters, the synapses where signals leap from cell to cell, and the nerve ﬁbers that convey pain or move our limbs. We know how our sense organs turn light rays and sounds waves into electrical signals, and we can trace the routes they follow to the specialized areas of cortex that respond to them. We know that such stimuli are weighed, valued, and turned into emotions by the amygdala—a tiny nugget of tissue that punches well above its weight. We can see the hippocampus retrieve a memory, or watch the prefrontal cortex make a moral judgment. We can recognize the nerve patterns associated with amusement, empathy— even the thrill of schadenfreude at the sight of an adversary suffering defeat. More than just a map, the picture emerging from imaging studies reveals the brain to be an astonishingly complex, sensitive system in which each part affects almost every other. “High level” cognition performed by the frontal lobes, for instance, feeds back to affect sensory experience—so what we see when we look at an object is shaped by expectation as well as by the effect of light hitting the retina. Conversely, the brain’s most sophisticated products can depend on its lowliest mechanisms. Intellectual judgments, for example, are driven by the body reactions that we feel as emotions, and consciousness can be snuffed out by damage to the humble brainstem. To confuse things further, the system doesn’t stop at the neck but extends to the tips of your toes. Some would argue it even goes beyond—to encompass other minds with which it interacts.
Neuroscientiﬁc investigation of the brain is very much a work in progress and no one knows what the ﬁnished picture will look like. It may be that the brain is so complicated that it can never understand itself entirely. So this book cannot be taken as a full description of the brain. It is a single view, from bottom to top, of the human brain as we know it today—in all its beauty and complexity. Be amazed.
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|December 4, 2019|
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