Brain

• 1. The Organ That Runs The Body

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  Inside the Areni-1 cave complex in Armenia, researchers found a brain estimated to be over 5,000 years old, preserved in the skull of a girl aged 12 to 14. It had shriveled, yet the cave's climate kept it remarkably intact. That ancient relic belonged to an organ that, in living animals, never rests. EEG recordings show the brain of a living creature is constantly active, even during sleep. This is the brain, the center of the nervous system in all vertebrates and most invertebrates. It sits typically in the head, near the organs for vision, hearing, and smell. How did such an organ arise from a simple worm-shaped ancestor? Why does a fruit fly carry several million synapses and a human roughly 100 trillion? And how do billions of individual cells, each understood in detail, somehow cooperate in ensembles that science still cannot fully explain? The answers stretch across hundreds of millions of years and the entire animal kingdom.

• 2. Worms, Octopuses, And Fruit Flies

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  All bilaterians are thought to descend from a common ancestor that appeared late in the Cryogenian period, between 700 and 650 million years ago, shaped like a simple segmented tubeworm. That basic worm form still echoes in modern bodies: a tube with a gut running from mouth to anus, and a nerve cord with a ganglion for each segment. The largest ganglion sits at the front, and that is the brain. Some worms, such as leeches, even carry a smaller "tail brain" at the rear end of the cord. Two invertebrate groups stand out for complexity. Arthropods carry a central brain called the supraesophageal ganglion, with three divisions and large optical lobes behind each eye. Cephalopods such as the octopus and squid have the largest brains of any invertebrate. A few bilaterians, including echinoderms and tunicates, lack a recognizable brain entirely, and scientists cannot yet say whether their ancestors ever had one. Certain small creatures became laboratory favorites because their nervous systems are convenient to study. The nematode Caenorhabditis elegans drew the attention of Sydney Brenner in the early 1970s. Its hermaphrodite nervous system contains exactly 302 neurons, always in the same places, wired identically in every worm. Brenner's team sliced worms into thousands of ultrathin sections, photographed each under an electron microscope, and matched fibers section by section to map the entire connectome. No other organism has been charted in such complete detail. The sea slug Aplysia californica became the model of choice for Nobel Prize-winning neurophysiologist Eric Kandel, who used its simple, accessible nervous system to probe the cellular basis of learning across hundreds of experiments.

• 3. Inside The Single Cell

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  A pyramidal cell in the human cerebral cortex carries an axon so long that, magnified until its cell body reached human size, the axon would become a cable a few centimeters across stretching more than a kilometer. Axons are the brain's wiring. Each is a thin protoplasmic fiber that projects from the cell body, often with many branches, toward targets nearby or far away. Signals travel as electrochemical pulses called action potentials. Each pulse lasts less than a thousandth of a second and races along the axon at 1 to 100 meters per second. Some neurons fire constantly at 10 to 100 pulses per second in irregular patterns; others stay quiet, then release a sudden burst. A single axon can form as many as several thousand synaptic connections. When an action potential reaches a synapse, it triggers the release of a neurotransmitter that binds to receptors on the target cell. Synapses are the points where the brain's essential business, cell-to-cell communication, takes place. Many synapses are dynamically modifiable, changing strength according to the patterns of signals passing through them. This activity-dependent change is widely believed to be the brain's primary mechanism for learning and memory. Most of the brain's space is filled with axons bundled into nerve fiber tracts. A myelinated axon wears a fatty insulating sheath that speeds signal propagation. Myelin is white, so fiber-packed regions appear as light white matter, in contrast to the darker grey matter where neuron cell bodies cluster densely.

• 4. A Shared Blueprint

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  In its earliest embryonic form, every vertebrate brain appears as three vesicular swellings at the front of the neural tube. These become the forebrain or prosencephalon, the midbrain or mesencephalon, and the hindbrain or rhombencephalon. At first the three are roughly equal in size. In fish and amphibians they often stay that way into adulthood. In terrestrial tetrapods such as mammals, the forebrain swells far larger, the hindbrain grows a bulky cerebellum, and the midbrain shrinks. Vertebrate brains rest in very soft tissue, pinkish outside and mostly white within. They are wrapped in connective membranes called meninges, with three layers: the dura, the arachnoid, and the innermost pia mater. Tightly joined endothelial cells in the cerebral blood vessels form the blood-brain barrier. It blocks many toxins and pathogens, but also blocks antibodies and some drugs, which complicates treating brain disease. Because of this barrier, brain metabolites are cleared largely by the bulk flow of cerebrospinal fluid through the glymphatic system rather than through venules. Neuroanatomists divide the vertebrate brain into six main subregions: the telencephalon, the diencephalon, the mesencephalon, the cerebellum, the pons, and the medulla oblongata. The midbrain, pons, and medulla are often grouped together as the brainstem. Each region carries a distinct job. The medulla handles vomiting, heart rate, and digestion. The pons governs sleep, respiration, swallowing, and posture. The cerebellum makes other systems precise: removing it leaves an animal able to act, but hesitant and clumsy. That precision is learned by trial and error, as in the muscle coordination of riding a bicycle. The cerebellum is 10% of the brain's volume yet holds 50% of all its neurons.

• 5. The Long Climb To The Cortex

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  The first vertebrates appeared over 500 million years ago, during the Cambrian period, perhaps resembling modern jawless fish like the hagfish and lamprey. Jawed vertebrates followed by about 445 million years ago, tetrapods by 350 million, amniotes by 310 million, and mammaliaforms by roughly 200 million years ago. Across this sweep, the same basic structures appear in every brain, though many remain rudimentary in the hagfish. Brain size tracks body size, but not in a straight line. Smaller animals tend to carry proportionally larger brains. For mammals, brain volume against body mass follows a power law with an exponent near 0.75. Primates defy that formula, with brains 5 to 10 times larger than it predicts, and predators tend to carry larger brains than the prey they hunt. Reptiles and mammals split from a common ancestor around 320 million years ago. There are 11,733 recognized reptile species against 5,884 extant mammals, and their brains vary widely. Crocodilians have the largest brain volume relative to body weight and the largest telencephalon, while snakes have the smallest telencephalon. Turtles carry the largest diencephalon per body weight; lizards have the largest mesencephalon. The mammalian forebrain is where the most dramatic change appears. A mammal's brain averages roughly twice the size of a bird's of the same body size, and ten times a reptile's. In non-mammalian vertebrates the surface of the cerebrum is a simple three-layered pallium. In mammals that pallium becomes the six-layered neocortex. As the cortex expanded, other regions shifted. The superior colliculus, which directs visual behavior in most vertebrates, shrank in mammals as the cortex took over its work. Placental mammals gained the corpus callosum, a broad nerve tract joining the two cerebral hemispheres.

• 6. What Makes Primates Different

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  Humans have an average encephalization quotient in the 7-to-8 range, while most other primates fall between 2 and 3. The encephalization quotient compares brain sizes while accounting for the nonlinear brain-to-body relationship. Dolphins score higher than non-human primates, but nearly every other mammal scores far lower, with the rat near 0.4 and the horse near 0.9. Most of the primate brain's enlargement comes from a massive expansion of the cerebral cortex, especially the parts devoted to vision and the prefrontal cortex. The visual processing network of primates includes at least 30 distinguishable brain areas, woven together by a complex web of connections. Visual areas have been estimated to cover more than half the total surface of the primate neocortex. The prefrontal cortex handles planning, working memory, motivation, attention, and executive control. It occupies a far larger share of the brain in primates than in other animals, and an especially large fraction in humans. This appetite for processing carries a metabolic price. Most vertebrate species spend between 2% and 8% of basal metabolism on the brain. In humans that figure rises to 20 to 25%, which is why large brains place severe demands on the animals that carry them.

• 7. Chemistry, Memory, And The Clock

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  With few exceptions, each neuron releases the same neurotransmitter at every synapse it makes, a rule known as Dale's principle. The two most widespread transmitters in the vertebrate brain are glutamate, almost always excitatory, and gamma-aminobutyric acid or GABA, almost always inhibitory. Because both appear nearly everywhere, drugs acting on them produce broad and powerful effects. Some general anesthetics work by reducing glutamate, while most tranquilizers calm by enhancing GABA. Other transmitters come from tightly confined sources. Serotonin, the target of many antidepressants, comes exclusively from a small brainstem region called the raphe nuclei. Norepinephrine, tied to arousal, comes only from the nearby locus coeruleus. The great majority of psychoactive drugs, from nicotine and heroin to cocaine and alcohol, act by altering specific neurotransmitter systems. The brain's central clock sits in the suprachiasmatic nucleus, a tiny part of the hypothalamus directly above where the optic nerves cross. Its neurons rise and fall over a period of about 24 hours, driven by rhythmic clock genes. The nucleus keeps time even when excised and placed in warm nutrient solution, though it normally takes light cues through the retinohypothalamic tract. The synaptic basis of memory was long suspected before it was proven. In the late 19th century Santiago Ramon y Cajal argued that learning lived in changing synaptic connections, yet evidence stayed scarce until 1970. In 1971 Tim Bliss and Terje Lomo published their paper on long-term potentiation, showing activity-induced synaptic changes that lasted at least several days. Brain-derived neurotrophic factor and physical activity both appear to help the process. Different forms of memory map to different structures: episodic memory leans on the hippocampus, whose severe damage can leave a person unable to form new long-lasting memories, while motor learning relies heavily on the cerebellum.

• 8. Reading The Living Brain

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  Because the brain contains no pain receptors, electrodes can record its activity in awake, behaving animals without causing distress. That single fact shapes much of how the organ is studied. Neuroscience draws on many disciplines, from psychology and neurology to psychiatry, computer science, and philosophy. The oldest method is anatomical, and until the middle of the 20th century progress came mostly from better stains and microscopes. The Golgi stain proved especially critical: it marks only a small fraction of neurons, but stains each one entirely, cell body, dendrites, and axon. In the hands of Camillo Golgi and the Spanish anatomist Santiago Ramon y Cajal, it revealed hundreds of distinct neuron types. The first real step toward modern understanding came from Luigi Galvani, who lived from 1737 to 1798. He found that a shock of static electricity applied to the exposed nerve of a dead frog made its leg contract. In the first half of the 20th century, Alan Hodgkin and Andrew Huxley worked out the biophysics of the action potential, while Bernard Katz studied the electrochemistry of the synapse. Genetic tools now reach deep into the living brain. In mice, genes can be knocked out or mutated, and Cre-Lox recombination can switch genes on or off in specific regions at specific times. In 2024, investigators studied almost 3 million nuclei from the human prefrontal cortex drawn from 388 individuals, annotating 28 cell types. They identified about half a million cell-type-specific regulatory elements and roughly 1.5 million single-cell expression quantitative trait loci, then built regulatory networks that revealed cellular changes in aging and neuropsychiatric disorders. The debate that once divided Aristotle, who placed the soul in the heart, and the author of On the Sacred Disease, who insisted joys and griefs come from the brain, has given way to a quieter and far harder question: how millions of cells cooperate to produce a mind.