The Brain

Pain-Free Tissue, Nighttime Plumbing Flushes, and the Twenty-Watt Power Budget

Living brain tissue contains zero nociceptors—the specialized nerve endings that detect cut, burn, or mechanical injury. That absence of pain receptors is why neurosurgeon Wilder Penfield could probe the exposed cortex of awake epilepsy patients under local scalp anesthesia. As his electrode touched specific cortical folds, patients calmly reported childhood memories or moved their fingers.

Weighing roughly 1,400 grams, the human brain represents just 2 percent of body mass yet consumes 20 percent of resting oxygen and glucose. The entire organ runs on a continuous power budget of 20 watts—less than a dim refrigerator bulb. Two-thirds of that electricity goes strictly toward operating microscopic $\text{Na}^+/\text{K}^+$-ATPase pumps that reset membrane voltages.

Because the brain lacks conventional lymphatic vessels inside the skull, neuroscientist Maiken Nedergaard wondered how neural tissue clears its daily metabolic trash. Lulu Xie and Maiken Nedergaard demonstrated in 2013 that during deep slow-wave sleep, the interstitial space between brain cells expands by 60 percent. That expansion opens perivascular glymphatic channels, allowing cerebrospinal fluid to flush beta-amyloid and tau proteins out of the cortex.

A single cubic millimeter of human cerebral cortex is roughly the size of one poppy seed. High-resolution electron microscopy reveals that this speck of tissue holds 50,000 neurons, 150 million synapses, and four kilometers of insulated axonal wiring. No semiconductor foundry on earth packs active switching elements and three-dimensional interconnects at that volumetric density.

Nerve impulses do not travel at the speed of light or even the speed of electricity in copper wire. Unmyelinated C-fibers carry dull ache signals at a sluggish one meter per second, roughly human walking pace. By contrast, thick motor axons wrapped in myelin insulation jump action potentials from gap to gap at 120 meters per second.

A six-month-old infant's brain contains nearly twice as many synaptic connections in its visual and prefrontal cortex as an adult brain. Early childhood learning does not proceed mainly by growing new connections. Instead, roving immune cells called microglia prune away half of the infant's excess synapses, carving crisp circuits out of an overgrown neural thicket.

Under the Monro-Kellie doctrine of neurosurgery, the adult skull is a rigid bone vault of fixed volume. Inside that sealed chamber, brain tissue occupies 80 percent, blood occupies 10 percent, and cerebrospinal fluid occupies 10 percent. Because water is incompressible, any expanding bleed or tumor must immediately displace fluid or crush the brainstem downward.

The blood-brain barrier is not a single wrapper stretched around the outside of the brain. It is a continuous molecular weld of tight junctions sealing endothelial cells along all 600 kilometers of cerebral capillaries. This microscopic barrier blocks 98 percent of small-molecule drugs and even keeps the body's own circulating adrenaline from entering neural tissue.

Your conscious experience of the present moment always lags roughly 80 milliseconds behind physical reality. When you tap your nose and your big toe at the same instant, the nerve signal from your nose reaches the cortex 60 milliseconds before the foot signal. The brain holds the fast signal in a buffer so both touches feel simultaneous.

Whenever your eyes dart from one word to the next—a rapid jump called a saccade taking 50 milliseconds—the visual world smears across your retina at 500 degrees per second. You never see that blur because the brain shuts off visual processing during the movement. Once the eye lands, the cortex back-fills the missing time with the destination image.

One-quarter of all cholesterol in the human body sits inside the brain, where oligodendrocytes wind it into myelin insulation around axons. Yet circulating blood lipoproteins cannot cross the blood-brain barrier. Consequently, the brain is isolated from dietary cholesterol and must synthesize 100 percent of its own supply locally inside star-shaped glial cells called astrocytes.

Popular folklore claims that human beings use only 10 percent of their brains. Positron emission tomography (FDG-PET) scans show that 100 percent of brain regions are metabolically active every day. However, the brain enforces a sparsity code in which fewer than 5 percent of cortical neurons fire simultaneously, because higher synchronization triggers an epileptic seizure.