Tukuk-OSThe modern count for human cerebral neurons is roughly 86 billion, each connecting with thousands of others — a wiring diagram whose synapse count exceeds a hundred trillion. Information moves as electricity along the axon, an impulse near 100 millivolts travelling anywhere from a slow 1 m/s to over 100 m/s depending on myelin, the fatty insulation wrapped around fast lines (the same insulation whose loss causes multiple sclerosis). At each junction the electrical trick becomes chemistry: a burst of molecules diffusing across a gap of 20 nanometres, caught by receptors that decide — fire or don’t.
The cost is startling for its size: at about 2% of body mass, the brain bills around 20% of resting energy, mostly to run the ion pumps that reset every electrical firing — the brain is an expensive machine kept perpetually cocked. It is also fenced: the blood–brain barrier tightly controls what crosses from blood, keeping out many useful medicines as readily as toxins — which is why treating brain illness is pharmacology’s hardest real estate.
Two properties turn the wiring into a life. Plasticity: connections that fire together strengthen, those unused are pruned — the reason childhood learning is cheap, adult skill is effortful, recovery after stroke is possible, and repeated practice physically rebuilds hardware. And the housekeeping nobody thought to look for: during deep sleep, the brain’s cells shrink slightly so fluid washes interstitial space, clearing metabolic debris including Alzheimer-linked proteins — the strongest current reason sleep is not a luxury but maintenance, booked nightly by every animal that has ever slept.