Your cortex holds billions; the cerebellum holds more
The human cerebral cortex packs roughly fourteen to sixteen billion neurons, while the cerebellum may hold fifty-five to seventy billion. Synapses may number near one hundred trillion. Action potentials flash along axons at one to one hundred meters per second, wiring a biological computer unlike any silicon chip.
Across vertebrates the brain is a cephalized nervous-system hub that receives sensory input, supports cognition, and coordinates movement and hormones. Embryonically it divides into forebrain, midbrain, and hindbrain; with the spinal cord it forms the central nervous system. Invertebrate brains arise from segmental ganglia, whereas vertebrate brains enlarge from the dorsal neural tube. Two broad cell classes dominate: neurons that signal, and glia that support, insulate, and guide development.
Neurons talk through synapses. An axon may branch to thousands of targets, releasing neurotransmitter that binds receptors—or, in some places, passing current through electrical synapses. Human brains are estimated at about one hundred trillion synapses; even a fruit fly brain has several million. Many synapses change strength with activity, the leading candidate mechanism for learning and memory. Myelinated axons look white; neuron-dense regions look grey. Magnify a cortical pyramidal cell to human-body size and its axon could stretch more than a kilometer.
Bilaterian animals share a deep body plan dating to a common ancestor late in the Cryogenian, some seven hundred to six hundred fifty million years ago: a tube with a gut and a nerve cord swollen into a front ganglion—the brain. Some bilaterians, including echinoderms and tunicates, lack a recognizable brain. Single-cell physiology is well mapped, yet how millions of cells cooperate remains an open problem. Modern models treat the brain as an information processor that acquires, stores, and transforms signals from the world.
Source: Brain
