Eighty-six billion neurons fit in roughly two percent of body weight
The adult human brain averages about 1.2 to 1.4 kilograms—only around two percent of body mass—yet holds more than eighty-six billion neurons plus a similar count of other cells. Wrapped in skull, fluid, and a blood–brain barrier, it still steers nearly everything you do.
Together with the spinal cord it forms the central nervous system: cerebrum, brainstem, and cerebellum. The cerebrum’s twin hemispheres carry white-matter cores and grey cortical shells; neocortex stacks six neuronal layers while allocortex keeps three or four. Four lobes—frontal, parietal, temporal, occipital—specialize loosely, with language often left-leaning and visual-spatial skill right-leaning, linked by the corpus callosum and other commissures. Typical volumes sit near 1260 cubic centimeters in men and 1130 in women, with wide individual ranges.
A stalk-like brainstem of midbrain, pons, and medulla bridges cerebrum to cord; the cerebellum (“little brain”) hangs behind on peduncles. Four ventricles circulate cerebrospinal fluid. Deep partners include thalamus, hypothalamus, pituitary, amygdalae, hippocampi, and basal ganglia. Three meninges—dura, arachnoid, pia—sheathe the soft, tofu-like tissue; white matter is about half of brain volume. Activity is neurotransmission along circuits. Trauma, stroke, Parkinson’s, dementias such as Alzheimer’s, multiple sclerosis, tumors, and psychiatric conditions including schizophrenia and depression can all implicate brain dysfunction.
Neuroanatomy maps structure; neuroscience maps function via animal histology, EEG, functional imaging, and lesion histories. Nineteenth-century phrenology wrongly pinned personality to cortical bumps; philosophy still wrestles with consciousness. One sleep study of 107 nap participants coupled slow oscillations with fast spindles in deep NREM, lighting thalamus and hippocampus and hinting at memory consolidation. Another line of work models how prediction error and uncertainty carve continuous experience into events. Soft gel, hard questions: a two-percent organ still outruns simple stories.
Source: Human brain
