What “86 Billion Neurons” Really Means Modern counts suggest the adult human brain contains about 86 billion neurons (not the old “100 billion” myth). These neurons are distributed roughly as: Alongside neurons are glial cells (astrocytes, oligodendrocytes, microglia) in comparable numbers. Glia support metabolism, insulate axons (myelin), modulate signaling, and help maintain and remodel synapses.…