Neurons in the Human Brain

The estimate of about 86 billion neurons comes from quantitative cell counts in adult human brains. It is not an exact, fixed count for every person: the foundational study reported a mean of 86.1 ± 8.1 billion in a small sample of adult men. Neurons are linked by synapses into networks that process information, but saying every neuron has hundreds or thousands of connections is an oversimplification; counts vary by cell type and brain region.