Approximately how many interneurons are there in the nervous system?

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Interneurons are local circuit neurons within the central nervous system that connect neurons inside the brain and spinal cord rather than sending signals to muscles or sensory organs. Because the CNS contains a very large number of neurons and interneurons form the bulk of these local networks, their total count lies in the tens of billions. A common and reasonable estimate for humans is about 20 billion interneurons, which fits the scale of CNS neuron populations better than the much smaller figures.

Numbers in the millions are far too low to account for the complexity of CNS wiring, and even a couple of billion is only a fraction of the interneuron population. The tens of billions figure aligns with how densely interconnected the brain and spinal cord are, supporting the idea that interneurons are the predominant local processors in the CNS.