Neurons in the brain

The number of neurons in the brain varies dramatically from species to species. One estimate puts the human brain at about 100 billion (1011) neurons and 100 trillion (1014) synapses. A lower 2012 estimate is 86 billion neurons, of which 16.3 billion are in the cerebral cortex, and 69 billion in the cerebellum. By contrast, the nematode worm Caenorhabditis elegans has just 302 neurons, making it an ideal experimental subject as scientists have been able to map all of the organism's neurons. The fruit fly Drosophila melanogaster, a common subject in biological experiments, has around 100,000 neurons and exhibits many complex behaviors. Many properties of neurons, from the type of neurotransmitters used to ion channel composition, are maintained across species, allowing scientists to study processes occurring in more complex organisms in much simpler experimental systems.