<P> Later discoveries yielded a few refinements to the simplest form of the doctrine . For example, glial cells, which are not considered neurons, play an essential role in information processing . Also, electrical synapses are more common than previously thought, meaning that there are direct, cytoplasmic connections between neurons . In fact, there are examples of neurons forming even tighter coupling: the squid giant axon arises from the fusion of multiple axons . </P> <P> Ramón y Cajal also postulated the Law of Dynamic Polarization, which states that a neuron receives signals at its dendrites and cell body and transmits them, as action potentials, along the axon in one direction: away from the cell body . The Law of Dynamic Polarization has important exceptions; dendrites can serve as synaptic output sites of neurons and axons can receive synaptic inputs . </P> <P> The number of neurons in the brain varies dramatically from species to species . The adult human brain contains about 85 - 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 . </P> <Table> <Tr> <Td> </Td> <Td> This section does not cite any sources . Please help improve this section by adding citations to reliable sources . Unsourced material may be challenged and removed . (February 2017) (Learn how and when to remove this template message) </Td> </Tr> </Table>

Where are neurons found in the human body