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Nervous System What is a neuron? Neuron is a cell How many neurons are in your nervous system? Answer There are estimated to be as many as 100 billion neurons in our nervous system! Anatomy of the neuron Physiology A typical neuron has all the parts that any cell would have, and a few specialized structures that set it apart. The main portion of the cell is called the soma or cell body. It contains the nucleus, which in turn contains the genetic material in the form of chromosomes. Physiology Neurons have a large number of extensions called dendrites. They often look likes branches or spikes extending out from the cell body. It is primarily the surfaces of the dendrites that receive chemical messages from other neurons. Physiology One extension is different from all the others, and is called the axon. The purpose of the axon is to transmit an electro-chemical signal to other neurons, sometimes over a considerable distance. In the neurons that make up the nerves running from the spinal cord to your toes, the axons can be as long as three feet! Physiology Longer axons are usually covered with a myelin sheath, a series of fatty cells which have wrapped around an axon many times. These make the axon look like a necklace of sausage-shaped beads. They serve a similar function as the insulation around electrical wire. Physiology At the very end of the axon is the axon ending, which goes by a variety of names such as the bouton, the synaptic knob, the axon foot, Physiology Between the axon ending and the dendrite of the next neuron is a very tiny gap called the synapse (or synaptic gap, or synaptic cleft), which we will discuss in a little bit. For every neuron, there are between 1000 and 10,000 synapses.