Nerve cells, also called neurons or simply nerve fibers, are the basic units of the nervous system. They transmit electrical impulses between different parts of the body and coordinate all its activities. The human brain alone contains about 100 billion nerve cells!
Nerve cells come in many shapes and sizes, but they all have one thing in common: they have a cell body (or soma), from which extend long, thin fibers called axons and dendrites. Axons carry signals away from the cell body, while dendrites bring signals towards it. Most nerve cells also have several small “branches” called dendritic spines sticking out from their dendrites. These spines increase the surface area available for receiving incoming signals.
The cell body of a typical neuron is about 20 micrometers (0.02 millimeters) across. But some neurons, like those that make up the optic nerve, can be much larger – up to 1 millimeter across! The longest axons in the human body are found in neurons that control our leg muscles; these axons can be over a meter long!
Most of a neuron’s volume is taken up by its cell body and its many branching dendrites. But even a “small” neuron has a surprisingly large amount of space inside it – more than enough room to fit an entire human hair! This space is filled with jelly-like material called cytoplasm, in which float various organelles such as mitochondria (the “powerhouses” of the cell). Cytoplasm also contains numerous other types of molecules required for cellular function.