The nervous system consists of the central nervous system (the brain and spinal cord), the peripheral nervous system (the sensory and motor neurons), and the autonomic nervous system (which regulates body processes such as digestion and heart rate).

All the divisions of the nervous system are based universally on the functions of neurons. Neurons are specialized cells that process information. Like all cells, they are unbelievably complicated in their own right. All nervous systems in all animal species have four basic types of functional cells:

  • Sensory neurons: These neurons tell the rest of the brain about the external and internal environment.

  • Motor (and other output) neurons: Motor neurons contract muscles and mediate behavior, and other output neurons stimulate glands and organs.

  • Communication neurons: Communication neurons transmit signals from one brain area to another.

  • Computation neurons: The vast majority of neurons in vertebrates are computation neurons. Computation neurons extract and process information coming in from the senses, compare that information to what’s in memory, and use the information to plan and execute behavior. Each of the several hundred brain regions contain very approximately several dozen distinct types of computational neurons that mediate the function of that brain area.

There are obvious structural differences between neurons and most other cells. While most non-neuronal cells resemble squashed spheroids, neurons typically have a “dendritic tree” of branches (or processes) arising from the cell body (or soma), plus a single process called an axon that also emanates from the cell body but runs for large distances (sometimes even up to several feet) before it branches. While the dendrites receive inputs from other cells, the axon sends the output of the cell to other cells.

Many axons are sheathed by glial cell processes that provide extra insulation. These are called myelinated axons. The gaps between the myelin wrappings are called nodes of Ranvier. This is where the action potential, the electrical pulses, repeat thus enabling the signal to maintain its strength over long distances. Myelinated axons have fast conduction velocities in which the action potentials travel at several hundred meters per second. Many smaller axons in the nervous system are unmyelinated and conduct action potentials more slowly.

The main structural parts of a neuron.
The main structural parts of a neuron.

What really distinguishes the nervous system from any other functioning group is the complexity of the neuronal interconnections. The human brain has on the order of 100 billion neurons, each with a unique set of about 10,000 inputs, yielding about a quadrillion synapses — a number even larger than the U.S. national debt in pennies! The number of possible distinct states of this system is virtually uncountable.