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, specialized cells that process information. Neurons generate nerve impulses that cause the release of chemicals in specialized spaces called synapses that allow different neurons to talk to each other. The proper function of neurons is dependent on specialized glial cells. All nervous systems in all animal species have seven 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 contains approximately several dozen distinct types of computational neurons that mediate the function of that brain area.
Myelin: Many axons are ensheathed by glial cell processes that provide extra insulation. This insulation is comprised of oligodendrocytes that form myelin, giving rise to myelinated axons. The gaps between the myelin wrappings are called nodes of Ranvier. This is where the action potential (electrical nerve impulse) repeats, 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.
Astrocytes: Astrocytes are star-shaped cells that provide metabolic support to neurons, as well as form the blood brain barrier. Astrocytes contribute significantly to synaptic function by maintaining the proper concentration of chemicals at the synapse and are also known to release gliotransmitters that can regulate synaptic transmission. The ability of astrocytes to integrate synaptic activity and their close physical location to synapses has given rise to the term tripartite synapse. The tripartite synapse refers to three entities: the neuronal pre-synaptic terminal, the neuronal post-synaptic terminal, and the adjacent process of an astrocyte.
Microglia: These cells are the only resident immune cells in the brain. They act as a first line of immune defense in the brain. Microglia are scavengers, removing dead cells and infectious agents by a process called phagocytosis. Although microglia can be activated in disease states to release harmful chemicals that injure neurons, microglia have been shown to provide trophic support to neurons. More recent studies have shown that microglia prune unnecessary synapses during development, which is required for proper central nervous system maturation.
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 synaptic inputs from other cells, the axon sends the output of the cell to other cells.
What really distinguishes the nervous system from any other functioning group is the complexity of the neuronal interconnections called synapses. The human brain has on the order of 100 billion neurons, each with a unique set of as many as 10,000 synaptic inputs, yielding a total of 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.