Every nerve cell, or neuron, is a tiny information processing unit with thousands of connections by which it can receive and send signals.  There are billions of nerve cells in the human brain and perhaps quadrillions of nerve connections.  No two nerves are exactly the same but most neurons contain some common features.  Nerve signals are received by branches called dendrites and these signals are passed to other cells by way of a long branched process called an axon.  The cell body contains the nucleus and the essential machinery for maintaining the function of the nerve cell.  Neurons are surrounded by special helper cells called glia.  Glia serve many purposes such as providing structural support and providing cushioning between cells.  Special glial cells (i.e., Schwann cells) wrap themselves around nerve fibers to provide insulation and help speed up nerve signals.

Nerve cell parts

Nerves come in many shapes and sizes.  Most nerve cells (neurons) have some common basic features.  The typical spinal motor neuron has 5 to 7 branched processes that extend from the cell body called dendrites.  A separate long fibrous projection extends from a thickened portion of the cell body.  This projection (called an axon) is wrapped by insulating Schwann cells and has branches at its end.  The branches are topped with knobs called terminal buttons.  Schwann cells wrap their membrane around neuron axons up to 100 times forming a myelin sheath.  The myelin sheath protects nerves and keeps nerves from "short circuiting" with other nerves.  This works just like the rubber coating on the outside of electrical wires.  The myelin sheath is locked in place by special proteins.  Small gaps between Schwann cells are called nodes of Ranvier.  These gaps speed up nerve signals by allowing electrical charges to reverse more quickly.  Read more about this in the nerve signals article.  In the central nervous system, most neurons have insulated (myelinated) axons, however instead of Schwann cells, they are myelinated by oligodendrogliocytes.  These cells are similar to Schwann cells however one oligodendrogiocyte may myelinate several axons of different neurons.

What are the types of neurons?

Neurons can be classified into three major groups: efferent  neurons, afferent neurons, and interneurons.

Efferent neurons send signals from the the central nervous system to the body.  Motor neurons are efferent neurons.  The motor neuron receives it's signal from the brain or spinal cord.  This nerve signal travels to a muscle where it stimulates the muscle to contract.

Afferent neurons send signals from the body to the spinal cord or brain.   These include nerves of sensation.  Nerves sensitive to touch send a signal from the skin to the spinal cord.  This signal is sent to the brain (except in a special case called a "reflex").

Interneurons are found only in the central nervous system (the brain and spinal cord).  They allow nerves to communicate with each other in complex ways.  These signals help with complex functions such as memory, thinking, and learning.

Neuron sizes

Nerves themselves can be seen without a microscope.  They often look like fleshy pink or white noodles among the body tissues.  Most neurons are too small to be seen without a microscope.   The dimensions of some nerve cells are truly amazing.  Imagine the cell body is the size of a tennis ball.  The dendrites would fill a large room with thousands of branches.  The axon would be one long cord about half an inch in diameter, but it would stretch almost a mile!

Nerve signals

Perhaps against common belief, nerves are not like telephone wires, passively transmitting electrical signals from one part of the body to the other.  Nerves pass signals actively through action potentials.  These action potentials are electrical charges that change in a wave-like pattern through the length of the nerve cell.  This process is somewhat like dominos standing in a row in which one domino falls and triggers another to fall and so on.  Like falling dominos, nerve signals are "all or nothing," meaning that either a domino falls or it doesn't.  Either a nerve "fires" or it doesn't.  A single nerve cell can only send one kind of signal and in only one strength.  The nerve cell membrane at rest has positive charges along the outside and negative charges on the inside.  When the nerve cell is stimulated to "fire," the positive and negative charges switch in a wave-like fashion down the length of the nerve.  The charges return to their normal resting state quickly after sending a signal.

Myelination, Schwann cells, and nodes of Ranvier

Nerve cells transmit nerve impulses quickly.  Nerve impulses (without the help of myelin) travel at speeds faster than the typical highway speed limit (55 miles per hour).  It turns out that this speed is still too slow for human function.  So, myelin to the rescue!  Myelin is nerve insulation that increases the speed of nerve signals up to 50 times faster than signals in unmyelinated nerves.  Myelin is typically made of cell membranes from special cells called Schwann cells or oligodendrogliocyte.  These cells wrap nerve axons with cell membrane up to 100 times per cell.  Nodes of Ranvier are narrow spaces between myelinated areas of axons that allow positive and negative charges to switch across the nerve cell membrane.  The nerve signal can jump from node to node, greatly increasing the speed of the signal.  This is called saltatory conduction.

 

 

Image used with permission from Wikimedia Commons.

Photo - Histology of normal anterior horn neurons of cervical spinal cord. Cresyl violet stain. Emory U./Dr. Karp. 1964. Used with permission.