Poliomyelitis is a viral disease. There are three types of poliovirus
and many strains of each type. The virus enters through the mouth and
multiplies in the throat and gastrointestinal tract, then moves into
the bloodstream and is carried to the central nervous system where
it replicates and destroys the motor neuron cells. Motor neurons control
the muscles for swallowing, circulation, respiration, and the trunk,
arms, and legs.
Human nerve cells have a protruding protein structure on their surface
whose precise function is unknown. When poliovirus encounters the nerve
cells, the protruding receptors attach to the virus particle, and infection
begins. Once inside the cell, the virus hijacks the cell’s assembly
process, and makes thousands of copies of itself in hours. The virus
kills the cell and then spreads to infect other cells.
Many types of human cells have receptors that fit the poliovirus; no
one knows why the virus favors motor neurons over other cells for replication.
For every 200 or so virus particles that encounter a susceptible cell,
only one will successfully enter and replicate.
In tissue culture, poliovirus enters cells and replicates in six to eight
hours, yielding 10,000 to 100,000 virus particles per cell.
||One way the human immune system protects itself is by producing antibodies that
engage the protein covering of the poliovirus, preventing the virus from
interacting with another cell.
||There are three types of poliovirus: 1, 2, and 3. Type 1 is the most
virulent and common. Both the Salk and Sabin vaccines are “trivalent” that
is, active against all three virus types. Type 2 poliovirus has not
been detected anywhere in the world since 1999.
||A person who gets polio is immune to future infection from the virus
type that caused the polio.
Poliovirus bound to a neuron receptor Illustration
courtesy of Link Studio
Scientifically accurate bronze models (without patina)
of the poliovirus created for the Smithsonian by Edgar Meyer, 2005.
These models are an adaptation of James Hogle’s image of the
poliovirus and were specially cast in bronze for the exhibit. They are
the first three dimensioanl representations of the poliovirus
Poliovirus Capsid Model and Scientific Art
Scientists use many types of models to visualize concepts about the real
world. Environmentalists and climatologists make computer graphics
models of the entire earth. In 1953, James Watson and Francis Crick
used a brass model of DNA's
structure—the so-called “double
helix”—as a physical analog of genes. All models are partly
right and partly wrong because they represent only a level of knowledge
at a given time. This bronze model of the poliovirus was made by for
the exhibition by biochemist/artist Edgar Meyer, based on the first
three-dimensional images of poliovirus that virologist James
Hogle at Harvard obtained from X-ray crystallography in 2000. While
the model represents the surface relief at a very high resolution,
the shell (capsid) in nature is more complex than artwork or X-ray
crystallography can show.
A Vaccine to Prevent Polio
Scientists could make vaccines even before they completely understood
how they functioned. Eventually researchers learned that vaccines
work by fooling the body’s immune system into producing antibodies
even though there is no disease. Jonas Salk
and Albert Sabin used this
knowledge to create two different kinds of polio vaccines.