somatic NERVOUS SYSTEMand the autonomic nerv-
ous system. The nerves of the somatic nervous
system are both sensory (conduct signals from the
body to the BRAIN) and motor (carry signals from
the brain to the structures of the body that are
under voluntary control). The nerves of the auto-
nomic nervous system regulate involuntary func-
tions such as HEART RATEand digestion as well as
the endocrine and exocrine glands. The auto-
nomic nervous system has two further subdivi-
sions: the sympathetic nerves (which serve the
thorax and lumbar region) and parasympathetic
nerves (which serve the head and sacral region.
For further discussion of the peripheral nervous
system within the context of the structures and
functions of the nervous system as a whole, please
see the overview section “The Nervous System.”
See also CENTRAL NERVOUS SYSTEM; ENDOCRINE
GLAND; PERIPHERAL NERVES.
persistent vegetative state An extended state of
unconsciousness in which higher BRAIN activity
(cerebral cortex function) is negligible or lost
though the brainstem continues to operate to sus-
tain the vital functions of living such as breathing,
HEART RATE, and BLOOD PRESSURE. Basic motor func-
tion, such as spontaneous though undirected
movement, may also occur as the brainstem is
responsible for some motor functions. The person
may also make sounds, move the eyes, and move
the MOUTH. However, there is no recognition of or
purpose to these actions, and the person cannot
follow instructions to move in certain ways and
does not speak, drink, or eat.
A person may remain in a persistent vegetative
state for months, years, or decades with adequate
nutritional support. Though in general the longer a
person remains in a persistent vegetative state the
less likely he or she will recover conscious function,
occasionally individuals emerge after extended
periods. The likelihood of recovery depends on the
extent and nature of damage to the cerebral cortex,
which imaging procedures such as MAGNETIC RESO-
NANCE IMAGING(MRI) and COMPUTED TOMOGRAPHY(CT)
SCANcan help assess. Persistent vegetative state
raises many medical, legal, and ethical concerns for
health-care providers as well as family members in
regard to how long to sustain life through support-
ive measures.
See also COMA; CONSCIOUSNESS; END OF LIFE CON-
CERNS; QUALITY OF LIFE.poliomyelitis A contagious viral INFECTION, often
called simply polio, that affects the nerves and
motor function throughout the body. Poliomyelitis
is rare today in the United States and other devel-
oped countries as a result of aggressive vaccination
programs. The first injectable VACCINEto prevent
poliomyelitis became available in 1955; a more
effective oral vaccine (modified live VIRUS) became
available in 1963. The last known “wild”
poliomyelitis infection occurred in the United
States in 1979. Subsequent poliomyelitis illness
resulted from infections acquired in other coun-
tries or from exposure of the nonvaccinated to the
oral vaccine. The switch to an enhanced inactive
(killed) poliovirus vaccine (IPV), capable of pro-
viding lifelong immunity, in 1987 eliminated the
latter as a cause of poliomyelitis. Oral poliovirus
vaccine is no longer available in the United States.
Three strains of poliovirus can cause infection. The
complete vaccination series consists of four doses
of vaccine, one for each strain and a final booster.Adults who travel to parts of the world
where poliomyelitis remains endemic
(notably Africa and Southeast Asia)
should receive either the complete inac-
tive poliovirus vaccine (IPV) series, if
never vaccinated or previous vaccina-
tion status is unknown, or an IPV
booster otherwise.Most poliomyelitis is either subclinical (no
symptoms) or nonparalytic (runs a course of ill-
ness with symptoms similar to those of INFLUENZA).
Paralytic poliomyelitis, which affects the BRAINand
SPINAL CORD, occurs in about 2 percent of infec-
tions. Among those who develop paralytic
poliomyelitis, the risk for death due to PARALYSISof
the muscles of BREATHINGand residual paralysis
after recovery from the infection are high. More
than 90 percent of people who develop nonpara-
lytic poliomyelitis, which affects PERIPHERAL NERVES,
recover without complications.
In the late 1970s health experts began tracking
the emergence of some polio-like symptoms, such
as MUSCLE weakness and generalized fatigue, inpoliomyelitis 277