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The human extrapyramidal system.
[locked-in syndrome]
The
term
"
extrapyramidal
"
originally
described
a
collection
of
tegmentospinal
pathways
that
conveyed
the
propagation
of
cortically-induced
seizures
to
the
spinal
cord
of
dogs
with
bilateral
transection
of
the
pyramidal
tracts
.
The
extrapyramidal
concept
developed
into
unwarranted
avenues
to
include
forebrain
structures
involved
in
motor
control
,
such
as
the
basal
ganglia
,
or
as
a
broad
synonym
of
clinical
syndromes
characterized
by
nonparalytic
abnormal
involuntary
movements
and
postures
.
Clinicians
and
pathologists
assumed
that
,
similarly
to
mammals
in
general
,
the
human
extrapyramidal
system
was
also
organized
in
parallel
with
the
pyramidal
tracts
.
Several
inconsistencies
of
this
model
,
as
applied
to
humans
,
were
overlooked
despite
compelling
clinicoanatomic
evidence
showing
that
(
i
)
bilateral
damage
to
the
pyramidal
tracts
at
any
level
from
the
pontomedullary
transition
to
the
motor
cortex
produces
the
locked-
in
syndrome
and
(
ii
)
the
extrapyramidal
tracts
are
sparse
in
man
.
In
the
present
essay
I
advance
the
hypothesis
that
the
human
extrapyramidal
system
is
fundamentally
different
from
its
mammalian
counterparts
both
anatomically
and
functionally
.
To
test
this
hypothesis
,
a
systematic
analysis
of
the
residual
motor
patterns
observed
on
a
natural
model
of
a
bilateral
section
of
the
human
pyramidal
tracts
,
the
locked-
in
syndrome
,
is
provided
.
This
analysis
reveals
that
the
human
extrapyramidal
system
underpins
the
organization
of
the
following
categories
of
motor
synergies
:
(
i
)
oculofacial
and
oculocephalic
,
(
ii
)
faciorespiratory
,
(
iii
)
axial
-appendicular
,
and
(
iv
)
plurisegmental
.
Anatomically
,
these
functional
repertoires
of
motor
integration
are
mediated
by
six
collections
of
axons
defined
by
their
brainstem
nuclei
of
origin
and
spinal
cord
destinations
:
reticulospinal
(
medial
and
lateral
)
,
vestibulospinal
(
medial
and
lateral
)
,
rubrospinal
(
lateral
only
)
,
and
tectospinal
(
medial
only
)
fiber
systems
.
I
conclude
that
the
extrapyramidal
concept
can
reliably
be
extended
to
humans
,
albeit
its
physiological
and
anatomical
scope
is
considerably
narrower
than
traditionally
assumed
.
Diseases
Validation
Diseases presenting
"pyramidal tracts"
symptom
locked-in syndrome
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