EXTRAPYRAMIDAL TRACTS
Descending tracts of
spinal cord other than pyramidal tracts are called
extrapyramidal tracts.
MEDIAL LONGITUDINAL
FASCICULUS
Situation
Medial
longitudinal fasciculus descends through posterior part of anterior
white column of the spinal cord.
Origin
Actually, this tract is
the extension of medial longitudinal fasciculus of
brainstem. Fibers of this tract take origin from four different areas in
brainstem:
i.
Vestibular nuclei
ii.
Reticular formation
iii.
Superior colliculus
iv.
Interstitial cells of Cajal.
Course
After entering the spinal
cord from the brainstem, the fibers of medial longitudinal
fasciculus descend through posterior part of
anterior white column of the same side. In the
spinal cord, this tract is well defined only in upper cervical segments. Below this level, the fibers run along with the fibers of anterior vestibulospinal tract.
Extent
Fibers of this tract
extend up to the upper cervical segments of spinal cord.
Termination
Fibers of this tract
terminate in anterior motor neurons of the spinal cord
along with fibers of anterior vestibulospinal tract either directly or through internuncial neurons.
Function
Medial
longitudinal fasciculus helps in the coordination of reflex ocular movements and the integration of ocular and neck movements.
Effects of Lesion
Reflex
ocular movements and reflex neck movements are affected in the lesion of this
tract.
ANTERIOR
VESTIBULOSPINAL TRACT
Situation
Anterior vestibulospinal
tract is situated in the anterior white column, along the
periphery of spinal cord lateral to tectospinal tract.
Origin
Fibers of this tract arise
from medial vestibular nucleus in medulla oblongata. In
fact, anterior vestibulospinal tract is the
extension of medial longitudinal fasciculus. Most of
the fibers are uncrossed.
Extent
Fibers run up to thoracic
segments of spinal cord.
Course
Fibers of this tract run
down from medulla into the anterior column of spinal
cord along the periphery. All
the fibers
are uncrossed
Termination
Along with
fibers of lateral vestibulospinal tract, the fibers of this tract terminate in
anterior motor neurons
directly
or through internuncial neurons.
Function
Function of
this tract is explained along with the function of lateral vestibulospinal tract.
LATERAL
VESTIBULOSPINAL TRACT
Situation
Lateral
vestibulospinal tract occupies the anterior part of lateral
white column of spinal cord.
Origin
Fibers of this tract take
origin from the lateral vestibular nucleus in medulla. This
nucleus is also
called
Deiter nucleus.
Extent
Fibers of this tract are
present throughout the spinal cord.
Course
From
Deiter nucleus, most of the fibers descend directly through lateral column.
Very few fibers cross to
the
opposite side before descending.
Termination
Fibers of this tract terminate
in the anterior motor neuron, either directly or via
internuncial neurons.
Functions
Vestibular nuclei receive
impulses concerned with muscle tone and posture from vestibular apparatus
and
cerebellum. Vestibular nuclei in turn convey the impulses to different
parts of the body through the
anterior
and lateral vestibulospinal tracts. Vestibulospinal tracts are
concerned with adjustment of position
of head and body during angular
and linear acceleration.
Effect of Lesion
Adjustment
of head and body becomes difficult during acceleration when the
vestibulospinal tracts are
affected by
lesion.
RETICULOSPINAL TRACT
Situation
Reticulospinal tract is
situated in the anterior white column, posterior to
anterior vestibulospinal tract.
Origin
Fibers of this tract arise
from the reticular formation of pons and
medulla. Pontine reticular fibers are
uncrossed
(direct) and descend in medial part of anterior column. Fibers from medullary
reticular
formation are predominantly
uncrossed and only few fibers are crossed. These fibers
descend in lateral part of anterior column and to some extend in the anterior part of lateral column.
Extent
Fibers of reticulospinal
tract extend up to thoracic segments.
Termination
Fibers of reticulospinal
tract terminate in gamma motor neurons of anterior gray horn through the
internuncial neuron.
Functions
Reticulospinal tract is
concerned with control of movements and maintenance of muscle
tone,
respiration
and diameter of blood vessels. Pontine and medullary fibers have opposite
effects on these
functions,
TECTOSPINAL TRACT
Situation
Tectospinal tract is
situated in the anterior white column of spinal cord.
Origin
Nerve
fibers of this tract arise from superior colliculus of midbrain.
Extent
Tectospinal tract extends
only up to lower cervical segments.
Course
After taking origin from superior
colliculus, the fibers cross the midline in dorsal tegmental decussation
and
descend in anterior column.
Termination
Fibers of tectospinal tract
terminate in the anterior motor neurons of spinal cord, directly or via internuncial
neurons.
Function
Tectospinal tract is
responsible for the movement of head in response to
visual and auditory stimuli.
RUBROSPINAL TRACT
Situation
Rubrospinal tract is
situated in the lateral white column of spinal cord.
Origin
Fibers of this tract arise
from large cells (nucleus magnocellularis) of red
nucleus in midbrain.
Extent
Nerve fibers of this tract appear in the spinal cord only up to
thoracic segments.
Course
After
arising from the red nucleus, the fibers cross the midline
in ventral tegmental decussation and descend into
spinal cord through the reticular formation of pons and medulla.
Termination
Fibers of rubrospinal tract
end in the anterior motor neurons of the spinal cord
via internuncial neurons.
Function
Rubrospinal
tract exhibits facilitatory influence upon flexor
muscle tone.
OLIVOSPINAL TRACT
Situation
Olivospinal tract is
present in lateral white column of spinal
cord.
Origin
The nerve
fibers of the olivospinal tract take origin from the inferior olivary nucleus, which is present in the medulla oblongata.
Termination
Fibers of this tract terminate
in the anterior motor neurons of spinal cord.
Function
Functions
of the olivospinal tract are not known clearly. It is believed that this
tract is involved in reflex movements arising from the proprioceptors.
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