DescendingTracts
 Bundles of nerve fibers in the white matter of the spinal cord
descending from the supraspinal nerve centers.
 Also called Upper Motor Neurons.
 They are concerned with Motor Activity.
 They originate from cerebral cortex and brain stem.
 Lower Motor Neurons (Neurons which innervate the skeletal
muscles).‫تكون باالخير‬
 Mainly present in the anterior gray column of the spinal cord.
DescendingTracts
Have 3orders :
*First-order Neuron: Cell body lies in the cerebral cortex
*Second-order Neuron:
An internuncial neuron, situated in the anterior gray column of the
spinal cord.
*Third-order Neuron:
Axon of 2nd order neuron is short and synapse with 3rd order
neuron,(Lower motor neuron), in the anterior gray column of the spinal
cord.
Axon of 3rd order neuron innervates the skeletal muscle through
anterior root and spinal nerve.
………………………………………………….
((Descending Tracts))
1-Corticospinal tract 2-Reticulospinal
Tract 3-Tectospinal Tract
4-Rubrospinal Tract 5- Vestibulospinal
Tract 6- Olivospinal Tract
Tract
Arise
spinal cord
Function
Corticospinal
Tracts
arise as Axons of
pyramidal cells
(situated in the 5th
layer of cerebral
cortex).
*On entering the Pons the tract is broken into many
bundles by transverse pontocerebellar fibers.
Concerned
Voluntary,
discrete and
Skilled
movements
(especially distal
parts of the
*1/3rd of the cells
arise form Primary
Motor cortex (Area 4).
*1/3rd of the cells
arise form Secondary
Motor cortex (Area 6).
*1/3rd of the cells
arise form Parietal
lobe (Area 3,1 and 2).
*The descending
fibers converge in
corona radiata.
*Then pass through
posterior limb of the
internal capsule and
reaches to midbrain.
*In medulla oblongata the bundle of fibers grouped
together anteriorly and form Pyramid (Pyramidal
Tract).
*Most of the fibers cross the midline at the
decussation of Pyramids (Motor Decussation) and
enter the lateral white column of spinal cord as
Lateral corticospinal tract.
limbs).
*The lateral corticospinal tract lies in the whole
length of spinal cord.
*The remaining fibers do not cross in the decussation
and descend in anterior white column as of spinal
cord as Anterior corticospinal tract.
*These remaining fibers eventually cross the midline
in the cervical and upper thoracic segments.
All the fibers(( terminate ))in the anterior gray
column of the spinal cord by synapsing with
internuncial or motor neurons).
Tract
Arise
spinal cord
terminal
Function
Reticulospinal
Tracts
Reticular Formation:
From Pons these neurons
send uncrossed axons down
into the anterior white
column of the spinal cord
and form Pontine (Medial)
Reticulospinal Tract.
anterior gray
columns of the
spinal cord.
Facilitate or
inhibit the
activity of Motor
neurons
Group of scattered
nerve cells and nerve
fibers in midbrain,
Pons and Medulla.
From medulla these neurons
send crossed and uncrossed
axons down into the lateral
white column of the spinal
cord and form Medullary
(Lateral) Reticulospinal Tract.
Function:
Facilitate or
inhibit voluntary
movements or
reflex activity.
Tract
Arise
spinal cord
terminal
Function
Tectospinal
Tract
superior Colliculus
of the Midbrain.
anterior white column of the
spinal cord.
anterior gray
column in the upper
cervical segments of
the spinal cord by
synapsing with
internuncial
Concerned with
reflex postural
movements in
response to
visual stimuli
Most of the fibers cross the
midline and descend through
the brainstem
neurons.
Red Nucleus: lies in
the tegmentum of
the midbrain at the
level of superior
colliculus.
It descends through Pons and
Medulla and enters in the
lateral white column of the
spinal cord.
Olivospinal
Tract
inferior olivary
nucleus
Vestibulospinal
Tract
Vestibular Nuclei:
Rubrospinal
Tract
Vestbul : Lies in the
Pons and Medulla.
synapsing with
internuncial neurons
in the anterior gray
column of the spinal
cord.
Facilitates the
activity of
flexors muscles
and inhibit the
activity of
extensors
muscles
descends in the lateral white
column
synapsing with
internuncial neurons
in the anterior gray
column of the spinal
cord
Play a role in
muscular activity
anterior white column
synapsing with
internuncial neurons
in the anterior gray
column of the spinal
cord.
Facilitates the
activity of
extensors
muscles and
inhibit the
activity of
flexors muscles.
Axons arising form red
nucleus cross the midline at
the level of the nucleus and
descend as Rubrospinal Tract