Descending Tracts
Dr Rania Gabr
Objectives
• Define the terms upper and lower motor neurons
with examples
• Describe the corticospinal (pyramidal) tract and
the direct motor pathways from the cortex to the
trunk and limbs.
• Briefly describe the indirect motor pathways from
the cortex to the trunk and limbs through
extrapyramidal tracts such as rubrospinal and
reticulospinal tracts..
• Compare the signs and symptoms of the upper
and lower motor neuron lesions.
Descending tracts
• Are segregated bundles of
nerve fibres in the white
matter of the spinal cord
descending from the
supraspinal centres
referred to as upper motor
neurons ( UMN )
• They are concerned with
somatic and visceral motor
activities.
• Their cells of origin lie in the
cerebral cortex and brain
stem
They regulate LMN activity:hey regulate the LMN activity
They are concerned with the control of movement, muscle tone,
spinal reflexes, spinal autonomic functions and the modulation
of sensory transmission to higher centers.
Upper motor neurons ( UMN )
• Are the descending supraspinal pathways that
influence the activity of the LMN
• They control :
1-voluntary motor activity
2-maintenance of posture & equilibrium
3- muscle tone and
4- reflex activity
• generally exert their effect on groups of
muscles ( not on one specific muscle )
reciprocally on agonist and antagonist muscle
group
UMN
cerebral cortex – midbrain - pons - medulla oblongata
descending tracts
sensory inputs
LMN
Lower motor neurons ( LMN )
Are motor neurons that innervate the voluntary muscles located in
1- anterior grey column of the spinal cord , and
2- motor nuclei of brainstem
form the final common pathway
LMN
Descending Tracts
1- Corticospinal tracts
(Lateral and anterior)
2- Corticobulbar tracts
3- Rubrospinal tract
4-Tectospinal tract
5- Reticulospinal tracts
(Lateral and medial)
6-Vestibulospinal tract
Corticospinal Tracts
The corticospinal tracts are particularly
concerned with the control of
voluntary, discrete, skilled movements,
especially those of the distal parts of
the limbs.
Corticospinal tract neurons arise from
cell bodies in the cerebral cortex.
The cells of origin are widely distributed
in the motor and sensory cortices,
including the precentral gyrus or
primary motor cortex.
large Betz cells give rise to the largest
diameter corticospinal axons.
Course of the tract
• 1- Corticospinal axons
leave the cerebral
hemispheres
:Pyramidal cells
• 2- They pass through
the the corona radiata
• 3-Then through post.
Limb of internal
capsule
• 4- They enter the crus
cerebri of the
midbrain. (Cerebral
peduncles)
• 5- They pass through the
ventral portion of the pons
• 6- Fibers reach the
medulla oblongata, where
they form two prominent
columns on the ventral
surface.
• They are called the
pyramids and for this
reason the term pyramidal
tract is used as an
alternative name for the
corticospinal tract.
• In the caudal medulla, the
fibers of the pyramids
undergo subtotal
decussation.
• ( Motor decussation)
• About 75-90% of fibers
decussate and enter the
contralateral lateral
corticospinal tract, which
is located in the lateral
part of the spinal white
matter.
• 10-25% of pyramidal fibres
remain ipsilateral and
enter the ventral
corticospinal tract located
lateral to the ventral
median fissure.
Rubrospinal Tract
• Originate from nerve cells in
red nucleus
nerve fibres / axons
– cross the mid line
– descend as rubrospinal
tract
• through pons and medulla
oblongata
• Terminate in the anterior grey
column of spinal cord
( facilitate the activity of flexor
muscles )
• The rubrospinal tract originates
from the red nucleus of the
midbrain tegmentum.
• It exerts control over the tone of
limb flexor muscles, being
excitatory to the motor neurons
of these muscles.
• Axons leave the cells of the red
nucleus course ventromedially
and cross in the ventral
tegmental decussation,
• after which they descend to the
spinal cord where they lie
ventrolateral to, the lateral
corticospinal tract.
• Red nucleus receives
afferent fibers from motor
cortex and from cerebellum.
• The rubrospinal tract
represents a non-pyramidal
route by which the motor
cortex and cerebellum can
influence spinal motor
activity.
• The tract facilitates the
activity of the flexors and
inhibits the activity of the
extensors and the
antigravity muscles.
Tectospinal tract
•
Tectospinal fibers arise from the
superior colliculus of the midbrain.
•
Axons pass ventromedially around the
periaqueductal grey matter and cross
in the dorsal tegmental decussation.
•
In the spinal cord, descending
tectospinal fibers lie near the ventral
median fissure and terminate
predominantly in cervical segments.
•
The superior colliculus receives visual
input and the tectospinal tract is
thought to mediate reflex movements
in response to visual stimuli.
Vestibulospinal Tracts
• nerve cells in the vestibular
nuclei (in the pons and
medulla oblongata
– receive afferents from the
inner ear and cerebellum
• axons descend uncrossed
– through medulla and
through the length of spinal
cord
• synapse with neurons in the
anterior gray column of the
• Vestibulospinal
fibres arise from the
vestibular nuclei
• They are Medial and
Lateral Vestibular
Nuclei
• They are situated in
the pons
The vestibular nuclei
receive input from the
labyrinthine system by
way of the vestibular
nerve and from the
cerebellum.
Axons arise from cells of
lateral vestibular nucleus
Descend ipsilaterally as the
lateral vestibulospinal tract.
Located in the ventral
funiculus.
Lateral vestibulospinal tract
fibres mediate excitatory
influences upon extensor
motor neurons.
They serve to control
extensor muscle tone in the
antigravity maintenance of
posture.
The medial vestibular
nucleus contributes
descending fibers to
the ipsilateral medial
longitudinal
fasciculus, also known
as the medial
vestibulospinal tract,
which is located
adjacent to the
ventral median
fissure.
Reticulospinal Tracts
The reticular formation of the pons and
medulla gives rise to reticulospinal fibres.
Axons arising from the pontine reticular
formation descend ipsilaterally as the
medial (or pontine) reticulospinal tract.
Axons from the medulla descend bilaterally
in the lateral (or medullary) reticulospinal
tracts.
Both tracts are located in the ventral and
lateral white columns respectively.
They influence the voluntary movement
• Pyramidal tract
refers to corticospinal tracts
• Extrapyramidal tract
other than corticospinal tract
( VeSt, ReSt, TeSt, RuSt )
Lesions of the Spinal Cord
• Lesions of the dorsal
columns
• The loss of proprioception leads
to a high steppage and unsteady
gait (sensory ataxia), which is
exacerbated when the eyes are
closed (Romberg's sign).
• Subacute combined
degeneration
• Deficiency of vitamin B12
• The degeneration of the
dorsal columns produces
sensory ataxia.
• The degeneration of lateral
columns produces
weakness and spasticity of
the limbs
• It is combined.
• Multiple Sclerosis
• Immune disease, leads
to damage of the
fasciculus cuneatus of
the cervical spinal cord
• loss of proprioception
in the hands and
fingers, causing
profound loss of
dexterity
• (Astereognosis).:is the
inability to identify an
object by touch without
visual input
Spinothalamic Tract Lesions
• Syringomyelia :Enlarged central canal
compressing adjacent nerve fibers.
• Neurons carrying pain and
temperature are damaged as they
decussate in the ventral white
commissure, close to the central
canal
• Selective loss of pain and
temperature below the lesion
• Dissociated sensory loss, since light
touch and proprioceptive sensation
are retained
Friedreich's Ataxia
Inherited degenerative disease (autosomal
recessive)
Affect the spinocerebellar tracts
Leads to incoordination of the arms (intention
tremor) and a wide-based, reeling or drunken gait
(ataxia).
Hereditary spastic paraparesis
• Inherited degenerative
disorder (autosomal
dominant)
• Degeneration of the lateral
columns, including the
lateral corticospinal tract
• It causes weakness of legs,
marked stiffness of gait.
• It results in spastic
paraparesis with
hyperreflexia and extensor
plantar responses
Poliomyelitis
• Acute viral infection of
ventral horn neurons
• Rapid paralysis and wasting
of the limb and respiratory
muscles.
Amyotrophic Lateral Sclerosis
Brown-Sequard syndrome
• Ipsilateral LMNL
at same level
• Degeneration of descending
• Ipsilateral UMNL
pathways
below the level
• weakness and spasticity of
the limb muscles
• Ipsilateral loss of
(Upper Motor Neuron Lesion) posterior column
• Degeneration of ventral horn
functions:propioception,vibration
Leads to weakness, wasting,
• Contralateral loss of
hypotonia
(Lower Motor Neuron Lesion) spinothalamic tract
functions: pain and temperature