pyramidal
And
Extrapyramidal
tracts
By: Dr. Khaled Ibrahim
THE DESCENDING MOTOR SYSTEMS
 For voluntary movements to occur, one needs 2 neurons:
1- Upper motor neurons
 whose cell bodies lie in the higher
motor centers in the brain and brain
stem, and their axons constitute the
descending motor pathways.
2- Lower motor neurons
 whose cell bodies lie in the spinal
ventral horns or the
cranial
motor
corresponding
nuclei , and include
both -and -MNs . Axons
of the
lower motor neurons proceed through
the
peripheral
somatic
innervate skeletal muscles.
nerves
to
Classification of descending motor systems
 The descending motor pathways have commonly been divided into “pyramidal”
and “extrapyramidal” tracts.
 This classification is based on the finding that the motor tract which originates
from the cerebral cortex and descends to the spinal cord (the corticospinal tract)
passes through the pyramids of the medulla, and therefore has been called the
“the pyramidal tract”.
 The rest of the descending motor pathways do not travel through the medullary
pyramids, and are therefore collectively gathered under the heading: “the
extrapyramidal tracts”.
 An alternative classification of the descending motor tracts is based on the
sites of termination of these tracts in the spinal cord.
 On this basis , the descending motor pathways can be classified, with respect
to their spinal sites of termination , into:
Medial system
Lateral system
 composed of tracts which terminate composed of tracts which terminate
primarily on the ventromedial neurons (or primarily on the dorsolateral neurons
their associated interneurons)
(or their associated interneurons)
 innervate the trunk (axial) muscles and  innervate the distal muscles of the
the proximal (girdle) muscles of the limbs
limbs.
 more concerned with postural control.
 more concerned with controlling fine
voluntary
extremities.
movements
of
the
There are five important sets of descending motor tracts, named according to
the origin of their cell bodies and their final destination:
1) the corticobulbospinal tract,
(= Pyramidal tract)
2) the rubrospinal tract,
3) the reticulospinal tracts,
4) the vestibulospinal tracts, and
5) the tectospinal tract.
Extrapyramidal tracts
The Corticobulbospinal Tract
Origin
1) 30% of the tract fibers come from the primary motor area (area 4),
2) 30% come from the premotor area and the supplemental motor area (both of
which constitute area 6).
3) The remaining 40% of the CBS tract fibers come from the somatic sensory
areas of the cerebral cortex.
Divisions:
1) The Corticobulbar Division
2) The Corticospinal Division
The Corticobulbar Division
Course and Termination
 Axons originate from the head and face regions of the cortical areas (4, 6, 3,
2, 1, 5, 7).
 The corticobulbar tract Terminate in the motor nuclei of :
3rd and 4th cranial nerves in the midbrain
5th, 6th, and 7th cranial nerves in the pons
9th, 10th, 11th, and 12th cranial nerves in the medulla
 Corticobulbar tract from one side of the brain terminates mostly in the
cranial motor nuclei of both sides of the brain stem. Except, the lower part of
the facial nerve nucleus, and the hypoglossal nerve nucleus receive only
contralateral innervation from the cerebral cortex.
The Corticospinal Division (= Pyramidal tract):
Course and Termination
fibers of the CBS tract descend from the cerebral cortex
collect together and descend through the posterior limb of the internal capsule
through the middle portion of the cerebral peduncles of the midbrain
The fibers are separated by transverse pontine fibers in the pons
In the upper medulla oblongata where they form the pyramids of the medulla
In the lower region of the medulla, most of the fibers cross to the opposite
site forming the “medullary decussation”
About 80% to 90% cross to the
opposite side of the spinal cord
fibers which do not decussate in
medulla (about 10%) descend on
the same side of the spinal cord
(contralaterally) and continue as the
(ipsilaterally) as the
“lateral corticospinal tract”.
“ventral corticospinal tract”.
Lateral corticospinal tract
 80-90% of fibers
Ventral corticospinal tract
 Remaining 10% of fibers
 Descends contralaterally in the  Descends ipsilaterally in the
lateral column of the opposite side) ventral column on the same side.
 Along the whole length of the  Till the mid-thorathic region only
spinal cord.
 Terminate gradually on the
laterally situated neurons on the
ventral horn directly or more
common
indirectly
through
interneurons.
 Terminate gradually by crossing
at various levels of the spinal cord
to terminate on the mediallysituated neurons directly or more
common
indirectly
through
interneurons.
 It is classified as a part of “lateral  It is classified as a part of “medial
motor system”.
motor system”.
Functions of the Corticobulbospinal Tract:
1) Initiation of Voluntary Movements
 This is particularly mediated by the CBS tract fibers that join the lateral motor
system. These fibers include:
1) The corticobulbar tract fibers that end in: (i) The lower part of the facial nerve
motor nucleus which supplies the muscles of the lower part of the face , and (ii)
The hypoglossal nucleus that innervate the tongue muscles. These facial and
tongue muscles are concerned with highly skilled movements such as those
involved in the act of speech.
2) The lateral corticospinal tract fibers that descend in the spinal cord for control
of muscles of the distal parts of the limbs, especially the hand and digits
muscles, which subserve fine skilled movements used in manipulation by hand
and fingers, and other accurate motor actions done by the limbs .
2) Role in Automatic and Postural Movements:
 This is mediated by small portion of the CBS tract fibers that join the medial
motor system. These fibers include:
1) Part of these fibers innervates some cranial nerve motor nuclei which supply
muscles of the head involved in such activities as closure of the eye lids,
chewing, swallowing, and phonation which often occur automatically.
2) Another part descends in the ventral corticospinal tract which exerts some
control on axial and girdle muscles, that are concerned mainly with postural
adjustments.
3) Facilitation of the muscle tone:
 CBS facilitates motor neurons, especially those innervating the distal flexor
muscles of the limbs.
The Rubrospinal Tract
Origin
red nucleus of the midbrain
Afferent
receives afferent connections from:
1) Ipsilateral cortical motor areas (corticorubral pathway),
2) Contralateral side of the cerebellum,
3) Basal ganglia.
Course and Termination
 On leaving the red nucleus, the fibers of the rubrospinal tract cross to the
opposite side -------> descend contralaterally through the brain stem and
the
lateral
column
corticospinal tract .
of the spinal white matter, very close to the lateral
 in the brain stem, some fibers of the rubrospinal tract terminate in various
nuclei of the brain stem, as well as the brain stem reticular formation.
 in the spinal cord, fibers leave it and terminate on the more laterally
situated motor neurons and their associated interneurons in the cord gray
matter, similar in this respect to the lateral corticospinal tract fibers.
Functions of the Rubrospinal Tract:
1) an additional pathway for transmission of cerebral cortical motor
commands to the lower motor neurons similar to those of the
corticospinal tract.
2) facilitatory to the - and -MNs of the distal flexor muscles, but they
are inhibitory to extensor muscles.
The Reticulospinal Tracts
Divisions:
1) Medial reticulospinal tract.
Medial reticulospinal tract
arises from neurons of
“pontine reticular formation”
2) Lateral reticulospinal tract.
Lateral reticulospinal tract
the arises from neurons in
“medullary reticular formation”
the
descends to all levels of the spinal its fibers descend to all levels of the
cord
spinal cord
terminate mainly on interneurons
in the spinal gray matter which
excite the medially situated - and
-MNs innervating the antigravity
muscles, that is, the muscles of the
vertebral column and the extensor
muscles of the lower limbs.
synapse with interneurons that
inhibit the - and -MNs of antigravity
and extensor muscles , but they
facilitate the - and -MNs of flexor
muscles.
Medial reticulospinal tract
Lateral reticulospinal tract
The pontine reticular formation The medullary reticular formation
has a high degree of natural receives afferent signals from: (i) the
excitability.
premotor area of cerebral cortex, (ii)
 In addition, it receives strong
the paleocerebellum, and (iii) red
excitatory
signals
from
the
vestibular
nuclei
and
the nucleus.
neocerebellum.
 Functions:
 Functions:
the
medial
(or
pontine) activate the medullary inhibitory
reticulospinal tract exerts a strong
system, which can thus counterfacilitatory effect on the motor
neurons of the antigravity and balance the facilitatory effect of the
extensor muscles to support the pontine reticular formation on the
body posture against gravity .
antigravity muscles.
 Both the facilitatory (pontine) and inhibitory (medullary) reticular
formations constitute together a controllable system that is regulated by
signals from the cerebral cortex, cerebellum, and other motor centers to
adjust the level of muscle tone in various muscles under different postural
conditions.
The Vestibulospinal Tracts
Divisions:
1) Lateral Vestibulospinal tract. 2) Medial Vestibulospinal tract.
Lateral Vestibulospinal tract
 originates
from
vestibular nucleus,
 descends
lateral originates
from
the
medial
vestibular nucleus
to all levels of the descends to the cervical and upper
spinal cord
 synapses
the
Medial Vestibulospinal tract
thoracic regions only.
on
interneurons synapses
on
interneurons
associated with the - and -MNs of
associated with the - and -MNs
antigravity and extensor muscles,
innervating muscles of the neck
which are medially situated in the
spinal ventral horn.
which regulate the head position.
Functions
Exposure to acceleratory
Change of head position in relation to
forces
the earth’s gravity
Vestibular nuclei
1) adjusting the tone Vestibulospinal
and contractionTract
of antigravity muscles to
maintain the body posture and equilibrium by LVST .
2) regulating the position of the head and upper limbs during
exposure to acceleration by MVST.
The Tectospinal Tract
Origin
Arises mainly from superior colliculi & to less extent from inferior
colliculi.
Course and Termination
The fibers of the tract cross the midline -----> descend only to the cervical
segments of the spinal cord ------> terminate on interneurons associated with
the motor neurons innervating the neck muscles .
Functions
Orienting responses that initiate reflex turning of the head in response to
visual or auditory stimuli
Example: turning the head to look at the source of a sudden visual stimulus
Finally, one can conclude that:
Lateral corticospinal tract + rubrospinal tract forms the “Lateral Motor System”
Whereas;
Medial corticospinal tract + reticulospinal tract + vestibulospinal tracts +
tectospinal tracts forms the “Medial Motor System”
The Primary Motor Area
Location
in the precentral gyrus of the frontal lobe, and corresponds to Brodmann’s
area 4 .
Body Representation
1) contralateral and inverted. However, several facial muscles are
represented bilaterally .
2) organized in a somatotopic manner with the feet at the upper medial
region of the gyrus and the face at the lower lateral region
3) Area of representation is proportional with the complexity of function
done by the muscle. So, muscles of hands and tongue occupies 50% of this
area
Neural Connections
A) Afferents
1)Thalamus and Somatic Sensory Area
sensory feed-back input from the muscle and joint proprioceptors
which provides the motor cortex with information about the actual motor
performance
2) The premotor and supplemental motor
of the same side providing higher control of its activity.
3) The basal ganglia and cerebellum
regulate and coordinate its motor activity.
4)The visual and auditory cortices
providing it with information about the spatial relations of the body to
the external environment.
5) The prefrontal region
appropriate course of motor action suitable with the surrounding
environment.
6) The motor areas of the opposite hemisphere
coordinating bilateral motor activities performed by both sides of the body .
B)
Efferents
1) About 30% of the axons of the corticobulbospinal tract.
2) large number of fibers that project onto the basal ganglia, establishing
a neural pathway for planning and programming of motor actions .
3) tremendous number of fibers which project to the cerebellum,
establishing pathways between the motor cortex and the cerebellum for
coordination and regulation of movements.
4) moderate number of fibers that pass to the red nucleus in the
midbrain, and also to the reticular formation
in the brain stem for
controlling their activity
Role in Movements
1) discharges the descending motor commands that produce voluntary
movements. It controls both “distal” and “proximal” muscles
2) facilitatory to the tone of distal muscles, particularly flexor muscles.
Premotor Area
Location
The premotor area lies immediately anterior to the lateral regions of the
primary motor area
 It occupies a large portion of area 6, and is bounded superiorly by the
supplemental motor area.
Neural Connections
With the Primary & supplemental motor areas:
With the Cerebellum
With the Basal Ganglia
Role in Movements
1) Enhancing the primary motor area to commence its activity.
2) Adjusting posture during performance of voluntary movements.
3) In association with the supplemental motor area, establishing the motor
programs necessary for execution of complex movements.
4) Inhibit grasp reflex
5) Influence autonomic activity as heart rate & arterial blood pressure
6) Inhibit muscle tone
7) A few highly specialized motor centers have been found in the premotor
areas of the human cerebral cortex :
Broca’s Area for Speech
The Frontal Eye Movements Area
located above Broca’s area in the frontal lobe
controls voluntary movements of the eyes toward different objects in
the visual field.
Head Rotation Area
located just above the eye movement area in the motor cortex .
directing the head toward different visual objects .
Area for Hand Skills
(Exner’s Area)