Pyramidal system.Upper motor neuron,Corticospinal tract, Corticobulbar tract,Giant pyramidal cells ,reticular formation , Babinski sign ,

1. By-Dr.Ranjeet Singha,PT(MPT in Neurology)
HAAD Licensed
Associate Professor,
College of Physiotherapy and Medical Sciences,
Guwahati,Assam.

2.  The pyramidal system is made up of three pairs
of descending motor tracts: (1) the corticobulbar
tracts, (2) the lateral corticospinal tracts, and (3)
the anterior corticospinal tracts (Fig. 103-1).The
corticobulbar tracts find their origin in the
primary motor cortex of the cerebrum and end
at the brainstem motor nuclei of cranial
nerves III, IV,VI,VII, IX, and XII, which are
responsible for control of eye movements, the
tongue, the muscles of facial expression, and the
more superficial muscles of the neck and back.

3.  consists of upper motor neurons extending
from the cortex to the brainstem or spinal
cord that make up two major pathways of
voluntary movement: the corticospinal and
corticobulbar tracts (sometimes called the
pyramidal tracts).

4.  a neuron that extends from the cerebral
cortex or brainstem to synapse with a lower
motor neuron (usually in the spinal cord).
Upper motor neurons control the activity of
lower motor neurons, which control the
activity of muscles to produce movement.

5.  primary pathway for producing voluntary
movement, the corticospinal tract is a large
collection of axons that travel from the
cerebral cortex down to the spinal cord and
synapse on neurons that can influence
muscle activity. Many of the axons that enter
the corticospinal tract originate in the
primary motor cortex, although other motor
areas also contribute to the pathway.

6.  pathway involved with voluntary movement
using muscles of the head, neck, and face.
The corticobulbar tract travels from
the cortex as part of the pyramidal
system (along with the corticospinal tract),
but it terminates on cranial nerve nuclei in
the brainstem instead of continuing down to
the spinal cord.

7.  The corticospinal tract represents the
highest order of motor function in humans
and is most directly involved in control of
fine, digital movements.This tract arises in
pyramidal neurons of layerV of the precentral
gyrus, the "primary motor cortex." Betz cells
are the largest of these pyramidal neurons.

8.  There is a motor homonculus in this gyrus,
with the feet represented near the
superomedial part of the motor cortex and
the leg, trunk, arm, hand and head
represented progressively further inferior on
the lateral side of the brain.

9.  Axons arising from neurons in the precentral
gyrus exit through the white matter and pass
through the internal capsule where they are
topographically arranged in the posterior
limb.

11.  The fibers controlling the lower extremity are
posterior to those of the upper limb.
Corticospinal fibers traverse the middle
portion of the cerebral peduncle of the
midbrain and then the basal pons.They enter
the pyramids of the medulla (from whence
they get their name).

12.  Over 90% of the axons in the pyramids
decussate just before reaching the upper
cervical spinal cord (the pyramidal
decussation) and they enter the lateral
funiculus of the spinal cord to become the
(lateral) corticospinal tract

13.  ). Most of these axons terminate in the
intermediate gray matter of the cord,
although some enter the dorsal horn (where
they can have an effect on sensory
transmission) and a few terminate directly on
alpha motor neurons, contributing to rapid
voluntary movement. Most of these fibers
terminate on interneurons of the spinal cord.

14.  These interneurons are responsible for
reflexes and, therefore, most motor activity
actually occurs by the regulation of reflex
excitability in the spinal cord.

15.  A few corticospinal axons descend the
anterior funiculus of the spinal cord as the
anterior (ventral) corticospinal tract.This is
more involved in axial (trunk and neck)
movements and terminates bilaterally.

16.  Origin. Giant pyramidal cells (30K) in the
precentral gyrus (cerebral cortex).
 Axon termination. Directly on
skeletomotor (alpha) and fusimotor
(gamma)
 motor neurons (55% in cervical region, 25%
lumbosacral region).

17.  Functional Significance
 1. Important in individual finger flexor
movements = fractionation of
 movements.
 2. Important in movements that require
speed, agility, adaptability.
 3.Terminate contralaterally on lower motor
neurons to distal or
 appendicular muscles, especially flexors of
upper limb.

18.  The course of the axon, which forms the tract,
is as follows:
 1. Pre-central gyrus (site of the upper motor
neuron cell body)
 2. Internal capsule (posterior limb, see below)
 3. Cerebral peduncle (crus cerebri) middle 3/5s
 4. Pons proper or basal pons
 5. Pyramid in the medulla
 6. Pyramidal decussation (how the left brain
controls the right body)
 7. Spinal cord
 (a)The lateral corticospinal tract, crossed. -
90%
 (b)The ventral or anterior corticospinal tract,
uncrossed 10%
 to axial muscles.

19.  Axons terminate onVentral Horn Cells =
Lower Motor Neurons.
 a. Crossed (decussation) axons terminate
contralaterally on motor
 neurons to appendicular muscles
 (b. Uncrossed axons terminate ipsilaterally on
axial muscles, they cross
 before terminating)

24.  Many projections from the cerebral cortex
terminate in the brain stem (generically
called corticobulbar projections).These
projections have several functions including
voluntary control over cranial nerves, relay to
the cerebellum, activation of other
descending pathways (i.e., "indirect
corticospinal projections") and modulation of
sensory processing.

25.  Many cranial nerve nuclei receive direct and
indirect (through the reticular formation)
cortical input via nerve fibers arising from the
motor cortex and traversing the genu of the
internal capsule. Most corticobulbar
connections are bilateral, meaning that
unless both sides of the nervous system are
affected, there is no loss of motor control.

26.  However, the facial nucleus to the lower face
receives only input from the contralateral
motor cortex and, therefore, there will be
weakness of voluntary movement of the
lower face on the side opposite damage to
corticobulbar neurons (with sparing of
movements of the upper face).

27.  The majority of corticobulbar projections
terminate in the ipsilateral basal pontine
nuclei.These nuclei relay to the cerebellar
cortex via projections that decussate in the
pons and enter the cerebellum through the
middle cerebellar peduncle (see below).
These represent, by far, the largest input to
the cerebellum.

28.  Bulbospinal projections
 There are several brain stem nuclei that
project to the spinal cord.The cerebral cortex
projects to most of these and, therefore, may
affect them as "indirect corticospinal
projections."These areas include the red
nucleus, which gives rise to the rubrospinal
tract that decussates in the midbrain and
descends the lateral funiculus near to the
location of the lateral corticospinal tract

29.  The reticular formation gives rise to several
descending pathways, one from the rostral
pons that helps pattern locomotion, one from
the caudal pons that can affect head
movement to coincide with eye movement
and one from the medulla that mostly inhibits
reflex activity in the spinal cord.

30.  This latter tract is excited by cortical input
and, therefore, cerebral motor cortical output
is mostly inhibitory to spinal cord reflexes via
this indirect pathway. For this reason,
interruption of corticobulbar projections
typically increases reflexes.

31.  Cerebral cortical projections also go to the
superior colliculus, a region that gives rise to
a tectospinal tract as well as projections to
eye movement centers.The superior
colliculus is mostly responsible for reflex head
and eye movement toward novel stimuli.The
cerebral cortical projections to the superior
colliculus may effect movement via these
projections.

34.  Bulb: the medulla + pons + mesencephalon
 The corticobulbar fibers are similar to
corticospinal fibers except instead of
terminating in the ventral horn of the cord, they
end in cranial nerve motor nuclei (EXCEPTTHE
EXTRAOCULAR NUCLEI III, IV, and VI to be
discussed later).The terms upper motoneuron
and pyramidal tract are often used collectively
as a term for both corticospinal and
corticobulbar axons. Hence we have upper
motor neurons that end on cranial motor

35.  . Origin -- Similar to that of the corticospinal
axons except face region of cortex.
 Course
 1. Internal capsule -- posterior limb (some books
say genu)
 2. Cerebral peduncle -- the axons either leave
the corticospinal fibers at this point or at a
slightly more caudal level and make their way to
the appropriate cranial nerve nuclei. Others
travel more diffusely in the tegmentum.There is
 no visible corticobulbar tract as there is for the
corticospinal tract.

36.  .Termination: Examples.
 1.Most muscles act together such as the
pharynx, larynx.They get input from
 both hemispheres.
 5.Termination on the hypoglossal nuclei
motoneurons is mostly crossed and
 can be useful in localizing lesions in the acute
state. Signs may disappear
 after a few days.
 6. Muscles of facial expression

38.  Corticobulbar tract to the facial nucleus lower
motor neurons.
 1.The lower motor neurons that innervate
muscles of the lower face receive
 only crossed corticobulbar axons from the cortex
of the opposite side.
 2.The motor neurons that innervate muscle of
the upper face receive both
 crossed and uncrossed corticobulbar axons.That
is that both hemispheres
 send cortical fibers to the nuclei on both sides of
the brainstem

39.  A. Motor cortex -- middle & anterior cerebral
arteries.
 B. Internal Capsule -- very variable
 1. Anterior limb -- lenticulostriate arteries from
middle cerebral artery.
 2. Posterior limb -- middle cerebral artery,
anterior choroidal artery and rarely branches
from posterior cerebral artery.
 C. Blood Supply of Cerebral Peduncle --
posterior cerebral artery (variable).
 D. Blood Supply of Pons -- Basilar artery.
 E. Blood Supply of Pyramid and lateral cord --
Anterior spinal artery.

40.  Most corticospinal lesions are in the internal
capsule or cerebral cortex in the distribution of
the middle cerebral artery and result in classic
signs.When the tract is lesioned in the brain
stem these signs are associated with cranial
nerve signs
 A. Babinski sign (extensor plantar or
dorsiflexion response or upgoing toe and
fanning of the other toes) is abnormal and
indicates damage to
 corticospinal tract.The big toe normally goes

41.  B. Exaggerated tendon reflexes (hyperreflexia
or increased DTRs).This includes clonus,
crossed adductor, jaw jerk. Early on flaccidity
may be due to "spinal" shock.
 C. Spasticity: increased resistance to passive
stretch. In upper extremity greater in flexors;
in lower extremity greater in extensors. Clinical
sign is the claspedknife response.
 D. Re-emergence of primitive reflexes, so-
called Frontal (lobe) release signs:
 snout, grasp, suck, root, palmomental and
glabellar

42.  Lesions of the upper motor neuron system
(Corticospinal-Corticobulbar System)
produce a different constellation of signs
than do lesions of the lower motor neuron
system (anterior
 horn cells).

43.  Initial weakness or
paralysis of muscles
of entire limb or side
of body and reduced
reflexes
 Spasticity of affected
muscles, clasped knife
 Hyperactivity of deep
tendon reflexes,
clonus
 Weakness or paralysis
muscles in discrete
area
 Flaccidity of affected
muscles
 Hypoactive or absent
deep tendon reflexes

44.  No muscle atrophy or
very slight from disuse
 No muscle
fasciculations
 Pathologic reflexes,
Babinski,
 Prominent muscle
atrophy
 Fasciculations present
 No pathologic reflexes
present