Specific Learning Objectives: At the end of session the students should be able to : Enumerate the descending tracts. Describe the origin, course, termination, collaterals of Pyramidal tract. Describe the functions of the pyramidal tract.

1. Descending Tracts
(Motor or Efferent Tracts)
1)Pyramidal (Corticospinal) Tracts
2)Extrapyramidal tracts

2. Specific Learning Objectives
At the end of session the students should be
able to :
•Enumerate the descending tracts.
•Describe the origin, course, termination,
collaterals of Pyramidal tract.
•Describe the functions of the pyramidal
tract.

3. 1)Pyramidal (Corticospinal) Tracts :
i)Lateral Corticospinal tract
(Crossed Pyramidal tract).
ii)Anterior Corticospinal tract
(Uncrossed Pyramidal tract).

4. 2)Extrapyramidal tracts:
i)Rubrospinal tract
ii)Tectospinal & Tectobulbar tracts
iii)Reticulospinal Tracts:
a)Medial or Pontine
b)Lateral or Medullary
iv)Vestibulospinal tract
v)Medial longitudinal fasciculus

5. Corticospinal tracts –
Pyramidal tracts
and corticobulbar tract

6. Introduction:
Longest tract –
Extends – Motor cortex to last segment of
spinal cord.
Present only in higher vertebrates & man
where cerebrum has developed.
So we are superior to other animals.

7. Composition:
Tracts on each side – One million nerve fibres.
Myelinated -60%,unmyelinated 40%.
Most of myelinated fibres are of small
diameter so the tract is slow conducting.

8. Origin:-
1.Primary motor cortex (area 4 ) – 30% (3% -
large pyramidal cells- Betz cells )
2.Premotor cortex (area 6 ) & area 8 - 30%
3.Somatosensory cortex (SI & SII) (3,1,2 & 40)
parietal lobe association areas (5,7) – 40%

9. Most impressive fibres from Giant Pyramidal
cells called as Betz cells (number 34,000)
i.e only 3% of the total fibres (myelinated but
of large diameter) so (fast conducting).
Remaining 97 % fibres of small diameter
conduct i) background tonic signals to the
motor areas of the cord,
ii)Feedback signals from cortex to incoming
sensory signals to control their intensities.

10. Motor cortex
Internal capsule
Ant.
Post.
C.N.
Thala.
Mid brain
Pons
Medulla
SP.Cord
Corona
radiata
Post limb
Cerebral
peduncles
Long.
fasciculi
Pyramids
decussation
Lat. CST Ant.CST
III, IV
V, VI, VII
IX,XI,XII

11. Motor cortex
Internal capsule
Corticobulbar fibers
Decussasion of pyramids
Ant. corticospinal
Lat. Cortico spinal
Ant. Motor fibers

13. Inputs of motor cortex :-
• Somatosensory, visual and auditory cortex
• Motor cortex of opposite side
•Thalamic sensory nuclei
• Basal ganglia & cerebellum through thalamus
• Intralaminar nuclei

14. Course :-
Genu & post limb of Internal capsule
Middle 3/5 of crus cerebri – face medially & legs
laterally, fibers to III, IV
Longitudinal fasciculi in pons,
fibers to V, VI, VII
Pyramids in upper part of medulla,
fibers to IX, XI, XII
Corona radiata in subcortical areas
80% cross
Lat. CST
20% - uncrossed
In lower part of medulla
Ant. CST In the spinal cord

15. Fibres in 1)Primary motor area(Precentral
gyrus) are arranged Upside down.
Toes –At the top,
Trunk – Middle &
Head - Below.
Motor homonculus.
2)Corona radiata – Fan shaped.
Projection type of fibres.

16. 3)Internal capsule:
Corticonuclear (corticobulbar) fibres – genu
Corticospinal fibres - Ant 2/3rd
of post limb.
Plane of fibres rotates from coronal to right
angle so Head fibres come ant & med while
Leg fibres lie posterior & lateral.

17. 4)Midbrain:
Corticobulbar fibres end on motor nuclei of
III & IV Cranial nerves of same & opposite
side.
Remaining fibres occupy middle 3/5th
of the
Crus cerebri or Cerebral peduncle.
Medial 1/5th
carries Fronto pontine & lateral
1/5th
Temporo pontine fibres.

18. 5)Pons:
Pyramidal fibres - most ventral part in front of
Trapezium.
Tract broken up into scattered bundles by
Nuclei Pontis & crossing fibres of Middle
cerebellar pedencle.

19. 6)Medulla:
Upper medulla: Scattered corticospinal fibres
reunite, occupy most ventral part forms a
bulge - Pyramid.
Lower medulla: 80 to 85 % fibres decussate,
cross to opposite side enter lateral white
column & descend down as
Lateral Corticospinal Tract.

20. 7)Spinal cord –
It extends throughout the cord & at each
spinal segment,some fibres leave the tract,
turn inward & end round the Ant horn
cells(motor neurons) either directly or
through interneurons.
Excites flexor motor neurons & inhibits
extensor motor neurons of digits.
So plantar reflex is Plantar flexion response.

21. Fibers ending in each segment turn medially
Directly on α- motor
neurons -10%
Innervating distal
groups of limb muscles
Through
interneurons
Fibers arising from
somatosensory
cortex
end on Dorsal
horn cells

22. Termination :-
Cervical region – 55%
Thoracic region – 20%
Lumbosacral region – 25%

23. Direct / ventral / uncrossed corticospinal tract
-15 to 20 % fibers without crossing in medulla
-directly descend in ant. white column near median
fissure as Anterior Corticospinal Tract.
-Origin - supplementary motor areas
- Extend up to mid thoracic region
-Termination - on the interneurons of the
same side or of the opposite side
- Function - Control of axial muscles on both sides.

24. Significance –
Lateral & Anterior Corticospinal Tracts
indicates that certain muscles in the
body must have bilateral control from
both cerebral hemisphere e.g –
Respiratory muscles.

25. To cerebellum
Collaterals of corticospinal tract
• To cortex for lat inhibition –
sharpness of boundaries.
• To
Red n. → rubrospinal
Reticular n. → reticulospinal
Vestibular n. → vestibulospinal
Pontine n. → pontocerebellar
Olivary n. → olivocerebellar
Caudate and lentiform nuclei,

26. Corticobulbar pathways :-
Formed by the fibers leaving corticospinal tract in
the brain stem and ending in the nuclei of the
motor cranial nerves on opposite side
Mid brain – III, IV
Pons – V, VI, VII, VIII
Medulla – IX, X, XI, XII
Functions – control vol. movement of
muscles of larynx, pharynx, palate, face, jaw
and eyes.

27. Functions of pyramidal tracts:-
1. Lat.CST –Controls voluntary, rapid, skillful
movements of fingers & hands of
contralateral side.
Signals coming from Primary motor cortex
excites a pattern of muscle activity.
2.Premotor area – develops a motor image
of total muscle movement to be performed.
Then sends signals to primary motor area to
exite the pattern of muscle activity.

28. 3.Forms part of pathway for superficial
reflexes such as cremasteric,abdominal &
plantar reflex.
4.Modify sensory input
Some of the fibres of the tract end
presynaptically on axon terminals of afferent
neurons or dendrites or cell bodies & thus
affect afferent system.

29. 5.Sensory motor co-ordination –
Somatosensory feedback to the motor
cortex helps to control the precision of
muscle contraction.

30. 6.Most of the CST fibres end on α motor
neurons.
But some fibres end on γ motor neurons.They
a)help in adjusting the length & excitation of
the extrafusal fibres &
b)also saves the energy of brain for further
stimulation of extrafusal fibres for damping of
movement.

31. 7.Corticobulbar /corticonuclear tracts are
responsible for voluntary control of muscles of
(head & neck) i.e larynx, pharynx, palate,
upper & lower jaw, eye etc.

32. Ant. CST–
Supplementary motor area thr Ant CST
controls bilateral postural movements.
Along with Premotor area provides fixation
movements of different segments of the
body,
positional movements of the head & eyes.
i.e background for finer motor control of
arms & hands.
e.g –bilateral grasping movements of
hands while climbing.

33. Applied aspects :-
1.Phylogenetically, ant. CST is older
Lat. CST - in human beings and few vertebrates
2.Myelination is complete after the age of 2 yrs.
3.Cortical neurons & certain brain stem nuclei with
their axons activate lower motor neurons-Upper
motor neurons.
4.Cranial nerve nuclei, Spinal motor neurons /Ant
horn cells with their axons activate directly
skeletal muscles -Lower motor neurons.

34. Damage to motor control system - Stroke or
apoplexy (sudden attack of paralysis).
Paralysis - (Loss of power of muscle group due to
absence of contraction of those muscles →loss of
movement).
Cause –
i)Ruptured blood vessel -Haemorrage in brain.
ii)Thrombosis of one of the major arteries supplying
the brain.
Commonest site is when CST passes through the
internal capsule.

35. Damage to motor cortex alone - Hypotonia.
Cause -Primary motor cortex exerts
continous tonic stimulatory effect on motor
neurons of spinal cord.
But lesions of motor cortex involve not only
the Primary motor cortex but also adjacent
cortical areas & deeper structures of
cerebrum (basal ganglia) - Damage to
Accessory pathways.

36. Accessory pathways normally inhibit
Vestibular & Reticular brain stem nuclei.
When the inhibition is gone they become
spontaneously active & cause excessive
stimulation of spinal motor neurons
→excessive/spastic tone in the involved
muscle areas of the body → Results in
muscle spasm or muscle spasticity on
opposite side of the body - Upper motor
neuron lesion.

37. Monoplegia - primary motor cortex
hemiplegia - internal capsule
quadriplegia or paraplegia – brain stem
paraplegia - spinal cord at thoracic level

38. Lower motor neuron lesion –
Results also in Paralysis but it is Flaccid
paralysis.
Clinical picture in Pyramidal tract lesions
-
Lesion above the level of sp. Cord – effects
on opposite side of body.
Lesion in the spinal cord – effects on the
same side.
Impairment of voluntary, skilled movements.

39. Upper motor neuron lesion :-
•Increased muscle tone - Hypertonia.
•Resistance to passive movements -Rigidity.
Rigidity - clasp knife (Spasticity), lead pipe or
Cogwheel rigidity.
•Group of muscles of opp side affected.
e.g:Flexors & extensors. So associated
movements affected.
•No atrophy (wasting of muscle).

40. •Superficial reflexes are absent.
Plantar reflex →Babinski’s sign positive.
•Deep /tendon reflexes are Exaggerated.
•Clonus.
•No tropic changes.

41. Toes down
(flexion)
Fanning
of toes
Normal plantar response Extensor plantar response
(Babinski sign)

42. Lower motor neuron lesion :
•Decreased muscle tone – Hypotonia.
•No rigidity but flaccid paralysis.
• Atrophy prominent (marked wasting of
muscles).
•Superficial reflexes absent.
•Deep reflexes absent.

43. •Individual muscles of same side are
affected.e.g:Poliomyletis.
•Trophic changes are seen.

44. Lesion to cranial nerve nuclei(LMN) –
Bulbar palsy.
Lesion to Corticonuclear tracts – (UMN)
Pseudobulbar palsy →Weakness or
paralysis of the muscles of pharynx,
larynx, palate, jaw & eye etc.

45. LMNP UMNP
Site of
lesion
α- or cranial
motor neuron
Neurons forming
descending tracts
Muscles
Involved
-single muscle on
the same side
Groups of muscles
on opposite side
Vol. movements lost lost
Tone of involved
muscles
- flaccidity Clasp-knife rigidity,
spastic paralysis

46. Planter reflex – lost only if
S1 is affected
LMNP UMNP
Tendon jerks – lost Exaggerated,
clonus+
Muscle atrophy – marked wasting absent
Superficial reflexes – lost lost
Babinski’s +ve

47. Motor area for face
Supranuclear lesion (UMN)
Facial nerve n.
Facial paralysis
Facial nerve
Corticobulbar tract