1. MOTOR PATHWAYS: CORTICOSPINAL & CORTICOBULBAR
Thomas H. McNeill, PhD
Depts. Cell and Neurobiology and Neurology
February 27, 2017

2. LEARNING OBJECTIVES
Understand the differences between lower motor and upper motor neurons.
Learn the pathways and innervation patterns of the corticospinal and corticobublar tracts.
Learn the signs and symptoms typical of upper and lower motor neuron lesions.
Be able to distinguish a facial paralysis from a central (UMN) lesion from a facial paralysis of a peripheral (LMN) lesion.
Understand the anatomical basis of the regulation of voluntary horizontal conjugate movements of the eyes.

3. Brain Structures Involved in the Regulation of Motor Activity
Brain structures that control voluntary motor activity are:
Motor cortex - initiates voluntary activity, esp. of the fine skilled variety
Subcortical motor nuclei in the brain stem - Regulate stereotyped voluntary activity for postural regulation and locomotion, largely under cortical control
Frontal Eye Fields – control conjugate movements of the eyes.
Brain structures that modulate motor activity are:
Basal Ganglia (action selection)
Cerebellum (action correction)

4. Principles of Organization
Motor pathways form a two neuron chain from the cortex to the target muscle
The two neuron chain is composed of a upper motor neuron and a lower motor neuron
Upper motor neurons are in the cortex or brain stem
Lower motor neurons are in the anterior horn of the spinal cord or cranial nerve nuclei whose axon innervates the target muscle
The motor pathway that controls movements of the face is separate from the motor pathway to controls movement in neck, trunk and limbs.
The motor pathway that controls horizontal conjugated eye movement is separate from the other motor pathways of the face.

5. Organization of the Motor Pathways
Motor Cortex
Higher level motor “association” cortices
The planning and programming of motor activity precedes the activation of the primary motor cortex
Motor planning occurs in the supplementary motor area (SMA) and the premotor area (PMA).
SMA and PMA projects to the primary motor cortex for execution of the planned movement

6. Primary motor cortex (PMC) exhibits somatotopy
Primary motor cortex (PMC) gives origin to the corticospinal and corticobulbar tracts.
Neurons of origin are found largely in the precentral gyrus and the paracentral lobule
The PMC has a somatotopic representation of body parts in an upside-down orientation
Head/Face area gives origin to the corticobulbar tracts. The rest of the primary motor cortex gives origin to the corticospinal tracts.

7. The Primary Motor Cortex has a Dual and Mostly Separated Blood Supply
Area of the cortex devoted to a body part is proportionate to the complexity of motor functions served
The vascular supply of primary motor cortex comes from the middle (MCA) and anterior (ACA) cerebral arteries. Each artery supplies specific somatotopic areas represented in the precentral gyrus or the paracentral lobule
ACA
MCA
PCA

8. Corticospinal Tract (CST)
Corona radiata
PLIC
Midbrain: Crus cerebri
Pons
Medulla:Pyramid
Medulla/
Cord junction
(Pyramidal decussation)
Spinal cord
Ant. Csp. T.
Lat. Csp. T.
Course of CST:
Corona radiata
internal capsule
crus cerebri
pons
medulla
Corticospinal Tracts (75-90%) cross the midline at the medulla/cord junction in the pyramidal decussation
In the spinal cord the crossed axons form the contralateral lateral CST
Uncrossed axons remain as the anterior CST
Lat. CST Ant. CST

9. Corticospinal tract (red line) is a one neuron pathway to the spinal cord
Internal capsule
Midbrain
(Crus cerebri)
Pons
Medulla
(pyramid)
Spinal cord
midbrain
The CSp. T descends through the subcortical white matter (internal capsule) and the three levels of the brain stem to reach the spinal cord. The cortical neurons and the tract are referred to as the “Upper motor neuron.”
pons
medulla
cross over
spinal cord

10. Somatotopy of CST Internal Capsule Midbrain
Somatotopy remains throughout the course of the CSTs

11. Location and Termination of CSTs in Spinal Cord
Lateral CST descends in the lateral funiculus and synapses ipsilaterally on LMNs that innervate the distal musculature of the limbs.
Anterior CST descends in the anterior funiculus and synapses bilaterally on proximal musculature of the limbs, trunk and neck.

12. REGULATION OF MOTOR ACTIVITY by UMN and LMN
UMNs (cortical)
Controls strength of contraction based on the number of LMNs and motor units activated
Controls voluntary motor activity, esp. the fine skilled, novel movements of distal parts of the limbs (use of the hand, independent movements of the thumb and other fingers as in playing the piano, learning to write/draw)
Controls muscle tone and generally keeps the Deep Tendon Reflexes suppressed under normal conditions.
LMNs
Final common pathway to skeletal muscles to elicit contraction in response to activation by the UMNs.
Provide trophic factors that sustain muscle fibers.
Essential for muscle tone = partial state of contraction of skeletal muscles.

13. Signs of Upper and Lower Motor Neuron Lesions
UMN
LMN
Weakness or Paralysis
YES
Atrophy NO
Fasciculation
Reflexes
INCREASED
DECREASED
Target Muscle

14. Terms to Describe Weakness or Loss of Movement
Definition
Example
Paresis
Weakness
Hemiparesis
- pelgia
No movement
Hemiplegia
Paralysis
upper limb paralysis
Palsy
Weakness or no movement
Facial Palsy
Hemi
One sided
Para
Both limbs
Paraplegia
Mono
One Limb
Monoparesis
Quad
All Limbs
Quadroplegia

15. Corticobulbar Tracts
The Corticobulbar tract controls the cranial motor nuclei that innervate the skeletal muscles of the head and neck(.i.e CN lll, lV, V, Vl, Vll, lX, X, Xl, Xll)
Corticobulbar axons descend through the internal capsule and leave the tract at all levels of the brain stem to terminate on cranial motor nuclei.
Corticobulbar tracts do not extend much below the medulla except to innervate the nucleus of XI in the upper cervical cord.

16. Location and Function of Cranial Nerve Motor Nuclei with a Somatomotor Component
NUCLEUS
LOCATION
FUNCTION (INNERVATION)
III*
Upper Midbrain
Muscles That Move the Eye
IV*
Lower Midbrain
Muscle That Moves the Eye V
Mid-pons
Muscles of Mastication
VI*
Lower Pons
VII
Muscles of Facial Expression
IX and X
(N. Ambiguus)
Upper Medulla
Muscles of the Larynx and Pharynx XI
Upper 5-6 segments of the spinal cord
SCM, Trapizious
XII
Muscles of the Tongue

17. Somatotopy of Corticobulbar Tract
Corticobulbar tract lies anterior to the CST in the posterior limb and close to the genu of the internal capsule and medial to the CST in the midbrain.

18. Termination of the Corticobulbar Tracts follow 3 Patterns: Bilateral, Contralateral or Both
1. Bilateral Pattern
The corticobulbar tracts for CN V, IX, X exert largely bilateral control over the target muscle since most of the muscles involved such as the jaw, laryngeal, pharyngeal and palatine muscles need to be activated simultaneously on both sides for the normal functioning of these structures.
Thus, unilateral damage to the corticobulbar tract will have little effect on the function of these muscles

19. 2. Contralateral Pattern for Hypoglossal Nuc. (Xll)
Although the corticobular tract for CN Xll ends bilaterally the contralateral input on the cranial motor neurons of the hypoglossal nucleus predominates.
The axons destined for the nuclei leave the tract immediately above the nuclei and synapse on LMN of CNXll.
Unilateral UMN lesions will result in deviation of the tongue away from the side of the lesion
Unilateral LMN lesions will result in deviation of the tongue toward the side of the lesion

20. 3. Bilateral & Contralateral Pattern for Facial Nerve (Vll)
The facial nucleus (Vll) is located in the lower pons and is functionally divided into two parts:
Neurons that innervate the upper face (forehead) receive bilateral innervation from the corticobulbar tracts
Neurons that regulate the lower face receive strictly contralateral input from the corticobulbar tract

21. BILATERAL & CONTRALATERAL CONTROL OF FACIAL NUCLEUS

22. UMN vs. LMN lesions affecting the facial nucleus/nerve1 2 3 4
Unilateral lesion
Motor Cortex
Post. limb, Int. Caps Effects muscles of lower face on the Crus Cerebri opposite side
VII nerve after stylomastoid foramen – Effects both upper and lower

23. Stroke vs. Bell’s palsy Unilateral UMN lesion
Unable to move
Unable to move
Unilateral UMN lesion
central facial paralysis
Unilateral LMN lesion of the VII nerve (nucleus)

24. Regulation of Conjugate Eye Movements
Voluntary eye movements involve CN lll, lV and Vl and can be in the horizontal or the vertical plane and normally occur in a conjugate manner, i.e., both eyes moving in the same direction

25. There are two major types of conjugate eye movements:
Saccades - are rapid movements that redirect gaze to each a target of interest to place it on the fovea.
Smooth pursuit movements - track moving objects to allow stable viewing of an object in motion
Pathways for horizontal saccades are the best understood. These involve muscles and cranial nerves that move the eyes laterally and medially.
For each eye the lateral rectus (LR) is the abductor and the medial rectus (MR) is the adductor.
The LR is innervated by CNV VI (Abducens) and MR by CN III (Oculomotor).

26. Pathway for Horizontal Conjugate Eye Movements
The motor commands for these movements are generated by the frontal eye fields (FEF) in the posterior part of the middle frontal gyrus.
- FEF
From FEF, a descending pathway reaches the pons by an undefined route

27. Left FEF L R
Right PPRF
In the pons, axons cross and end on the opposite side in the "horizontal (lateral) gaze center" in the paramedian pontine reticular formation (PPRF).
PPRF sends axons to activate the VI nucleus on the same side MR LR
CN lll
MLF
CN Vl

28. Some axons of VI nucleus leave by way of the VI nerve to innervate the LR muscle of the same side.
Some axons cross to the opposite side to enter the medial longitudinal fasciculus (MLF) to reach III nucleus on the opposite side. This results in contraction of the opposite MR muscle.
Activation of left frontal eye fields will result in simultaneous contractions of right LR and left MR - Eyes to the right!
Left FEF L R
Right PPRF MR LR
CN lll
MLF
CN Vl

29. Predict the effects of lesions A-C on horizontal conjugate gaze
LEFT C
Left
RIGHT
Right PPRF B

30. Questions?