The motor system consists of lower and upper motor neurons. Lower motor neurons originate in the spinal cord and brainstem and directly innervate skeletal muscles, controlling movement. Upper motor neurons originate in the motor cortex and brainstem and project to lower motor neurons. Upper motor neurons are involved in encoding movement intention, expressing emotions, skilled motor behavior, and setting the gain of reflexes through pathways like the corticospinal tract. Damage results in spasticity and hyperreflexia, while sparing muscle strength.

1. Motor System
• Lower motor neuronal control;
• Upper motor neuronal control.
by Dr. M. Kornieieva

2. Central Nervous System

3. The Lower Motor System
SKELETAL MUSCLE CONTRACTION is
initiated by “lower” motor neurons in the spinal
cord and brainstem.
Cell Bodies in the:
1) Ventral horn of the spinal
cord gray matter;
2) Motor nuclei of the cranial
nerves in the brainstem
Axons: pass directly to skeletal muscles
α motor neurons
Control of movement:
1
2
3
4

4. Lower Motor Neuron – Spinal Cord
α motor neurons

5. Lower Motor Neuron – Brainstem
α motor neurons
The term “lower” regarding neurons indicates
relation to muscle rather then level of localization.

6. Motor Neuronal Pools
MCS
LCS

7. γ-Motor neuron
Specialized muscle fibers
(sensory receptors - spindles)
Sensory axons 1a
Send information about length of
the muscle to the SC and BS
Muscle spindles in
striated muscle
α-Motor
neuron
Extrafusal
fibers
Contraction

8. Myotatic (stretch) reflex
Ia

9. Myotatic (stretch) reflex

10. Golgi Tendon Organs
Localization – muscular tendon
Structure – network of nerve endings
interdigitating with collagen fibers
Activation – tension of muscle contraction
Function – suppression of the related
⍺-motor neuron output
Negative feedback system

11. Golgi Tendon Organs
Works via the inhibitory interneuron

12. Flexion-
withdrawal
(crossed
extensor)
reflex

13. Spinal Cord Circuitry and Locomotion
Central pattern generator
(in spinal cord)
Stance phase
Swing phase
Mesencephalic
locomotor region
Speed

14. Lower Motor Neuron Syndrome
• Muscle atrophy
• Fibrillations and fasciculations
• Muscle paralysis (loss of volitional control) or paresis
(profound weakness)
• Areflexia (loss of myotactic reflexes)
• Profound reduction in muscle tone

15. Upper Motor Neuronal
Control of Movement

16. Upper Motor system
• Encoding of movement intention
• Expression of emotions (facial
expression, posture)
• Skilled motor behavior
Motor cortex:
Brainstem:
• Settings of posture
• Setting the gain of reflexes

17. Somatotopy in the spinal cord (LMN)
Lateral white matter
(axons from motor cortex)
Medial white matter
(axons from brainstem)
UMN
Parallel pathways
bilateral contralateral

18. Parallel
pathways:

19. Primary Motor Cortex
Lateral view Medial view

20. Primary Motor Cortex
Large motor neurons in the
layer 5 called the Betz cells.
Betz cells give rise to corticospinal and
corticobulbar tracts => lower motor circuits.
Precentral gyrus

21. Somatotopy in the
Motor Cortex
What exactly is represented here?
• movements (not muscles!) or movement intentions
• multiple dimensions of movements (force, direction,
amplitude)
• movements that engage hand, lower face and hand-to-
mouth coordination
• skilled manual behavior in central person space
• lesions impair “fractionated” finger/facial movements
(i.e. manual and facial skills)

22. Encoding of Movement Intention
in Central Space in M1

23. Mosaic organization
Medial part Lateral part
Premotor Cortex
monkey
L
M

24. Brainstem Upper Motor Neurons
L/M
Vestibulo-
spinal tracts
Adjustment of position
of body regarding the
position of the head
Feedback operation
on posture
Anticipate adjustment
of posture
Forebrain
Spinal cord
Ready..
Set…

25. Reticular Formation

26. Pyramidal tracts
1) Corticospinal tract
2) Corticobulbar tract
Cortex
α-motor neuron
Cortex
Basal ganglia
Red nucleus
Reticular formation
Olivary nucleus

27. The Corticospinal and
Corticobulbar p/ws
Internal capsule

28. The Corticospinal and Corticobulbar tracts
Corticopontine
tract
Opposite
cerebellar
hemisphere
Cerebellar
peduncle
All parts of
motor cortex

29. The Corticospinal and Corticobulbar tracts
Midbrain
Pons Cerebral
peduncles
of midbrain
Pontine nuclei
Lower pons
Upper Medulla
Cortico-
spinal
tract
Medullary
Pyramids
Corticobulbar, -pontine, -
spinal fibers
Caudal medulla
Decussation of the pyramids

30. The Corticospinal Pathway
90-95%
5-10 %

31. Control of Facial Expression
Facial nerve, VII
Lower motor neuron
UMN

32. Emotional
Motor
System
Primary Motor Cortex
Somatosensory Cortex
Supplementary motor cortex
Cingulate motor cortex
Pyramidal smile Emotional smile

33. Speech Motor System

34. Red nucleus
Olivary nucleus Lat. vestibular nucleus
Superior
colliculus
Reticular
formation
Paracentral gray

35. Extrapyramidal tracts1. Rubrospinal tract
2. Pontine reticulospinal tract
3. Medullary reticulospinal tract
4. Lateral vestibulospinal tract
5. Tectospinal tract
Look for details at Clinical Neuroanatomy 7th ed. by R. Snell (pp.157-160)
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36. Upper Motor Neuron Syndrome