Cortical Control of Motor Activity – The Corticospinal Tract

Question 1

Cortical structures:

Answer 1

• primary motor cortex
• prefrontal cortex
• somatosensory and parietal association cortex

Question 2

Subcortical structures:

Answer 2

• basal ganglia
• cerebellum
• thalamus

Question 3

Types of Movement:

Answer 3

1. Reflexes
   
   • Stereotyped responses to stimuli
2. Voluntary
   
   • Goal-directed
   • Internally generated
   • Improve with practice
3. Automatic Postural Adjustments

Question 4

Motor Pathways: Two Main Systems

Answer 4

1. Lateral:
   
   • Lateral Corticospinal Tract
   • Rubrospinal Tract
2. Medial:
   
   • Anterior Corticospinal Tract
   • Lateral Vestibulospinal Tract
   • Medial Vestibulospinal Tract
   • Corticotectal and Tectospinal Tract
   • Reticulospinal Tract

Question 5

What is the most important system for voluntary movement?

Answer 5

Coritcospinal tract

Question 6

Coritcospinal Tract:

Answer 6

• One neuron
  
  • Upper Motor Neuron
• Projects to:
  
  • Lower motor neurons (LMN) in ventral horn of ALL levels of spinal cord
• Function
  
  • Voluntary movement of distal extremities
  • Skilled movements
  • Excites flexor m.’s, inhibits extensor m.’s

Question 7

Corticospinal Tract:

Route

Answer 7

• CST axons arise from:
  
  1. Primary motor cortex (area 4) (33% of fibers)
  2. Betz cells (large pyramidal neurons in lamina V of the precentral gyrus) (3% of fibers).
  3. Premotor cortex (area 6) and Supplementary motor area (area 6) (33% of fibers)
  4. Parietal lobe (areas 3, 1 and 2) (33% of fibers)
• CST axons pass through:
  
  1. corona radiata
  2. posterior limb of internal capsule
  3. the middle of the cerebral peduncle (crus cerebri)
  4. medullary pyramids

Question 8

Upper Motor Neuron (UMN) Route:

Answer 8

1. Cerebral cortex
2. Corona radiata
3. Internal capsule
   
   • Posterior limb
4. Crus cerebri
5. Ventral pons
6. Medullary pyramid
   
   • Pyramidal decussation
7. Lateral funiculus

Question 9

Primary Motor Cortex (M-I):

Answer 9

• Somatotopically Organized
• Tasks that require great precision:
  
  • ​Structures have disproportionately larger representations
  • motor homunculus
• Motor map exhibits overlap & plasticity
  
  • can change with increased use or disuse​

Question 10

Lateral Corticospinal Tract:

Answer 10

• 90% of fibers decussate in pyramidal decussation
• descend in lateral funiculus to all sp cd levels

Question 11

Anterior / Ventral corticospinal tract:

Answer 11

• 10% of fibers do NOT decussate in pyramids
• descend in the anterior funiculus
• decussate in ventral white commissure to thoracic sp cd.

Question 12

1. Lesions ABOVE the pyramidal decussation result in _____________ weakness
2. Lesions BELOW the pyramidal decussation result in ___________ weakness

Answer 12

1. Above ⇒ contralateral
2. **Below ⇒ **ipislateral

Question 13

What signs would you expect to find with an UMN lesion?

Answer 13

• Paresis (weakness) or paralysis
• Spasticity
• Hyperreflexia
• Loss of abdominal reflexes
• Babinski sign:
  
  • ​stroking the plantar surface of the foot along the lateral border ⇒ dorsiflexion of the great toe (normal response is plantar flexion)

Question 14

What signs would you expect to find with an LMN lesion?

Answer 14

• Muscle atrophy
• Fasciculations
• Hypotonia
• Hyporeflexia

Question 15

Lesion in the Cortex:

Answer 15

Contralateral paresis of a particular body part corresponding to area of cortical damage.

Question 16

Lesion in the Posterior Limb of Internal Capsule:

Answer 16

Contralateral hemiplegia

Question 17

Where would a lesion that causes Weber’s Syndrome be located? What is a possible cause?

Answer 17

• Midbrain
  
  • Cerebral peduncle (Crus cerebri)
• Possible Cause: Occlusion of PCA

Question 18

Lesion in the Cerebral Peduncle (Crus cerebri):

Answer 18

• Weber Syndrome
  
  • Possible Cause: Occlusion of PCA
• Corticospinal Tract lesion ⇒
  
  • Contralateral paralysis of lower face, tongue, arm and leg
• CN III injury ⇒
  
  • Ipsilateral oculomotor palsy
  • eye deviates laterally, ptosis, pupil is dilated and fixed

Question 19

Where is the lesion that causes medial medullary syndrome located?

Answer 19

• Rostral Medulla
  
  • Medullary Pyramid
• Possible Cause: occlusion of vertebral a. or anterior spinal a.

Question 20

Lesion in the Medullary Pyramid:

Answer 20

1. Medial Medullary Syndrome
   
   • Possible Cause: occlusion of vertebral a. or anterior spinal a.
2. Corticospinal Tract lesion ⇒
   
   • Contralateral hemiparesis of arm and leg, face is spared
3. CN XII injury ⇒
   
   • Ipsilateral paralysis and atrophy of tongue
4. Medial Lemniscus injury ⇒
   
   • Contralateral loss of touch, vibration and joint position sensation from half of the body

Question 21

Lesion in the Spinal Cord:

Answer 21

• Corticospinal Tract lesion ⇒
  
  • Ipsilateral spastic paralysis
  • Ipsilateral Babinski sign