D8 - Major pathways of the brain and the spinal cord

Question 1

General about tracts in the CNS

Answer 1

• Pathways of the CNS cannot be visualized anatomically but are functional units that consist of ascending or descending axons travelling together and conveying information from one location or another
• Their function and location have been determined by assessing the results of experimentally induced damage to certain parts of the CNS

Question 2

What can the tracts of the CNS can be divided into?

Answer 2

• Ascending pathways (sensory)
  
  • ​Medial lemniscus
    
    • ​Spinal lemniscus (trunk and limbs)
    • Trigeminal lemniscus (sensory nerves of the head)
  • Extralemniscal system
• Descending pathways (motoric)

Question 3

Where are the tracts found in the spinal cord

Answer 3

• In the white matter, divided nito 3 paired funiculus:
  
  1. Funiculus dorsalis (Ascending pathway)
  2. Funiculus lateralis (Descending & ascending pathway)
  3. Funiculus ventralis (Descending pathway)

Question 4

Ascending pathways

Answer 4

• Sensory
• Transmits information from the skin, muscle, tendons → peripheral nerves → CNS (→ spinal cord → brain)
• More and more fibers are controlled towards the brain

1. ​Tr. spinocorticalis
   
   • ​Fasciculus gracilis
   • Fasciculus cutaneous
2. Tr. spinocellebellaris
3. Tr. spinotectalis
4. Tr. spinothalamicus
5. Tr. spinolivaris

Question 5

Ascending pathways

Funiculus dorsalis

Answer 5

• Tr. spinocorticalis
  
  • Fasciculus gracilis (medial segement of funiculus dorsalis)
  • Faciculus cutaneus (lateral segment of funiculus dorsalis) → cerebellum → primary sensory cortex (gyri postcentralis)
• Tractus pyramidalis = corticospinalis dorsalis

Question 6

Ascending pathways

Funiculus lateralis

Answer 6

• Tr. spinocerebellaris
  
  • Stretch receptors (tendons)
  • Signals to cerebellum
    
    • Pathway goes to the muscles
  • Highest subcortical
  • Cerebellar ataxia: if this is not working properly
• Tr. spinotectalis
  
  • Tectum mesencephali
  • ​→ Visual and auditory
• Tr. spinothalamicus
  
  • ​Comes mostly from the skin
  • ​→ thalamus ​→ primary sensory cortex
  • Noxious stimuli

Question 7

Ascending pathways

Funiculus ventralis

Answer 7

• Tr. spinoolivaris
  
  • ​Termination of nucl. olivaris
  • → nucl. olivia → cerebellum

Question 8

Ascending pathways

Afferent pathways of the sense organs

Answer 8

Visual pathways

• Retina contains the receptor of visual information
• Retina → n. opticus → optic chiasm → tr. opticus: synapses in the optic thalamus → visual cortex of the occipital lobe
• Some fibers contine from tr. opticus to colliculus rostralis in caudate nucleus

Vestibular and auditory pathways

• Fibers from both vestibular and auditory organs enter the brainstem within the n. vestibulocochlearis → trapezoid body
• Fibers of the vestibular organ can go in one of the directions:
  
  • Vestibular organ → vestibular nuclei → cerebellum
  • Vestibular organ → cerebellum
    
    • ​Directly via the caudal cerebral peduncles
• Auditory area lies in the temporal lobe
• Cochlear fibers forms synapses with dorsal and ventral cochlear nuclei → ascend in the lateral lemniscus → caudal colliculi of the midbrain

Question 9

Descending pathways

Answer 9

• Motoric
• Descending autonomic system
  
  • Parasympathetic to organs
    
    • Sweat glands
    • Intestinal glands
• Descending motor pathways
  
  • Tr. corticospinalis / tr. pyramidales
  • Extrapyramidal system

1. Tr. corticospinalis lateral
2. Tr. corticospinalis ventral
3. Tr. reticulospinalis
4. Tr. tectospinalis
5. Tr. rubospinalis
6. Tr. reticulospinalis
7. Tr. tectospinalis
8. Tr. vestibulospinalis

Question 10

Descending pathways

Pyramidal tract

Answer 10

• Mostly developed in primates
• Origin: cells of the motor cortex of the neopallium
• Role: movement of muscles
• It comprises three types of fibers:
  
  • Corticospinal fibers
    
    • ​​Cerebral cortex → spinal cord
  • Corticobulbar fibers
    
    • ​​Cerebral cortex → various nuclei of contralateral cranial nerves
  • Corticopontine fibers
    
    • ​Cerebral cortex → nuclei in the pons
• Include:
  
  • Tr. corticobulbar
  • Tr. corticospinalis

Question 11

Descending pathways

Extrapyramidal tracts

Answer 11

• Involuntary activation
• Multisynaptic pathway originating from:
  
  • Corpus striatum
  • Subthalamic nuclei
  • Substantia nigra
  • Nuclei ruber
  • Reticular formation
• Location:
  
  • Pons
  • Medulla
• Target lower motor neurons in the spinal cord that are involved in reflexes, locomotion, complex movements, and postural control
• The extrapyramidal tracts include:
  
  • Tr. rubospinalis
    
    • ​Connects nucl. ruber to cornu ventralis
    • Walk, run, jump, swim
  • Tr. reticulospinalis
    
    • ​= formatio reticularis
    • Location: medulla oblongata
    • Coordinate basic movement
  • tr. vestibulospinalis lateralis
    
    • ​Muscle contraction, balance
  • Tr. tectospinalis

Question 12

Draw the spinal cord with the tracts

Answer 12

Question 13

Descending pathways

Tr. corticospinalis

Answer 13

• Pathway:
  
  1. Primary motor cortex
  2. Gyrus precentralis
  3. Pedunculus cerebri
  4. Tr. pyramidalis
     
     • Crossing (lateral):
       
       • Eq: 50%
       • Ca: 75%
     • Non-crossing:
       
       • Eq: 50%
       • Ca: 25%