Sensory Tracts

Summary

• Introduction to tracts

• The four ascending (sensory) tracts:
  • Spinothalamic tract
  • Dorsal column
  • Spinocerebellar tract
  • Trigeminothalamic tract

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Introduction to Tracts

• Tracts are collections of the cell bodies of neurones that make up the white matter of the spinal cord, communicating information to/from the SC and brain
  1. Ascending tracts convey sensory information from SC to brain
  2. Descending tracts convey motor information from brain to SC

• Tracts receive the sensory axons from cells in the dorsal root ganglia (DRG)

• There are 4 main sensory tracts:
  • Each is responsible for a different type of sensation

 

 

1. Spinothalamic tract
   • Perception of
     • Pain
     • Temperature
     • Crude touch
     • Firm pressure
   • Includes two parts:
     1. Lateral spinothalamic tract
     2. Anterior/ventral  spinothalamic tract
   • Enters the spinal lemniscus in brainstem
2. Dorsal columns
   • Perception of discriminative (fine)touch
   • Enters medial lemniscus in brainstem
3. Spinocerebellar tract
   • Perception of proprioception
   • Goes to cerebellum
4. Trigeminothalamic tract
   • Takes all 4 senses from the face (CNV)
   • Visceral afferents from CNV, CNVII, CNIX, CNX

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Spinothalamic tract

• 2 parts with different functions:
  1. Lateral spinothalamic tract
     1. Pain
     2. Temperature
  2. Ventral spinothalamic tract
     1. Crude touch
     2. Firm pressure

• The lateral spinothalamic tract has ends in two places
  1. First terminates in the reticular formation of brainstem
     • This induces reticular alerting response in the entire nervous system
     • This therefore initiates a reflex resposne
  2. It then reports to the limbic system
     • Indicates nature of the stimulus

• Pathway:

  1. Primary neurone
     • Cell body in DRG
     • Synapses at ipsilateral dorsal horn
       • May ascend/descend a little at Lissauers Tract
  2. Secondary neurone
     • Decussates at the level it enters
       • Lesion in SC will therefore cause contralateral loss of sensation
     • The impulse travels in the ventral or lateral spinothalamic tract
     • Synapses in the thalamus (in the ventral posterolateral nucleus)
  3. Tertiary neurone
     • Starts in thalamus/VPL
     • Travels in the spinothalamic lemniscus
     • Synapses in the primary sensory cortex

       

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Dorsal Column

• This is responsible for sensation of:
  • Conscious proprioception
  • Discriminative/fine touch

• 2 parts – both run in the dorsal column of white matter
  • Both carry the same function – but from different levels
  • Both are named according to the nucleus they synapse at

1. Fasciculus Gracilis  – Medial dorsal column
   • Afferents come from lower limb (Sacral/lumbar)
   • Synapses in Nucleus gracilis in medulla
2. Fasciculus Cuneatus – Lateral dorsal column
   • Fibres come from from upper limb (thoracic/cervical)
     • Therefore present throughout spinal cord
   • Synapses in Nucleus cuneatus in medulla

• An aid to remember this is that ‘Major League Gaming needs LUC‘
  • I.e. Medial, Lower limb, Gracilis => Lateral, Upper limb, Cuneatus

 

• Pathway:

  1. Primary neurone
     • Cell body lies in DRG
     • Travels in either fasciculus up to medulla
     • Synapses in the CNS at the medulla either at:
       1. Nucleus gracilus
       2. Nucleus cuneatus
  2. Secondary neurone
     • Forms Internal Arcuate Fibres
       • NB These decussate in medulla
     • Synapses in the VPL in thalamus
       • Travels to thalamus as the medial lemniscus
  3. Tertiary neurone (same as spinothalamic tract)
     • Starts in thalamus/VPL
     • Synapses in the primary sensory cortex

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Spinocerebellar Tract

• Communicates 2 sensations with the cerebellum:
  1. Unconscious proprioception
  2. Whole limb + postural movement

• 2 pathways:
  1. Ventral spinocerebellar pathway
  2. Dorsal spinocerebellar pathway

• Only 2 neurons in both pathways
  1. Primary neurone
     • Identical for both pathways (unlike the secondary neurone)
     • Cell body lies in DRG
     • Synapses in the dorsal horn (Nucleus Dorsalis)
  2. Secondary neurone (Ventral SCP)
     • Travel up spinal cord as Clarke’s column
       • Decussate in spinal cord at level of entry
     • Pass into cerebellum at midbrain via superior cerebellar peduncles
     • Pass back into pons and decussate again into other cerebellum hemisphere via middle peduncles
  3. Secondary neurone (Dorsal SCP)
     • Travel up Clarke’s column
     • Enter cerebellum at medulla via inferior cerebellar peduncle
     • NB, does not decussate at all

 

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Trigeminothalamic Tracts

• Input into the trigeminothalamic tract comes from several cranial nerves:
  1. CNV 
     • This receives general afferents from the skin of the face/head/neck
     • Synapse in trigeminal ganglion
  2. CNVII, CNIX, CNX (visceral afferents from the rest of the body)
     • Received via the spinal nucleus
     • Synapse at parasympathetic nuclei in medulla

• The Trigeminal nucleus has 3 parts with different afferent types in each part (NB this is different to the trigeminal ganglion)
  1. Mesencephalic nucleus (proprioception)
  2. Chief/primary nucleus (pressure, touch) – ventral trigeminothalamic pathway
     • Essentially dorsal column for the face
  3. Spinal nucleus (pain, temp) – dorsal trigeminothalamic pathway
     • Essentially spinothalamic tract for the face

       

• Pathway:

  • Primary neurone
    • Cell body lies in trigeminal ganglion
      • This is analogous to the  DRG in the spine
      • Except Mesencephalic nuclei (see below)
        • Cell body in mesencephalic nucleus itself – not in trigeminal ganglion
    • Synapses in the trigeminal nucleus in brainstem according to sensation (e.g. pain in the spinal nucleus)
      • E.g. pain/temp in spinal nucleus
      • Touch/pressure in chief nucleus
  • Secondary neurone
    • Decussates at level of entry into brainstem
    • Travel up as trigeminal lemniscus
    • Synapses in the VPM  in thalamus
  • Tertiary neurone
    • Starts in VPM
    • Passes through internal capsule
    • Synapses in primary sensory cortex (to face division – more lateral than limbs)