Sensory Tracts

Summary

  • Introduction to tracts
  • The four ascending (sensory) tracts:
    • Spinothalamic tract
    • Dorsal column
    • Spinocerebellar tract
    • Trigeminothalamic tract
This table summarises the key sensory tracts.

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
Summary of the afferent (sensory) and efferent (motor) tracts and their positions in the spinal cord’s white matter
  • 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

 

This diagram indicates the key factors that can change with each tract.

 

  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

Spinothalamic tract

  • 2 parts with different functions:
    1. Lateral spinothalamic tract
      1. Pain
      2. Temperature
    1. 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

        Pathway of the spinothalamic tract.

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


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

 

Image showing the pathways of the ventral and dorsal spinocerebellar tracts.

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

        The trigeminal nucleus complex. Note the difference between the trigeminal nucleus and trigeminal ganglion.
  • 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)

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