🎯 What are tracts?

• the spinal cord has various pathways known as tracts, which are bundles of nerve fibres that transmit impulses between the brain and the body

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💡 Types of tracts

• the tracts in the spinal cord can be categorised into two types:

  • ascending tracts: transmit sensory impulses from the spinal cord to the brain

    • important ascending tracts:

      • spinothalamic tract

      • dorsal column-medial lemniscal (DCML) pathway

      • spinocerebellar tract

  • descending tracts: transmit impulses from the brain to the spinal cord

    • important descending tracts:

      • corticospinal tract

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⬆️ Spinothalamic tract

• function

  • conveys crude touch (cannot be localised), pain and temperature

• location of tract in the spinal cord

  • anterolateral white matter

• course

  1. electrical impulses are generated from sensory nerve endings of the sensory neuron (this is the first-order neuron)

  2. the sensory neuron synapses with a second-order neuron at the dorsal horn of the spinal cord

  3. axons of the second-order neuron decussate via the anterior commissure to enter the contralateral spinothalamic tract which is located within the anterolateral white matter (note that as second-order neurons decussate, they ascend two to three spinal segments before reaching the contralateral spinothalamic tract)

  4. second-order neurons synapse with third-order neurons at the thalamus, specifically at the ventral posterior lateral (VPL) nucleus

  5. third-order neurons project to the primary somatosensory cortex (via the internal capsule) which corresponds to the post-central gyrus

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⬆️ Dorsal column-medial lemniscal (DCML) pathway

• function

  • conveys discriminative touch (can be localised), vibration and proprioception (joint position sense)

• location of tract in the spinal cord

  • posterior columns, which are divided into two regions:

    • laterally: fasciculus cuneatus (carries information from the arms, neck and upper trunk)

    • medially: fasciculus gracilis (carries information from the legs and lower trunk)

• course

  1. electrical impulses are generated from sensory nerve endings of the sensory neuron (this is the first-order neuron)

  2. first-order neurons enter the spinal cord at the dorsal horn of grey, and will subsequently ascend in the dorsal columns

  3. sensory neurons from the upper body will ascend in the fasciculus cuneatus while neurons from the lower body will ascend in the fasciculus gracilis

  4. fibres from the fasciculus cuneatus and gracilis will synapse with second-order neurons at nucleus cuneatus and gracilis respectively at the level of the lower (caudal) medulla

  5. second-order neurons decussate as the internal arcuate fibres

  6. once decussated, these fibres become the medial lemniscus which project to the thalamus, specifically the ventral posterior lateral (VPL) nucleus to synapse with third-order neurons

  7. third-order neurons project to the primary somatosensory cortex via the internal capsule

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⬇️ Corticospinal tract

• function

  • initiation of voluntary movement

• location of tract in the spinal cord

  • note that there are two descending corticospinal tracts:

    • laterally: lateral corticospinal tract

    • anteromedially: anterior corticospinal tract

• course

  1. upper motor neurons arise from the primary motor cortex (which corresponds to the precentral gyrus)

  2. axons descend to the cerebral peduncles in the midbrain via the internal capsule

  3. axons continue down the brainstem until they reach the medullary pyramids where they decussate

     • approximately 85% of the corticospinal fibres decussate, and these make up the lateral corticospinal tract

     • the rest of the corticospinal tract continues as the anterior corticospinal tract

  4. corticospinal tract fibres enter the spinal cord and synapse with lower motor neurons at the ventral horn

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🧠 Homunculus

• the whole body is represented in the primary motor cortex and primary somatosensory cortex

• from lateral to medial:

  • face —> arms —> trunk —> legs​