Ascending & Descending tracts of spinal cord

General overview

• Ascending tracts
• Sensory
• Descending tracts
• Motor

General arrangement of both tracts

• 1st order neuron
• 2nd order neuron
• 3rd order neuron

The only difference is the different locations where each order of neuron ends.

Decussation is the cross-over of the tract from one side to the other. Therefore, there are instances where the left side of the body is controlled by the right brain hemisphere. Decussation occurs at different locations for each tracts.

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Descending tracts (Motor)

General arrangement of descending tracts

• 1st order neuron
• First-order neurons conduct impulses from receptors of the skin and from proprioceptors (receptors located in a join, muscle or tendon) to the spinal cord or brain stem, where they synapse with second-order neurons. First-order neuron’s cell bodes reside in ganglion (dorsal root or cranial).
• starts at the cerebral cortex in the somatomotor area
• 2nd order neuron
• 2nd neuron to carry an order. The order could be a sensory stimulus or a motor stimulus.
• axon of the 1st order neuron will synapse with the 2nd order neuron at the level of the brain stem, which commonly decussate (crosses over) to the opposite side.
• 3rd order neuron
• The 3rd order neuron is located in the ventral horn of the spinal cord, which will exit with the spinal nerve to supply the muscle.

Types of descending tracts:

• Lateral corticospinal tract
• Anterior corticospinal tract

Therefore, the descending tract is also known as corticospinal tract.

Flow of arrangement of the corticospinal tract:

• Corticospinal tract arise from
• long axons of the pyramidal cells (extrapyramidal layer) of the precentral gyrus (primary motor centre of the cerebral cortex)
• lies in front of the central sulcus
• Homunculus arrangement
• arranged upside down
• the finer the movement, the more the cortical representation
• fingers, face, tongue – more
• trunk, lower limbs – less
• medial surface: lower limbs
• superolateral surface: everything else

1) 1st order neuron

• Fibres of the 1st order neuron arise from the precentral gyrus
• These fibres converge and enter a small area
• internal capsule
• like a bunch of flowers with a ribbon tied around it
• ALL the fibers (from ascending & descending tracts) converge here
• Function: separates the caudate nucleus and the thalamus from the lenticular nucleus
• Ques: location of internal capsule
• bounded medially by the thalamus and caudate nucleus
• bounded laterally by the lenticular nucleus
• Parts of internal capsule (not homunculus arrangement, normal head to toe)
• anterior limb
• head & neck fibres most anterior
• posterior limb
• lower limb fibres most posterior
• The descending fibres passes through the
• LATERAL half of the posterior limb of internal capsule
• After the internal capsule, the fibres enter the brain stem
• midbrain
• pons
• medulla

2) 2nd order neuron

• Fibres of the 1st order neuron ends when it enters the brain stem and synapse with the 2nd order neuron
• The fibres pass through the brainstem
• 1st – through the (mid 5th) crus cerebri of midbrain
• 2nd – through the anterior part of the pons
• 3rd – in the medulla oblongata
• 80-85% of the fibres cross to the opposite side
• Motor decussation
• uncrossed fibres
• Enters the spinal cord

3) 3rd order neuron

• 2nd order neuron fibres in the medulla oblongata enters the spinal cord and synapse with the 3rd order neuron
• Motor decussation
• in the spinal tract, the crossed tract descend as the lateral corticospinal tract
• Therefore, the motor cortex of the cerebral hemisphere controls the opposite side of the body (L – R, R – L)
• contra-lateral side
• In upper motor neuron lesions:
• above the motor decussation (above medulla)
• opposite side of body affected
• below the motor decussation
• same side of body affected
• ipsilateral side
• Uncrossed fibres
• in the spinal tract, the uncrossed tract descent as the anterior corticospinal tract
• its fibres cross at spinal level?

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Ascending tracts (sensory)

Types of ascending tracts:

• Spinothalamic tracts
• Lateral
• pain & temperature
• Anterior
• light touch & pressure
• Dorsal column tract
• deep touch & pressure
• proprioception
• vibration sensation
• Spinocerrebellar tract
• posture & coordination

Flow of arrangement of Spinothalamic tracts:

1st order neuron:

• Arise from sensory receptors of the body
• The fibres enter the white mater and ends at the substantia gelatinosa
• tip of posterior gray horn

2nd order neuron:

• The fibres of 1st order neuron synapse with the 2nd order neuron at the substantia gelatinosa
• These fibres then cross to the opposite side
• Pain & temperature fibres
• enters the lateral spinothalamic tract
• Light touch & pressure fibres
• enters the anterior spinothalamic tract
• These tracts ascends to brainstem
• to medulla oblongata, pons and midbrain
• tracts flattened in the brainstem
• spinal lemniscus
• Reaches the ventral posterolateral nucleus of the thalamus
• ends here

3rd order neuron:

• The 3rd order neurons arise from the thalamus and pass through the internal capsule
• thalamocortical fibres pass through the medial part of the posterior limb of the internal capsule
• Enters the postcentral gyrus
• sensory cortex of the cerebrum
• behind the central sulcus
• Same homunculus arrangement
• more sensitive areas in the body have a greater representation

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Flow of arrangement of the dorsal column tracts:

1st order neuron:

• Arise from the sensory receptors of the body
• Fibres enter the dorsal column of the SAME side (post column of spinal cord)
• ascends to the medulla oblongata
• (does not synapse and end here like spinothalamic tract)
• Enters medulla oblongata
• ends in the gracile and cuneate nucleus

2nd order neuron:

• Starts at the gracile & cuneate nucleus of the medulla oblongata
• These fibres crosses to the opposite side of the medulla oblongata
• Ascends through the brain stem
• as flattened bundle
• medial lemniscus
• Ends in the ventral posterolateral nucleus of the thalamus

3rd order of nucleus:

• Arise from the thalamus
• Pass through the internal capsule
• medial aspect of the posterior limb of internal capsule
• Reaches the postcentral gyrus
• ends here

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Flow of arrangement of Spinocerebellar tract:

1st order neurons:

• Arise from the sensory receptors of the body
• Enters the spinal cord
• Ends in the Clarke’s Column of the posterior grey horn
• synapse

2nd order neurons:

• Arise from the Clarke’s Column
• synapse with 1st order neurons
• Ascends in the spinocerebellar tracts
• Enters the cerebellum
• through the interior and superior cerebellar peduncles
• the only tract that enters the cerebellum

Actual decussation of these tracts:

• These tracts decussate 2 times
• therefore cerebellum controls same side of body
• ipsilateral
• eg. right spinocerebellar tract controls the right side
• vice versa

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Clinical anatomy:

• Lesion of one half of the spinal cord may lead to
• Opposite side of body
• loss of pain, temperature, light touch, pressure sensations
• Same side of body
• loss of the other sensations
• Sensory cortex of the cerebral hemisphere controls the opposite side of th
  e body
• contralateral
• Lesion above the sensory decussation
• all the sensations of the OPPOSITE side of the body are lost
• Lesion below the sensory decussation
• sensations of the SAME side of the body are lost
• Cortical lesions
• affected areas are usually limited
• paralysis/parasthesia is localized
• Internal capsule lesions
• all ascending & descending tracts are affected
• hemiplegia/hemiparasthesia