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)

image

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:

image

  • 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

image

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

image

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

image

image

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?

image

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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:

image

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

image

image

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

image

_____________________________________________________________________

Flow of arrangement of the dorsal column tracts:

image

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

image

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

image

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

_____________________________________________________________________

Flow of arrangement of Spinocerebellar tract:

image

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:

image

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

_____________________________________________________________________

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 the 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
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What Is Faith? "Now faith is the substance for all things hoped for, the evidence of things not seen" (Hebrews 11:1)
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58 Responses to Ascending & Descending tracts of spinal cord

  1. marsman says:

    useful and clear.
    Thanks a lot

  2. H199 says:

    This has cleared up what lecture notes and wikipedia and textbooks couldnt!! thankyou so so much!! :)

  3. terrichan says:

    You’re welcome! That’s why I started this blog! :)

  4. rehnuma says:

    wonderful collection

  5. syuhada eddiyah says:

    thanks a lot for this lecture notes. it helps me a lot. =)
    hope to share more notes from this page…thank you….^_^

  6. Dr.McCarroll says:

    Nice review

  7. Med-Student says:

    Thank you! This has proved very helpful for a PBL session. :)

  8. RABICA says:

    IT WAS GREAT AND USEFUL BUT I COULDNT UNDERSTAND THE SPINOCEREBELLAR TRACT ESP THE LAST DIAGRAM HOW IT MEETS THE RIGHT N LEFT MID BRAIN AS MID BRAIN LIES ABOVE THE CEREBELLAM

    • simiyu says:

      This is kind of late but here goes; it is a 2D image think of it in 3D. That’s why it enters the cerebellum through the anterior and superior peduncles.

  9. Captain John Longcock says:

    “internal capsule
    Function: separates the caudate nucleus and the thalamus from the lenticular nucleus”

    “heart
    Function: separates the left lung from the right lung”

    7/10 would read again

  10. simiyu says:

    Oh well not all points were vague. For maximum benefit use with some other site. gives you a guide.

  11. the theory was quite good but can it be given in little details please for easy understanding..thank you so much for the information….

  12. Lebari jnr. says:

    Am greatful for this note, it of helped to me. Thanks.

  13. MT Kennedy says:

    Can you explain how the cerellar tract crosses twice? is that the ventral/anterior tract? Have you diagrams of the other ascending/decending tracts? Many many thanks

    • simiyu says:

      Ventral spinocerebellar tracts cross immediately they enter spinal cord and again in the white matter of the cerebellum(crossing a road twice) dorsal spinocerebellar tracts don’t decussate at all.

  14. MT Kennedy says:

    meant cerebellar

  15. Sandra lumamba says:

    Specific functions

  16. mokwe magdalene says:

    hmmm! At least i can now to some xtend the ascending and descending tracts of the spinal cord. Tanks alot

  17. ramsha says:

    OH THANKS

  18. Sarah says:

    I love you. None of my textbooks have anything about any tracts in them!! Useless. And wikipedia was a bit useless too! This is amazing. Thank you so much!

  19. bibals2000 says:

    An interestin piece. Thanxx a lot!

  20. it is little bt helpful…………

  21. florence says:

    This was very helpfull thankyou

  22. jihye yang says:

    Thank you so much!

  23. Toni says:

    Thank you, now can we just have you as our teacher and we will be fine!

  24. Huy says:

    Thanks to your notes now I understand what “cross” means. Keep up the good work.

  25. Mnangat says:

    It was a superb review. Thanks.

  26. sangita says:

    explained in a simple and interesting way

  27. ganesh says:

    all concepts cleared…. thanks a lot….

  28. ConcernedStudent says:

    Whoa! There is a massive mistake in this article! In the section titled: “General arrangement of descending tracts” it says that first order neurons collect impulses from the skin and proprioceptors”. The problem is that you’re describing ascending tracts! First order descending tracts start in the brain, not in the periphery. If they start in the skin, or proprioceptors and go to the brain, they would be ASCENDING neurons. Motor neurons start in the brain and DESCEND to the muscles and organs. The funny thing is, the diagram that you used directly above the text is accurate, it shows the first order neuron in the motor cortex. That’s all I’ve read so far.

  29. narmeen anees says:

    thankquuuuuuuuuu soooooooooooooooooooo much

  30. Aleena bajwa says:

    Help full for my presentation thanksssssss too

  31. Doc Raney says:

    Good forum but I have a question. According to my reading I have seen that the corticospinal tracts (which are descending) consist of only 2 neurons (Upper and Lower) and 1 synapse (in the anterior horn of the spinal column) as compared to the ascending sensory tracts which contain 3 neurons and 2 synapses. My question lies in the above explanation which say that there are 3 neurons in the corticospinal tracts with 2 synapses.

    Anyone can clarify?

  32. sabita lamsal says:

    Really it is one of the best study materials of ascending and descending tract
    Thank u so much for this

  33. thanks!
    it is really helpful.

  34. sumanta says:

    it is very helpful….wish u could cover other topics also

  35. cphy says:

    Really helped me a lot! I struggled for a year with the tracts, now it’s so much clearer. Thank you! But just to clarify, for the spinocerebellar pathway , it crosses from the left clarke’s column to the right then up the spinocerebellar tract to the RIGHT mid brain then crosses to the left midbrain to the cerebellum.

  36. hardlife zambezi says:

    tatenda ………thank you

  37. dr.mariyah says:

    could you please write more on cerebellar tracts?it would help me understand without going through the boring text.

  38. navid says:

    really helped me a lot! thank you so much

  39. Dr. Meshkat says:

    The article made me clear of tracts.thnx and gratitude for article..

  40. Yasmeen says:

    awesome work

  41. saddamhussainkhann says:

    its very easy for understanding thanks a lot

  42. moyin says:

    Fantastic it just solved my prob good job

  43. Mayeji says:

    it was real helpful to me in my studies,thanks..!

  44. Joyce says:

    Thank you its of real good help

  45. Dean says:

    I would like some clarification on pathway,vs, faniculus vs, column, vs tract

  46. Mindfreak says:

    Wow….amazingly written….simple yet informative…Thanx a lot for taking the effort to summarize and also sharing with everyone!!

  47. sapna says:

    thanku so much its really helpful for me

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