Medicine – Spinal Cord Anatomy

The spinal cord contains major descending motor pathways and ascending sensory pathways. For clinical neurology, the three most important tracts to understand are the corticospinal tract, dorsal columns, and spinothalamic tracts.

Their functions and sites of decussation are especially important because spinal cord lesions produce different patterns of weakness and sensory loss depending on which tract is affected.

1. Corticospinal Tract

The corticospinal tract, also called the pyramidal tract, is the major descending pathway responsible for voluntary movement.

It carries motor signals from the cerebral cortex down through the brainstem and spinal cord to lower motor neurones.

2. Function of the Corticospinal Tract

The corticospinal tract is particularly important for:

Voluntary movement.

Fine skilled movements.

Fractionated movements of the distal limbs, especially the hands and fingers.

Damage to this tract produces upper motor neurone signs below the level of the lesion.

3. Origin of Corticospinal Fibres

Fibres arise mainly from the:

Primary motor cortex.

They also arise from premotor and somatosensory cortical areas.

From the cortex, the fibres descend through the:

Corona radiata.

Internal capsule.

Cerebral peduncles of the midbrain.

Pons.

Medullary pyramids.

4. Decussation of the Corticospinal Tract

The original note states that the corticospinal tract decussates in the midbrain, but this is not correct.

Most corticospinal fibres cross in the:

Lower medulla at the pyramidal decussation.

Approximately the majority of fibres cross here and then descend in the contralateral spinal cord as the lateral corticospinal tract.

Therefore:

Corticospinal tract decussation = lower medulla, not midbrain.

5. Lateral Corticospinal Tract

After crossing in the medulla, most fibres descend in the lateral corticospinal tract.

Because they have already crossed, a lesion of the lateral corticospinal tract within the spinal cord produces:

Ipsilateral upper motor neurone weakness below the level of the lesion.

For example:

Right spinal cord corticospinal lesion → right-sided UMN weakness below the lesion.

6. Corticospinal Tract Lesion

Damage produces typical upper motor neurone signs:

Weakness.

Increased tone or spasticity.

Hyperreflexia.

Clonus.

Extensor plantar response.

Early after an acute spinal cord lesion, however, there may temporarily be flaccidity and reduced reflexes due to spinal shock.

7. Major Ascending Sensory Pathways

Two major ascending sensory pathways are particularly important:

Dorsal column–medial lemniscus pathway.

Spinothalamic pathway.

They carry different sensory modalities and cross at different levels.

This difference is crucial for lesion localisation.

8. Dorsal Columns

The dorsal columns, also called the posterior columns, carry highly organised sensory information from the body toward the brain.

Their main modalities are:

Joint-position sense.

Vibration sense.

Fine discriminative touch.

Two-point discrimination.

They are therefore especially important for proprioception.

9. Dorsal Column Pathway

Peripheral sensory fibres enter the spinal cord and ascend ipsilaterally in the posterior columns.

They do not immediately cross within the spinal cord.

They ascend all the way to the medulla.

10. Dorsal Column Synapse

The first major synapse occurs in the lower brainstem, specifically in the medulla.

Fibres from the lower body synapse in the:

Nucleus gracilis.

Fibres from the upper body synapse in the:

Nucleus cuneatus.

After this synapse, the second-order neurones cross.

11. Dorsal Column Decussation

The dorsal column pathway crosses in the medulla.

After synapsing in the gracile and cuneate nuclei, fibres decussate as internal arcuate fibres and then ascend as the:

Medial lemniscus.

Therefore:

Dorsal columns ascend ipsilaterally in spinal cord → synapse in medulla → then decussate.

12. Clinical Effect of Dorsal Column Lesions

A unilateral dorsal column lesion in the spinal cord causes:

Ipsilateral loss of vibration and proprioception below the lesion.

The patient may develop sensory ataxia because the brain receives impaired information about limb position.

13. Sensory Ataxia

Dorsal column dysfunction can cause:

Unsteady gait.

Difficulty walking in the dark.

Loss of joint-position sense.

Loss of vibration sense.

Positive Romberg sign.

Vision can partially compensate for lost proprioception, so closing the eyes worsens balance.

14. Organisation of the Dorsal Columns

The dorsal columns are divided into two main fasciculi.

The fasciculus gracilis carries information mainly from the lower trunk and lower limbs.

The fasciculus cuneatus carries information mainly from the upper trunk and upper limbs and is present above approximately the mid-thoracic level.

15. Spinothalamic Tracts

The spinothalamic tracts are part of the anterolateral sensory system.

They primarily transmit:

Pain.

Temperature.

Crude touch and pressure are also carried within the broader anterolateral system.

16. Spinothalamic Pathway

Pain and temperature fibres enter the spinal cord through the dorsal roots.

They may travel up or down a short distance within Lissauer’s tract before synapsing in the dorsal horn.

The second-order neurones then cross to the opposite side.

17. Decussation of the Spinothalamic Tract

Unlike the dorsal columns, spinothalamic fibres cross very early.

They decussate through the:

Anterior white commissure of the spinal cord.

This usually occurs within approximately one or a few spinal segments after entry.

Therefore:

Spinothalamic pathway → crosses soon after entering spinal cord.

18. Clinical Effect of Spinothalamic Lesions

Because the fibres cross within the spinal cord, a unilateral lesion of the spinothalamic tract causes:

Contralateral loss of pain and temperature below the lesion.

The sensory loss often begins a few segments below the actual lesion because the incoming fibres travel briefly before crossing.

19. Why Spinothalamic Loss Starts Below the Lesion

Pain and temperature fibres may ascend or descend approximately one to two spinal levels before synapsing and crossing.

Therefore, if the spinothalamic tract is damaged at a particular level, sensory loss may begin:

One or a few dermatomes below the lesion.

This is an important localisation principle.

20. Lamination of Spinothalamic Fibres

The spinothalamic tract has an organised arrangement, or somatotopy.

Fibres from different body levels occupy different positions within the tract.

A useful simplified arrangement is:

Sacral fibres are more lateral.

Lumbar fibres lie medial to sacral fibres.

Thoracic fibres lie further medial.

Cervical fibres are relatively medial.

A common mnemonic is:

SALT

Sacral → lateral.

21. Clinical Importance of Spinothalamic Lamination

The laminated arrangement can help explain particular patterns of sensory loss.

For example, an expanding central spinal cord lesion may initially damage the crossing spinothalamic fibres near the central canal while sparing the more peripheral tract.

This is seen classically in:

Syringomyelia.

22. Syringomyelia and the Spinothalamic Pathway

A syrinx expands around the central canal and may damage fibres crossing through the anterior white commissure.

This produces bilateral segmental loss of:

Pain.

Temperature.

while initially preserving:

Vibration.

Joint-position sense.

This is called dissociated sensory loss.

23. Key Difference Between Dorsal Columns and Spinothalamic Tracts

The major distinction is where they cross.

Dorsal columns:

Ascend ipsilaterally through the spinal cord.

Cross in the medulla.

Spinothalamic tract:

Synapses in the spinal cord.

Crosses within one or a few spinal segments.

Then ascends contralaterally.

24. Hemicord Lesion and Brown-Séquard Pattern

The differing decussations explain the classic findings of a spinal cord hemisection.

A unilateral spinal cord lesion can cause:

Ipsilateral UMN weakness below the lesion from corticospinal tract damage.

Ipsilateral loss of vibration and proprioception below the lesion from dorsal column damage.

Contralateral loss of pain and temperature beginning slightly below the lesion from spinothalamic tract damage.

This combination is called the Brown-Séquard pattern.

25. Corticospinal Tract – Note Form

Type: descending motor pathway.

Function: voluntary movement.

Origin: cerebral motor cortex.

Course: cortex → internal capsule → midbrain → pons → medulla → spinal cord.

Decussation: lower medulla at pyramidal decussation.

Spinal cord lesion: ipsilateral UMN weakness below lesion.

Signs of damage: weakness, spasticity, hyperreflexia, clonus and extensor plantars.

26. Dorsal Columns – Note Form

Type: ascending sensory pathway.

Carries: vibration, joint-position sense and fine discriminative touch.

Spinal cord course: ascends ipsilaterally.

First major synapse: gracile and cuneate nuclei in medulla.

Decussation: medulla after synapse.

Spinal cord lesion: ipsilateral loss of vibration and proprioception below lesion.

Clinical result: sensory ataxia and positive Romberg sign.

27. Spinothalamic Tract – Note Form

Type: ascending sensory pathway.

Carries: pain and temperature.

Synapse: dorsal horn of spinal cord.

Decussation: anterior white commissure, usually within one or a few segments.

After crossing: ascends contralaterally.

Spinal cord lesion: contralateral loss of pain and temperature below lesion.

Lamination: sacral fibres relatively lateral and cervical fibres relatively medial.

Key Clinical Pattern

Remember the three major pathways as:

Corticospinal → MOTOR → crosses in MEDULLA.

Dorsal columns → VIBRATION + POSITION → ascend ipsilaterally, cross in MEDULLA.

Spinothalamic → PAIN + TEMPERATURE → crosses almost immediately in the SPINAL CORD.

The most important correction to the original note is:

The corticospinal tract does NOT decussate in the midbrain. Most fibres cross at the pyramidal decussation in the lower medulla.

A very useful lesion rule is:

Spinal cord hemisection → ipsilateral weakness + ipsilateral loss of vibration/proprioception + contralateral loss of pain/temperature.