Spinalcord tracts
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Here is a complete overview of the spinal cord tracts, sourced from Gray's Anatomy for Students.
Spinal Cord Tracts
The white matter of the spinal cord is organized into three funiculi (columns) on each side - anterior, lateral, and posterior - each carrying specific ascending (sensory) or descending (motor) tracts.
ASCENDING (Sensory) TRACTS
Sensory pathways use a 3-neuron chain: 1st-order (peripheral receptor → spinal cord/medulla), 2nd-order (spinal cord/medulla → thalamus), 3rd-order (thalamus → cortex).
1. Anterolateral Pathways (Anterior & Lateral Funiculus)
Carry pain, temperature, and crude touch.
Three tracts make up this system:
| Tract | Modality | Target |
|---|---|---|
| Spinothalamic | Pain, temperature, crude touch | VPL nucleus of thalamus → somatosensory cortex |
| Spinoreticular | Emotional/arousal aspects of pain | Reticular formation (brainstem) |
| Spinomesencephalic | Nociceptive modulation | Periaqueductal gray (midbrain) |
Course of the Spinothalamic Tract:
- 1st-order neuron: cell body in dorsal root ganglion → enters posterior horn
- Axons either synapse immediately in laminae I & V, or travel 1-2 segments in Lissauer's tract before synapsing
- 2nd-order neuron: crosses obliquely over 2-3 segments via the anterior white commissure to the contralateral anterolateral tract
- Ascends to VPL nucleus of thalamus
- 3rd-order neuron: projects via posterior limb of internal capsule to primary somatosensory cortex
Clinical key: Because crossing occurs over 2-3 segments, a hemisection of the cord (Brown-Sequard syndrome) causes contralateral pain/temperature loss 2-3 levels below the lesion.
2. Posterior Column-Medial Lemniscal Pathway (Posterior Funiculus)
Carries fine/discriminative touch, vibration, and conscious proprioception.
Two fasciculi:
- Fasciculus gracilis - lower limb and trunk (medial, present all levels)
- Fasciculus cuneatus - upper limb and neck (lateral, present from T6 upward)
Course:
- 1st-order neuron: cell body in dorsal root ganglion → axon ascends ipsilaterally in posterior column
- Synapses in nucleus gracilis / nucleus cuneatus in the caudal medulla
- 2nd-order neuron: crosses as internal arcuate fibers → forms medial lemniscus in contralateral medulla
- Ascends to VPL nucleus of thalamus
- 3rd-order neuron: projects via posterior limb of internal capsule → primary somatosensory cortex
Clinical key: Crosses in the medulla (not the cord). Ipsilateral proprioception/vibration loss occurs with cord lesions (e.g., subacute combined degeneration in B12 deficiency, tabes dorsalis).
3. Spinocerebellar Tracts (Lateral Funiculus)
Carry subconscious proprioception to the cerebellum.
| Tract | Origin | Decussation | Side |
|---|---|---|---|
| Dorsal spinocerebellar | Clarke's nucleus (C8-L3) | Does NOT cross | Ipsilateral |
| Ventral spinocerebellar | Lumbosacral cord | Crosses twice (net ipsilateral) | Ipsilateral (double-cross) |
DESCENDING (Motor) TRACTS
Descending tracts are divided into the lateral motor system (fine voluntary movement) and the medial motor system (posture, balance, axial movements).
LATERAL MOTOR SYSTEM
1. Lateral Corticospinal Tract (Lateral Funiculus)
The most clinically important tract - controls voluntary movement of the extremities.
Course:
- Upper motor neuron (UMN): cell body in primary motor cortex
- Axons converge in corona radiata → descend via posterior limb of internal capsule → crus cerebri of midbrain → anterior pons → pyramid of medulla
- ~85% decussate at caudal medulla (pyramidal decussation) → form lateral corticospinal tract
- Descend in lateral column → synapse on LMN cell bodies in lateral anterior horn
- Lower motor neuron exits via anterior root
2. Rubrospinal Tract (Lateral Funiculus)
- Origin: red nucleus (midbrain)
- Crosses at ventral tegmental decussation → descends in lateral column
- Only descends to cervical levels
- Facilitates flexor and inhibits extensor activity of the upper limb
MEDIAL MOTOR SYSTEM
Regulate axial/truncal muscles for posture, balance, and automatic movements. Project bilaterally.
| Tract | Origin | Extent | Function |
|---|---|---|---|
| Anterior corticospinal tract | Motor cortex (fibers that did NOT decussate at medulla) | To upper thoracic cord | Bilateral axial/proximal limb control |
| Tectospinal tract | Superior colliculus (dorsal midbrain) | Cervical cord only | Reflex postural responses to visual stimuli |
| Medial vestibulospinal tract | Medial vestibular nucleus | To thoracic cord, bilateral | Head/neck posture relative to vestibular input |
| Lateral vestibulospinal tract | Lateral vestibular nucleus (Deiters) | Entire cord, ipsilateral | Extensor muscle tone, balance |
| Reticulospinal tracts | Reticular formation (pons & medulla) | Entire cord | Modulate muscle tone, autonomic functions |
Summary Comparison Table
| Tract | Type | Location | Crosses? | Modality |
|---|---|---|---|---|
| Fasciculus gracilis | Ascending | Post. funiculus | In medulla | Fine touch, vibration, proprioception (LL) |
| Fasciculus cuneatus | Ascending | Post. funiculus | In medulla | Fine touch, vibration, proprioception (UL) |
| Spinothalamic | Ascending | Anterolateral | In cord (2-3 seg) | Pain, temperature, crude touch |
| Dorsal spinocerebellar | Ascending | Lateral funiculus | No | Subconscious proprioception (LL) |
| Ventral spinocerebellar | Ascending | Lateral funiculus | Double cross | Subconscious proprioception (LL) |
| Lateral corticospinal | Descending | Lateral funiculus | In medulla (~85%) | Voluntary movement (extremities) |
| Anterior corticospinal | Descending | Anterior funiculus | In cord (bilateral) | Axial/proximal voluntary movement |
| Rubrospinal | Descending | Lateral funiculus | In midbrain | UL flexion |
| Vestibulospinal (lateral) | Descending | Anterior funiculus | No | Extensor tone, balance |
| Reticulospinal | Descending | Anterior/lateral | Mixed | Tone, autonomic |
| Tectospinal | Descending | Anterior funiculus | In midbrain | Head/neck reflexes to visual stimuli |
Source: Gray's Anatomy for Students, Neuroanatomy section (eFigs. 9.39-9.47)
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