Sensory pathways
Outline the major somatosensory pathways of the body
Somatosensory information from the periphery is carried to the brain via three major ascending systems. Each pathway is specialised for different modalities of sensation and follows a characteristic 3-neurone chain (except spinocerebellar pathways, which do not reach conscious perception).
The main somatosensory pathways are:
- spinothalamic tracts
- dorsal column–medial lemniscus (DCML) system
- spinocerebellar tracts
| Spinothalamic tract | Dorsal column–medial lemniscus (DCML) | Spinocerebellar tracts | |
|---|---|---|---|
| Peripheral receptors | |||
| Primary function |
|
| Unconscious proprioception |
Receptor types |
|
|
|
| First-order neurone | |||
Fibre type |
| Aβ (thick myelinated) fibres | Aα (thickest myelinated) fibres |
| Conduction velocity | ~20 m/s (Aδ), ~2 m/s (C) | ~40–80 m/s | ~80–120 m/s |
| Cell body location | Dorsal root ganglion | Dorsal root ganglion | Dorsal root ganglion |
Path | Peripheral nerves enter via dorsal root and synapse in laminae I–VI of the ipsilateral dorsal horn | Lower body neurones:
| Synapse in Clarke’s nucleus (ipsilateral dorsal grey matter) |
| Second-order neurone | |||
Decussation | Immediate (in the spinal cord) | Medulla | Dorsal tract does not decussate |
Ascending pathway |
| Medial lemniscus → thalamus | Spinocerebellar tracts → cerebellum |
| Termination | Thalamus + brainstem (PAG, reticular formation, tectum) | Thalamus (VPL) | Cerebellar cortex |
| Conduction velocity | ~40 m/s | ~80 m/s | ~120 m/s |
| Third-order neurone | |||
| Projection | Thalamus → somatosensory cortex | Thalamus → somatosensory cortex | No third-order neurone |
Summary
Somatosensory information is carried via three specialised ascending systems:
- spinothalamic tract → pain, temperature, crude touch
- DCML system → fine touch, vibration, proprioception
- spinocerebellar tracts → unconscious proprioception
Together, they integrate sensation, perception, and motor coordination, forming the foundation of somatic sensory processing.