Pyramidal (motor) decussation at the cervicomedullary junction
Decussation · 85-90% cross cervicomedullary jn
At the junction of the medulla and spinal cord about 85 to 90 per cent of pyramidal fibres cross to the opposite lateral funiculus as the lateral corticospinal tract; the remaining 10 to 15 per cent continue uncrossed as the anterior corticospinal tract. This is the reference point for the laterality of weakness.
Traced from the start
- Primary motor cortex (precentral gyrus, Brodmann area 4)
- Upper motor neurone axon: corona radiata, posterior limb of internal capsule, crus cerebri, basis pontis and medullary pyramid
- Pyramidal (motor) decussation at the cervicomedullary junction
Detail
- Crossing
- 85-90% cross to form the lateral corticospinal tract
- Uncrossed
- 10-15% continue as the anterior corticospinal tract
- Level
- Cervicomedullary junction, caudal to the sensory decussation
When it goes wrong
UMN lesion above the decussation (cortex, internal capsule, brainstem)
Contralateral hemiparesis
UMN lesion below the decussation (spinal cord)
Ipsilateral UMN weakness below the level of the lesion
Practise this structure
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