decussation

Decussation refers to the anatomical crossing of neural pathways from one side of the nervous system to the other. This crossing occurs at various levels of the brain and spinal cord and helps explain why sensory and motor information often projects to or controls the opposite side of the body.

Key examples include:

- Pyramidal decussation: at the caudal medulla, fibers of the corticospinal tract cross to the opposite side, so voluntary motor control is generally contralateral to the brain hemisphere that initiates it.

- Dorsal column–medial lemniscus pathway: fibers carrying fine touch and proprioception ascend on the same side of the body and cross in the medulla (internal arcuate fibers) to form the medial lemniscus, resulting in contralateral perception of these modalities.

- Spinothalamic tract: pain and temperature fibers cross in the spinal cord near entry, usually within one or two segments, and then ascend contralaterally.

- Optic chiasm: nasal retinal fibers cross at the optic chiasm, causing partial decussation of the visual fields and contributing to contralateral visual field representation.

The term decussation reflects a functional pattern where information or commands cross to the opposite side. Clinically, lesions above a decussation typically produce contralateral deficits, whereas lesions below it yield ipsilateral deficits for the affected modalities. Decussation patterns vary by tract and are essential for interpreting neuroanatomical localization in clinical practice.