Question

Why is it important that fibers cross over or decussate in the brain stem?

Answer 1

Decussation is an X-shaped crossing of homonymous nerves or bands of nerve fibers, connecting corresponding parts on opposite sides of the brain or spinal cord.

It occurs just before the junction between the medulla oblongata and the spinal cord.

Decussation is the reason that brain injuries and strokes on one side of the head typically cause paralysis on the other side of the body.

The reason behind decussation may be that in the early development of vertebrates, shortly after gastrulation, the anterior forehead region twists anti-clockwise about the body axis, whereas more caudal body part twist clockwise. As a result the fore head region including forebrain, eyes and olfactory organs, is crossed laterally and inverted dorsoventrally.