Cerebral Hemispheres and Corresponding Areas on the Body.

[aj47]

Why is it that certain areas of the Cortex on one Cerebral hemispheres will only receive information from the opposite side of the body.

For example if a person has a stroke damaging areas of thier left motor cortex, than movement on the right hand side of the body will be affected. Surely having to cross over requires more neurons giving a slight, but measurable difference in reaction time?

[aj47]

Anyone?