Cerebral hemispheres' control of opposite sides of the body

  • Thread starter grosquet
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I am curious to find out why the 2 parts of the brain control the opposite sides of the body. This is the best I’ve found so far:

“For some as-yet-unknown reason, nearly all of the signals from the brain to the body and vice-versa cross over on their way to and from the brain. This means that the right cerebral hemisphere primarily controls the left side of the body and the left hemisphere primarily controls the right side. When one side of the brain is damaged, the opposite side of the body is affected. For example, a stroke in the right hemisphere of the brain can leave the left arm and leg paralyzed.“
( from http://www.ninds.nih.gov/disorders/brain_basics/know_your_brain.htm)

Does anyone know how far back this goes? I mean, did it get started with the most primitive vertebrate?


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Through both vertebrate and invertebrate nervous systems, there are many sets of fiber that cross the mid-line to the opposite side of the body. However not all fibers do this. This at least some crossing fibers are required to coordinate the opposite sides of the body.
An example is during walking, one legs goes forward as the other goes back. This requires anti-coordinating the motor neuron pools for leg muscles.
At higher levels, signals cross to generate avoidance responses, such as bending the body to move away from a loud noise, vibration, or touch.
Visual inputs from the retina to the midbrain (lateral geniculate or optic tectum) cross the body to the opposite side.
In frogs, the optic tectum receives also inputs from other sense modalities and brain areas and is involved in some orientation reflexes and startle reflexes, which are carrier out by via to motor and motor control areas in the hindbrain and spinal cord. some of these will be crossed fibers (to the other side) and some not.
In mammals, where cortex is greatly expanded, with new functions. The visual areas receive large inputs from thalamic visual areas on the same side.

The nervous system is mix of crossing and non-crossing fibers and assessing why a particular step is on one side and not the other is a difficult problem, but it does not seem to me to be surprising.

A related issue might be the orientation (top-bottom or nasal-temporal in the visual field axis vs. dorsal-ventral or medial-lateral in the tissue) of the visual field in the nervous system vs. in the real world. The representation in the nervous system does not directly match the external sensory sources, but this does not matter functionally, since the nervous system can use similar transformations to generate the appropriate movements.

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