What causes seasickness (motion sickness in general)

Its interesting how sometimes our bodies are still stuck with their age old beliefs and struggle to cope up with the new environments we have put ourselves in. Motion sickness is one such thing where the body's defense mechanism kicks in when the body is put in an unnatural environment like a ship or a car!

Many of us experience at least mild motion sickness which happens mostly on ships, but also in cars, air planes etc. and usually causes the person to vomit. Though it feels like just another sickness, the reasons behind motion sickness are interesting.

Before we delve into what causes motion sickness lets quickly look at the input mechanisms our body uses to determine its own orientation and whats happening around.

1. Vision: The most obvious one. The brain receives signals from eyes which it uses to determine the body's current state - if it is stationary, falling, moving etc. 

2. Proprioceptors: Deep muscle tissues that register stimuli like toning, stretching, contraction etc. These signals are in turn used by brain to determine the current state of the body. 

3. Inner Ear: This is the most important one. While we usually think of ears as sound aids, inner ear has one other important function. The inner ear has some complex mechanisms (fluids and canals) which can detect gravity, orientation and rotation of ones head, linear acceleration etc. In effect, the inner ear can be considered an advanced sensor that tells the brain if the body is moving forward, backward, falling, rotating etc. It is this constant feedback that helps the brain to keep the body in a balanced position.

Cause of motion sickness:
The brain coordinates the information from the above three sources to determine the overall state of the body. The problem arises though, when the brain detects anomalies between the input it receives from these sources. 

Lets say you are travelling in a closed cabin on a ship sailing in not-so-calm waters. The vision input says everything is stationary in the cabin and the brain thinks the body is stationary. However, the inner ear senses motion because the ship is drifting on waters and so the brain thinks it is moving. It is these conflicting signals that trigger motion sickness. (Similarly the other combinations like the vision input saying that the body is moving while the proprioceptors saying the body is stationary etc.)

It is hypothesized that when the brain receives conflicting signals about the state of the body, it suspects neurotoxins in the body and the part of the brain (postrema) responsible for triggering vomiting when the body is poisoned kicks in. The brain also puts the body in defense mode and suspends many normal body activities like digestion. 

Mitigating motion sickness:
Given this knowledge it is an easy guess how to mitigate motion sickness. One needs to convince the brain that there is no dissonance and ask the brain to take a chill pill. In the sea sickness example mentioned above, the solution would be to stay on the deck of the ship where the eyes can also detect motion or at least stay in rooms with windows that show the horizon. This would save the brain from the conflicting signals and resume normal body activities. 

Did the simple science for kids in this post make you a little smarter?
If you have any ideas / questions you want to learn about, please leave a note in the comments section. 
Would be glad to know if there are any inaccuracies in the article.

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