Motion Sickness
(Sea Sickness, Car Sickness)

• Motion sickness is caused by a disturbance of the inner ear that affects the organ of balance and equilibrium.
• The symptoms of motion sickness usually, but not always, stop when the motion that causes it ceases.
• Medications are available for the prevention and treatment of motion sickness.

Motion sickness is a very common disturbance of the inner ear that is caused by repeated motion such as from the swell of the sea, the movement of a car, the motion of a plane in turbulent air, etc. In the inner ear (which is also called the labyrinth), motion sickness affects the organs of balance and equilibrium and, hence, the sense of spatial orientation.

Motion is sensed by the brain through three different pathways of the nervous system that send signals coming from the inner ear (sensing motion, acceleration, and gravity), the eyes (vision), and the deeper tissues of the body surface (proprioceptors). When the body is moved intentionally, for example, when we walk, the input from all three pathways is coordinated by our brain. When there is unintentional movement of the body, as occurs during motion when driving in a car, the brain is not coordinating the input, and there is thought to be discoordination or conflict among the inputs from the three pathways. It is hypothesized that the conflict among the inputs is responsible for motion sickness.

For example, when we are sitting and watching a picture that depicts a moving scene, our vision pathway is telling our brain that there is movement, but our inner ear is telling our brains that there is no movement. Thus, there is conflict in the brain, and some people will develop motion sickness in such a situation (even though there is no motion).

The cause of motion sickness is complex, however, and the role of conflicting input is only a hypothesis, or a proposed explanation, for its development. Without the motion-sensing organs of the inner ear, motion sickness does not occur, suggesting that the inner ear is critical for the development of motion sickness. Visual input seems to be of lesser importance, since blind people can develop motion sickness. Motion sickness is more likely to occur with complex types of movement, especially movement that is slow or involves two different directions (for example, vertical and horizontal) at the same time.

The conflicting input within the brain appears to involve levels of the neurotransmitters (substances that nerves within the brain use to communicate with one another) histamine, acetylcholine, and norepinephrine. Many of the drugs that are used to treat motion sickness act by influencing or affecting the levels of these compounds within the brain.

Source: MedicineNet.com
http://www.medicinenet.com/motion_sickness/article.htm