Motion sickness is thought to occur when there's a disconnect between the systems the body uses to orient itself in space. These include the inner ear, the visual system and specialized nerve cells called proprioceptors. However, these three components may not have equal importance. Blind people can develop motion sickness, so input from the eyes is not critical. The labyrinth, by contrast, appears to be essential--motion sickness does not occur without a functioning labyrinth.
Anatomy 101: the Inner Ear
In addition to the cochlea, the shell-shaped organ that contains nerve cells for hearing, the inner ear includes a structure called the labyrinth. This consists of three fluid-filled tubes called semi-circular canals, which tell the brain the direction of motion, whether side to side, up and down or forward and back. It's also sensitive to gravity and acceleration.
Anatomy 101: the Visual System
The eyes transmit information via the optic nerves and optic tracts, with signals ending up in the occipital cortex at the back of the brain. The visual system monitors the direction of motion, like the labyrinth, but also tells the brain where you are relative to your environment---for example, whether you're right-side up or not.
Anatomy 101: Proprioceptors
These are nerve cells in the skin, muscles, tendons and joints that sense which parts of the body are up or down, touching the ground or other surfaces, or moving. They include stretch receptors in muscle, golgi tendon organs in tendons and pacinian corpuscles in skin, and sense small changes in position, pressure and the like.
What Goes Wrong
It's thought that motion sickness arises when the brain can't make sense of the various inputs it's receiving. During voluntary, intentional movement, the brain is in charge and things are coordinated, but this doesn't happen when the plane you're in suddenly hits a patch of turbulence.
What to Avoid
Complex movement, like amusement park rides that spin, tilt and go up and down all at once, is a setup for motion sickness. Reading or sitting in a seat facing backward while traveling, or consuming alcohol or foods that aren't easily digestible beforehand, can increase the risk of motion sickness. Greasy or spicy foods, strong smells and smoking are triggers for some people.
What to Do
Try to see the same thing your body and inner ear are feeling. Look at the horizon when you're on a boat, or into the distance when in a train or car. Sitting where there's the least motion helps, too. In a plane this is generally near the wing. In a car, the front seat is better for passengers (driving is best). Keep your head still, and adjust vents to blow cool air on your face. If you're prone to severe motion sickness, talk to your doctor about preventive medication before your next trip.