Causes of motion sickness

The vestibular apparatus & maintenance of balance

The basis for motion sickness lies deep within the ear in the vestibular apparatus. The vestibular system is a complex neural system that provides balance and posture related information to the brain.

The system has tiny organs, tubes, fluids and a complex system of nerve channels to the brain. This system helps the brain know the position of the body, its motion relative to the environment and give out signals that help in maintaining a posture without falling. 1-4

The vestibular system consists of the vestibular apparatus that has three strategically placed semi-circular tubes connected with each other. These tubes contain tiny hair like cells on their inner linings. The tubes are further filled with a fluid called endolymph.

As the head moves the fluid moves and stimulats the hair cells. These hair cells are connected to a tiny but extremely sensitive network of nerves. These carry the signals about the motion to the brain.

The vestibular apparatus senses linear acceleration that may be gravitational and inertial (vertical and horizontal movements respectively). The brain has nerve projections into the spinal cord, and back of the brain (cerebellum) that adjusts the body posture in relation to the perceived position.

Apart from information from the vestibular apparatus the eyes also send in information regarding the position of the body and its relative motion by visual cues. The whole circuit of nerves is extremely complex and operates largely without conscious thought.

Pathophysiology of motion sickness

The most accepted theory of causation of motion sickness is that the inputs from the vestibular apparatus and the eyes get jumbled and confuse the brain.

The brain is constantly updated about the body position and movement by the eyes and vestibular system. If for example a person is travelling by car the vestibular apparatus due to lack of movement tells the brain that the body is relatively still, however, the eyes tell the brain that the car is moving.

Vice versa if a person is attempting to read something while moving in a car, the eyes tell the brain that the body is stationary and the bumpy ride reminds the brain via the vestibular apparatus that the body is moving. This mismatch of information can lead to the symptoms of motion sickness, such as nausea and vomiting.