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Medical Causes of Motion Sickness

The most commonly accepted explanation is the “sensory conflict theory”, known medically as the Neural Mismatch Theory.

Put forth by Reason and Brand (1975) and supported by later studies, this theory suggests that motion sickness is triggered by conflicting and confusing messages to the brain about the body’s position, movement, and balance.

The Anatomy

  Inner Ear (vestibular system)
Your internal “gyroscope” for balance and movement

Eyesight signals
Your position and orientation in space

Spinal Cord & Peripheral Nervous system

Sensory receptors (proprioceptors) in skeletal muscle tissue, tendons, & joints

Medulla (lower brain stem)
CTZ (Chemoreceptor Trigger Zone)
Vomiting Center

Vagus Nerve
Sends signal to your stomach that produces discomfort, then nausea


Nerve Fibers
Small diameter (unmyelinated) Nerve Fibers – carry signals slower
Large diameter (myelinated) Nerve Fibers – carry signals faster

The Physiology

Your brain receives information about your body’s position, balance, equilibrium, and movement from three primary sources:

  1. The Eyes (Visual system)
  2. The Inner Ear (Vestibular System)
  3. Sensory receptors (Nervous System) in skeletal muscle tissue, tendons, and joints, called proprioceptors

The Eyes sense our location, orientation, and if things are moving. The Semi-Circular Canals in the inner ear sense if we are moving, and if we’re balanced. The Proprioceptors under the skin tell the brain (by sensing pressure) if we are standing, sitting, leaning against something, etc. - When everything’s fine, all three are sending the same message to the brain, and all the neural signals “match”.

But when there is a prolonged “mismatch” between these three signals, for instance when the eye perceives no motion but the inner ear senses you’re dropping (air turbulence) or accelerating forward (a car or train), the brain receives conflicting messages about the body’s equilibrium.

More over, neural signals from these three systems will reach the brain at slightly different times. Signals from the eyes, for instance, travel to the brain quicker than signals from the inner ear. This is because large diameter nerve fibers carry neural signals much faster than small diameter nerve fibers. -- So when motion sickness conditions occur, the brain receives an “OK” signal, then a moment later receives a “Not OK” signal. And a moment later receives an “OK” signal, etc. It’s no surprise that sensory conflict and confusion is the result.

The brain’s response to this confusion takes place in the Medulla (lower brain stem) which contains the “Chemoreceptor Trigger Zone”, and a part that’s actually called “The Vomiting Center” (I wonder what that does?). – As the “mismatch” of signals builds up in the brain, it affects the Chemoreceptor Trigger Zone, which sends information to the Vomiting Center. If the mismatched signals persist, the Vomiting Center will send sickness-inducing signals through the Vagus Nerve to the stomach.

The result is nausea (and often vomiting) – the classic symptoms of what’s called “Motion Sickness”.

Academic Citations:
Reason JT, Brand JJ. Motion sickness. London, Academic Press, 1975.

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