Re: what are the physiological + neurological effects of motionsickness ?

Dear Friend,

Your question about the neuronal mechanisms underlying motion sickness is 
interesting. Unfortunately, these mechanisms are not known yet. What can 
be done is identify structures of the nervous system that possibly play a 
role in that phenomenon and formulate hypothesis about a mechanism. An 
idea could be that vomiting (either caused by intoxication or motion 
sickness) is triggered by a single mechanism. 

Early studies have identified a region of the brain stem (area postrema) that is 
important in producing vomiting. However, this region is most probably not 
important for motion sickness to occur (Wilpizeski et al., 1986 ; Laryngoscope 
96, 1221-1225). The only structure that seems essential to produce motion 
sickness is the vestibular system. As you probably know, the vestibular 
system is a part of the inner ear that is formed by semicircular canals 
and otolith organs that contain hair cells. People with bilateral 
vestibular dysfunctions are prevented from motion sickness. Hair cells are 
connected to axons that send projections to the vestibular nucleus and 
possibly to the nucleus tractus solitarius. This last nucleus is also 
known to produce vomiting. This pathway (vestibulosolitarius) could be a 
putative route from the inner hear to regions of the brain stem that 
control vomiting. 

There is also the idea that it is the conflict between 
actual experience produced by sensory inputs (ex. visual), vestibular as 
well as non-vestibular (ex. visceral graviceptors) and the experience that 
the person expects on Earth, based on his/her past experience. For 
example, one can feel sick if he/she views a moving visual environment 
without the accompanying vestibular signals (i.e. that head is stable 
whereas it should not be). I believe that this so-called conflict theory 
is interesting since the vestibular nucleus receives both visual and 
vestibular inputs. Finally, there are also many chemical agents as well as 
several other brain structures that may be involved in motion sickness. 
Increasing in vasopressin rate may be correlated with onset of nausea but 
it remains unclear whether this increase can trigger nausea or if it 
occurs in parallel with nausea. Drugs such as antihistamines may also 
reduce motion sickness but what remains to be demonstrated is whether the 
effect is produced by specific action on the receptors of vestibular 
nucleus neurons or through a general suppression of activity of various 
brain cells.

See also a recent review :

Yates et al. (1998), Brain Res. Bull. 47(5) 395-406.

For sex differences, see also :

Collins W.E. & Lentz J.M. (1977) Aviation Space & Environmental Medicine.  
48(7):587-94.

Hope that helps,
Eric