A model for the immediate adaptation of the nervous system to changes in gravity
Author: Florian P.M. Kohn, Claudia Koch and Ramona Ritzmann.
~Christian Hoffmann 2/2019
The following model integrates the results from the various experiments that have been carried out in the past decades from cellular level up to the neuromuscular interface. To avoid long-term adaptation processes, only immediate effects have been taken into account. The model was designed in a bottom-up approach, starting at the very base level of gravity dependence.
Therefore, it can be used as a framework for future—more complex data—as long-term adaptation processes and the gravity dependence of for example, the human brain.
Micro- and hyper gravity change the biophysical properties of biological membranes in every cell in the body. This is not due to some biological effect or process, it is a change in thermodynamic properties of biological membranes; therefore, this can be seen as the basic principle of how gravity affects cells as neuronal cells, for example.
The Effect of Gravity on the Nervous System
On Earth, it is well known that the properties of membrane-integrated proteins as ion channels depend on the physical state of the membrane. Lateral pressure or membrane fluidity is an important component, for example, the open state of alamethicin pores clearly depends on the lateral pressure of the membrane, and the pore activity increases with an increased lateral pressure. An increased lateral pressure can be interpreted as decreased membrane fluidity.
This was also shown for other ion channels, for example, the closed-state probability of nicotinic acetylcholine receptor channels increases (the open-state probability decreases) toward decreased membrane fluidity.
The pore activity of alamethicin and the open-state probability of ion channels is also gravity dependent. In microgravity, the open-state probability decreases, whereas in hyper gravity, it increases.
As membrane fluidity is affected by gravity and due to the fact that ion channels are affected by membrane fluidity, the first part of the model can be described as follows: In microgravity, the membrane fluidity is increased. This changed membrane fluidity decreases the open-state probability of ion channels. This effect is inversed in hyper gravity: membrane fluidity decreases and the open-state probability of ion channels increases (Figure 1).
It was shown that cells slightly depolarize in microgravity—the membrane potential gets more positive—and they hyperpolarize in hyper gravity. With a light depolarization of the resting potential, the threshold to trigger action potentials is reached more easily. This effect was demonstrated in spontaneous active leech neurons. The rate of APs increased in microgravity.
With these findings, the model of gravity dependence on the molecular level can be extended to explain the cellular gravity dependence of single (neuronal) cells (Figure 2).
For further information on this research please feel free to read following:
Effects of gravity on the Nervous system