Facilitation, Inhibition and Rebound-Effects

Involved in how cells respond

During the time I worked for my PhD in Neurobiology at the Australian National University 40  years ago under the guidance of Professor G.A. Horridge F.R.S., I really enjoyed electrophysiology. One needed a lot of patience, sophisticated, equipment and a certain amount of skill to successfully penetrate a photoreceptive cell or a neuron in the brain of an insect and to place the tip of an electrode in it to record the cell’s response to a brief flash of light. Of course it depended very much on the species one experimented with and because it was not exactly easy to achieve to get into the cells of the carcass beetle or sawfly larvae that I worked with (and there could be several days when I got no result at all), the moments I did manage to hold a cell long enough to check all kinds of parameters like absolute, spectral and polarization sensitivities, were always “eureka” events and gave me great satisfaction.

The photoreceptive cells of the insect’s retina always responded with a depolarization to a flash of light: a small depolarization if the light was either dim or if it was of a colour that the insect possessed no visual pigment for. However, bigger depolarizations occurred if the stimulating light consisted of a brighter flash or a colour that the insect eye could perceive. I was surprised that sometimes I recorded from cells in which the second flash of the stimulating light gave a larger response than the one that had occurred in response to the preceding (earlier) flash. Of course, this could have been an artefact with the electrode being somewhat better positioned during the second flash. But it could also have been ‘facilitation’, in which the first flash made it easier for the cell to respond the second time it was stimulated. An initial stimulus causing a follow up stimulus to be responded to more promptly, faster, or stronger can physiologically be explained in several ways (involving membrane properties, ion transfers, intracellular cellular messengers, etc.)  For whole organisms one could argue that facilitation means that one organism’s survival can profit from the presence or actions of another organism. A large number of salmon in a small holding tank, for example, will facilitate the spread of fish lice from fish to fish.

Deeper in the insect’s head, further away from the eye towards the brain, some cells I recorded from reacted differently. These cells were producing regular spikes of a specific height that would not change at all even if the stimulus intensity was changed. It was the number of the spikes (i.e. the frequency) that increased in response to a bigger stimulus or decreased when the stimulating light was made dimmer. However, I also came across some cells that totally stopped producing spikes for the duration of the stimulating flash of light hitting the eye. Obviously, the flash of light was inhibiting the cells’ production of neuronal spikes. When the light was off and it became dark again, the train of spikes re-appeared. But not only that: for a short while following the inhibition period, the number of spikes drastically exceeded the normal rate of spikes. It was as if the cell ‘knew’ it had to ‘catch up’ what it had missed during the inhibition period. The effect to compensate for what had been missing during the inhibition period is called the “rebound effect”.

This so-called rebound can of course also be observed in whole animals and is not restricted to cells responding to light.  If, for example, you always sleep for 7 hours and then for whatever reason are forced to only sleep 4 hours one night, you will sleep more than your normal quota the next day. Or, another example, if your dog gets a regular bowl of dog food every evening, but you forget to feed it one day, then the doggie will eat a lot more the day after it had to starve. A response increases when it had paused for a while. Rebound can also mean that symptoms re-emerge after they seemed to have disappeared and this is particularly important (and annoying) when it comes to certain diseases, allergies, etc. However, when after a sad phase in life, happiness returns the rebound effect can be wonderful and does not need a physiological explanation   –  although there is one!

© Dr V.B. Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com, 2021.
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