Temporal Summation

Last semester I took a biology class. One day at the laboratory they had us doing some experiments on the reflexes (contraction of the pupil, etc.). I was then trying to perform the I’m sure very familiar procedure of the following picture to my partner.

 

 

It is a very easy procedure, so of course I was really pissed when I could not get it right. No matter how hard I hit her knee, it just would not lift. It was until few days ago that I realize what I was doing wrong.

When I was doing the experiment I was not told about temporal summation. Since I do not want you to make the same mistake whenever you find yourself in the necessity of performing the same procedure, I will explain the concept.

At the beginning of the 20^th century, the scientist Charles Scott Sherrington made an experiment with a dog. He would suspend the dog above the ground and pinch one of its feet. Then he observed its reactions which were to flex the pinched leg and extend the others. Seems easy and basic right? However, from this experiment, Sherrington discovered 3 things and won a novel prize.

He discovered (1)that reflexes are actually slower than conduction along an axon, (2)that few weak stimuli presented at short time intervals or slightly different locations produce a stronger reflex than a single stronger stimulus, and (3)that when a set of muscles becomes excited, another relaxes.

 

He observed that when he would pinch the dog’s foot several times really fast but not hard, he would cause a reflex. He assumed then that it was because the continuous stimuli must accumulate in a single axon causing it to reach its threshold and begin an action potential. He named this temporal summation. He also noticed that pinching in different spots close by in the dog’s foot would also cause a reflex, which means that stimuli in different spaces also accumulates. He named this spatial summation.

 

So, Why did my experiment not work? Because I was trying to produce the reflex in one single blow. I should have tried with a few continuous. Or maybe not. Maybe I am just very untalented for those kind of experiments. Try it yourself! Do the experiment and let me know what was easier: one strong blow, or few weaker hits.