MadSci Network: Neuroscience
Query:

Re: How do you explain the phenomenon of reflex enhancement??

Date: Fri Sep 24 11:22:58 1999
Posted By: Brenda Hefti, Grad student, Neuroscience Pgm/Physiology Dept
Area of science: Neuroscience
ID: 936754223.Ns
Message:

This question stumped me for a little while!  As for me, I kind of like to 
be puzzled...I guess I'm in scientific research for a reason.

Your question of reflex enhancement used the specific example of clenching 
your teeth.  I looked and looked through both my texts and some medical 
texts for some reason why clenching your teeth might enhance the reflex at 
your kneecap, but I couldn't find any good evidence for how this could 
occur.

Then I tried it myself.  When all else fails -- repeat the experiment!

Maybe anyone who's curious about the answer should try it too.  Clench your 
teeth -- if this was done in front of a classroom, chances are that the 
person put on a good show for the class and did it really hard, so you 
clench your teeth hard, too.

What do you feel?

Well, when I did it I noticed that my whole upper body tensed -- especially 
my neck and shoulders, and my legs tensed, too.  Clenching your teeth hard 
is difficult to do without tensing other body parts.

So in order for me to give you a satisfactory answer to your question, I'm 
going to assume that the person's leg muscles were somewhat tensed when 
the experiment was repeated.  I've seen this same experiment, and the same 
result was gotten when the person was told to try to prevent the reflex -- 
the reflex got bigger (presumably because they were flexing their leg 
muscles to try to prevent the "kick").

The patellar reflex is a stretch reflex.  It isn't caused by hitting your 
patella (kneecap), it's caused by hitting the tendon just below the 
patella.  When this tendon is hit, it and the muscle it is attached to is 
quickly stretched, and the nervous system responds by contracting the 
muscle.  The trigger for the reflex is the sudden change in muscle length 
that is caused when you hit the tendon attached to the muscle.  There is a 
special sensory organ in the muscle called a muscle spindle which tells the 
spinal cord and brain how long the muscle is (whether is is long -- 
relaxed -- or short -- contracted).  It is not very active when the muscle 
is relaxed.  When the tendon is hit, it senses the muscle's change in 
length.  It sends that information to the spinal cord as a sudden increase 
in activity.  This information is sent directly to a motor neuron which 
contracts the muscle to try to compensate for this sudden change.  That 
contraction makes your leg kick.

When the muscle is already slightly flexed, the muscle spindle is already 
more active.  So when the tendon is hit, the signal sent from the muscle 
spindle to the motor neuron is bigger, and therefore, the motor neuron's 
reaction to the signal is bigger, the muscle contracts more, and the kick 
you see is increased.

I realize that I based my answer on a question you only indirectly asked, 
but I think it is the most likely explanation for what you described.  I am 
fairly confident that clenching your teeth alone, without tensing your 
body, will not increase this reflex.  

Or, I could be wrong.

Try it again, but try to use better controls for your experiment.  Make an 
effort not to tense your whole body when you clench your teeth.  As another 
control, try it while clenching your fist instead.  Make sure, also, that 
you hit the tendon with the same force every time.

Let us know what you find, and if you still see this increase in the 
patellar stretch reflex, I will look into it further.

Good luck,

Brenda




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