MadSci Network: General Biology
Query:

Re: why do we have muscle spasms?

Date: Fri Jun 29 09:20:37 2001
Posted By: David Burton, Post-doc/Fellow, Physiology, University of Oxford
Area of science: General Biology
ID: 990030709.Gb
Message:

Hello Josh,

The types of muscle spasms that you are referring to are caused by a 
repetitive twitching of the muscle.  These can usually be stopped quite 
easily simply by using the affected muscle.    To be precise I will only 
talk about muscles that one has conscious control of the movement, which 
are called skeletal muscles.  The body contains many other non voluntary 
muscles called cardiac (heart) and smooth muscles but I will not talk about 
these since they have different regulatory systems and will only help to 
confuse the issue.

In the absence of any conscious stimulation the tension in skeletal muscles 
should remain constant at all times.  In the body skeletal muscles do not 
relax completely and under resting conditions there is a small amount of 
tension but this is constant.  Muscles are activated by electrical impulses 
that travel down nerves to the muscle.  When an impulse reaches the muscle 
it stimulates a contraction.  The twitching that you experienced in your 
eye is caused by a repetitive electrical impulse being conducted by a nerve 
that stimulates the muscle repeatedly.  For details on how nerve impulses 
stimulate muscle contraction you may like to look at the answer to ano
ther Mad Scientist question but I will summarise this briefly.

A single nerve impulse will result in a single quick twitch of a muscle.  
Repetitive impulses will result in repetitive muscle twitches.  If the 
impulses are close enough together the following twitch may be larger than 
the previous one, and this can go on up to a rate of simulation where the 
twitches all combine together to form a tetanus, which is what we are 
normally aware of as a muscle contraction.  So the spasms that you 
experience are repetitive stimulations by a nerve that are not close enough 
together to result in single contraction and are experienced as a 
repetitively twitching muscle.

These twitches are very common and not normally an indication of anything 
wrong.  Occasionally they can become problematic for people and this is an 
indication of an underlying condition that causes these tremors.

What makes the nerve rapidly fire in this manner is not known in all cases 
but in more severe cases of muscle tremor underlying causes have been 
identified.  In a syndrome known as hemifacial spasm the muscles on one 
side of the face undergo this tremor due to the facial nerve being trapped 
and squeezed.  The squeezing of this nerve results in the rapid firing of 
electrical stimuli to the muscle resulting in spasm.

Another form of painful muscle spasm is called reflex muscle spasm.  This 
is caused by disease in the spinal chord or joints.  One common reflex you 
may be aware of is the knee jerk or patellar reflex, where one strikes a 
tendon just below the knee resulting in a lower leg reflex.  This reflex 
action occurs by the activation of a nerve circuit that starts with a 
stretch receptor in the tendon below the knee, goes via the spinal chord to 
the thigh muscle to stimulate contraction.  There are many of these reflex 
circuits in the body and if one of these gets repetitively stimulated due 
to disease it can become painful.

However I should point out that the occasional tremor in the eye or leg is 
nothing to worry about and is not an indication of anything wrong.  Only 
when a tremor becomes persistent and painful is it a problem.

Thank you for your interesting question, and good luck in the future.

Dave Burton





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