Hey all:
I was wondering if someone could explain frequency summation (muscle) to me.
TPR explains it as having "the amount of time between successive stimulations that must be greater than the duration of the refractory period, but brief enough so that the sarcoplasmic [Ca2+] has not been returned to a [low]."
I'm hoping someone can explain this in a better and simpler way. I'm just not understanding the "time between successive stimulations."
Thanks!
I think that the question (I wish that you had posted it) above is getting at frequency summation in a single muscle fiber. Here is a good blurb from the follwing website which answers your specific question,
http://people.eku.edu/ritchisong/301notes3.htm. This website is excellent for reviewing skeletal muscle, btw.
"Wave Summation - an increase in the frequency with which a muscle is stimulated increases the strength of contraction. This is illustrated in (b). With rapid stimulation (so rapid that a muscle does not completely relax between successive stimulations), a muscle fiber is re-stimulated while there is still some contractile activity. As a result, there is a 'summation' of the contractile force. In addition, with rapid stimulation there isn't enough time between successive stimulations to remove all the calcium from the sarcoplasm. So, with several stimulations in rapid succession, calcium levels in the sarcoplasm increase. More calcium means more active cross-bridges and, therefore, a stronger contraction."
----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
As related to a complete muscle (by me):
Multiple fiber summation and frequency summation are the reasons why every time you pick up beer you don't contract your biceps with the same amount of force that you use when you do dumbbell curls, which would result in tossing your beer over your shoulder and undoubtedly splashing it all over the biggest guy in the bar--wham bam, pow pop, blackeye! Your body arranges the muscle fibers (muscle fiber = muscle cell = myocyte) into motor units--groups of fibers that are always activated simultaneously when they receive stimulation from a motor nerve (a nerve that controls muscle contraction).Different motor units have different sizes (more or less fibers) and different sensitivities to stimuli from their motor nerve--the small motor units (the weaker ones) are activated first and greater stimuli (e.g. a blast from the motor nerves that your brain commands when it tells your arm to lift a dumbbell) recruit the larger, and then the largest, motor units = weak contraction when that's all you need and strong contraction when you're picking up something heavy.
So that's mechanism #1 and it's called multiple fiber summation -- more stimulation recruits more (and larger) motor units. To understand frequency summation you need to appreciate that the contraction of a muscle fiber is really fast -- the calcium ions from the sarcoplasmic reticulum that result in the sliding of the filaments (recall: thin filaments = actin; thick filaments = myosin -- review the sarcomere because it is high yield for the MCAT) only hang out in the sarcoplasm for a fraction of a second! But anything that you use a muscle for generally lasts longer than milliseconds, even lifting a beer, so frequency summation is needed and this is how it works. Frequency summation is actually pretty simple--the motor nerves that simulate the motor units (and the muscle fibers/cells/myocytes that compose the motor units) fire over-and-over again, causing the sarcoplasmic reticulum to keep releasing calcium. Eventually, you reach a point where the firing of the nerves causes calcium to be released at a rate that is faster than the rate of the active pumping of that calcium back into the sarcoplasmic reticulum. The result is a high enough concentration of calcium in the sarcoplasm to result in sustained contraction even between action potentials from the motor nerve -- tetany.
I think that the question (I wish that you had posted it) above is getting at frequency summation in a single muscle fiber. Here is a good blurb from the follwing website which answers your specific question,
http://people.eku.edu/ritchisong/301notes3.htm. This website is excellent for reviewing skeletal muscle, btw.
Wave Summation - an increase in the frequency with which a muscle is stimulated increases the strength of contraction. This is illustrated in (b). With rapid stimulation (so rapid that a muscle does not completely relax between successive stimulations), a muscle fiber is re-stimulated while there is still some contractile activity. As a result, there is a 'summation' of the contractile force. In addition, with rapid stimulation there isn't enough time between successive stimulations to remove all the calcium from the sarcoplasm. So, with several stimulations in rapid succession, calcium levels in the sarcoplasm increase. More calcium means more active cross-bridges and, therefore, a stronger contraction.
Hope this helps,
Dr. Leonardo Noto
www.leonardonoto.com
