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|2. The [sarcomere] length-tension relation|
The above section describes how passive and total (& hence active tension) varies as a function of the length of the muscle. Maximal active force is generated at lengths close to the resting length of the muscle. In the 1970's workers succeeded in dissecting out small bundles of viable muscle fibres that enabled them to set the length of the fibres, accurately measure the sarcomere length and record the passive and total force generated at each length. As you can probably imagine, this resulted in a similar length-tension relation, but this time the x-axis units will be in microns. You should recognise the graph as one used in the lectures:
|Important! Tension measured as a function of sarcomere length||Top|
Having a measurement of sarcomere length with which to correlate with passive and total tension (& thereby the active tension), enabled workers to corroborate the sliding filament theory and estimate very accurately the length of the thick and thin filaments.
As part of your course you are required to be able to understand the structure of the sarcomere and to be able to interpret the graph in figure 4 in order to calculate the lengths of the thick and thin filaments.
To do this you may need to sketch out the sarcomere & consider where the actin and myosin filaments are within the sarcomere, before reading further.
To help you to understand the problem and how to solve it, I've provided additional graphs based on figure 4 but with cartoons of a stylized sarcomere superimposed on the graph. Remember two things here:
It should be obvious from the cartoon that the actin filaments do not overlap the myosin filament - no cross bridges can form and so no contraction is possible. The arrow from the right hand z-disk is projected down to enable you to easily read off the sarcomere length at this point and also on down to illustrate on the graph that no active force is generated by muscle fibres at such extreme lengths.
|Maximal force at resting sarcomere length!!!||Top|
The muscle has a narrow range of optimal lengths over which tension does not vary with length and active force generation is maximal. The reason for this is illustrated by the next figure:
Notice that maximum force is achieved at lengths ranging from 2 mm to about 2.35 mm. In life the sarcomere length in a resting muscle is about 2.2 mm.
STOP! It is important that you realize that skeletal muscle in your body, when at resting length, is at its optimal length for development of force. Moreover, sarcomere length is reduced by only about 10% (to a little under 2 mm) by a maximal shortening of the muscle. So, even after the muscle has shortened maximally (determined and limited by the articulation of the joint) the muscle is still at its optimal length.
Your perception that you can exert more force when your limbs are partly flexed reflects the mechanics (mechanical efficiency of levers) of the joint and not the power output of the muscle at different lengths.
|What happens at short sarcomere lengths?||Top|
The remaining portion of the length-tension relation tells us that active force declines as the sarcomere becomes shorter than this 'optimum' length. Why?
The answer is not so easy to visualize. At moderately short lengths it seems that the ends of the actin filaments, which are 1 m m in length begin to meet in the middle of the sarcomere. In some text-books this is said to impare the proper association of myosin with the appropriate actin filament. Any cross bridges that form between the myosin filament and the wrong filament will result in wasted effort as the movement of the myosin head will not act to shorten the sarcomere. Thus, active stress falls away progressively as the sarcomere is made shorter and shorter. This is the interpretation I tend to favor.
At a point however the rate at which tension declines with ever decreasing sarcomere lengths changes abruptly. The next figure illustrates the probable cause for this.
Although it is perhaps not completely obvious from the cartoon, one can imagine that at some point the myosin filament and the Z-disks that delimit the sarcomere will collide. The myosin filament is 1.6 m m in length. Look again at figure 7. The inflection occurs at almost exactly 1.6 m m! This was indeed the evidence that was used to suggest the length of the thick filament. It has since been measured directly using electron microscopy and these measurements also fix the length of the thick filament to be 1.6 m m.
There is still some active force generated at still shorter sarcomere lengths. How can this be?
There is a lot of energy being provided by all the myosin heads and this is able to deform and buckle the thick filament to some extent. There is a limit to this, however, and active force falls to zero when the sarcomere is about 1.2 m m in length. The stiffness of the thick filament is resisting any further shortening and none of the energy from the cross bridges is being done on the external load, the transducer.
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|Copyright © 1998 University of Bristol. All rights reserved.
Author: Phil Langton
|Last modified: 30 Nov 1999 23:17
Authored in CALnet