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1. The sarcomere Logo

Understanding the organisation of striated muscle at the level of the sarcomere and the contractile proteins that give rise to sarcomere striations is crucial if one is to understand how striated muscle functions and to appreciate the important differences between skeletal and cardiac and smooth muscles.

The following sections underscore the important structures and their interrelationships. I am grateful to Dr Mike Fenenczi (Mill Hill) for the use of several graphic files that are reproduced here with permission.


The sarcomere Top

As you should know by now, skeletal muscle as seen under the light microscope has a striated appearance, as seen in the picture of a myofibril:
A single myofibril dissected from a muscle fiber. The myofibrils are about 1 micrometer thick,

Figure 1 - A single myofibril dissected from a muscle fiber. The myofibrils are about 1 micrometer thick,




It turns out that the highly ordered structure of striated muscle that contributed significantly in our (Huxley and contempories) attempts to understand the basic cellular mechanism that underlay muscle contraction.

As you can see in the next few figures, there is a very close correlation between the striations, that one can see with instruments not much more sophisticated than the class lab microscopes, and the appearance and organisation of the contractile proteins themselves.



Image of microfibril with cartoon representation of contractile proteins overlaid

Figure 3 - Image of microfibril with cartoon representation of contractile proteins overlaid
cartoon labelled with structures you should recognise (sarcomere is Z-line to Z-line)

Figure 4 - cartoon labelled with structures you should recognise (sarcomere is Z-line to Z-line)


The filaments Top

The striated appearance of is due to the arrangement of actin (thin filament; green) and myosin (thick filament; orange) and the fact that the thick and thin filaments have different refractive indices in the light microscope (remember the why the bands are called A and I). The myofibrils are also birefringent. The dark bands in the micrograph represent regions of overlap between the thin and thick filaments. The Z-line which ties the thin filaments together can also be seen. Stretching to scale two of the sarcomeres above shows the relationship between the bands and the filament arrays:


A sarcomere from a mammalian muscle is about 2.4 micrometer long at rest. It can be extended reversibly to more than 3 micrometers, and it can shorten to less than 2 micrometer. The appearance of the striations change during shortening allowing us to examine the relationship between active force and sarcomere length (see self assessment exercises). The shortening of the sarcomere is achieved by the actin and myosin filaments sliding over one another. That sliding movement of the sarcomere is shown below:
Animation of sarcomere showing overlap of filaments

Figure 5 - Animation of sarcomere showing overlap of filaments


The contractile proteins Top

This area does not tend to give you problems and I haven't written this section up accordingly. If you are particularly interested or you can't get enough of this sort of thing, you could take a look at Mike Ferenczi's excellent Web pages: Mike Ferenczi's Web Site (Mill Hill, London)

For some of you it is difficult to visualise how this all comes together to produce contraction of a muscle cell. Is this is the case you may wish to view the video clip below. It shows contraction of an isolated cardiac ventricular myocyte. The cell has obvious sarcomeric striations which, if you look carefully, appear to bunch up as the cell contracts ie. the sarcomeres shorten.

video clip of contraction heart cell

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Copyright © 1998 University of Bristol. All rights reserved.
Author: Phil Langton 		Last modified: 9 Jun 1999 20:12
Authored in CALnet