A vital part of the human body, the muscle enacts movement with the environment, as well as internal function in digestion, blood flow and respiration. The best example to understand muscle structure comes from the muscles that make us move.
These skeletal muscles are connected to the skeleton (with tendons). They form part of the mechanical system that moves the limbs and other parts of the body. The structure of a muscle is ordered in a decreasing hierarchy, best described alongside a diagram:
Skeletal muscles, such as the biceps above, are made up of many neighboring muscle-ﬁber bundles – bound together by connecting tissues. Each bundle contains a few single muscle ﬁbers, which are cells with their own (multiple) nuclei – along a plasma membrane.
The muscle ﬁbers themselves are made up of many parallel and elongated contractile threads known as myoﬁbrils. These myoﬁbrils are made up of two, even smaller, threadlike sub-units: thin ﬁlaments (consisting of coiled-up actin molecules [two] and two other regulatory proteins) and thick ﬁlaments (made up of many staggered myosin molecules) – together, these two polymers are grouped as myoﬁlaments. (Campbell & Reece, 2005) Their organization is orderly, and repeated under a unit called ‘sarcomere’ – beginning and ending at each Z-line, as marked on the diagram (and microscope-image) above.
To summarize, another word for skeletal muscle is striated muscle, which is deﬁned as “muscle tissue in which the contractile ﬁbrils in the cells are aligned in parallel bundles, so that their different regions form stripes visible in a microscope [as in the photo above]. Muscles of this type are … under voluntary control”. (“Striated Muscle”, 2007) The contractile action of the muscle is controlled by an overlapping (and non-overlapping) and movement of the myoﬁlaments.
– Campbell, N.A., Reece, J.B. (2008) Biology, 8e Int’l. San Francisco: Pearson/Benjamin
– “Striated Muscle” (2007) New Oxford American Dictionary. Macintosh OS X Dictionary,
Version 2.0.2 (51.4).