The General Anatomy of a Skeletal Muscle Help
Introduction to The General Anatomy of a Skeletal Muscle
We have been considering the specific names and appearances of various skeletal muscles. But there is also a general anatomy that practically all skeletal muscles share.
Figure 14.5 includes examples of fascia ( FASH -ee-uh). A fascia is a thin “band” (fasci) of fibrous connective tissue surrounding or penetrating a muscle. Consider, for instance, the epimysium ( ep -uh- MIS -ee-um), which is a sheet or band of fascia that is “present” (-urn) “upon” (epi-) the entire “muscle” (mys). [ Study suggestion: Look at a raw piece of chicken. Peel the skin back slightly from the flesh, and you will see a milky looking membrane lying upon the meat. What is the name of this membrane?]
Cutting the muscle in half, we can view the perimysium ( pair -uh- MIZH -ee-um). The perimysium penetrates deeply into the muscle organ and subdivides it into fascicles ( FAS -uh-kuls) – “little bundles” of muscle fibers that are surrounded by sheets of fascia. The perimysium, therefore, is the fascia present “around” (peri-) each bundle or fascicle of muscle fibers.
Finally, the endomysium ( en -doh- MIZH -ee-um) is the fascia present “within” (endo-) each bundle or fascicle, and between its individual muscle fibers. You will note from Figure 14.5 that the skeletal muscle fibers, themselves, are striated ( STRY -ay-tid) or “furrowed,” that is cross-striped with blackish lines.
Structures Within The Muscle Fibers
An important question for us to ask is, “Okay, but so far we haven’t learned how the internal anatomy of a skeletal muscle explains how it contracts (shortens) and provides the pulling force for body movements.” Ultimately, the answer to this question will require us to examine the inner anatomy of an individual skeletal muscle fiber, which is actually a long, fiber-shaped cell.
As Figure 14.6 reveals, the striated (cross-striped) muscle cell or fiber contains numerous myofibrils ( my -uh- FEYE -brils). The word myofibril literally means “little fiber” (fibril) of a “muscle” (my). Each of these myofibrils is actually a slender, fiber-shaped, cell organelle. The myofibrils have a dark-and-light banding pattern. The dark bands are called the A bands, while the light bands are called the I bands. The striations ( stry - AY -shuns) or cross-stripes of each muscle fiber, then, in reality just represent the dark A bands of their myofibrils, stacked one upon the other to make a stripe.
Within the middle of each light I band is a dark, zig-zagging Z-line. These dark lines mark off a series of sarcomeres ( SAR -koh- meers ). A sarcomere is a short “segment” (-mere) of “flesh” (sarc): that is, a region of myofibril between two Z-lines. Hence, each myofibril organelle within a muscle fiber basically consists of a series of sarcomeres, attached end-to-end.