skeletal muscles have nerves, muscle fibers, blood vessels, and connective tissues. Let’s see: one example of a skeletal muscle is the biceps – the one that we flex to make our arm muscle bulge. Now, this biceps is composed of maybe hundreds, even thousands of cylindrically shaped cells called the muscle fibres, which are surrounded by a connective tissue called endomysium. The muscle fibers will be grouped together in bundles or compartments called “fascicles”. The fascicles will determine the strength, shape, range of motion, and form of the skeletal muscle. If each muscle fibre is covered by endomysium, remember that each compartment on the other hand is protected by another connective tissue called perimysium. Now, the compartments or bundles will come together to form the biceps and will now be covered by the connective tissue, epimysium. To summarize, there will be epimysium on the outermost layer of the skeletal muscle (let’s say biceps), then the perimysium will cover each bundle of muscle fibres. Now, each muscle fibre will be protected by another type of connective tissue called endomysium. Another interesting fact, all these three connective tissues will extend beyond the flesh of the muscles to form what thick, rope-like structure called tendons or flat sheet called aponeurosis. Commonly, we only refer to the tendons are the structure responsible for muscle-bone attachment; however, there are times when aponeurosis takes the tendon’s place because it can cover wider areas. More of this attachment will be discussed in the next section.
Microscopy of Skeletal Muscles
Earlier we have tackled the three types of muscle tissues, and then we got over the foundations of the skeletal muscle structure. The basics now give you the ammunition to learn more about skeletal muscles at a microscopic level.
Understanding the cellular level of the skeletal muscles means we have to talk about the muscle fibres. As discussed earlier, muscle fibres are actually cells. Since they are cells, it follows that they have the same structure as any autosomal cells that we have, only we’ll be more specific in naming them. For an instance, muscle fibres have plasma membrane which we call “sarcolemma” – this is the covering of the fibre itself and is different from the connective tissue endomysium. Inside the muscle fibre is the cytoplasm, but we will call it “sarcoplasm”. In the sarcoplasm are other organelles like mitochondria and nucleus. It also contains glycogen and myoglobin, a pigment similar to haemoglobin the supplies muscle cells with oxygen.
The most notable thing that can be found in the sarcoplasm is the myofibrils. These are cylindrical protein structures that run from one end of the fibre to the other side. There could easily be hundreds to thousands myofibrils in each cell, making them so packed inside that they squish the organelles and push the nucleus close to the sarcolemma, so that it is more visible. This is why skeletal muscles appear to be “multinucleated”.
The myofibrils – which occupy almost 80% of the muscle fibre – are contractile elements because they contain myofilaments. These myofilaments do not run from end to end of the fibre; instead they are enveloped in units called sarcomeres. These sarcomeres on the other hand are separated by dense material called Z-lines. Myofilaments can be Thick or Thin. Thick myofilaments contain the myosin while the thin myofilaments contain the protein actin. These two proteins, myosin and actin, make muscle contraction possible. While the myosin is rod-like and has two round heads called cross bridges, the actin is a double-stranded coil and contains tropomysin and troponin, which are regulatory proteins.
----Credits-----
Music: Pond5.com
Visuals: canstockphoto.com
Contributing writers and video editors:
Lorraine Bunag
Daniel Boyko
Michael Kostikin
Victor Onyino
0 Comments