Understanding Skeletal Muscle: Structure, Growth, and Atrophy

The human body contains over 650 individual muscles that are all responsible for certain things (The Library of Congress).

There are three main muscle categories in the human body, which are cardiac, skeletal, and smooth (The Library of Congress). Cardiac muscles are responsible for carrying out the contraction of the heart (The Library of Congress). Smooth muscles make up the walls of structures in the human body, including the intestines, uterus, blood vessels, and internal muscles of the eye (The Library of Congress). Skeletal muscles are different from both cardiac and smooth muscles because skeletal muscles are responsible for helping carry out movements throughout the human body (The Library of Congress).

Skeletal muscles are the most observable when they grow and degrade because the only cardiac muscle we have is our heart, which is located inside our rib cage, and smooth muscle sizes are at the cellular level (The Library of Congress). When a muscle grows, that muscle is going through a process which is called muscular hypertrophy (Miklush).

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On the other hand, when a muscle degrades, that muscle is going through a process called muscular atrophy (Medline Plus). Skeletal muscles have many different structures that make them up that can carry out both muscle hypertrophy and muscle atrophy (Medline Plus, Miklush, National Cancer Institute).

Skeletal muscles are responsible for skeletal movements in the human body (Seer Training Modules).

Skeletal muscles are considered organs of the muscular system and are connected to a bone or more of the skeleton (Seer Training Modules). Every skeletal muscle consists of skeletal muscle tissue, connective tissue, nerve tissue, and blood or vascular tissue (Seer Training Modules). These muscles vary in size, shape, and arrangement of muscle fibers (Seer Training Modules).

The most basic unit of a muscle are the actin and myosin strands (Avatar Nutrition). Actin and myosin are two groups of proteins that help the muscle contract (Dictionary.com). These strands are located in segments called sarcomeres (Avatar Nutrition). These sarcomeres are located within myofibrils, which are basic threads in a muscle cell (Avatar Nutrition). Myofibrils are located within muscle cells, which are also called muscle fibers (Avatar Nutrition). One muscle can be composed of hundreds or thousands of these fibers, which are just individual cydrillical muscle cells (Seer Training Modules). The muscle fibers are soft and fragile just like the other cells in your body, but the connective tissue that surrounds every individual fiber called the endomysium provides support and protection to the fibers (Seer Training Modules). The endomysium also helps the muscle fibers to withstand the force of contraction (Seer Training Modules). The next biggest part of the muscle is called a fascicle (Seer Training Modules). A fascicle is the name for a bunch of muscle fibers bundled up (Seer Training Modules). Around every fascicle is another layer of connective tissue called a perimysium which surrounds it (Seer Training Modules). Many fascicles are squeezed together tightly by the outer layer of the muscle called the epimysium (Seer Training Modules).

The epimysium is yet again another layer of connective tissue that surrounds a part of the muscle, but it surrounds the whole muscle in this case (Seer Training Modules). Parts of the epimysium shoot inwards into the muscle to divide it into compartments (Seer Training Modules). Skeletal muscles contain many blood vessels and nerves, which all penetrate the epimysium (Seer Training Modules).

The blood vessel, or nerve, then branches off into smaller parts into the other areas of the muscle (Seer Training Modules). The reason there are blood vessels and nerves in skeletal muscles relates to the contraction of the muscle (Seer Training Modules). The nerve cell must send impulses to the muscle for it to contract (Seer Training Modules). Finally, the outermost layer of the muscle is called the fascia (Seer Training Modules). The fascia is what covers each muscle (Merriam Webster). All of the connective tissue that is located within the muscle comes to a point where it combines together into a ropelike strand, then connects to the bone, thus the name skeletal muscle (Seer Training Modules). Typically the muscle spans a joint with one end fixed to the joint, and the other end(s) able to move as a result of muscle contraction (Seer Training Modules). Skeletal muscles may have many shapes and sizes, but each one is composed of the same components (Seer Training Modules).

Every skeletal muscle is able to grow through a process called skeletal muscle hypertrophy (Miklush). When a cell in the body goes through hypertrophy, it gains size and mass (Miklush). The prefix “hyper” means excessive, and the root “trophy” means stimulation from nutrition, hormones, or other factors of growth (Miklush). If you put the two definitions together, hypertrophy means excessive stimulation from nutrition, hormones, or other factors of growth (Miklush).

When a cell goes through hypertrophy, it produces more organelles, plasma, and protein (Miklush). When lifting weights and exercising, cells are stimulated by the increased workload and adapt by getting bigger (Miklush). Hypertrophy can eventually be visually observed (Damas, Libardi, Ugrinowitsch). It is estimated that with four or less resistance sessions, the biggest part of the muscle grows a little bit mainly from muscle damage induced swelling (Damas, Libardi, Ugrinowitsch). It is also estimated that after 10 resistance sessions that muscle hypertrophy starts to occur, but it is after 18 of these sessions that you can visibly see muscle hypertrophy (Damas, Libardi, Ugrinowitsch). This growth of the muscles happens because the body adds myosin filaments to the individual muscle fibers in a muscle (Avatar Nutrition). Building muscle requires proteins including mTOR, which stands for the mammalian target of rapamycin (Avatar Nutrition and National Cancer Institute). Mammalian target of rapamycin is a complex protein that helps control several cell functions and plays a part in cell division and survival (Seer Training Modules).

When other proteins and mTOR start to work together they start to build muscle (Avatar Nutrition). This process is known as muscle protein synthesis or MPS (Avatar Nutrition). Muscle protein synthesis requires protein to happen (Avatar Nutrition). The protein gives the muscle essential amino acids to help it grow (Avatar Nutrition). There is also a counteract to MPS called muscle protein breakdown (Avatar Nutrition). To grow a muscle, an individual must have a higher rate of muscle protein synthesis than muscle protein breakdown (Avatar Nutrition). If the rate is the same, the individual will neither gain, nor lose any muscle (Avatar Nutrition). If the rate is in favor of muscle protein breakdown, your muscles will start breaking down (Avatar Nutrition).

The more protein an individual has, the more energy it gives to their body and muscles (Avatar Nutrition). The higher an individual’s protein count is, the higher chance their muscles will go through muscle protein synthesis (Avatar Nutrition). The process of MPS is essential in the entire process of skeletal muscle hypertrophy. The more protein someone consumes and the more their muscles go through muscle protein synthesis, they have a better chance of having their muscles carry out with muscle hypertrophy (Avatar Nutrition). It is estimated that someone has 1.4 to 2 grams of protein per kilogram of your body per day (0.7 to 1 gram of protein per pound of your body per day) to maximize their chance of carrying out with muscle hypertrophy (Avatar Nutrition). Skeletal muscles require the two main steps of resistance training and consuming lots of protein to carry out with muscle hypertrophy (Sandri and Avatar Nutrition).

Skeletal muscles can grow, but they can also shrink (Medline Plus). The name for the wasting away or loss of skeletal muscle tissue is muscle atrophy (Medline Plus). When a person has muscle atrophy their strength and muscle mass decrease (Lumen Learning). Muscle atrophy results in a decrease in protein count, fiber diameter, force production, and fatigue resistance (Robert W. Jackman and Susan C. Kandarian). Little is known about how muscles break down at the molecular level, but decrease of muscle protein synthesis and increase of protein degradation have been shown to contribute to the loss of protein and size in muscles (Robert W. Jackman and Susan C. Kandarian).

There are three types of muscle atrophy which are physiologic atrophy, pathologic atrophy, and neurologic atrophy (Medline Plus). Physiologic muscle atrophy is caused by the lack of muscle movement ,and it can be reversed with better exercise and nutrition (Medline Plus). Pathologic muscle atrophy is seen with aging, starvation, and diseases (Medline Plus). Neurologic muscle atrophy is the most severe type of muscle atrophy and it is caused by an injury to a muscle or a disease if a nerve that connects to a muscle (Medline Plus). Some other causes of muscle atrophy include malnutrition, diseases, burns, and arthritis (Medline Plus). These are not all of the causes of muscle atrophy, as there are many other less common causes (Medline Plus). There are multiple treatment options to help with muscle atrophy including physical therapy, electrical stimulation, nutrition plans, and surgery (Eske). Skeletal muscles atrophy because of multiple causes, little is known about the process, and a few treatment options have been discovered (Lumen Learning, Medline Plus, Eske, Robert W. Jackman and Susan C. Kandarian).

Skeletal muscles are composed of many different structures (Seer Training Modules). They also can go through both processes of hypertrophy and atrophy (Medline Plus, Miklush). Skeletal muscles are observable because of their size and placement in the human body (The Library of Congress). These muscles in everyone’s body are constantly changing no matter the circumstance (Avatar Nutrition). Skeletal muscles are necessary for all movements the human body makes (The Library of Congress).