Human Skeletal Muscle Fiber Overview

Muscles! Muscles!!, you must have seen my previous posts on Muscles, and in my post today, we will be talking about the Skeletal Muscle Fiber types. There are three types of skeletal muscle fibers in humans. In the fascicle of the muscle is where muscle fibers exist and these muscle fibers differ. We have Type 1, Type 2a, and Type 2x which are the types of muscle fibers in humans, other animals have Type B muscle fibers.^ncbi

The Type 1 muscle fiber is also known as the Slow Oxidative Muscle Fiber, and some others call it Red Slow Oxidative fiber. It has the least fiber diameter of all muscle fibers, It has a lot of blood vessels or capillary supply taking blood into the muscle giving a reddish color as a result of its high capillary density. I possess a significant amount of mitochondria which are responsible for ATP production, taking glucose, amino acid, and fat in the presence of oxygen to produce energy in Aerobic respiration. The cells produce energy on aerobic cellular respiration undergoing the KREB cycle, electron transport train reaction, and other oxidative reactions to produce ATP under Oxidative phosphorylation. The Type 1 muscle fiber has a very slow contractile speed as a result of a decreased myosin ATPase activity. The Myosin which is in the thick filament has an enzyme known as Myosin ATPase, which breaks the bond of ATP to ADP and inorganic phosphate (you can check out Myosin in my previous post). This process of converting the ATP to ADP is done slowly, causing the detachment of the myosin from the Actin slowly causing the contraction speed to decrease but because it contracts slowly, it can increase the longevity of the activity of the muscle thanks to the availability of energy being produced. This will cause the muscle to be fatigue resistance and so it is not strong, but it is the first muscle to be recruited when involved with heavy carrying. The energy of the Slow Oxidative muscle fiber is stored in form of triglycerides which is a storage molecule, undergoing beta-oxidation in the mitochondria. In the muscle, there is a protein known as Myoglobin which temporarily holds onto oxygen, to be sent to the muscles whenever it is needed. This muscle is useful in marathon running, maintaining body posture and positions against gravity.^{ncbi, aps, ncbi}

The next type of muscle fiber I will be discussing is the type 2X muscle fiber also known as the fast/glycolytic muscle fiber. It is an intermediate fiber in diameter, bigger than Type 1 and Type 2a in humans. It usually appears white or palish pink because it doesn't have many capillaries or blood vessels going into these muscle cells, making it have a low capillary density. It doesn't have many mitochondria since there is very little oxygen being delivered to the muscle, and it doesn't utilize aerobic cellular respiration, rather it depends on anaerobic respiration. It possesses a significant amount of glycogen granules known as Glycosome which will break down glycogen into glucose which can be converted into pyruvate producing little ATP. it utilizes the Creatine phosphate pathway where the Creatine Kinase enzyme binds the Creatine phosphate and the ADP to produce ATP in little amounts. Energy is produced by anaerobic respiration through Creatine phosphate pathways and glycolysis. The Myosin ATPase activity is very fast producing ATP very fast and producing very fast contractility. It has a very low fatigue resistance wearing out very fast but producing the largest amount of power, so it is the third muscle fiber to be recruited. It is used for very powerful but short-timing purposes such as throwing punches, hitting, and weight lifting.^{ncbi 1, ncbi 2, ncbi 3}

The third type of muscle fibers I will discuss is type 2a muscle fibers which are known as Fast Oxidative Muscle fibers. It is the largest muscle fiber in diameter, but this is not the same for other animals but in humans, it is the biggest muscle fiber in diameter. It is dependent on oxygen which means it requires a lot of capillaries through it making it highly dense but because it is large its reddish color isn't full, giving it a reddish-pink color. It utilizes a lot of energy, and so it has lots of mitochondria utilizing a lot of oxygen and producing a lot of ATP. This muscle can switch between aerobic and Anaerobic conditions as it possesses a lot of glycogen which can be broken down to glucose, and can also exhibit glycolysis to produce pyruvate and ATP. The type 2a muscle fibers also possess Creatine kinase which binds to Creatine Phosphate and ADP to produce ATP very fast but not much, the process also produces creatine. Its contraction is fast as the myosin ATPase quickly cuts ATP to ADP, allowing for the fast release of Myosin from Actin. It is moderately fatigue resistant lasting for about 30 minutes producing power. It is a second recruitment order muscle fiber allowing for carrying short bursts of energy for an average time for activities such as walking, sprinting, and skeletal activities above 1 minute. ^{ose, physiopedia, pubchem, ncbi}

Depending on the type of sport and how long you have been participating in such sport will determine the muscle fibers that will be utilized more. Some activities require type 1, others require type 2a while some utilize type 2x.

Image 1 || Wikimedia Commons || Organization of Muscle Fiber