Gross Structure – what are the endomysium, perimysium, epimysium, fascicle, muscle fiber?

Anaerobic exercise is an intense form of physical exercise in which lactate formation occurs. It is generally used in non-endurance sports for promoting strength, speed, power and to build muscle mass. Anaerobic energy systems are of two types : 1) high energy phosphates, adenosine triphosphate and creatine phosphate; and 2) anaerobic glycolysis. Adenosine triphosphate (ATP) is a molecular unit which performs function of intracellular energy transfer. ATP can be used as the source of energy for the contraction during exercise. ATP can be produced by creatine phosphate, glycogen and aerobic respiration. Creatinine phosphate (CP) is also stored in the muscle cells like ATP. In the process of breaking down of CP, large amount of energy gets released and this energy gets utilised for the resynthesise of ATP. ATP can provide energy for 3 seconds on the other hand CP can provide energy for up to 8 seconds with high rate as compared to ATP. Lifting heavy weight, sprinting fast for 50 – 100 metres and punching boxing bag hardly for 2 – 3 punches are the activities which use ATP-PC. ATP–CP is also known as phosphagen system. Total amount of energy storage in ATP-CP system is less, however this phosphagen form of energy can be utilized as rapid availability of energy (Plowman and Smith, 2013).  

In glycolysis, glucose gets converted into pyruvate and free energy gets released. Released free energy can be useful for the formation of ATP and NADH (reduced nicotinamide adenine dinucleotide). Glucose, ADP and NAD are the substrates and pyruvate, ATP and NADH are the end products in glycolysis. Glycolysis has low capacity for about 30 seconds to 3 minutes and high power but not very powerful. Glycolysis occurs in cytosol which is a liquid found inside the cells. Hexokinase, phosphofructokinase and pyruvate Kinase are the three kinases plays important role in regulation of glycolysis. Hexokinase regulates first step which is a conversion of glucose to glucose-6-phosphate. Phosphofructokinase regulates third step which is conversion of fructose-6-phosphate into fructose-1,6-biphosphate. Pyruvate Kinase regulates final step which is conversion phosphoenolpyruvate into pyruvate. In anaerobic glycolysis, NADH plays role in the production of lactate from the pyruvate. Pyruvate is the final product of glycolysis. In this process of cellular respiration ATP gets produced. Glycolysis is required in the weight training, sprinting, jogging, high rep swings or snatches (Porcari et al., 2015).

Aerobic metabolism take place in the mitochondria and in this process more amount of energy gets released. In aerobic metabolism acetyl COA, hydrogen and glucose get oxidized. Pyruvate dehydrogenase complex oxidizes pyruvate to acetyl-CoA and CO₂. In the first step, pyruvic acid formed from the glucose and in the second step this pyruvic acid formed in the glycolysis enters mitochondria. Isocitrate dehydrogenase is the rate limiting enzyme in the Krebs Cycle, TCA, Citric Acid cycle. Dehydrogenase enzymes eliminate electrons from the intermediate products and these enzymes gets transported to coenzyme NAD which result in the NADH. Electrons can be taken to the inner membrane of mitochondria by these coenzymes for use in the electron transport chain. Dehydrogenase is regulated by its substrate pyruvate and product acetyl CoA. Glycolysis, Krebs cycle and electron transport chain produces 4, 2 and 32 ATPs produces respectively. Overall in aerobic metabolism 38 ATPs gets produced. Aerobic metabolism includes less intensity exercises which can be performed for longer duration. It includes walking, long slow runs, rowing, and cycling. In aerobic glycolysis, NADH transported into the mitochondria and gets oxidized to NAD (Boone, 2013).

How is blood supplied to the muscle?

Electron transport chain occurs in the inner mitochondrial membrane. In oxidative phosphorylation, ATP gets formed due to transfer of electrons from NADH or FADH ₂ to O₂ through series of electron carriers. Electrons get transferred from electron donors like NADH or FADH ₂ to electron acceptors like oxygen through redox reaction. This electron transfer is associated with development of proton gradient across the membrane and transfer of protons through the membrane. Due to this electron transfer under the influence of ATP synthase, ATP and water produced during electron transport chain. NADH + H+ from glycolysis can enter the ETC by entering in the mitochondria. 34 ATP molecules can be produced from NADH + H+ and FADH₂. Oxygen is the final acceptor of electron in electron transport chain. Glycerol 3-phosphate shuttle is the shuttle for NADH + H+ (Connes et al., 2010).

Triglycerides are the main fat present in the blood and it is taken up in the adipose tissue which can be used as energy for the next time. Saturated and unsaturated fatty acids are types of two common fatty acids. Beta oxidation is the catabolic process in which breaking of fatty acids take place to produce acetyl-CoA. This acetyl-CoA enters into the citric acid cycle, and NADH and FADH_2. These co-enzymes are useful in the electron transport chain. 7 NADH + H+ and 7 FADH₂ molecule gets produced during beta oxidation. Remaining acetyl-CoA and acyl-CoA enters citric acid cycle by combining with oxaloacetate to form citrate. It is called beta oxidation because beta carbon of  the fatty acids gets converted into carbonyl group due to oxidation. One fatty acid will generate more ATP as compared to glucose when it gets fully oxidised. Glucose produces more CO₂ per O₂ as compared to the fatty acids. Proteins can be oxidised to get energy. Phosphagen pathway provides most of the energy at the start of the exercise. Aerobic metabolism is a slow process and it utilizes carbohydrate and fat as the energy source. Hence, it can’t be used as energy source at the start of the exercise (MacLaren and Morton, 2011).

Epimysium is the layer of connective tissue in which outermost layer of the muscle fiber gets wrapped. Endomysium is the connective tissue layer in which individual muscle fiber gets encased inside the fascicle. Perimysium is the middle layer connective tissue which bound to long muscle fibers of every fascicle. Fascuculus is the bundle of skeletal muscles surrounded by connective tissue like perimysium. Muscle fibers are building blocks of skeletal muscles and these are made up of myofibrils which contains actin and myosin filament. Skeletal muscles are closely intertwined by blood vessels and every muscle is supplied with multiple blood capillaries. Sarcolemma is present on the outside because it is plasma membrane and sarcoplasm is present inside because it is cytoplasm of the skeletal muscle. Sarcomere is section between two neighbouring Z-lines. Z-line is disc between the I bands and it is dark in colour. I-band is the region which surrounds z-line and it is zone of thin filaments. A-band is present after I-band and it contains single thick filament. In between A-band a paler region is present which is called as H-zone. Sarcoplasmic reticulum is defined as special type of smooth endoplasmic reticulum. Sarcoplasmic reticulum regulates concentration of calcium ions in the cytoplasm of striated muscles. Transverse tubules (T-tubules) are the extension of cell membrane which enter towards centre of the cells. T-tubules which are closely associated with specific part of the sarcoplasmic reticulum are called as cisternae. Actin is a group of globular multi-functional proteins which form microfilaments. Myosins are the ATP-dependent superfamily of motor proteins which plays role in muscle contraction. Tropomyosin is the essential part of the actin filaments which plays important role in the regulation of actin filaments in both muscle and nonmuscle cells. Troponin is a complex of troponin C, troponin I, and troponin T and plays essential role in the muscle contraction. Actin filaments are majorly present in I band and it extends upto A-band. Myosin filaments are present throughout A-band. Actin filaments bound to the Z-line. H-zone is associated with thick filament myosisn (Richter et al., 2013).

ATP is the source of energy for contraction and relaxation. ATPase is present on myoin protein. ATP is in mitochondria. Acetylcholine stimulates sarcoplasmic reticulum for release calcium through the muscle which stimulates contraction. Toponin and tropomycin binds to the calcium which makes myosin cross bridges to attach to actin which brings contraction in muscles. Excitation signal come from the brain through nerve impulse. This excitation potential arrives at nerve terminal and releases acetylcholine. Acetylcholine travels through neuromascular junction which stimulates sarcoplasmic reticulum to release calcium through the muscle. Myosin swivels and actin slides. Relaxation occurs when nerve impulse stops and calcium gets back in the sarcoplasmic reticulum which causes breaking actin and myosin link. According to sliding filament theory, muscle contraction occurs in four stages like muscle activation, muscle contraction, recharging by ATP resynthesise and relaxation which will be helpful in the improving performance of athletes (Kraemer et al., 2011)

References :

Boone. (2013). Introduction to Exercise Physiology. Jones & Bartlett Publishers.

Connes, P. Hue, O., and Perrey, S. (2010). Exercise Physiology: From a Cellular to an Integrative Approach. OS Press.

Kraemer, W. J., Fleck, S. J., Deschenes, M. R. (2011). Exercise Physiology: Integrating Theory and Application. Lippincott Williams & Wilkins.

MacLaren, D., and Morton, J. (2011).  Biochemistry for Sport and Exercise Metabolism. John Wiley & Sons.

Plowman, S. A., and Smith, D. L. (2013). Exercise Physiology for Health Fitness and Performance. Lippincott Williams & Wilkins.

Porcari, J., Bryant, C., and Comana, F. (2015). Exercise Physiology. F.A. Davis.

Richter, E. A., Kiens, B., Galbo, H., and Saltin, B. (2013). Skeletal Muscle Metabolism in Exercise and Diabetes. Springer Science & Business Media.