Muscle Contraction & Energetics
Exercise Physiology | Muscle Contraction | Muscle Fibers | Muscle Adaptations | Exercise Fuels | CHO Metabolism | Fat Metabolism | Oxygen Uptake | Cardiovascular Exercise | Respiratory Responses | VO2 Max | Temperature Regulation | Heat | Fluid Balance | Fatigue | Sprinting | Endurance | Genes | Practical Case Example
Muscle Contraction and Energetics
Our first module will focus on skeletal muscle, how it contracts, and the fuels that it utilizes during exercise. We’re not going to have extensive coverage of how skeletal muscle functions. If you’re uncertain about the basics of skeletal muscle physiology, I suggest you go back and review that. In any standard textbook of human physiology. Skeletal muscle is characterized by its specialized structure, and at various levels, microscopic levels, you can see the well-organized structure of skeletal muscle. Ultimately the actin and the myosin filaments, within skeletal muscle interact to generate force and that’s under the control of the nervous system.
This lecture module focuses on skeletal muscle energy metabolism, utilization of adenosine triphosphate (ATP) in the cross-bridge cycle, and creatine phosphate as it serves as a buffer for adenosine triphosphate levels during very intense exercise. You will learn about metabolic pathways that involve a carbohydrate, where it is degraded through glycolysis, and then either converted to lactate or into the mitochondria, where the Krebs Cycle and the electron transport chain can generate adenosine triphosphate. Along with ATP and carbohydrate, and to a minor extent, protein, the other important fuel utilized during exercise is fat, and this fat can either come from the adipose tissue, triglyceride stores, and the plasma fatty acids, or intramuscular triglycerides. When these fats are broken down, the fatty acyl-CoA’s, transfer into the mitochondria across the mitochondrial membrane. This is a process known as beta-oxidation expressed as β-oxidation. Where these fatty acids can be degraded into the Kreb Cycle, and in much the same way, for carbohydrates generate ATP through the electron transport chain.
- Sequence of Events in Muscle Contraction
- ATP Dependent Process in Skeletal Muscle
- ATP Store & Turnover Rates During Exercise
- Skeletal Muscle ATP Generation
- Overview of Skeletal Muscle Energy Metabolism
Sequence of Events in Muscle Contraction
You should review the basic steps and sequence of events involved in muscle contraction. It starts in the motor cortex with activation of the motor centers, that control various muscles of the body. There’s descending motor activation through the various motor pathways and finally excitation, the alpha-motoneurons that go to specific muscles. Transmission of the electrical impulse across the neuromuscular junction is mediated by the transmitter acetylcholine, and ultimately, the excitation of the motor nerve is transferred to excitation of the muscle membrane and the extension of the muscle membrane into the muscle, the T-tubule. The fundamental event in muscle contraction we refer to as excitation-contraction coupling. Whereby depolarization of the t-tubule and activation of the calcium-release channels from the sarcoplasmic reticulum, results in calcium release into the Sarcoplasm. Calcium then binds with Troponin C, allows the interaction of actin and myosin, and we get the generation of force. ATP or adenosine triphosphate, the high energy molecule that’s utilized at various steps in muscle contraction is very important, and we’ll spend some time looking at the energetics of muscle, and the fuels for exercise in our next module.
This figure summarizes those processes in graphical form. And you can see the various steps involved beyond the neuromuscular junction, in the skeletal muscle. We’ll come back to this when we look at muscle fatigue in our fifth module, and you can refer back to this figure, to see where parts of this process may be affected during fatigue. So again, just to reiterate some of the key steps. We have excitation of the muscle membrane and the T-tubule, release of calcium from the cytoplasmic reticulum, and during recovery, the resequestration of calcium back into the cytoplasmic reticulum. The binding of calcium to the thin filament’s complex and the interaction of actin and myosin and the hydrolysis of ATP.
ATP Dependent Process in Skeletal Muscle
There are a number of processes within muscle contraction that are dependent upon ATP. Notably, the myosin ATPase is important in the cross-bridge cycle. But other important sources or other important sites of ATP utilization include the sodium-potassium ATPase, which is very important in maintaining the excitability of the sarcolemma. Particularly in returning potassium to the muscle, after each action potential. Another important ATPase is the calcium ATPase on the cytoplasmic reticulum, and during relaxation when calcium is pumped back into the cytoplasmic reticulum, this enzyme is very active. Studies in rodents with selective inhibitors of the myosin ATPase have been used to reduce the force output of muscle but during activation, there’s still activity of the sodium-potassium-ATPase and the calcium-ATPase, suggest that the ATP utilization, due to the myosin ATPase of the force production is about 50 to 60% of the total ATP turnover. But all of these processes are critically dependent upon ATP.
ATP Store & Turnover Rates During Exercise
We turn our attention to how ATP is utilized during exercise, we need to just reflect on how, in exercise science, we express exercise intensity and that’s normally done relative to the maximal oxygen uptake. We’ll come back to that concept in our third module on oxygen uptake. But for the moment, if you think of VO2 max, as a signpost, or a, a marker of exercise intensity. Anything up to 100% of VO2 max is largely aerobic, with a small contribution from the so-called anaerobic energy stores. If you exercise at an intensity higher than VO2 max, then clearly the other energy systems, the non-aerobic energy systems, are contributing to that power generation. The muscle ATP concentration is in the order of five to six millimole per kilo of wet muscle. If you’re exercising at 75% of your VO2 max, reasonably moderate intense exercise, if you relied solely on the ATP within your muscle, you’d only be able to go for about 15 seconds. If you increased the exercise intensity to a higher level, say 140% of VO2 max, you’d go for about 6 seconds. And then if you went as hard as you possibly could, for a very short period, relying only on the ATP in your muscle, you’d last less than 2 seconds. So clearly, other metabolic pathways in skeletal muscle are activated, to generate ATP, to maintain muscle contraction.
Skeletal Muscle ATP Generation
These energy systems traditionally have been divided into two types. The first refers to substrate-level phosphorylation where the ATP is regenerated by the interactions and the exchange of phosphate during a series of reactions directly at the substrate level. The first of these is using creatine phosphate, which is another high energy phosphate compound stored in skeletal muscle. The other pathway, is the so-called anaerobic glycolysis, where glucose units, either from blood glucose or muscle glycogen, are broken down in a series of steps, to pyruvate, which can then either be oxidized or converted to lactate, and at high rates of energy expenditure, the conversion of pyruvate to lactate is the dominant pathway. The other metabolic pathway is oxidative phosphorylation. And this, pathway the metabolism of carbohydrate and fat generate electron donors which then interact with molecular oxygen. To the electric transport chain, to establish a, an electromotive gradient which drives the re-synthesis of ATP. This figure summarizes those energy systems, the important processes I mentioned earlier where ATP is utilized in muscle contraction. And then the other pathways, that regenerated ATP from ADP during the exercise of varying intensity and various duration. In our next module, on fuels and exercise, we’ll focus on these various energy systems in more detail. For the moment, however, creatine phosphate and the degradation of carbohydrate to lactate the so-called anaerobic pathways, and then in terms of oxidative metabolism, the utilization of carbohydrate lipid and to a very small extent, protein.
Overview of Skeletal Muscle Energy Metabolism
It’s worth just reflecting on an overview of skeletal muscle energy metabolism, and we’ll come back to this as I said, in the second module on fuels. Here is the utilization of ATP in the cross-bridge cycle. Creatine phosphate serves as a buffer for, ATP levels during very intense exercise. The other metabolic pathways involve carbohydrates. Where it’s degraded through glycolysis, and then either converted to lactate or into the mitochondria, where the Krebs Cycle and the electron transport chain can generate ATP. The other important fuel during exercise is fat, and this fat can either come from the adipose tissue, triglyceride stores, and the plasma fatty acids, or the intramuscular triglycerides. These are broken down, the fatty acyl-CoA’s, transfer into the mitochondria across the mitochondrial membrane. There’s a process known as beta-oxidation. Where these fatty acids can be degraded into the Kreb Cycle, and in much the same way, for carbohydrates generate ATP through the electron transport chain. As I said, we’ll come back to this topic in much more detail in our second module on fuels for exercise..
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