Human Physiology: Muscle Energy and Types

Creatine Metabolism, High Energy Phosphates, and Muscle Energy Supply

• Creatine metabolism, high energy phosphates, and muscle's energy supply are interconnected.
• Muscle is the primary biochemical transducer that converts chemical energy into mechanical energy.
• Muscles maintain a constant supply of chemical energy, primarily ATP and creatine phosphate.
• Neural control of muscles regulates speed, force of contraction, duration of activity, and return to original state.

Muscle Types

• Muscles are categorized into smooth, skeletal, and cardiac.
• Skeletal muscles (attached to bone for movement) are striated.
• Cardiac muscles are striated, responsible for heart function.
• Smooth muscles (lining digestive tract and blood vessels) are non-striated.
• Skeletal muscles are under voluntary nervous control; smooth and cardiac muscles are involuntary.

Skeletal Muscle Structure

• Skeletal muscle fibers run the length of muscles and are multinucleated.
• These fibers contain numerous mitochondria for ATP production, crucial for muscle contraction and relaxation.
• Muscle fibers consist of myofibrils arranged in parallel (sarcomeres).
• Myofibrils are embedded in sarcoplasm (cytoplasm of fiber) and surrounded by sarcolemma (excitable plasma membrane).
• Sarcoplasm contains glycogen, glycogenolysis enzymes, glycolytic enzymes, ATP, and creatine phosphate.

Skeletal Muscle Classification

• Muscle fibers are classified as slow-twitch (Type I) or fast-twitch (Type II).
• Slow-twitch fibers are rich in mitochondria and myoglobin (red color) and are highly oxidative (aerobic).
• They contract slowly, sustain contractions, and resist fatigue, suitable for repeated low-intensity activities (jogging, walking).
• Fast-twitch fibers (Type II) are further subdivided into Type IIa and Type IIb.
• Type IIa fibers are intermediate, and faster contracting, and have high myoglobin content (red).
• Type IIb fibers have few mitochondria and low myoglobin (white), rely primarily on anaerobic glycolysis (fast, but unsustainable), and fatigue quicker.
• Fast-twitch fibers are used for short bursts of high-intensity activities (sprints, heavy weightlifting).

Creatine Synthesis

• Creatine is not an essential nutrient, naturally produced in the human body from amino acids (glycine and arginine) or found in meat in the diet.
• Creatine synthesis begins in the kidneys and is completed in the liver.
• Glycine combines with arginine in the kidneys to form guanidinoacetate, which then travels to the liver.
• In the liver, guanidinoacetate is methylated by S-adenosyl methionine to create creatine.
• Creatine then travels in the bloodstream to tissues (brain, heart, skeletal muscle).
• Creatine reacts with ATP to form creatine phosphate.
• The reaction is catalyzed by creatine phosphokinase (CK) and is reversible, enabling cells to use creatine phosphate to regenerate ATP.
• The amount of phosphocreatine is proportional to muscle mass.

Creatine Kinase (CK)

• CK is necessary for converting creatine to phosphocreatine.
• Different CK isoenzymes (CK-MM, CK-MB, CK-BB) are located in various tissues.
• CK levels are clinically important; increased levels indicate damage or injury in skeletal muscle or heart tissue (e.g., crush injuries, myocardial infarction).

Creatinine Metabolism

• Creatine phosphate is unstable, spontaneously converting to creatinine.
• Creatinine cannot be further metabolized and is excreted in urine.
• Daily creatinine excretion is constant, directly related to muscle mass.
• Decreased muscle mass (e.g., muscular dystrophy, paralysis) leads to lower creatinine excretion.
• Kidneys play a key role in creatinine metabolism. Damaged kidneys cause creatinine to build up in the blood. (Serum creatinine is used to assess kidney health).

ATP Metabolites

• During exercise, ATP is broken down to ADP.
• Some ADP is reconverted to ATP through adenylate kinase reaction forming ATP and AMP (AMP is an auxiliary source of energy).
• AMP is further broken down to IMP and ammonia.
• AMP can also be dephosphorylated into adenosine, which acts as vasodilator increasing blood supply to the muscle.

Energy Systems

• The body uses various energy systems (ATP-PC, anaerobic glycolysis, aerobic) depending on the activity's intensity and duration.
• ATP-PC is predominant during short-duration, high-intensity exercises (e.g., sprints).
• Anaerobic glycolysis (lactic acid system) is the primary supplier of energy for short-term, high-intensity activities (e.g., 400 m sprint).
• Aerobic energy system provides energy for longer-duration, low-to-moderate intensity activities (e.g., endurance activities).

Explore the intricacies of muscle energy supply and metabolism in this quiz. Learn about the various muscle types, including skeletal, cardiac, and smooth muscles, and their physiological roles. Test your knowledge on how chemical energy is converted into mechanical energy and the structure of skeletal muscles.