Physiology of Muscle PDF

**Physiology of Muscle** Muscles are essential for movement, posture, and a variety of vital bodily functions. This unit covers the classification of muscles, the detailed physiology of skeletal muscle contraction, muscle metabolism, different types of muscle fibres, and the distinctions between sk...

**Physiology of Muscle** Muscles are essential for movement, posture, and a variety of vital bodily functions. This unit covers the classification of muscles, the detailed physiology of skeletal muscle contraction, muscle metabolism, different types of muscle fibres, and the distinctions between skeletal, cardiac, and smooth muscle. **1. Classification of Muscles** Muscles can be classified based on their structure and function into three main types: - **Skeletal Muscle:**\ Attached to bones, skeletal muscles are responsible for voluntary movements. These muscles are striated (having a banded appearance) and are under conscious control. - **Cardiac Muscle:**\ Found exclusively in the heart, cardiac muscle is also striated but operates involuntarily. It is responsible for pumping blood throughout the body and has unique features such as intercalated discs, which allow for synchronized contractions. - **Smooth Muscle:**\ Located in the walls of hollow organs (such as blood vessels, intestines, and the bladder), smooth muscle is non-striated and operates involuntarily. It is responsible for movements such as peristalsis in the digestive tract and vasoconstriction in blood vessels. **2. Levers in the Muscular System** Muscles work with bones to create movement through lever systems. Levers are classified into three types based on the relative positions of the fulcrum (joint), effort (muscle force), and load (resistance). **First-Class Lever:** - **Structure:** The fulcrum is located between the effort and the load. - **Example:** The head acting on the atlas vertebrae. The neck muscles apply effort to lift the head, which is the load, with the fulcrum being the joint between the skull and the spine. **Second-Class Lever:** - **Structure:** The load is located between the fulcrum and the effort. - **Example:** Standing on tiptoes. The ball of the foot acts as the fulcrum, the body weight is the load, and the effort is applied by the calf muscles. **Third-Class Lever:** - **Structure:** The effort is applied between the fulcrum and the load. - **Example:** Flexing the forearm at the elbow. The elbow joint is the fulcrum, the load is the weight of the forearm and hand, and the effort is applied by the biceps muscle. **3. Physiology of Skeletal Muscle Contraction** Skeletal muscle contraction is a complex process involving multiple molecular interactions and steps, primarily dependent on ATP, calcium ions, and the regulatory proteins tropomyosin and troponin. **1. Structure of a Sarcomere:** - **Sarcomere:** The functional unit of a skeletal muscle fibre, defined by the area between two Z discs. It consists of thick filaments (myosin) and thin filaments (actin). - **Myosin:** The motor protein with heads that form cross-bridges by attaching to actin during contraction. - **Actin:** The protein that forms the backbone of the thin filament, providing binding sites for myosin heads. **2. The Role of ATP:** - **ATP Binding:** ATP binds to the myosin head, causing it to detach from actin, a process necessary for cross-bridge cycling. - **ATP Hydrolysis:** ATP is hydrolysed by myosin ATPase, providing energy to \"cock\" the myosin head into a high-energy state. - **Cross-Bridge Formation:** Myosin binds to actin, forming a cross-bridge, and the release of ADP and phosphate causes the power stroke, sliding the filaments and shortening the muscle. **3. The Role of Calcium Ions (Ca²⁺):** - **Release from Sarcoplasmic Reticulum:** An action potential triggers the release of Ca²⁺ from the sarcoplasmic reticulum into the cytoplasm. - **Binding to Troponin:** Ca²⁺ binds to troponin, causing a conformational change that moves tropomyosin away from actin's binding sites, allowing myosin to attach to actin. **4. Troponin and Tropomyosin:** - **Tropomyosin:** A protein that blocks the myosin-binding sites on actin when the muscle is relaxed. - **Troponin:** A complex of three proteins (troponin C, I, and T) that regulates the interaction between actin and myosin by controlling the position of tropomyosin. **5. The Sliding Filament Theory:** - **Cross-Bridge Cycling:** Myosin heads repeatedly bind to actin, pull the thin filaments toward the centre of the sarcomere, release, and reset. This process shortens the sarcomere, leading to muscle contraction. - **Relaxation:** When stimulation ceases, Ca²⁺ is actively pumped back into the sarcoplasmic reticulum, tropomyosin re-covers the binding sites on actin, and the muscle relaxes. **6. Types of Muscle Contraction:** - **Isometric Contraction:** Muscle tension increases without a change in muscle length. This occurs when the load is greater than the force the muscle can generate, such as holding a heavy object stationary. - **Isotonic Contraction:** Muscle tension remains constant while the muscle length changes. Isotonic contractions can be further classified into: - **Concentric Contraction:** The muscle shortens as it contracts, such as lifting a weight. - **Eccentric Contraction:** The muscle lengthens while maintaining tension, such as lowering a weight slowly. **7. Muscle Twitch and Summation:** - **Muscle Twitch:** A single, brief contraction resulting from a single action potential in a muscle fibre. - **Summation:** Increased force of contraction resulting from the rapid succession of action potentials that prevent the muscle from fully relaxing between stimuli. - **Incomplete and Complete Tetanus:** - **Incomplete Tetanus:** Muscle fibres do not completely relax before the next stimulus, leading to a higher tension with each successive stimulus. - **Complete Tetanus:** Muscle fibres are stimulated at a high frequency, resulting in a sustained contraction with no relaxation. **4. Muscle Metabolism** Muscle contraction requires energy, primarily provided by ATP. The sources of ATP for muscle contraction include: **1. Creatine Phosphate:** - **Phosphagen System:** Creatine phosphate donates a phosphate group to ADP to quickly regenerate ATP, providing energy for the first few seconds of intense activity. **2. Glycolysis:** - **Anaerobic Pathway:** Glucose is broken down into pyruvate, producing a small amount of ATP. In the absence of oxygen, pyruvate is converted to lactate, leading to muscle fatigue. **3. Aerobic Respiration:** - **Oxidative Pathway:** Occurs in the mitochondria, where glucose, fatty acids, and amino acids are oxidized in the presence of oxygen to produce a large amount of ATP. This pathway is sustainable for prolonged, lower-intensity activities. **5. Types of Muscle Fibbers** Skeletal muscle fibres are categorized based on their contraction speed and metabolic characteristics: **Type I Fibbers (Slow-Twitch, Oxidative):** - **Characteristics:** These fibres contract slowly, have high endurance, and rely primarily on aerobic metabolism. They contain a large number of mitochondria, myoglobin (which gives them a red colour), and capillaries. - **Function:** Type I fibres are suited for activities requiring endurance, such as long-distance running. **Type IIa Fibbers (Fast-Twitch, Oxidative-Glycolytic):** - **Characteristics:** These fibres contract quickly and are moderately resistant to fatigue. They can utilize both aerobic and anaerobic metabolism. - **Function:** Type IIa fibres are used in activities that require both endurance and power, such as middle-distance running. **Type IIb Fibbers (Fast-Twitch, Glycolytic):** - **Characteristics:** These fibres contract very quickly and powerfully but fatigue rapidly. They rely primarily on anaerobic metabolism and have fewer mitochondria, myoglobin, and capillaries. - **Function:** Type IIb fibres are suited for short bursts of power and speed, such as sprinting or weightlifting. **6. Differences Between Skeletal, Cardiac, and Smooth Muscle** **Skeletal Muscle:** - **Structure:** Striated, with multiple nuclei per cell, and under voluntary control. - **Function:** Responsible for voluntary movements, posture, and heat production. - **Contraction Mechanism:** Controlled by the somatic nervous system, with rapid contraction and relaxation phases. **Cardiac Muscle:** - **Structure:** Striated, with one or two nuclei per cell, and interconnected by intercalated discs that facilitate synchronized contractions. - **Function:** Pumps blood throughout the body by rhythmic, involuntary contractions. - **Contraction Mechanism:** Controlled by the autonomic nervous system and intrinsic conduction system (pacemaker cells). Cardiac muscle cells exhibit automaticity and rhythmicity. **Smooth Muscle:** - **Structure:** Non-striated, with a single nucleus per cell, and found in the walls of hollow organs. - **Function:** Controls involuntary movements such as peristalsis in the digestive tract and vasoconstriction in blood vessels. - **Contraction Mechanism:** Controlled by the autonomic nervous system, hormones, and local factors. Smooth muscle contractions are slow and sustained, often involving a \"latch\" mechanism that allows for prolonged contraction with minimal energy expenditure. **Multiple Choice Questions (MCQs)** 1. **Which type of muscle fibre is most resistant to fatigue and relies primarily on aerobic metabolism?** - a\) Type I - b\) Type IIa - c\) Type IIb - d\) Type IIc 2. **Which of the following describes a third-class lever?** - a\) The effort is between the fulcrum and the load. - b\) The fulcrum is between the effort and the load. - c\) The load is between the fulcrum and the effort. - d\) The effort is applied at the fulcrum. 3. **What triggers the release of calcium ions from the sarcoplasmic reticulum during skeletal muscle contraction?** - a\) Binding of ATP to myosin - b\) Action potential traveling down the T-tubules - c\) Formation of cross-bridges between actin and myosin - d\) Hydrolysis of ATP 4. **Which of the following is not a characteristic of cardiac muscle?** - a\) Striated appearance - b\) Involuntary control - c\) Multinucleated cells - d\) Presence of intercalated discs 5. **Which process produces the most ATP during prolonged muscle activity?** - a\) Glycolysis - b\) Creatine phosphate breakdown - c\) Aerobic respiration - d\) Anaerobic respiration **Clinical Cases** **Case 1: Muscle Fatigue in a Marathon Runner** **Presentation:**\ A 35-year-old marathon runner experiences muscle fatigue and cramping during long-distance races. Despite adequate training, these symptoms persist, particularly during the latter stages of the race. **Discussion:** - **Question:** Explain how muscle fibre types and energy metabolism contribute to muscle fatigue during prolonged exercise. What strategies could help improve the runner\'s performance? - **Answer:** Type I fibres, which rely on aerobic metabolism, are more resistant to fatigue and are primarily used during long-distance running. However, as glycogen stores deplete and reliance on Type II fibres increases, fatigue sets in due to lactic acid build up from anaerobic metabolism. The runner could benefit from nutritional strategies to maintain glycogen levels, as well as training to enhance aerobic capacity and efficiency. **Case 2: Delayed Onset Muscle Soreness (DOMS) in a Weightlifter** **Presentation:**\ A 28-year-old weightlifter complains of muscle soreness and stiffness 24-48 hours after an intense training session. The soreness is most pronounced in the muscles that were heavily loaded during eccentric contractions. **Discussion:** - **Question:** Describe the physiological basis of delayed onset muscle soreness (DOMS) and the role of eccentric contractions in this condition. What measures can be taken to alleviate and prevent DOMS? - **Answer:** DOMS is thought to result from microscopic damage to muscle fibres and connective tissues, particularly during eccentric contractions where the muscle lengthens under tension. This damage triggers inflammation and a repair process, leading to soreness. Measures to alleviate DOMS include adequate warm-up, gradual progression of training intensity, and post-exercise recovery techniques such as stretching and massage.