Topic 3: Chapt 12 Flashcards
What functions do muscles serve in the body?
Muscles serve two common functions: generating motion and generating force. Additionally, skeletal muscles contribute significantly to the homeostasis of body temperature by generating heat, such as through shivering when cold conditions threaten homeostasis.
How many types of muscle tissue are found in the human body, and what are they?
The human body has three types of muscle tissue: skeletal muscle, cardiac muscle, and smooth muscle.
What distinguishes skeletal muscle from cardiac and smooth muscle in terms of appearance?
Skeletal and cardiac muscles are classified as striated muscles due to their alternating light and dark bands seen under the light microscope, while smooth muscle lacks obvious cross-bands.
What is the signal to initiate muscle contraction, and what motor protein is involved?
The signal to initiate muscle contraction is an intracellular calcium signal, and movement is created when a motor protein called myosin uses energy from adenosine triphosphate (ATP) to change its conformation.
Despite differences, what do skeletal, smooth, and cardiac muscles have in common?
All three muscle types share certain properties, including the initiation of muscle contraction by an intracellular calcium signal and the involvement of the motor protein myosin using ATP to create movement.
What percentage of total body weight do skeletal muscles constitute?
Skeletal muscles make up about 40% of total body weight.
What are tendons made of?
Tendons are made of collagen.
What is the origin of a muscle, and what is its insertion?
The origin of a muscle is the end of the muscle that is attached closest to the trunk or to the more stationary bone, while the insertion of the muscle is the more distal or more mobile attachment.
Define flexor and extensor muscles and their respective movements.
A muscle is called a flexor if the centers of the connected bones are brought closer together when the muscle contracts, resulting in flexion. Conversely, a muscle is called an extensor if the bones move away from each other when the muscle contracts, resulting in extension.
What are flexor-extensor pairs called, and why?
Flexor-extensor pairs are called antagonistic muscle groups because they exert opposite effects.
Provide an example of an antagonistic muscle pair in the arm.
An example of an antagonistic muscle pair in the arm is the biceps brachii, which acts as the flexor, and the triceps brachii, which acts as the extensor.
Describe the movements associated with the contraction of the biceps brachii and the triceps brachii.
When the biceps brachii contracts, the hand and forearm move toward the shoulder, resulting in flexion. Conversely, when the triceps brachii contracts, the flexed forearm moves away from the shoulder, resulting in extension.
What is the structure of a skeletal muscle fiber?
A skeletal muscle fiber is a long, cylindrical cell with up to several hundred nuclei near the surface of the fiber.
What are satellite cells, and what is their role?
Satellite cells are committed stem cells located just outside the muscle fiber membrane. They become active and differentiate into muscle when needed for muscle growth and repair.
Describe the arrangement of muscle fibers within a muscle.
The fibers in a given muscle are arranged with their long axes in parallel, bundled together into units called fascicles.
What is the connective tissue sheath that encloses the entire muscle?
The entire muscle is enclosed in a connective tissue sheath that is continuous with the connective tissue around the muscle fibers and fascicles and with the tendons holding the muscle to underlying bones.
What are the main intracellular structures in striated muscles, and what is their function?
The main intracellular structures in striated muscles are myofibrils, highly organized bundles of contractile and elastic proteins that carry out the work of contraction.
What is the function of the sarcoplasmic reticulum (SR) in muscle fibers?
The sarcoplasmic reticulum concentrates and sequesters calcium ions (Ca2+) with the help of a Ca2+ ATPase in the SR membrane. Calcium release from the SR creates calcium signals that play a key role in contraction in all types of muscle.
What are t-tubules, and what is their function?
T-tubules are a branching network of transverse tubules that are adjacent to and closely associated with the terminal cisternae of the sarcoplasmic reticulum. They allow action potentials to move rapidly from the cell surface into the interior of the fiber, facilitating simultaneous calcium release from the terminal cisternae and muscle contraction.
What is the role of glycogen and mitochondria in muscle fibers?
Glycogen serves as a reserve source of energy, while mitochondria produce much of the ATP required for muscle contraction through oxidative phosphorylation of glucose and other biomolecules.
What are myofibrils, and what is their composition?
Myofibrils are contractile structures found within muscle fibers. They are composed of several types of proteins, including myosin, actin, tropomyosin, troponin, titin, and nebulin
Describe the structure and function of myosin.
Myosin is a motor protein consisting of two identical protein chains, each with one large heavy chain plus two smaller light chains. It forms thick filaments in muscle fibers and contains a motor domain that uses energy from ATP to create movement, as well as binding sites for actin.
What is actin, and what role does it play in muscle contraction?
Actin is a protein that makes up the thin filaments of muscle fibers. It forms long chains or filaments that, in skeletal muscle, twist together to create the thin filaments. Actin provides binding sites for myosin crossbridges, facilitating muscle contraction.
What is the structure and function of a sarcomere?
A sarcomere is the functional unit of a myofibril, responsible for muscle contraction. It consists of alternating light and dark bands and includes the following structures:
-Z disks: Zigzag protein structures serving as attachment sites for thin filaments.
-I bands: Light bands occupied only by thin filaments.
-A bands: Dark bands encompassing the entire length of thick filaments.
-H zones: Central regions of A bands occupied by thick filaments only.
-M lines: Bands representing proteins that form attachment sites for thick filaments, dividing A bands in half.
What is the role of titin in muscle physiology?
Titin is a large elastic protein that stretches from one Z disk to the neighboring M line, stabilizing the position of contractile filaments and returning stretched muscles to their resting length. It plays a crucial role in muscle elasticity.
How does nebulin contribute to muscle function?
Nebulin is an inelastic giant protein found alongside thin filaments, attaching to the Z disk. It helps align actin filaments within the sarcomere, ensuring proper filament alignment during muscle contraction
What is muscle tension, and what role does it play in muscle physiology?
Muscle tension is the force created by contracting muscle fibers, enabling movement or resistance against a load. It is a crucial aspect of muscle physiology, allowing for various physical activities.
Describe the major steps leading up to skeletal muscle contraction.
The major steps leading up to skeletal muscle contraction are as follows:
- Events at the neuromuscular junction: This involves the conversion of an acetylcholine signal from a somatic motor neuron into an electrical signal in the muscle fiber. It initiates the process of muscle excitation.
- Excitation-contraction (E-C) coupling: This process translates muscle action potentials into calcium signals within the muscle fiber. These calcium signals trigger the contraction-relaxation cycle.
- Contraction-relaxation cycle at the molecular level: The sliding filament theory of contraction explains how actin and myosin filaments interact within the muscle fiber, leading to muscle contraction and relaxation.
What is the sliding filament theory of muscle contraction, and how was it developed?
The sliding filament theory of muscle contraction proposes that overlapping actin and myosin filaments of fixed length slide past one another in an energy-requiring process, resulting in muscle contraction. This theory was developed by scientists Andrew Huxley and Rolf Niedergerke in 1954, who discovered that the length of the A band of a myofibril remains constant during contraction, leading them to propose this alternative model.
Describe the changes that occur in a sarcomere during muscle contraction according to the sliding filament theory.
During muscle contraction, the Z disks of the sarcomere move closer together as the sarcomere shortens. The I band and H zone, regions where actin and myosin do not overlap in resting muscle, almost disappear. Despite the shortening of the sarcomere, the length of the A band remains constant. These changes are consistent with the sliding of thin actin filaments along the thick myosin filaments as the actin filaments move toward the M line in the center of the sarcomere.
How does the sliding filament theory explain muscle contraction without movement?
The sliding filament theory explains how a muscle can contract and create force without necessarily creating movement. For example, when pushing against a wall, tension is generated in the muscles without moving the wall. According to the theory, tension generated in a muscle fiber is directly proportional to the number of high-force crossbridges between the thick and thin filaments, allowing for force generation without movement.
How does the movement of myosin crossbridges contribute to muscle contraction?
The movement of myosin crossbridges provides force that pushes the actin filament during contraction. Similar to a competitive sailing team raising a heavy mainsail, myosin heads bind to actin molecules (the “rope”), initiating a power stroke where the myosin crossbridges swivel and push the actin filaments toward the center of the sarcomere. This process repeats many times as the muscle fiber contracts.
What initiates the power stroke in muscle contraction?
The power stroke in muscle contraction is initiated by a calcium signal. When calcium ions bind to troponin, it triggers a conformational change in tropomyosin, exposing the myosin-binding sites on actin. This allows myosin crossbridges to bind to actin and initiate the power stroke.
How is the energy for the power stroke generated in muscle contraction?
The energy for the power stroke in muscle contraction comes from ATP. Myosin is an ATPase (myosin ATPase) that hydrolyzes ATP to ADP and inorganic phosphate (Pi). The energy released by ATP hydrolysis is stored as potential energy in the angle between the myosin head and the long axis of the myosin filament. This potential energy is then converted into kinetic energy during the power stroke, moving actin filaments.
How does a calcium signal initiate muscle contraction?
A calcium signal initiates muscle contraction by binding to troponin C, which is part of the troponin complex. When calcium binds to troponin C, it causes a conformational change in the troponin complex, pulling tropomyosin away from actin’s myosin-binding sites. This exposes the binding sites, allowing myosin heads to form strong crossbridges with actin filaments and initiate the power stroke, leading to muscle contraction.
What is the role of troponin in muscle contraction?
Troponin is a calcium-binding complex consisting of three proteins. It controls the positioning of tropomyosin, an elongated protein polymer that wraps around actin filaments and partially covers actin’s myosin-binding sites in resting muscle. When calcium binds to troponin C, troponin undergoes a conformational change that moves tropomyosin away from actin’s myosin-binding sites, allowing muscle contraction to occur
How does muscle relaxation occur after contraction?
Muscle relaxation occurs when cytosolic calcium concentrations decrease. As calcium unbinds from troponin, tropomyosin returns to its “off” position, covering most of actin’s myosin-binding sites. This prevents myosin heads from forming strong crossbridges with actin filaments. During relaxation, the filaments of the sarcomere slide back to their original positions with the help of titin and elastic connective tissues within the muscle.
Describe the molecular events of a contractile cycle in skeletal muscle
the contractile cycle in skeletal muscle starts with the rigor state, where myosin heads are tightly bound to G-actin molecules without ATP or ADP bound to myosin. The cycle proceeds as follows:
- ATP binds and myosin detaches: An ATP molecule binds to the myosin head, causing myosin to release from actin due to decreased actin-binding affinity.
- ATP hydrolysis and cocking of myosin head: ATP hydrolysis occurs, providing energy for the myosin head to rotate and reattach to actin. The myosin head forms a 90° angle with the filaments, storing potential energy.
- Power stroke: Calcium binding to troponin uncovers the myosin-binding sites on actin, allowing the myosin head to swivel and form strong, high-force bonds. The myosin head and hinge region tilt from a 90° angle to a 45° angle, sliding the attached actin filament along with them.
- Myosin releases ADP: At the end of the power stroke, myosin releases ADP, returning to the rigor state and preparing for the next cycle.
What happens in the rigor state of muscle contraction?
In the rigor state, no ATP or ADP is bound to myosin, and myosin heads are tightly bound to G-actin molecules. This state is brief in living muscle fibers, as ATP quickly binds to myosin once ADP is released. However, after death and when ATP supplies are exhausted, muscles remain in the rigor state, leading to rigor mortis where muscles “freeze” due to immovable crossbridges between actin and myosin.
What challenges exist in studying muscle contraction at the molecular level?
Studying muscle contraction at the molecular level faces challenges due to the complexity of the process and limitations of research techniques. Techniques such as crystallization of molecules and electron microscopy cannot be used with living tissues. Additionally, the movement of molecules in a myofibril is difficult to observe, and progress in understanding muscle contraction relies on snapshots of the beginning and end stages of contraction. However, ongoing research aims to overcome these challenges, offering the potential for a more comprehensive understanding of muscle contraction in the future.
Describe the process of excitation-contraction coupling in skeletal muscle.
Excitation-contraction coupling in skeletal muscle involves the following steps:
- Release of Acetylcholine (ACh): Acetylcholine is released from the somatic motor neuron and binds to ACh receptor-channels on the motor end plate of the muscle fiber.
2.Generation of Endplate Potential (EPP): ACh-gated channels open, allowing Na+ and K+ to cross the membrane. Na+ influx exceeds K+ efflux, depolarizing the membrane and generating an endplate potential (EPP).
3.Initiation of Muscle Action Potential: The EPP reaches threshold and initiates a muscle action potential, which propagates along the surface of the muscle fiber and into the t-tubules.
- Calcium Release from Sarcoplasmic Reticulum: The muscle action potential triggers the opening of voltage-gated Na+ channels, leading to Ca2+ release from the sarcoplasmic reticulum.
- Calcium Binding to Troponin: Elevated cytosolic Ca2+ levels cause Ca2+ to bind to troponin, leading to a conformational change in tropomyosin and exposure of myosin-binding sites on actin.
- Initiation of Contraction: With the myosin-binding sites exposed, cross-bridge formation occurs between actin and myosin, initiating muscle contraction.
How is calcium release triggered in excitation-contraction coupling?
Calcium release is triggered in excitation-contraction coupling when the muscle action potential reaches the t-tubules. Voltage-gated L-type calcium channels (DHP receptors) in the t-tubule membrane change conformation in response to depolarization, leading to the opening of ryanodine receptors (RyR) in the sarcoplasmic reticulum. This allows stored calcium to flow into the cytosol, initiating muscle contraction.
What is the role of tropomyosin in muscle contraction?
Tropomyosin plays a crucial role in muscle contraction by regulating the exposure of myosin-binding sites on actin. In the absence of calcium, tropomyosin blocks these binding sites, preventing cross-bridge formation and muscle contraction. When calcium binds to troponin, tropomyosin undergoes a conformational change, allowing myosin to bind to actin and initiate contraction.
What is the latent period in excitation-contraction coupling?
The latent period is the short delay between the muscle action potential and the beginning of muscle tension development during excitation-contraction coupling. This delay represents the time required for calcium release from the sarcoplasmic reticulum and its binding to troponin. Once contraction begins, muscle tension increases steadily to a maximum value before decreasing during relaxation.
Explain how skeletal muscles obtain ATP for contraction.
Skeletal muscles obtain ATP for contraction through various metabolic pathways and energy sources:
- Phosphocreatine (PCr) System: Muscles store a small amount of ATP, which is quickly used up during contraction. Phosphocreatine, another high-energy molecule stored in muscle cells, can rapidly regenerate ATP from ADP during intense exercise. Creatine kinase catalyzes the transfer of phosphate from phosphocreatine to ADP, forming ATP.
- Glycolysis: Glucose is metabolized through glycolysis to produce ATP. This process occurs both aerobically (in the presence of oxygen) and anaerobically (in the absence of oxygen). Aerobic glycolysis yields more ATP per glucose molecule compared to anaerobic glycolysis. However, during strenuous exercise when oxygen supply is limited, anaerobic glycolysis becomes the primary source of ATP production.
- Oxidative Phosphorylation: Pyruvate, the end product of glycolysis, enters the citric acid cycle (Krebs cycle) in the presence of oxygen. This process, known as oxidative phosphorylation, generates a large amount of ATP from the oxidation of glucose.
- Fatty Acid Oxidation: Skeletal muscles can also utilize fatty acids as an energy source, especially during rest and low-intensity exercise. Fatty acids are oxidized through beta-oxidation to produce acetyl-CoA, which enters the citric acid cycle to generate ATP.
- Protein Metabolism: While proteins are not the primary energy source for muscle contraction, amino acids can be metabolized to produce ATP under certain conditions. However, protein metabolism is generally limited compared to carbohydrate and lipid metabolism.
What role does phosphocreatine play in muscle contraction?
Phosphocreatine (PCr) serves as a rapid source of ATP regeneration during intense muscle contraction. Creatine kinase catalyzes the transfer of a phosphate group from phosphocreatine to ADP, forming ATP. This process helps replenish ATP levels quickly, allowing muscles to sustain high-intensity contractions.
