Muscles 1/3 - Textbook Ch. 6 Flashcards
Study for quz 3 and final
LO 1 Compare and contrast microtubules and microfilaments in terms of primary, secondary, tertiary, and quaternary structural levels.
Microtubules and microfilaments are key components of the cytoskeleton in cells, differing significantly in their structural organization across various levels. At the primary structural level, microtubules are composed of tubulin proteins, specifically α-tubulin and β-tubulin, which form heterodimers. Microfilaments, on the other hand, are primarily made up of actin monomers. At the secondary structural level, these tubulin dimers align head-to-tail to form protofilaments, a linear polymer, while actin monomers similarly polymerize into a double helical structure. The tertiary structure of microtubules involves the assembly of about 13 protofilaments into a hollow tube, providing structural support and pathways for intracellular transport. Microfilaments, while also helical, are much thinner and primarily function in muscle contraction, cell motility, and maintaining cell shape. In terms of quaternary structure, microtubules are more involved in complex assemblies like the mitotic spindle and are capable of interacting with various motor proteins (e.g., kinesin and dynein), whereas microfilaments form complex networks and bundles aided by cross-linking proteins (e.g., filamin), playing critical roles in cellular dynamics and architecture. These differences highlight the specialized roles that microtubules and microfilaments play in cellular organization and function.
LO 1 Distinguish between the motor proteins in terms of structure and function.
Motor proteins, crucial for cellular movement and transport, vary in structure and function, primarily exemplified by kinesin, dynein, and myosin. Kinesin typically moves along microtubules, transporting cellular cargo towards the microtubule’s plus end (typically away from the cell center), and is structurally characterized by two heavy chains with motor domains that have ATPase activity, and two light chains that bind cargo. Dynein, also microtubule-based, moves towards the minus end (towards the cell center), and is larger and more complex than kinesin, featuring a heavy chain with a motor domain that powers movement via ATP hydrolysis, and several intermediate and light chains that help attach to cargo and regulate activity. Myosin, mainly found in muscle fibers but also in other cell types, interacts with actin filaments instead of microtubules. It is involved in muscle contraction, cytoplasmic streaming, and vesicle transport within cells. Myosin’s structure typically includes a head that binds to actin and hydrolyzes ATP to facilitate movement, along with a tail region that determines the specific cellular function by binding to different cargoes and proteins. Each of these motor proteins is adapted to specific cellular tasks, reflecting their distinct structural attributes and functional roles in intracellular transport and mobility.
LO 2 What is the role of energy in construction and use of the cytoskeleton?
The cytoskeleton, an intricate network of filaments within cells, plays a vital role in maintaining cell shape, enabling movement, and facilitating intracellular transport, all of which require energy. The construction and remodeling of the cytoskeleton are energy-dependent processes, primarily involving the hydrolysis of ATP. For example, the polymerization and depolymerization of actin filaments and microtubules are regulated by ATP and GTP hydrolysis, respectively. This energy release not only provides the necessary power for adding or removing subunits from these structures but also drives the dynamic instability that characterizes microtubule behavior. Additionally, motor proteins like kinesin, dynein, and myosin, which move along these filaments, also rely on the hydrolysis of ATP to facilitate their movements and transport of cellular components. Thus, energy, in the form of nucleotide triphosphates like ATP and GTP, is fundamental to the construction, maintenance, and functionality of the cytoskeleton, affecting both its stability and its ability to undergo rapid reorganization in response to cellular needs.
LO 2 Describe the different ways the actin cytoskeleton can be used to control cell structure and shape.
The actin cytoskeleton is fundamental to cellular structure and dynamics, facilitating a range of functions crucial for cell shape, movement, and internal organization. It achieves this through mechanisms like polymerization and depolymerization, which allow cells to extend and retract projections for movement or environmental adaptation. Actin’s cortical meshwork underpins the cell’s mechanical strength and shape, while stress fibers within the cell facilitate contractility and adhesion. During cell division, actin forms a contractile ring that assists in the physical separation of daughter cells. It also plays roles in processes such as endocytosis, ensuring efficient internalization and trafficking of cellular materials, and in signal transduction, helping to relay and amplify signals within the cell. Collectively, these activities highlight the actin cytoskeleton’s critical role in maintaining and modifying cellular architecture and responsiveness.
LO 3 Contrast the properties exhibited by myosins that walk on microfilaments with those of thin filaments.
LO 3 Discuss the structure and function of myosin within the thick filament.
Myosins and thin filaments (primarily composed of actin) are essential components of the cytoskeleton with distinct but complementary roles in cellular mechanics. Myosins are motor proteins that “walk” along actin filaments using ATP-driven movements, which enable them to transport cellular cargo, generate force, and participate in muscle contraction. They interact dynamically with actin filaments, exerting force through a power stroke mechanism that is crucial for processes such as cell motility and cytokinesis. In contrast, actin filaments, or thin filaments, provide structural pathways for myosin movement and serve as integral structural elements that stabilize and shape the cell. They do not inherently generate force or perform active transport but rather form a stable yet dynamic scaffold that supports various cellular functions. This relationship highlights a complex interplay where myosin converts chemical energy into mechanical work on a static but adaptable actin framework, essential for numerous cellular activities.
LO 4 What is the relationship between muscle filaments (thick and thin), sarcomeres, myofibrils, and myofibers?
Myosin, a crucial motor protein, forms the backbone of thick filaments primarily found in muscle cells. Each myosin molecule consists of a tail and two heads, which bind to actin filaments and ATP molecules. The heads undergo conformational changes that enable them to perform power strokes, essential for muscle contraction. These thick filaments are arranged in a staggered array, with the heads projecting outward at regular intervals, ready to interact with actin thin filaments during contraction cycles. The collective sliding of these myosin heads along actin filaments, driven by ATP hydrolysis, results in the shortening of muscle fibers, fundamental to muscle contraction and movement. Thus, myosin in thick filaments is integral not only for structure but also for converting chemical energy into mechanical force in muscle cells.
LO 4 Discuss the role of -binding proteins in muscle contraction.
Calcium-binding proteins play a pivotal role in muscle contraction by regulating the interaction between actin and myosin, the primary proteins responsible for muscle contraction. In skeletal muscle, the most notable calcium-binding protein is troponin. When calcium ions bind to troponin, it causes a conformational change in another protein called tropomyosin, which normally blocks the myosin-binding sites on actin filaments. The shift in tropomyosin’s position exposes these binding sites, allowing myosin heads to attach to actin and initiate contraction through their power-stroke movement. This mechanism is essential for controlling the start and stop of muscle contraction in response to neural stimuli and calcium ion levels, highlighting the critical regulatory role of calcium-binding proteins in muscle physiology.
LO 5 What are the main differences between cardiac and skeletal muscle?
Cardiac and skeletal muscle differ in several key aspects, including structure, control mechanisms, and function. Skeletal muscle is made up of long, cylindrical fibers that are multinucleated and striated, arranged in parallel bundles that contribute to voluntary movements controlled by the somatic nervous system. In contrast, cardiac muscle cells are shorter, branched, and interconnected by intercalated discs which facilitate rapid and synchronized contraction essential for heart function. Cardiac muscle contractions are involuntary and regulated by the autonomic nervous system and the heart’s intrinsic pacemaker cells, allowing it to maintain consistent, rhythmic contractions to pump blood throughout the body. Additionally, cardiac muscle has a higher density of mitochondria and relies almost exclusively on aerobic respiration for energy, reflecting its need for continuous and relentless activity.
LO 5 Contrast the ways smooth and striated muscle rely on thick versus thin filament regulation.
Smooth and striated muscles differ in how they regulate contraction through their thick and thin filaments. In striated muscle, which includes skeletal and cardiac muscle, contraction is regulated primarily by the thin filaments. This regulation is achieved through the troponin-tropomyosin complex which, in response to increases in intracellular calcium levels, exposes myosin-binding sites on actin, allowing thick (myosin) filaments to interact with thin (actin) filaments to facilitate contraction. Conversely, in smooth muscle, the regulation is centered more on the thick filaments. Here, contraction is controlled by the phosphorylation of the light chain of myosin, which is dependent on calcium-calmodulin interactions. This phosphorylation activates myosin ATPase, enabling the interaction of myosin with actin (thin filaments) to initiate contraction. This difference underscores the diverse functional requirements of these muscle types in the body.
LO 6 Compare the contractile properties of sonic and locomotor muscles of fish.
Sonic muscles in fish, which are used to produce sound, and locomotor muscles, which facilitate movement, exhibit distinct contractile properties due to their different functional demands. Sonic muscles are highly specialized for rapid contraction and are among the fastest contracting muscles found in vertebrates. They can contract and relax at very high frequencies, enabling fish to generate sound for communication or mating purposes. These muscles often have a higher density of mitochondria and specific adaptations in their sarcoplasmic reticulum and motor neuron innervation to support these rapid, repeated contractions. In contrast, locomotor muscles are designed for a variety of speeds and endurance levels, supporting swimming through sustained, slower contractions that provide propulsion. They typically have a more varied fiber type composition, allowing them to efficiently perform both sustained and burst activities, depending on the fish’s need for speed or endurance during movement.
LO 6 What are muscle fiber types? How do animals alter muscle fiber types in response to physiological challenges?
Muscle fiber types are categorized based on their speed of contraction, metabolic pathways, and endurance capabilities, commonly referred to as slow-twitch (Type I) and fast-twitch (Type II) fibers. Slow-twitch fibers are highly efficient at using oxygen to generate fuel for sustained activities and are fatigue-resistant, whereas fast-twitch fibers are better suited for short bursts of speed and power but fatigue more quickly. Animals can alter their muscle fiber types in response to various physiological challenges through a process known as muscle plasticity. For instance, endurance training can induce a transformation from fast-twitch to slow-twitch fibers, enhancing efficiency and fatigue resistance. Conversely, activities that require quick, powerful movements, like sprint training, can promote a shift towards more fast-twitch fibers. This adaptability allows animals to optimize muscle performance in response to changes in activity patterns or environmental demands, ensuring survival and reproductive success.
What genomic and genetic events might have contributed to the expansion of the myosin II family in vertebrates?
The expansion of the myosin II family in vertebrates can be attributed to several genomic and genetic events, primarily gene duplication and subsequent diversification. Gene duplication events provide a means for new genetic material to evolve without disrupting the function of the original genes, allowing for the evolution of new functions or the specialization of existing ones. In vertebrates, these duplications likely occurred during two rounds of whole-genome duplication that are thought to have taken place in the early evolution of this group. This genomic duplication would have created multiple copies of myosin II genes, which could then diverge and specialize, leading to the variety of myosin II types observed today. These specialized myosin II proteins could adapt to different muscle types and functions, contributing to the complexity and versatility of vertebrate muscular systems. Further, point mutations and regulatory changes would fine-tune the expression and function of these duplicated genes, enhancing their adaptation to new functional demands in various environmental and physiological contexts.
What enables the cell to produce so many configurations of actin filaments, given that microfilaments and thin filaments are composed of simple repeats of actin?
The ability of cells to produce a diverse array of actin filament configurations, despite their composition of simple actin repeats, is facilitated by the versatility of actin-binding proteins and the intrinsic properties of actin itself. Actin monomers polymerize to form filaments that can twist into various shapes and lengths, depending on the binding and influence of different actin-binding proteins. These proteins, such as formins, Arp2/3, and tropomyosin, control the nucleation, elongation, branching, and stabilization of actin filaments. Additionally, the ATPase activity of actin provides dynamic instability, allowing rapid assembly and disassembly in response to cellular needs. This intricate regulation by numerous modifiers enables the cell to reorganize actin filaments in various configurations, supporting diverse cellular functions like movement, division, and maintaining cell shape.
Describe the molecular processes of neuromuscular excitation, from the sites of neurotransmitter synthesis to release within the muscle.
How do animals use muscle in physiological systems?
Neuromuscular excitation begins with the synthesis of neurotransmitters, primarily acetylcholine, in the motor neuron. These neurotransmitters are then packaged into vesicles and stored at the presynaptic terminal of the neuron. Upon receiving an action potential, voltage-gated calcium channels open, allowing calcium to enter the neuron and trigger the fusion of neurotransmitter vesicles with the presynaptic membrane. The neurotransmitters are released into the synaptic cleft and bind to acetylcholine receptors on the muscle cell membrane, causing it to depolarize. This depolarization triggers an action potential in the muscle cell, leading to muscle contraction through the sliding filament mechanism.
Muscles play a vital role in various physiological systems beyond locomotion. They assist in the cardiovascular system by helping pump blood through the heart and veins, aid in the respiratory system by managing the volume of the lungs for breathing, and support the digestive and excretory systems by helping propel substances through various organs. Additionally, muscle activity generates heat, which is crucial for maintaining body temperature in homeothermic animals.
Hummingbird hearts beat extremely quickly, at about 30 Hz. Predict what you would find if you examined the structure of a hummingbird cardiomyocyte.
Given the extremely rapid heartbeat of a hummingbird, at about 30 Hz, one would predict that the structure of a hummingbird cardiomyocyte would be highly specialized for rapid contraction and relaxation. These cells likely have a very high density of mitochondria to meet the enormous energy demands required for such rapid beating. Additionally, hummingbird cardiomyocytes might exhibit an extensive network of sarcoplasmic reticulum for efficient calcium handling, ensuring quick release and reuptake crucial for rapid muscle relaxation and contraction. The myofibrils themselves are probably densely packed and aligned to facilitate quick and powerful contractions. This specialized cellular architecture supports the hummingbird’s need for a high metabolic rate and the ability to sustain its energetically demanding flight patterns.
The main pathways of energy production are glycolysis and mitochondrial oxidative phosphorylation. Discuss how these metabolic pathways integrate into the EC coupling patterns of different muscles.
The integration of glycolysis and mitochondrial oxidative phosphorylation into excitation-contraction (EC) coupling varies among different muscle types, reflecting their distinct energy demands and functional roles. Fast-twitch muscle fibers, which are adapted for quick, powerful bursts of activity, primarily rely on glycolysis for rapid ATP production, though this pathway is less efficient and results in quicker fatigue. These muscles store more glycogen and have a higher capacity for anaerobic metabolism, aligning with their rapid response to excitation and swift contraction requirements. In contrast, slow-twitch fibers, which are designed for endurance and continuous activity, predominantly utilize oxidative phosphorylation, a more efficient and sustainable energy pathway. These fibers contain more mitochondria and myoglobin, enhancing their capacity for sustained ATP production during prolonged contractions. This differentiation in energy pathways is crucial for the specific EC coupling patterns in muscle fibers, catering to their unique physiological demands and ensuring optimal muscle function across various activities.
Striated muscle cells are postmitotic and can live for the lifetime of the organism. Discuss how this property affects muscle biology, both normally and in disease.
Striated muscle cells, including those in skeletal and cardiac muscle, are postmitotic, meaning they do not divide after their initial formation. This longevity allows muscle cells to provide sustained function throughout an organism’s life but also poses unique challenges in both health and disease. In normal physiology, the longevity of these cells contributes to the stability and endurance of muscle function, crucial for ongoing motor activity and cardiac output. However, their postmitotic nature complicates regeneration processes; when muscle cells are damaged, they cannot simply divide to replace themselves. Instead, repair is primarily managed by satellite cells that differentiate into new muscle cells. In diseases such as muscular dystrophies or in cardiac injury like myocardial infarctions, the inability of the muscle cells to proliferate limits the regeneration capacity, often resulting in scar formation or functional impairment. Thus, understanding and manipulating satellite cell function and other regenerative mechanisms are critical in addressing the deficits caused by the loss or dysfunction of these vital cells.
Which motor proteins work with microtubules?
Motor proteins that interact with microtubules include dyneins and kinesins, which are crucial for intracellular transport and cell division processes. Dyneins are responsible for retrograde transport, moving cargoes toward the minus-end of microtubules, typically oriented towards the cell center, and are essential for ciliary and flagellar beating. Kinesins mostly facilitate anterograde transport, moving cargoes toward the plus-end of microtubules, which generally points toward the cell periphery, and play key roles in mitosis by helping to segregate chromosomes and elongate the spindle. These proteins convert chemical energy from ATP hydrolysis into mechanical work, enabling them to “walk” along microtubules and transport various cellular components, including vesicles, organelles, and protein complexes.
Which plant alkaloids disrupt animal cytoskeleton function? What do they do for the plants? Why might they have no effect on the cytoskeleton in the plants?
Plant alkaloids such as colchicine, vinblastine, and taxol disrupt animal cytoskeleton function by interfering with microtubule dynamics. Colchicine binds to tubulin and prevents its polymerization, vinblastine binds to tubulin as well but leads to tubulin aggregation, and taxol stabilizes microtubules in such a way that they cannot depolymerize. These alkaloids serve as defense mechanisms for plants, deterring herbivores and pathogens by their toxicity. They likely have no effect on the plant cytoskeleton because plants have evolved specific transporters or metabolic pathways that either modify these compounds to render them inactive within their own cells or actively prevent their accumulation in cytoskeletal-active forms, allowing them to selectively target and affect animal cells without harming themselves.
