Muscles 1/3 - Textbook Ch. 6

Question 1

LO 1 Compare and contrast microtubules and microfilaments in terms of primary, secondary, tertiary, and quaternary structural levels.

Answer 1

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.

Question 2

LO 1 Distinguish between the motor proteins in terms of structure and function.

Answer 2

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.

Question 3

LO 2 What is the role of energy in construction and use of the cytoskeleton?

Answer 3

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.

Question 4

LO 2 Describe the different ways the actin cytoskeleton can be used to control cell structure and shape.

Answer 4

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.

Question 5

LO 3 Contrast the properties exhibited by myosins that walk on microfilaments with those of thin filaments.

Question 6

LO 3 Discuss the structure and function of myosin within the thick filament.

Answer 6

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.

Question 7

LO 4 What is the relationship between muscle filaments (thick and thin), sarcomeres, myofibrils, and myofibers?

Answer 7

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.

Question 8

LO 4 Discuss the role of -binding proteins in muscle contraction.

Answer 8

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.

Question 9

LO 5 What are the main differences between cardiac and skeletal muscle?

Answer 9

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.

Question 10

LO 5 Contrast the ways smooth and striated muscle rely on thick versus thin filament regulation.

Answer 10

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.

Question 11

LO 6 Compare the contractile properties of sonic and locomotor muscles of fish.

Answer 11

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.

Question 12

LO 6 What are muscle fiber types? How do animals alter muscle fiber types in response to physiological challenges?

Answer 12

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.

Question 13

What genomic and genetic events might have contributed to the expansion of the myosin II family in vertebrates?

Answer 13

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.

Question 14

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?

Answer 14

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.

Question 15

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?

Answer 15

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.

Question 16

Hummingbird hearts beat extremely quickly, at about 30 Hz. Predict what you would find if you examined the structure of a hummingbird cardiomyocyte.

Answer 16

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.

Question 17

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.

Answer 17

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.

Question 18

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.

Answer 18

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.

Question 19

Which motor proteins work with microtubules?

Answer 19

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.

Question 20

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?

Answer 20

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.

Question 21

What factors influence the assembly and disassembly of microtubules and microfilaments?

Answer 21

The assembly and disassembly of microtubules and microfilaments are influenced by several factors, including the concentration of tubulin and actin monomers, which must exceed a certain threshold to enable polymerization. Additionally, various binding proteins play critical roles; for microtubules, proteins like MAPs (microtubule-associated proteins) stabilize the structure, while catastrophe factors promote disassembly. In the case of microfilaments, profilin promotes actin assembly by adding to the growing filament, and cofilin helps in disassembly by severing filaments. Moreover, cellular conditions such as pH, ionic strength, and the presence of ATP (for actin) or GTP (for tubulin) also crucially impact the dynamics of these cytoskeletal elements, with nucleotide hydrolysis driving changes in polymer stability. Temperature and regulatory signals from within the cell can further modulate the assembly processes, aligning cytoskeletal behavior with cellular needs and environmental cues.

Question 22

What is meant by polarity with respect to microfilaments and microtubules? Why is it important to structure and function?

Answer 22

Polarity in microfilaments and microtubules refers to the structural orientation of these cytoskeletal elements, where each end distinctly differs in terms of its behavior and chemical properties. Microtubules have a “plus” end that grows rapidly and a “minus” end that grows more slowly, whereas microfilaments feature a “barbed” (plus) end that elongates faster than the “pointed” (minus) end. This polarity is crucial because it determines the directionality of growth and shrinkage, essential for processes like cell division, where microtubules must organize into a spindle to segregate chromosomes accurately. In cellular motility, polarity allows microfilaments to push or pull the cell in specific directions. Additionally, the directional transport of vesicles and organelles along these structures is facilitated by motor proteins that recognize and move directionally along the polarity of these filaments, underscoring the functional importance of this characteristic in cellular organization and dynamics.

Question 23

Describe duty cycle and unitary displacement in relation to nonmuscle and muscle myosin activity.

Answer 23

Duty cycle and unitary displacement are two critical parameters that describe the activity of myosin motors, both in muscle and nonmuscle cells. The duty cycle of a myosin motor refers to the proportion of the total cycle time during which the myosin head remains attached to the actin filament. This is crucial for understanding how long a myosin can generate force before it releases and reattaches. Muscle myosins typically have a high duty cycle, meaning they spend more time attached to actin, which is necessary for sustained muscle contraction. In contrast, nonmuscle myosins may have a shorter duty cycle, allowing for more dynamic rearrangements of the actin cytoskeleton in cellular processes like migration or division. Unitary displacement describes the distance a myosin head moves along an actin filament in a single power stroke. Muscle myosins often exhibit larger unitary displacements, facilitating efficient sliding of actin filaments for contraction, while nonmuscle myosins might have shorter displacements, suitable for their roles in cell tension and maintenance rather than gross movement. Together, these parameters help define the mechanical efficiency and roles of different myosin types across cellular activities.

Question 24

How does the organization of the sarcomere influence contractile force?

Answer 24

The organization of the sarcomere, the fundamental unit of muscle contraction in striated muscle, plays a critical role in influencing contractile force. Each sarcomere is bordered by Z-discs to which actin filaments are anchored, with myosin filaments positioned in between. The precise, alternating arrangement of these thick and thin filaments allows for maximum interaction during muscle contraction. When a muscle contracts, the myosin heads bind to actin filaments and pull them toward the center of the sarcomere, effectively shortening the sarcomere and generating tension. The greater the overlap between actin and myosin filaments, the more cross-bridges can form, which enhances the contractile force. Additionally, the regular, repeating structure of the sarcomere ensures uniform force transmission along the muscle fiber, optimizing the muscle’s overall efficiency and power in contraction. This organized structure allows muscles to effectively convert biochemical energy into mechanical work.

Question 25

Compare the constraints on myosin function in vesicle traffic versus the contractile apparatus.

Answer 25

Myosin proteins play versatile roles in cellular functions, but they operate under different constraints when involved in vesicle traffic compared to their roles in the contractile apparatus. In vesicle trafficking, myosin motors, particularly those from the myosin I and V families, facilitate the transport of vesicles along actin filaments within cells. These myosins are adapted to carry small loads over long distances and often work in a processive manner, meaning they can take multiple steps along an actin filament without detaching. This requires high duty ratios to maintain constant contact with actin filaments, ensuring smooth transport of vesicles to precise locations within the cell. Conversely, in the contractile apparatus of muscle cells, such as in sarcomeres, myosin II motors function under the requirement to generate large forces and perform rapid, powerful contractions. These myosins operate with a different set of mechanical properties; they have lower duty ratios, which allow for quick attachment and detachment from actin to facilitate fast, pulsatile contractions. This is critical for efficient muscle function where rapid and synchronized contractions are necessary. Thus, the functional constraints on myosin in these two systems reflect adaptations to their specific cellular roles – sustained transport versus rapid force generation.

Question 26

Is muscle activity more accurately described as cellular movement or a change in cell shape? What types of cells need to move within the vertebrate body?

Answer 26

Muscle activity can be more accurately described as a change in cell shape rather than cellular movement. Muscle cells, or myocytes, primarily contract and relax, altering their length and shape to generate force and movement of the body or body parts, without the cells themselves relocating. This change in shape is crucial for functions ranging from locomotion to the pumping of the heart. In contrast, cellular movement, where entire cells traverse within the body, is critical in other cell types such as immune cells (e.g., leukocytes) that need to navigate through the bloodstream and tissues to reach sites of infection or injury. Additionally, embryonic cells move during development to form various structures, and metastatic cancer cells move as they spread from their original location to other parts of the body. These processes involve intricate mechanisms of cellular motility and adhesion, differing fundamentally from muscle activity which is predominantly about shape change to facilitate movement.

Question 27

What channels determine depolarization and repolarization in muscles?

Answer 27

In muscle cells, the depolarization and repolarization phases of the action potential are primarily determined by the activity of specific ion channels. Depolarization is typically initiated by the opening of voltage-gated sodium (Na+) channels, which allow an influx of Na+ ions into the muscle cell, causing the membrane potential to become less negative. This rapid influx of Na+ leads to the depolarized state necessary for muscle contraction. Repolarization, the process that restores the resting membrane potential, is achieved mainly through the opening of voltage-gated potassium (K+) channels. These channels facilitate the efflux of K+ ions from the muscle cell, returning the membrane potential to its more negative resting state. Additionally, voltage-gated calcium (Ca2+) channels also play a role, particularly in cardiac and smooth muscle cells, where they help in maintaining a prolonged depolarization phase necessary for longer contractions. These ionic movements through specific channels are crucial for the cyclic process of contraction and relaxation in muscle tissue.

Question 28

How does the refractory period differ between skeletal and cardiac muscle? Why is this important to the function of the muscle?

Answer 28

The refractory period in skeletal and cardiac muscles serves distinct functions and differs in duration, which is crucial for their respective roles. In skeletal muscle, the refractory period is relatively short, allowing for rapid and frequent stimulation of muscle fibers, which is essential for voluntary and finely controlled movements. This short refractory period enables skeletal muscles to relax quickly after contraction, preparing them for the next voluntary contraction. In contrast, cardiac muscle has a longer refractory period, preventing the reinitiation of an action potential too soon after the first. This characteristic is vital to ensure that the heart chambers have enough time to refill with blood after each contraction, supporting efficient blood pumping. The longer refractory period in cardiac muscle prevents tetanus (a sustained contraction), which is essential for maintaining a rhythmic and effective heartbeat. This difference in refractory periods underscores the adaptation of each muscle type to its specific functional demands in the body.

Question 29

What is the role of T-tubules? Which type of muscle would have abundant T-tubules?

Answer 29

T-tubules, or transverse tubules, play a critical role in ensuring that an action potential rapidly penetrates the interior of the muscle cell, triggering a uniform and synchronized contraction. They are invaginations of the sarcolemma (muscle cell membrane) that facilitate the deep and rapid spread of electrical signals into the muscle fiber, ensuring that calcium ions are released uniformly from the sarcoplasmic reticulum across the cell. T-tubules are especially abundant in skeletal and cardiac muscle, where quick and coordinated contractions are essential. Skeletal muscle fibers, which require rapid and forceful contractions for movement, and cardiac muscle cells, which must contract in a coordinated manner to efficiently pump blood, both rely heavily on well-developed T-tubule systems to function effectively.

Question 30

What is the role of terminal cisternae? Which type of muscle would have abundant terminal cisternae?

Answer 30

Terminal cisternae are specialized regions of the sarcoplasmic reticulum in muscle cells that are crucial for storing and releasing calcium ions, which are essential for muscle contraction. When a muscle is stimulated to contract, calcium ions are rapidly released from the terminal cisternae into the surrounding muscle cytoplasm to initiate contraction. This process is facilitated by the close association between the terminal cisternae and T-tubules, which transmit the action potential from the surface to the interior of the muscle fiber. Terminal cisternae are particularly abundant in skeletal muscle, which requires quick and robust responses to stimulation for rapid, forceful contractions. This structural arrangement allows for efficient and synchronized muscle contractions necessary for movement and stability.

Question 31

Distinguish between depolarization-induced release and -induced release. What role do -binding proteins play in contraction and relaxation?

Answer 31

Depolarization-induced calcium release occurs when an action potential triggers the opening of voltage-sensitive channels in the sarcoplasmic reticulum of muscle cells, leading to the release of calcium ions into the cytosol. This type of release is primarily associated with skeletal muscles, where rapid and direct electrical stimulation is needed for contraction. In contrast, calcium-induced calcium release happens when the influx of calcium ions through channels in the plasma membrane triggers further release of calcium from the sarcoplasmic reticulum; this mechanism is typical in cardiac muscle cells, supporting the rhythmic contractions of the heart. Calcium-binding proteins play pivotal roles in both contraction and relaxation of muscles. During contraction, these proteins, such as troponin and calmodulin, bind calcium ions, initiating structural changes in muscle fibers that enable contraction. For relaxation, these proteins help in pumping calcium back into the sarcoplasmic reticulum and removing calcium from the cytosol, thus stopping the contraction process. This dynamic regulation of calcium levels ensures that muscles can efficiently cycle through contraction and relaxation phases.

Question 32

Does smooth muscle have actin and myosin? Does it have thick and thin filaments? Does it have sarcomeres?

Answer 32

Smooth muscle indeed contains actin and myosin, which are crucial for its contraction. These proteins are organized into thick and thin filaments, similar to those in skeletal and cardiac muscles. However, unlike these other muscle types, smooth muscle does not have sarcomeres, the regular arrangement of repeating structural units that characterize striated muscle. Instead, the actin and myosin filaments in smooth muscle are distributed throughout the cell in a network-like arrangement, which allows for slow, sustained, and uniform contractions across the muscle tissue. This structure is well-suited to the functions of smooth muscle, which include controlling the diameter of blood vessels and moving contents through the gastrointestinal tract.

Question 33

Discuss the regulation of smooth muscle contractile properties through and -independent mechanisms.

Answer 33

Smooth muscle contraction is regulated both by calcium (Ca²⁺)-dependent and Ca²⁺-independent mechanisms. Ca²⁺-dependent pathways involve the increase of intracellular Ca²⁺, which binds to calmodulin, activating myosin light chain kinase (MLCK) that phosphorylates myosin to initiate contraction. In Ca²⁺-independent regulation, contraction is controlled through the phosphorylation of myosin light chain by various kinases, like Rho-kinase, which can modulate myosin's interaction with actin independent of Ca²⁺ levels. These pathways allow smooth muscle to maintain tone and regulate contractile force in response to diverse physiological signals, ensuring versatility in functions such as vasodilation, peristalsis, and maintaining arterial pressure.

Question 34

Compare and contrast EC coupling in synchronous and asynchronous insect flight muscles.

Answer 34

In insects, excitation-contraction (EC) coupling in flight muscles can be either synchronous or asynchronous, each adapted to different flight mechanisms. Synchronous flight muscles, found in insects like bees and flies, contract once for each nerve impulse received, directly linking wing beats to neural signals. This type of muscle depends on precise timing between nerve impulses and muscle contractions for rapid but controlled wing movements. Asynchronous flight muscles, found in insects like dragonflies, can contract multiple times for a single neural impulse, utilizing an internal mechanism that allows the muscle to "ratchet" up to higher frequencies than those of the neural inputs. This enables very rapid wing beats independent of direct neural control, allowing for both high speed and energy efficiency. The key difference lies in how the muscle's internal properties and neural control mechanisms are tuned for either direct control or speed and efficiency.

Question 35

What are EPSPs?

Answer 35

Excitatory postsynaptic potentials (EPSPs) are temporary increases in the membrane potential of a postsynaptic neuron in response to the reception of neurotransmitters released by a presynaptic neuron. These neurotransmitters bind to receptor molecules on the postsynaptic neuron, leading to the opening of ion channels that typically allow positively charged ions to enter the neuron. This influx of positive ions makes the inside of the neuron less negatively charged relative to the outside, thus depolarizing the postsynaptic membrane. If the depolarization is sufficient and reaches a certain threshold, it triggers an action potential, which can lead to neural firing. EPSPs are crucial in the process of synaptic transmission and neural communication, influencing how signals are propagated through neural circuits.

Question 36

Briefly describe three different muscles that are able to maintain a contraction for long periods: tonic muscle, smooth muscle (latch), and a mollusk catch muscle.

Answer 36

Tonic muscle, smooth muscle in latch state, and mollusk catch muscle are three types of muscle that can maintain contraction for prolonged periods without fatiguing quickly. Tonic muscles, found in vertebrates, maintain posture and slow, sustained contractions by slowly cycling their cross-bridges, which requires less energy than fast-twitch fibers. Smooth muscles in a latch state, common in organs like the uterus or blood vessels, maintain tension over extended periods through a mechanism where dephosphorylated myosin heads remain attached to actin filaments longer than usual, effectively "latching" to maintain force without constant ATP expenditure. Mollusk catch muscles, present in bivalves like clams, can stay contracted for long durations by a unique mechanism involving a regulatory protein that locks myosin and actin together, thus avoiding fatigue while keeping the shell closed against predators or environmental stressors. Each of these muscle types has adapted to their specific functional demands by evolving mechanisms that allow them to sustain force using minimal energy over extended times.

Question 37

Why are heater organs and electric organs considered modified muscles?

Answer 37

Heater organs and electric organs are considered modified muscles because they have evolved from muscle tissue to perform specialized functions. Heater organs, found in some species of fish like the opah and certain sharks, have modified skeletal muscle that generates heat through rapid, repetitive muscle contractions, helping to maintain a higher body temperature which can enhance metabolic processes and swimming speed. Electric organs, seen in fish such as electric eels and rays, derive from muscle cells that have evolved into electrocytes. These specialized cells lose their contractile ability but instead stack in columns to generate bioelectric fields used for navigation, communication, or defense by rapidly altering the electrical potential across their membranes. Both organs exemplify the remarkable adaptability of muscle tissue to meet diverse ecological needs beyond locomotion.

Question 38

Most cells gather the ends of microtubules near the nucleus of the cell at the ___________________ (MTOC)

Answer 38

Most cells gather the ends of microtubules near the nucleus of the cell at the _microtubule-organizing center_ (MTOC)

Question 39

α-tubulin vs β-tubulin?

Answer 39

α-tubulin and β-tubulin are two closely related proteins that form the fundamental building blocks of microtubules, which are key components of the cytoskeleton in eukaryotic cells. Both α-tubulin and β-tubulin bind to GTP and assemble into heterodimers, which are the subunits that polymerize head-to-tail to form the hollow cylindrical structures of microtubules. While α-tubulin is permanently bound to GTP, β-tubulin can hydrolyze GTP to GDP after polymerization. This GTP hydrolysis is critical for the dynamic instability of microtubules, allowing them to rapidly assemble and disassemble, which is essential for many cellular processes including cell division, intracellular transport, and cell shape maintenance. Despite their structural similarities and cooperative function, these proteins are encoded by different genes, allowing for subtle variations in their roles and regulation within the cell.

Question 40

protofilament?

Answer 40

A protofilament is a linear chain of tubulin dimers that forms the structural basis of a microtubule, an essential component of the cytoskeleton in eukaryotic cells. Each protofilament consists of alternating α-tubulin and β-tubulin subunits, linked head-to-tail, creating a polar structure with distinct plus and minus ends. In microtubules, typically thirteen protofilaments align parallel to each other and join laterally to form a hollow tube. This arrangement provides stability and allows for dynamic assembly and disassembly, which are vital for cellular processes such as mitosis, intracellular transport, and maintenance of cell shape and structure. The protofilaments are crucial for the polymerization dynamics of microtubules, particularly through their interactions with various microtubule-associated proteins that regulate their stability and function.

Question 41

microtubule-associated proteins?

Answer 41

Microtubule-associated proteins (MAPs) play crucial roles in regulating the structure and function of microtubules within the cytoskeleton of eukaryotic cells. These proteins affect microtubule dynamics by stabilizing or destabilizing microtubules, mediating their interactions with other cellular components, and influencing their assembly and disassembly. MAPs are diverse, with some like tau and MAP2 primarily stabilizing microtubules in neuronal axons and dendrites, respectively, enhancing microtubule assembly and promoting stability against depolymerization. Other MAPs, such as kinesin and dynein, are motor proteins that transport cellular cargo along microtubule tracks, critical for intracellular trafficking and proper cell function. This regulation is vital for various cell activities, including cell division, cell signaling, and maintenance of cell shape.

Question 42

kinesin vs dynein

Answer 42

Kinesin and dynein are motor proteins that play essential roles in intracellular transport by moving along microtubule tracks, but they differ significantly in their directionality and function. Kinesin generally moves cargo toward the plus end of microtubules, which typically corresponds to the periphery of the cell, making it crucial for transporting materials like vesicles, organelles, and proteins from the cell center towards the outer regions. In contrast, dynein moves toward the minus end of microtubules, directing cargo towards the cell nucleus and generally involved in retrograde transport. This directional movement is essential for processes like mitosis, organelle positioning, and vesicle trafficking. Additionally, kinesin usually has a straight, processive motion, while dynein is known for its more complex and forceful action, often working in large complexes and capable of transporting larger cargoes over long distances.

Question 43

true or false? Cilia and flagella are composed of microtubules

Answer 43

true! Cilia and flagella are similar structures with diverse roles in animal physiology. For example, flagella propel sperm toward the egg, while cilia allow epithelial cells to push mucus over the cell surface, as occurs in the respiratory tract.

Question 44

axoneme?

Answer 44

The axoneme is the structural core of cilia and flagella, essential for their motility and sensory functions. It typically consists of a "9+2" arrangement of microtubules: nine outer doublet microtubules surrounding a central pair of single microtubules. This structure is linked by various proteins, including nexin links and dynein arms, which facilitate the sliding of microtubules against each other to produce bending movements. The axoneme's organized structure and dynamic capabilities are crucial for processes such as cell motility, fluid movement across epithelial surfaces, and the functioning of sensory cells.

Question 45

Filopodia?

Answer 45

Filopodia are thin rodlike extensions of cells formed by actin fibers. Cells build filopodia for many purposes. For example, nerve cells use filopodia to make physical contact with neighboring cells, which is an important step in the embryonic development of the nervous system. Digestive epithelia use filopodia to build microvilli.

Question 46

microfilaments vs microtubules?

Answer 46

Microfilaments are composed of long strings of the protein actin. These actin monomers are called G-actin, because of the globular structure of the protein. Microfilaments and microtubules are key components of the cytoskeleton in eukaryotic cells, playing critical roles in maintaining cell shape, enabling movement, and facilitating intracellular transport. Microfilaments, composed primarily of actin, are thin and flexible, crucial for muscle contraction, cell division, and cytokinesis. They provide structural support, particularly in the cell cortex, and are integral to cellular motility through structures like lamellipodia and filopodia. Microtubules, on the other hand, are thicker and more rigid, made from tubulin subunits. They form a structural framework used for intracellular transport of organelles and vesicles, with motor proteins like kinesin and dynein moving along them. Microtubules also play a vital role in cell division by forming the mitotic spindle, which segregates chromosomes during mitosis.

Question 47

Lamellipodia

Answer 47

Lamellipodia resemble the pseudopodia found in protists, but they are thinner and more sheetlike.

Question 48

acrosome reaction?

Answer 48

The acrosome reaction is a crucial process in fertilization, where the sperm releases digestive enzymes to penetrate the egg's outer layers. This reaction occurs when the sperm comes into contact with the zona pellucida, a glycoprotein layer surrounding the egg. The acrosome, a cap-like structure over the anterior part of the sperm's head, contains these enzymes. Upon activation, the acrosome undergoes a series of changes that result in the fusion of the outer acrosomal membrane with the sperm's plasma membrane, releasing the enzymes. This release allows the sperm to digest a path through the zona pellucida, enabling it to reach and fuse with the egg membrane, initiating fertilization.

Question 49

Myosin I and V vs Myosin II jobs?

Answer 49

Myosin I and Myosin V are used for intracellular traffic! Myosin II is "muscle myosin", but can be found in other tissues.

Question 50

myosin light chains?

Answer 50

Myosin light chains are essential components of the myosin motor protein complex, primarily found in muscle cells, where they play a critical role in muscle contraction. These light chains can be divided into two types: regulatory and essential light chains. The regulatory light chains modulate myosin activity in response to changes in calcium ion concentrations, thereby influencing muscle contraction and relaxation. The essential light chains, on the other hand, stabilize the lever arm of the myosin head, which is crucial for the power stroke that drives muscle contraction. By attaching to the neck region of the myosin head, these light chains enhance the efficiency and control of the contraction cycle.

Question 51

sliding filament model

Answer 51

The sliding filament model is a well-established explanation for how muscles contract to produce movement. According to this model, muscle contraction occurs when the thin filaments (actin) and thick filaments (myosin) within the muscle fiber slide past each other, shortening the overall length of the sarcomere—the functional unit of a muscle fiber. Myosin heads on the thick filaments bind to actin on the thin filaments and pull them inward, which causes the sarcomere to shorten and generate force. This process is powered by ATP and is regulated by calcium ions and troponin-tropomyosin complexes that control the binding sites on actin. This mechanism allows muscles to contract smoothly and efficiently, driving movement across a wide range of activities.