UNIT 2 – Actin Filaments (Q&A) | MZO-001 MSCZOO | IGNOU

SAQ 1

a) Microfilaments, composed of the protein ……………………play a key role in cell movement and contraction.

b) The cytoskeleton is essential for the proper segregation of chromosomes during ……………………

c) The cytoskeleton is crucial for the maintenance of ……………………the balance of solutes inside and outside the cell. Answer: homeostasis.

d) The dynamic nature of actin filaments involves constant ……………………and depolymerization.

e) Actin-binding proteins like ……………………regulate the branching of actin filaments.

f) In treadmilling, the addition of……………………to the barbed end occurs while monomers are simultaneously removed from the pointed end.

Answers: (a) actin, (b) mitosis, (c) homeostasis, (d) polymerisation, (e) Arp2/3 complex, (f) G-actin

SAQ 2 

a) What is the cortical cytoskeleton?

The cortical cytoskeleton is a specialized and dynamic part of the cell's cytoskeleton located just beneath the plasma membrane. It is primarily composed of actin filaments (microfilaments) along with associated proteins like spectrin, ankyrin, filamin and ERM (ezrin, radixin, moesin) proteins. This region forms a dense network of filamentous proteins that help maintain the cell's shape, provide mechanical support and regulate interactions with the external environment. The cortical cytoskeleton plays a key role in cellular processes like endocytosis, cell motility, signal transduction and adhesion, especially in animal cells.

Structure of the Cortical Cytoskeleton

The main structural element of the cortical cytoskeleton is F-actin, which forms a thin, mesh-like layer closely attached to the inner surface of the plasma membrane. This layer is cross-linked by actin-binding proteins such as filamin and is anchored to membrane proteins via adaptor proteins like spectrin and ankyrin. The actin network is not static; it is continuously undergoing polymerization and depolymerization, which allows the cell to adapt its shape and mechanical properties in response to internal and external signals. The thickness of this actin-rich layer varies with the type and condition of the cell but typically remains between 100–500 nm.

Functions of Cortical Cytoskeleton

There are five main functions of the cortical cytoskeleton:

1. Maintenance of Cell Shape:
  • It provides mechanical strength to the cell membrane and helps the cell resist deformation, especially during external pressure or stress.
2. Cell Motility and Migration:
  • By controlling actin polymerization at the cell edges, it enables amoeboid or crawling-type movement in cells like leukocytes, fibroblasts, and epithelial cells.
3. Endocytosis and Exocytosis:
  • It facilitates vesicle formation and membrane trafficking by coordinating with clathrin and dynamin complexes during endocytosis and vesicle docking during exocytosis.
4. Cell Adhesion and Cortical Tension:
  • It stabilizes cell junctions and generates cortical tension required for tissue integrity and cell positioning within tissues.
5. Signal Transduction:
  • It acts as a platform for membrane-bound signaling complexes and transduces signals from the external environment to the internal cytosol.

b) Which proteins are involved in tropomyosin movement?

Tropomyosin is a long, coiled-coil protein that wraps around the length of actin filaments in both muscle and non-muscle cells. Its primary role is to regulate the accessibility of myosin-binding sites on actin filaments. In resting muscle cells, tropomyosin covers these binding sites and prevents interaction between actin and myosin. However, during muscle activation, tropomyosin shifts its position to uncover the sites, allowing contraction to occur. This movement of tropomyosin is not spontaneous or random because it is highly regulated and controlled by several proteins. These regulatory proteins can be classified into two major types based on the nature of their involvement: proteins with a direct role, which actively cause the movement of tropomyosin and proteins with an indirect role, which support or influence this process without directly causing the shift. Understanding which proteins fall into each category helps clarify the precise molecular events involved in muscle contraction and cytoskeletal regulation.

1. Proteins with Direct Role in Tropomyosin Movement

There is only one main protein complex that directly moves tropomyosin i.e., the troponin complex. This complex is unique to striated muscles and plays an active role in calcium-regulated muscle contraction. The troponin complex is composed of three subunits:
  1. Troponin C (TnC): This subunit binds calcium ions during muscle excitation.
  2. Troponin I (TnI): This inhibitory subunit normally prevents the interaction between actin and myosin by stabilizing tropomyosin's blocking position.
  3. Troponin T (TnT): This subunit binds to tropomyosin and anchors the entire troponin complex to the actin-tropomyosin filament.
When calcium ions are released from the sarcoplasmic reticulum during muscle contraction, they bind to TnC. This binding triggers a conformational change that shifts the position of TnI and TnT, pulling tropomyosin away from the myosin-binding sites on actin. This exposure allows myosin heads to bind actin and generate contraction. Thus, troponin complex is the only known protein complex that causes the active and direct movement of tropomyosin.

2. Proteins with Indirect Role in Tropomyosin Movement

In non-muscle cells, where troponin is absent, tropomyosin movement is passive and largely depends on the reorganization of the actin cytoskeleton by proteins such as formin, Arp2/3 complex, cofilin, profilin, nebulin, tropomodulin and actin itself.

Although these proteins do not move tropomyosin by themselves, they influence its movement or positioning indirectly by modifying the structure or arrangement of the actin-tropomyosin complex.
  • Myosin: Myosin cannot move tropomyosin, but it binds to actin only after tropomyosin has moved. Therefore, its activity depends on tropomyosin displacement.
  • Actin: Tropomyosin is bound along actin filaments, so any structural change in actin (such as polymerization or branching) may indirectly reposition tropomyosin.
  • Tropomodulin: This protein caps the minus end of actin filaments and binds tropomyosin, helping to stabilize its linear alignment.
  • Nebulin: Found mainly in skeletal muscle, nebulin serves as a scaffold that helps keep tropomyosin in the correct position along actin filaments.
  • Formin and Arp2/3 complex: These actin-nucleating proteins regulate the growth and branching of actin filaments in non-muscle cells, indirectly changing tropomyosin’s location.
  • Cofilin and Profilin: These proteins modify actin dynamics in non-muscle cells and can influence where tropomyosin binds.

c) Discuss the structure of sarcomere.

The sarcomere is the basic contractile unit of striated muscle fibres, including skeletal and cardiac muscles. It is a highly organized, repeating structural unit of the myofibril, extending from one Z-line to the next. Its detailed architecture was clarified with the introduction of the sliding filament theory in 1954 by A.F. Huxley and R. Niedergerke and also H.E. Huxley and J. Hanson, who independently explained the mechanism of contraction based on filament sliding. The unique arrangement of thick and thin filaments within the sarcomere provides both its structure and function, making it the key unit responsible for muscle contraction.

Each sarcomere is composed of distinct regions, named based on their appearance under light and electron microscopy, and these regions directly relate to the arrangement of actin and myosin filaments.

Components of Sarcomere

The sarcomere spans from one Z-line to another Z-line and its structure is divided into several key components, each with a unique function. The primary components of the sarcomere include:

1. Z-line (Z-disc):

  • The Z-line is a dense structure that forms the boundary of each sarcomere. It anchors the thin (actin) filaments. The Z-line is crucial for maintaining the structural integrity of the sarcomere and ensuring the proper alignment of the contractile filaments. Z-lines help in transmitting the force generated during muscle contraction.

2. A-band:

  • The A-band is the region where both thick (myosin) and thin (actin) filaments overlap. It appears dark under a microscope due to the overlap of these filaments. The A-band remains constant in length during muscle contraction, as the thick and thin filaments do not change in size but slide past each other.

3. I-band:

  • The I-band is the region that only contains thin (actin) filaments. This band appears light under a microscope because it lacks the thick myosin filaments. The I-band shortens during muscle contraction as the actin filaments slide toward the M-line, reducing the space between Z-lines.

4. H-zone:

  • The H-zone is the central part of the A-band where only thick (myosin) filaments are present. During muscle contraction, the H-zone decreases in size as the actin filaments slide over the myosin filaments.

5. M-line:

  • The M-line is the middle of the sarcomere, located in the center of the H-zone. It holds the thick myosin filaments in place and helps maintain the structure of the sarcomere during contraction.

Filamentous Proteins in Sarcomere

The sarcomere, as the basic contractile unit of muscle fibers, contains intricate protein structures essential for its function. These filamentous proteins, mainly actin and myosin, interact to facilitate muscle contraction through the sliding filament mechanism. Additionally, several other structural proteins help maintain the organization and stability of the sarcomere, ensuring efficient muscle movement.

1. Actin (Thin Filament):

  • Actin is a globular protein that polymerizes into long, thin filaments. It forms the thin filaments in the sarcomere, extending from the Z-line toward the M-line. Actin interacts with myosin during muscle contraction, leading to the sliding filament mechanism.

2. Myosin (Thick Filament):

  • Myosin is a motor protein that forms the thick filaments in the sarcomere. Each myosin molecule has a tail and two globular heads. The heads bind to actin filaments and use ATP hydrolysis to generate the force needed for muscle contraction.

3. Titin:

  • Titin is a large structural protein that spans from the Z-line to the M-line. It plays a key role in maintaining the sarcomere's structural integrity, acting as a spring that helps the sarcomere return to its resting state after contraction.

4. Nebulin:

  • Nebulin is another structural protein that is associated with actin filaments. It helps regulate the length of the thin filaments and ensures proper alignment of actin filaments in the sarcomere.

Supporting Structural Proteins

1. α-actinin:

  • α-actinin is found in the Z-line, where it anchors actin filaments. It plays a key role in stabilizing the Z-line structure and contributes to the overall organization of the sarcomere.

2. Myomesin:

  • Myomesin is located in the M-line and is responsible for anchoring the thick myosin filaments in place. It helps maintain the structural integrity of the sarcomere during contraction.

3. CapZ:

  • CapZ is a protein that caps the plus end of the actin filaments at the Z-line. This prevents the addition of new actin subunits to the filaments, thus helping to maintain their length during muscle contraction.

4. Tropomodulin:

  • Tropomodulin caps the minus end of the actin filaments, preventing the disassembly of actin at the Z-line. It helps stabilize the actin filaments and maintains their length.

5. Desmin:

  • Desmin is an intermediate filament protein that links adjacent myofibrils at the Z-line. It provides structural support and helps to transmit the contractile force across the muscle fibers.
The sarcomere is the basic contractile unit of striated muscle fibres, including skeletal and cardiac muscles. It is a highly organized, repeating structural unit of the myofibril, extending from one Z-line to the next. Its detailed architecture was clarified with the introduction of the sliding filament theory in 1954 by A.F. Huxley and R. Niedergerke and also H.E. Huxley and J. Hanson, who independently explained the mechanism of contraction based on filament sliding.


d) What is the role of actin and myosin in muscle contraction?

Actin and myosin are two primary filamentous proteins that play a direct and central role in muscle contraction. These proteins are located inside the sarcomere, which is the basic contractile unit of striated muscle fibers. Their interaction is responsible for producing the force and movement necessary for muscle contraction, based on the Sliding Filament Theory proposed independently by Huxley and Niedergerke, and Huxley and Hanson in 1954.

Role of Actin

Actin forms the thin filaments of the sarcomere. It is a polymer of globular actin (G-actin), which forms a helical structure called filamentous actin (F-actin). Each actin filament contains binding sites for the heads of myosin. These sites are normally blocked by a regulatory protein called tropomyosin, which is held in place by a calcium-sensitive complex known as troponin. When a nerve impulse stimulates the muscle, calcium ions are released from the sarcoplasmic reticulum. These ions bind to troponin-C, leading to a conformational change that moves tropomyosin away from the actin binding sites. This unblocking allows myosin heads to attach to actin and initiate contraction.

Role of Myosin

Myosin forms the thick filaments of the sarcomere. It is a motor protein that has two heads and a long tail. Each myosin head contains an ATPase enzyme and a binding site for actin. When the actin-binding sites are exposed, myosin heads bind to actin, forming a cross-bridge. Using energy from ATP hydrolysis, the myosin head undergoes a conformational change called the power stroke, which pulls the actin filament toward the center of the sarcomere (M-line). The myosin head then detaches, re-cocks using a new ATP molecule, and repeats the cycle.

Together, these repeated cycles of cross-bridge formation and power strokes shorten the sarcomere length, resulting in contraction of the entire muscle fiber. Thus, actin provides the track and regulatory control, while myosin provides the driving force of contraction.
Actin and myosin are two primary filamentous proteins that play a direct and central role in muscle contraction. These proteins are located inside the sarcomere, which is the basic contractile unit of striated muscle fibers.

e) Define the following terms: I band, A band and H-zone.

In striated muscle fibers such as skeletal and cardiac muscles, the basic contractile unit is the sarcomere, which is the region between two Z-lines. Each sarcomere contains two main types of filaments: thin filaments (actin) and thick filaments (myosin). These filaments are arranged in a specific, overlapping pattern that produces the characteristic alternating light and dark bands seen under the microscope. Based on the arrangement and degree of filament overlap, the sarcomere displays three main horizontal zones: I band, A band, and H-zone. Understanding these zones is essential for grasping how muscles contract and how structural changes are translated into mechanical force.

1. I Band (Isotropic Band):

The I band is the lighter region of the sarcomere that contains only thin filaments (actin). It does not have any thick filaments, and for this reason, it appears less dense under the microscope. The I band stretches from the end of one thick filament in a sarcomere to the beginning of a thick filament in the adjacent sarcomere. This band is bisected by the Z line (or Z disc), which serves as the point of anchorage for the thin filaments from two neighboring sarcomeres. During muscle contraction, the I band shortens significantly as the thin filaments slide over the thick filaments, bringing the Z lines closer together.

2. A Band (Anisotropic Band):

The A band is the dark band seen in a sarcomere and represents the entire length of the thick filaments (myosin). This region also includes areas where thick and thin filaments overlap, which contributes to its darker appearance. Unlike the I band, the A band remains unchanged in length during contraction because the thick filaments do not change their length. The central part of the A band contains only thick filaments (forming the H-zone), while the lateral parts include overlapping thin filaments.

3. H-Zone (Hensen's Zone):

The H-zone is the lighter central part of the A band where there is no overlap between thick and thin filaments. It contains only thick filaments (myosin) and is visible only when the muscle is in a relaxed state. During muscle contraction, as the actin filaments slide inwards, the H-zone becomes narrower or even disappears because thin filaments move into this region and begin to overlap with the thick filaments.

SAQ 3 

a) Define the term: "rigor mortis".

Rigor mortis is a postmortem physiological phenomenon in which the muscles of a dead body become stiff and rigid due to a biochemical condition that prevents the relaxation of muscle fibers. It is directly related to the structure and function of sarcomeres, especially the role of actin and myosin filaments in muscle contraction. Under normal living conditions, muscle contraction is an active process that requires ATP for both the contraction and relaxation phases. After death, the production of ATP ceases completely, leading to an irreversible binding between the actin and myosin filaments in the sarcomere, causing the muscles to remain in a contracted state. This permanent cross-bridge formation without ATP results in the stiffness that characterizes rigor mortis.

The process of rigor mortis begins approximately 2 to 6 hours after death, depending on environmental conditions like temperature, cause of death and metabolic activity at the time of death. The stiffness first appears in the smaller muscles such as those of the face and jaw, and gradually spreads to the larger muscles of the trunk and limbs. The condition reaches its maximum within 12 hours, during which the entire musculature becomes stiff and inflexible. After this peak, rigor mortis begins to resolve or disappear within 24 to 48 hours as cellular enzymes (like lysosomal enzymes) start breaking down the muscle proteins during decomposition, a process called autolysis.

From a structural perspective, rigor mortis gives a practical demonstration of how critical ATP is for normal muscle function. In the living body, ATP breaks the actin-myosin cross-bridge after contraction, allowing the muscle to relax. In the absence of ATP, the myosin head remains attached to actin, locking the muscle in a contracted position.

In forensic medicine, rigor mortis is a key tool used to estimate the time since death, known as the postmortem interval (PMI). By observing the degree and location of rigor mortis in a body, forensic pathologists can estimate how long a person has been dead, especially in the early postmortem period. Thus, rigor mortis not only has physiological relevance but also major practical significance in postmortem examinations.

b) What filaments that move to shorten muscle according to sliding filament theory of muscle contraction?

The filament that moves to shorten a muscle during contraction is actin.

Actin is a thin protein filament that slides inward during the contraction process. According to the sliding filament theory, the thick filament called myosin stays mostly in place and forms cross-bridges with actin. These cross-bridges pull the actin filaments toward the center of the sarcomere. As actin slides inward, the sarcomere becomes shorter which leads to overall muscle shortening. This means that actin is the filament that actually changes position during contraction. Myosin only helps in pulling actin by using ATP but it does not move itself. That is why actin is the correct answer for the filament responsible for shortening the muscle.

c) What is the most fundamental unit of muscle contraction?

The most fundamental unit of muscle contraction is the sarcomere. 

Sarcomere is the basic structural and functional unit of striated muscle fibers. A sarcomere is the segment of a myofibril that lies between two Z-lines. It contains organized arrangements of actin (thin) and myosin (thick) filaments. During muscle contraction, these filaments slide past each other, leading to the shortening of the sarcomere. As sarcomeres contract together in large numbers, the entire muscle fiber shortens, resulting in muscle contraction. Therefore, sarcomeres are the smallest repeating units responsible for the contractile activity in skeletal and cardiac muscles.

TERMINAL QUESTIONS 

1. What is the cortical cytoskeleton? Describe its significance.

The cortical cytoskeleton is a specialized layer of cytoskeletal elements located just beneath the plasma membrane of eukaryotic cells. It is primarily composed of actin filaments, though it may also involve other cytoskeletal components such as intermediate filaments and microtubules in some cases. This structure provides mechanical support to the cell, helping it maintain its shape, and is involved in various cellular processes such as cell movement, division and response to external signals. The cortical cytoskeleton plays a vital role in maintaining cell integrity and enabling cells to interact with their environment.

Significance of Cortical Cytoskeleton

1. Maintaining Cell Shape

  • The cortical cytoskeleton plays a major role in determining and stabilizing the shape of the cell. It acts as a scaffold that supports the plasma membrane and prevents excessive deformation under mechanical stress. This network ensures that the cell maintains its structure and resilience, which is vital for its survival.

2. Cell Movement and Motility

  • The actin filaments in the cortical cytoskeleton are essential for cellular movement. They allow the cell to extend pseudopodia, form lamellipodia (broad, sheet-like protrusions), or create filopodia (finger-like projections) during cell migration. This ability to move is crucial during processes like wound healing, immune cell trafficking and the development of the embryo.

3. Cell Division (Cytokinesis)

  • During cell division, the cortical cytoskeleton is essential for cytokinesis, the final phase of cell division, where the cytoplasm divides. The actin filaments form a contractile ring at the center of the cell that pinches the cell into two daughter cells. This ensures that the division process is smooth and accurate, maintaining proper cell numbers.

4. Intracellular Transport and Signal Transduction

  • The cortical cytoskeleton is also involved in intracellular transport. It helps in organizing vesicles, organelles and other materials to their correct locations within the cell. In addition, it supports signal transduction pathways, allowing the cell to respond to external signals like hormones, growth factors and other environmental stimuli.

5. Neuronal Function and Synaptic Plasticity

  • In neurons, the cortical cytoskeleton contributes to the formation and maintenance of dendritic spines, small protrusions on dendrites where synapses are located. These spines are involved in synaptic plasticity, a process crucial for learning and memory. The ability of neurons to change and adapt their connections is, therefore, directly influenced by the cortical cytoskeleton.

2. How do different drugs affect the actin filament?

The effect of different drugs on actin filaments plays a crucial role in understanding their impact on various cellular functions, such as cell motility, division and the overall structural integrity of the cell. Actin filaments are an essential component of the cytoskeleton, providing mechanical support and helping in cellular processes. Drugs can influence the dynamics of actin filaments by either promoting or inhibiting their assembly, stability, or disassembly. The regulation of actin filaments by drugs is important for manipulating cellular functions. 

There are two major ways drugs can affect actin filaments:
  1. by inhibiting actin polymerization
  2. by stabilizing the filaments

1. Drugs that inhibit actin polymerization

These drugs prevent the polymerization of actin monomers into long, functional actin filaments, which is crucial for many cellular processes like cell shape maintenance, movement and division.

Cytochalasins:

  • This family of drugs binds to the barbed end of actin filaments, blocking the addition of new actin monomers. This inhibition of actin polymerization prevents the formation of new actin filaments and disrupts cellular processes like cytokinesis and cell motility. Cytochalasins can also cause the disassembly of actin filaments that are already present, leading to changes in cell shape and function.

Latrunculin:

  • This drug works by binding to actin monomers, preventing them from polymerizing into filaments. Latrunculin is particularly used in experimental settings to study the effects of actin depolymerization on cell movement, morphology and signaling. It can lead to the disassembly of actin filaments and significantly affect processes like cell migration and wound healing.

2. Drugs that stabilize actin filaments

Some drugs help stabilize the structure of actin filaments, promoting the assembly or preventing their disassembly. These drugs affect the dynamic nature of the actin cytoskeleton.

Phalloidin:

  • Phalloidin, derived from the deadly Amanita mushroom, binds tightly to actin filaments, stabilizing them and preventing their disassembly. While this stabilization helps in maintaining the structure of the cytoskeleton, it also interferes with the normal turnover of actin filaments that is essential for cellular functions like cell movement and division. The drug is often used in research to visualize actin filaments, as it enhances the stability of actin and makes it easier to observe.

Jasplakinolide:

  • This drug stabilizes actin filaments by promoting their polymerization and inhibiting their depolymerization. Jasplakinolide induces the formation of actin filaments and stabilizes them, which can impact cellular processes like migration, morphogenesis and apoptosis. It is used in research to study the role of actin dynamics in various cellular processes.

3. What is the correct order of muscle contraction from beginning to end?

Muscle contraction is a fundamental biological process responsible for generating force, enabling movement, and supporting various physiological functions like breathing, blood circulation and voluntary movements. This process involves the interaction of actin and myosin filaments, which slide past each other to shorten muscle fibers. This mechanism is explained through the sliding filament theory, which describes how actin (thin) and myosin (thick) filaments work together within the sarcomere to produce contraction.

The contraction is initiated by an electrical signal from the nervous system and follows a series of steps that ultimately lead to muscle shortening and force generation. These steps encompass the interaction of actin and myosin, regulated by calcium ions, which trigger the sliding of the filaments and cause the muscle to contract. 

The process of muscle contraction involves following key steps:
  1. Nerve Impulse and Acetylcholine Release at Neuromuscular Junction
  2. Action Potential Propagation Through Sarcolemma and T-Tubules
  3. Calcium Release from Sarcoplasmic Reticulum
  4. Calcium Binding to Troponin and Exposure of Actin Binding Sites
  5. Cross-Bridge Formation Between Myosin and Actin
  6. Power Stroke and Sliding of Filaments
  7. ATP Binding and Cross-Bridge Detachment
  8. ATP Hydrolysis and Reactivation of Myosin Head

1. Nerve Impulse and Acetylcholine Release at Neuromuscular Junction

Muscle contraction begins when a motor neuron sends an electrical signal (nerve impulse) to the muscle fiber. At the neuromuscular junction, this impulse causes the release of a neurotransmitter called acetylcholine (ACh) into the synaptic cleft. ACh binds to receptors on the sarcolemma (muscle cell membrane), triggering the next step.

2. Action Potential Propagation Through Sarcolemma and T-Tubules

Binding of ACh to its receptor causes depolarization of the sarcolemma, generating an action potential. This electrical signal quickly spreads along the sarcolemma and deep into the muscle fiber through the T-tubules (transverse tubules). This ensures that the signal reaches all parts of the muscle fiber simultaneously.

3. Calcium Release from Sarcoplasmic Reticulum

As the action potential travels through the T-tubules, it stimulates the sarcoplasmic reticulum (SR) to release calcium ions (Ca²⁺). The SR is a special organelle that stores calcium, which is essential for muscle contraction. The calcium ions are released into the cytosol of the muscle fiber.

4. Calcium Binding to Troponin and Exposure of Actin Binding Sites

The released Ca²⁺ ions bind to a protein called troponin, which is present on the thin filament (actin). This causes a conformational change in the troponin-tropomyosin complex, which moves tropomyosin away from the myosin-binding sites on actin filaments. These exposed sites are now ready for interaction with myosin heads.

5. Cross-Bridge Formation Between Myosin and Actin

Once the actin binding sites are exposed, the myosin heads from thick filaments (which are already energized by ATP hydrolysis) attach to them. This physical connection between actin and myosin is known as a cross-bridge. This is a key moment where the mechanical part of contraction is initiated.

6. Power Stroke and Sliding of Filaments

After cross-bridge formation, the myosin head pivots, pulling the actin filament toward the center of the sarcomere. This movement is called the power stroke. As a result, the thin filaments slide over the thick filaments, shortening the sarcomere and generating muscle contraction. The ADP and Pi are released during this process.

7. ATP Binding and Cross-Bridge Detachment

To detach the myosin head from actin, a new ATP molecule must bind to the myosin head. This binding causes the cross-bridge to break, allowing the myosin head to release the actin filament. Without ATP, the myosin would remain stuck to actin, as seen in rigor mortis after death.

8. ATP Hydrolysis and Reactivation of Myosin Head

The ATP bound to the myosin head is now hydrolyzed to ADP and Pi. This reaction re-energizes the myosin head and returns it to its original high-energy position. If calcium is still present, the cycle repeats, leading to continuous contraction. When calcium is removed and ACh is broken down, the muscle relaxes.
Muscle contraction is a fundamental biological process responsible for generating force, enabling movement, and supporting various physiological functions like breathing, blood circulation and voluntary movements. This process involves the interaction of actin and myosin filaments, which slide past each other to shorten muscle fibers. This mechanism is explained


4. Find the right combination by matching the columns:

[A] P (a); Q (d); R (c); S (b) 
[B] P (d); Q (c); R (a); S (b) 
[C] P (a); Q (b); R (c); S (d) 
[D] P (c); Q (d); R (b); S (a)

Answer: [B] P (d); Q (c); R (a); S (b) 






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Actin and myosin are two primary filamentous proteins that play a direct and central role in muscle contraction. These proteins are located inside the sarcomere, which is the basic contractile unit of striated muscle fibers. Their interaction is responsible for producing the force and movement necessary for muscle contraction, based on the  Sliding Filament Theory  proposed independently by Huxley and Niedergerke, and Huxley and Hanson in 1954. Role of Actin Actin forms the  thin filaments  of the sarcomere. It is a polymer of globular actin (G-actin), which forms a helical structure called  filamentous actin (F-actin).  Each actin filament contains binding sites for the heads of myosin. These sites are normally blocked by a regulatory protein called  tropomyosin,  which is held in place by a calcium-sensitive complex known as  troponin.  When a nerve impulse stimulates the muscle, calcium ions are released from the sarcoplasmic reticulu...
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Fluid mosaic model of the plasma membrane

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The fluid mosaic model is one of the most widely accepted and foundational concepts that explains the structural organization of the plasma membrane in living cells. It was first proposed by  S.J. Singer and Garth L. Nicolson in 1972  and remains relevant today, though slightly refined with modern findings. Before we move into the components and structure, it is important to understand that this model helps explain how the membrane remains flexible, selectively permeable, and functionally active for processes like transport, cell signaling and communication. Basic Concept of the Fluid Mosaic Model According to the fluid mosaic model, the plasma membrane is viewed as a dynamic, semi-fluid bilayer of lipids with proteins embedded in it, much like boats floating in a sea. The word  "fluid"  refers to the lateral movement of lipids and some proteins within the membrane, while  "mosaic"  refers to the patchwork arrangement of various proteins that float freely o...
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Write the name of two glands are the main secretary gland of brain

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The brain is not only the central organ of the nervous system but also contains two major secretory glands that are directly involved in endocrine functions. These are: Pituitary gland (Hypophysis) Pineal gland (Epiphysis cerebri) These glands are part of the neuroendocrine system, which links the brain and hormonal regulation. They help in maintaining homeostasis, regulating growth, metabolism, stress response, sleep cycle, reproduction and many other vital processes. 1. Pituitary Gland: The Master Gland The pituitary gland is called the  "master gland"  because it controls the activity of almost all other endocrine glands in the body. It is located at the base of the brain, in a bony cavity called the sella turcica of the sphenoid bone. It is connected to the  hypothalamus  by a stalk called the  infundibulum,  which carries signals from the brain to control hormone secretion. The pituitary gland has two main lobes: i. Anterior Pituitary (Adenohypophysis)...
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What is the cortical cytoskeleton?

The cortical cytoskeleton is a specialized and dynamic part of the cell's cytoskeleton located just beneath the plasma membrane. It is primarily composed of  actin filaments  (microfilaments) along with associated proteins like spectrin, ankyrin, filamin and ERM (ezrin, radixin, moesin) proteins. This region forms a dense network of filamentous proteins that help maintain the cell's shape, provide mechanical support and regulate interactions with the external environment. The cortical cytoskeleton plays a key role in cellular processes like endocytosis, cell motility, signal transduction and adhesion, especially in animal cells. Structure of the Cortical Cytoskeleton The main structural element of the cortical cytoskeleton is  F-actin,  which forms a thin, mesh-like layer closely attached to the inner surface of the plasma membrane. This layer is cross-linked by actin-binding proteins such as filamin and is anchored to membrane proteins via adaptor proteins like spec...
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Write the name of neurotransmitter which act as neuromodulator as well as inhibitor of neurotransmitter

One of the best-known neurotransmitters that shows both neuromodulatory and inhibitory functions is  GABA (Gamma-Aminobutyric Acid).  It is a major chemical messenger in the central nervous system (CNS) and plays a vital role in regulating brain activity. GABA is unique because it not only participates in fast synaptic transmission as an inhibitory neurotransmitter but also acts more broadly as a neuromodulator that controls the excitability of entire neural circuits. 1. GABA as a Neuromodulator As a neuromodulator, GABA works at a slower and more widespread level than fast synaptic transmission. Instead of targeting a single postsynaptic neuron, it can influence large groups of neurons or entire regions of the brain. Neuromodulatory action of GABA usually happens through GABA-B receptors, which are metabotropic and linked with second messenger systems. Through these pathways, GABA can regulate the activity of other neurotransmitter systems like: Glutamate (main excitatory neu...
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