Muscle Fiber Contraction (B)

ASE109 Structure and Function of the Human Body

The Muscular System

Part B: Muscle Fiber Contraction

Overview of Muscle Fibers

• Definition: A muscle cell is known as a fiber.

• Characteristics of Skeletal Muscle Fiber:

  • Size: A skeletal muscle fiber can be as large as 100 µm in diameter and up to 50 cm in length.

  • Cellularity: Each muscle fiber is multinucleated.

Anatomy of a Muscle Fiber (1)

• A muscle fiber has the usual cellular components, but its terminology differs:

  • Plasma Membrane: Referred to as sarcolemma (where "sarco-" means "muscle").

  • Cytoplasm: Called sarcoplasm.

  • Endoplasmic Reticulum (ER): Called sarcoplasmic reticulum, which stores calcium ions.

Anatomy of a Muscle Fiber (2)

• Unique anatomical characteristics include the T system:

  • T Tubules: Formed by the sarcolemma dipping down into the fiber, which come into contact with expanded portions of the sarcoplasmic reticulum that serve as calcium storage sites.

Anatomy of a Muscle Fiber (3)

• Contains hundreds to thousands of myofibrils (approximately 1 µm in diameter), the contractile components of muscle fibers.

• Other organelles such as mitochondria are found in the sarcoplasm between myofibrils.

• The sarcoplasm holds glycogen for energy and myoglobin, a red pigment that binds oxygen necessary for muscle contraction.

Structure of Skeletal Muscle Fibers

• A skeletal muscle fiber (or skeletal myocyte) is cylindrical in shape.

• Myofibrils: Grouped inside these fibers; they run the entire length of the muscle fiber.

• Myofilaments: Each myofibril is made up of smaller cylinders called myofilaments, leading to a structure of small cylinders within larger muscle fiber cylinders.

Structure of the Myofibril

• Myofibrils consist of smaller filaments arranged in repeating units called sarcomeres.

• Each sarcomere contains:

  • Thick Filaments: Comprised of the protein myosin.

  • Thin Filaments: Comprised of the protein actin.

• Striations: Under the electron microscope, sarcomeres display alternating light and dark bands due to the arrangement of myofilaments:

  • Dark bands represent overlapping myosin and actin, while light bands represent gaps between them.

Summary of Muscle Fiber Anatomy

Name and Function

• Sarcolemma: Plasma membrane of a muscle fiber that forms T tubules.

• Sarcoplasm: Cytoplasm of a muscle fiber containing organelles, including myofibrils.

• Myoglobin: A red pigment that stores oxygen for muscle contraction.

• T Tubule: Extends from the sarcolemma into the muscle fiber and conveys impulses that trigger calcium release from the sarcoplasmic reticulum.

• Sarcoplasmic Reticulum: Smooth ER of a muscle fiber that stores calcium ions.

• Myofibril: A bundle of myofilaments that contracts.

• Myofilament: Composed of actin and myosin filaments responsible for muscle striations and contraction.

Myofibrils and Contraction

• Responsible for muscle fiber contraction as it leads to the shortening of the entire muscle fiber.

• Before understanding how muscles contract, the specific arrangements of myofibrils within a sarcomere must be examined. - Myofibrils are the fundamental rod-like units within muscle fibers, primarily responsible for muscle contraction. These structures are composed of tightly packed cylindrical proteins that enable the muscle fiber to contract and relax efficiently. - The contraction process of muscle fibers occurs when myofibrils shorten, resulting in the overall shortening of the entire muscle fiber. This contraction is a critical mechanism for facilitating movement in the body. - To fully appreciate how muscles contract, it is essential to examine the specific arrangements and organization of myofibrils within a structural unit called the sarcomere. Sarcomeres are the basic repeating units of myofibrils, consisting of various proteins that interact to produce contraction. - Within the sarcomere, myofilaments, including thick and thin filaments (comprised of myosin and actin, respectively), are strategically arranged. When these filaments slide past each other during contraction, they cause the sarcomere to shorten, ultimately leading to the shortening of the entire adjacent muscle fiber. This interaction demonstrates the sliding filament model of muscle contraction, which is crucial for normal muscular function and movement.

Sarcomere Structure

• A Band: A wide dark band of overlapping thick and thin filaments situated in the center of the sarcomere.

• H Zone: A narrow light region found within the A band, indicating the space between two sets of thin filaments.

• M Line: Central line in the H zone consisting of proteins linking adjacent thick filaments.

• I Band: Light band containing portions of thin filaments not overlapping thick filaments.

• Z Line: The point at which two sets of thin filaments are anchored, marking the boundaries of a sarcomere.

Filament Composition

• Thick Filaments: Comprised of several hundred molecules of myosin, resembling a golf club where the straight portion is like the handle and the globular head forms cross bridges with actin

• Thin Filaments: Made of two intertwining strands of actin.

• During contraction, the thin filaments slide over the thick filaments.

Sliding Filament Model of Muscle Contraction

• As the thin filaments slide past the thick filaments, the sarcomere shortens, leading to muscle fiber contraction.

• Filaments remain the same length during contraction; however, the I band shortens, and the Z lines move inward while the H band nearly disappears. ### ASE109 Structure and Function of the Human Body #### The Muscular System ##### Part B: Muscle Fiber Contraction #### Detailed Sliding Filament Model of Muscle Contraction - **Mechanism of Sliding:** As the thin filaments (actin) slide past the thick filaments (myosin), the sarcomere shortens, leading to muscle fiber contraction. This process is orchestrated by the cyclical attachment and detachment of myosin heads on specific binding sites along the actin filaments. - **Structural Changes During Contraction:** While the individual filaments (actin and myosin) remain the same length, the macroscopic appearance of the myofibril changes significantly: - **I Band:** This region, containing only thin filaments, shortens as thin filaments are pulled toward the center. - **Z Lines:** These boundaries move inward, closer to one another, reducing the overall length of the sarcomere. - **H Zone:** Located in the center of the A band, the H zone contains only thick filaments; it narrows and can nearly disappear as thin filaments meet or overlap in the center. - **A Band:** Notably, the total width of the A band remains constant because the length of the thick filaments does not change. - **Energy and Regulation:** - **ATP Contribution:** $ATP$ supplies the chemical energy required for the mechanical work of contraction. Myosin filaments act as an ATPase, breaking down $ATP \rightarrow ADP + P$ to energize the cross-bridge pull. - **Regulatory Proteins:** The sliding is regulated by the troponin-tropomyosin complex. In a resting state, tropomyosin blocks the binding sites. Upon stimulation, $Ca^{2+}$ binds to troponin, causing a conformational change that shifts tropomyosin and allows the myosin heads to grab the actin. - **The All-or-None Principle:** At the cellular level, once a muscle fiber is stimulated beyond its threshold by acetylcholine ($ACh$) at the neuromuscular junction, it contracts to its fullest extent relative to the available $Ca^{2+}$ and $ATP$.

• Energy Contribution: ATP supplies the energy for muscle contraction, with myosin filaments breaking down ATP as their cross-bridges pull actin filaments toward the center of the sarcomere.

Mechanism of Muscle Fiber Contraction

Motor Neuron Stimulation

• Muscle fibers contract in response to stimulation by motor neurons

  • The axon terminals of motor neurons do not directly touch the sarcolemma but are close to it, separated by the synaptic cleft.

• This region is called the neuromuscular junction.

Neuromuscular Junction

• Each motor neuron has several branches ending in axon terminals, allowing one motor neuron to stimulate multiple muscle fibers.

• Neurotransmitter Release: Axon terminals contain synaptic vesicles filled with acetylcholine (ACh), which is released into the synaptic cleft upon nerve signals reaching the axon terminal.

Muscle Fiber Contraction: Role of Acetylcholine

• Binding of ACh to receptors on the sarcolemma generates electrical signals that spread across the sarcolemma and down the T tubules.

• Ca2+ Release: Signals from T tubules cause calcium ions to release from the sarcoplasmic reticulum, leading to sarcomere shortening and muscle fiber contraction. #### Neuromuscular Junction - Each motor neuron has several branches ending in axon terminals, allowing one motor neuron to stimulate multiple muscle fibers. - **Neurotransmitter Release:** Axon terminals contain synaptic vesicles filled with **acetylcholine (ACh)**, which is released into the synaptic cleft upon nerve signals reaching the axon terminal. - **Function of the Synaptic Cleft:** The synaptic cleft is the small gap between the axon terminal and the muscle fiber's sarcolemma, facilitating communication between the nerve and muscle. - **Role of Acetylcholine (ACh):** ACh binds to specific receptors on the sarcolemma, initiating a series of events that leads to muscle contraction. This binding creates an action potential in the muscle fiber that propagates along the sarcolemma and into the T-tubules. - **Calcium Ion Release:** The action potential triggers the release of calcium ions from the sarcoplasmic reticulum into the sarcoplasm, which is essential for muscle contraction. - **Termination of Signal:** The action of ACh is terminated by the enzyme acetylcholinesterase (AChE), which breaks down ACh in the synaptic cleft, preventing continuous stimulation of the muscle fiber. This process is crucial for muscle relaxation and preventing spasms.

Structure of the Thin Filament

• Associated proteins include tropomyosin and troponin:

  • Tropomyosin: Threads winding around actin filaments.

  • Troponin: Occurs at intervals along actin filaments, with binding sites for myosin on actin covered by tropomyosin.

Muscle Fiber Contraction: Role of Calcium

1. Released calcium ions combine with troponin.

2. This interaction causes tropomyosin to shift, exposing myosin binding sites on actin.

3. Myosin heads bind to the exposed sites on actin, forming cross-bridges.

Cross-Bridge Mechanism

• Upon binding:

  • Power Stroke: The bending of myosin heads moves actin filaments towards the center of the sarcomere.

  • Each power stroke requires ATP.

• Steps of Hydrolysis and Attachment:

  1. ATP is hydrolyzed to ADP + P when myosin heads are unattached.

  2. ADP + P bind to myosin as it attaches to actin, forming a cross-bridge.

  3. Release of ADP + P causes myosin heads to change position, facilitating the power stroke.

  4. Binding of fresh ATP detaches the myosin head from actin, returning it to a resting position. #### Power Stroke - **Power Stroke:** The bending of myosin heads results in the pulling of actin filaments towards the center of the sarcomere. This movement is essential for the overall contraction of the muscle fiber, enabling the generation of force necessary for bodily movements. - Each power stroke is fueled by adenosine triphosphate (ATP), the primary energy carrier in cell metabolism. The hydrolysis of ATP to adenosine diphosphate (ADP) and inorganic phosphate () releases energy that is harnessed for the muscle contraction process. #### Steps of Hydrolysis and Attachment 1. **ATP Hydrolysis:** When myosin heads are in a relaxed, unattached state, ATP binds to them and is hydrolyzed to ADP and inorganic phosphate (P). This process not only releases energy but also primes the myosin head for the next attachment to actin. 2. **Cross-Bridge Formation:** In the presence of calcium ions, the ADP and P remain attached to the myosin head as it binds to an exposed binding site on the actin filament, forming a cross-bridge. This binding initiates a series of events that will culminate in muscle fiber contraction. 3. **Power Stroke Activation:** The release of ADP and P from the myosin head causes it to undergo a conformational change, pivoting and pulling the attached actin filament inward toward the center of the sarcomere. This shortening of the sarcomere results in muscle contraction, generating the necessary force for movement. 4. **Detachment and Resetting:** After the power stroke, a new molecule of ATP binds to the myosin head. This binding causes myosin to release from the actin filament, detaching the cross-bridge and allowing the myosin head to return to its original position, ready to start the cycle anew. This resetting of the myosin head prepares it for the next cycle of contraction, ensuring the muscle can rapidly respond to additional stimulation.