10.3 Muscle Fiber Contraction and Relaxation
3 min read•june 18, 2024
Muscle fibers are the building blocks of our movement. They contract and relax through a complex dance of proteins, ions, and energy molecules. This intricate process allows us to do everything from lifting weights to typing on a keyboard.
The explains how muscles work. Thick and thin filaments slide past each other, powered by ATP and regulated by calcium. This microscopic movement translates into the macroscopic contractions we see and feel in our daily lives.
Muscle Fiber Contraction and Relaxation
Components of muscle fiber contraction
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- functions as the basic contractile unit of a muscle fiber
- Composed of thick and thin filaments that slide past each other during contraction
- Thick filaments primarily contain protein (motor protein)
- Thin filaments primarily contain , , and proteins (regulatory proteins)
- Composed of thick and thin filaments that slide past each other during contraction
- interacts with to generate force during muscle contraction
- Myosin heads are the globular portions of myosin that bind to actin (cross-bridge formation)
- Actin forms the backbone of thin filaments and provides binding sites for myosin heads
- complex regulates the interaction between actin and myosin
- binds (Ca2+) which initiates muscle contraction
- inhibits actin-myosin interaction in the absence of Ca2+
- binds to and helps position it on the thin filament
- Tropomyosin covers the actin-binding sites for myosin when the muscle is relaxed
- (SR) stores and releases Ca2+ during muscle contraction and relaxation
- are enlarged portions of the SR close to the (transverse tubules)
- T-tubules are invaginations of the sarcolemma that conduct action potentials into the muscle fiber
- , an oxygen-binding protein in muscle fibers, facilitates oxygen delivery during contraction
Sliding filament theory in muscle function
- Sliding filament theory explains how muscle fibers contract and relax
- During contraction, thin filaments slide past thick filaments, shortening the length ()
- During relaxation, thin filaments slide back to their original position, lengthening the sarcomere
- describes the series of events that occur during muscle contraction
- Myosin heads bind to actin forming cross-bridges
- Myosin heads pivot, pulling the thin filaments towards the center of the sarcomere (power stroke)
- Myosin heads detach from actin (cross-bridge detachment)
- Myosin heads return to their original position, ready to bind actin again ()
- is required for muscle contraction and relaxation
- ATP binding to myosin allows the myosin head to detach from actin after the power stroke
- ATP hydrolysis provides energy for the myosin head to return to its original position (recovery stroke)
- Creatine phosphate serves as a rapid energy source for ATP regeneration during intense muscle activity
Neural stimulation to muscle relaxation sequence
- Action potential arrives at the from the motor neuron
- is released from the motor neuron terminal into the synaptic cleft
- ACh binds to (nAChRs) on the sarcolemma (muscle cell membrane)
- Sarcolemma depolarizes, generating an action potential that propagates along the surface and into the T-tubules
- Depolarization of the T-tubules activates voltage-gated Ca2+ channels (, DHPRs) in the terminal cisternae of the SR
- Activated DHPRs cause the opening of (RyRs) in the SR membrane
- Ca2+ is released from the SR into the sarcoplasm (cytoplasm of the muscle cell)
- Ca2+ binds to troponin C, causing a conformational change in the troponin complex
- Tropomyosin moves, exposing the actin-binding sites for myosin heads
- Myosin heads bind to actin, forming cross-bridges (muscle contraction begins)
- Cross-bridge cycling continues as long as Ca2+ remains bound to troponin C
- Sarco/endoplasmic reticulum Ca2+- () pumps actively transport Ca2+ back into the SR
- As Ca2+ is removed from the sarcoplasm, it dissociates from troponin C
- Tropomyosin returns to its original position, blocking the actin-binding sites for myosin
- Cross-bridge cycling stops, and the muscle fiber relaxes to its resting state
Types of Muscle Contractions
- : Muscle length changes while tension remains constant
- : Muscle generates force without changing length
- : Postmortem muscle stiffening due to the depletion of ATP, preventing myosin-actin detachment
Key Terms to Review (42)
Acetylcholine: Acetylcholine is a neurotransmitter that plays a crucial role in the communication between neurons, the activation of muscle fibers, and the regulation of various physiological processes in the body. It is a key player in the functioning of the nervous system, muscle tissues, and the autonomic nervous system.
Acetylcholine (ACh): Acetylcholine is a neurotransmitter in the nervous system that plays a crucial role in stimulating muscle contractions and is involved in various brain functions including memory and learning. In the context of skeletal muscle, it is essential for transmitting nerve signals to muscle cells, leading to muscle movement.
Actin: Actin is a globular protein that forms long, thin filaments which are a major component of the cytoskeleton in all eukaryotic cells and is crucial in the contraction of skeletal muscles. It works together with myosin to convert chemical energy into mechanical energy, leading to muscle movement.
Actin: Actin is a globular protein that is a key structural component of the cytoskeleton in eukaryotic cells. It is involved in a wide range of cellular processes, including muscle contraction, cell motility, cell division, and the maintenance of cell shape and integrity.
ATP (Adenosine Triphosphate): ATP, or adenosine triphosphate, is the primary energy currency of the cell. It is a high-energy molecule that stores and transports the chemical energy needed to power a wide variety of cellular processes, from muscle contraction to protein synthesis. ATP is central to the functions of human life, chemical bonds, chemical reactions, organic compounds, cellular organelles, protein synthesis, muscle contraction, respiration, metabolism, and fluid balance.
ATP synthase: ATP synthase is an enzyme complex embedded in the mitochondrial membrane that facilitates the synthesis of ATP (adenosine triphosphate), the primary energy carrier in cells, from ADP (adenosine diphosphate) and inorganic phosphate during the process of oxidative phosphorylation within carbohydrate metabolism. It acts as a molecular generator, converting an electrochemical gradient into energy stored in the form of ATP.
ATPase: ATPase is an enzyme that catalyzes the hydrolysis of ATP, breaking it down into ADP and inorganic phosphate. This process releases energy that can be used to drive various cellular processes, such as active transport, muscle contraction, and nerve impulse transmission.
Calcium Ions: Calcium ions (Ca2+) are essential mineral ions that play crucial roles in various physiological processes, including muscle contraction, nerve impulse transmission, and cellular signaling. These positively charged ions are involved in a wide range of functions throughout the body, making them a key topic in the study of anatomy and physiology.
Cross-bridge cycle: The cross-bridge cycle is the process through which muscle fibers contract by the interaction between actin and myosin filaments. This cycle involves a series of steps where myosin heads bind to actin, pull, and then release, generating force and enabling muscle shortening. Understanding this cycle is crucial for grasping how muscles contract and relax during physical activities.
Dihydropyridine receptors: Dihydropyridine receptors are specialized voltage-gated calcium channels primarily located in the membranes of skeletal muscle cells, playing a crucial role in muscle contraction. These receptors act as sensors for changes in membrane potential, leading to calcium influx, which is essential for initiating the muscle contraction process. They are part of a complex signaling mechanism that connects electrical signals to mechanical responses in muscle fibers.
Excitation-contraction coupling: Excitation-contraction coupling is the physiological process by which an electrical stimulus leads to muscle contraction. It bridges the gap between the generation of an action potential in a muscle fiber and the onset of contraction.
Excitation-Contraction Coupling: Excitation-contraction coupling is the process by which an electrical signal (excitation) in a muscle fiber triggers the mechanical contraction of that fiber. It is a crucial link between the nervous system and the muscle system, enabling voluntary movement and reflex responses.
Isometric Contraction: An isometric contraction occurs when a muscle generates force without changing its length. This type of contraction is crucial for maintaining posture and stabilizing joints, as the muscle tension increases without any visible movement. Isometric contractions are different from isotonic contractions, where the muscle changes length while producing movement.
Isotonic contraction: Isotonic contraction is a type of muscle action where the muscle changes length while the tension remains constant, commonly occurring during most everyday activities. This process facilitates movement of the body or objects by shortening (concentric contraction) or lengthening (eccentric contraction) of the muscle.
Isotonic Contraction: Isotonic contraction is a type of muscle contraction where the muscle shortens while maintaining a constant tension throughout the movement. This is in contrast to isometric contraction, where the muscle length remains constant during the contraction.
Myofibril: A myofibril is a long, cylindrical organelle found within the cytoplasm of a muscle cell. Myofibrils are the basic contractile units of skeletal and cardiac muscle, responsible for the muscle's ability to generate force and produce movement.
Myoglobin: Myoglobin is a muscle protein that binds oxygen, facilitating oxygen storage and transport within muscle cells. This protein is essential for muscle metabolism, especially in skeletal muscle, where it aids in the efficient use of oxygen during muscular contractions. Myoglobin's role becomes particularly important when muscles require sustained energy, as it helps maintain oxygen availability to meet the high metabolic demands.
Myosin: Myosin is a type of motor protein found in muscle cells that, through its interaction with actin, plays a crucial role in muscle contraction and movement. It converts chemical energy in the form of ATP to mechanical energy, thus enabling the muscles to contract.
Myosin: Myosin is a motor protein that is essential for muscle contraction and movement. It is a key component of the contractile apparatus within muscle fibers and interacts with the actin filaments to generate the force required for muscle movement and locomotion.
Neuromuscular Junction: The neuromuscular junction is the site where a motor neuron from the nervous system connects with and transmits signals to a muscle fiber, enabling muscle contraction. It is a critical interface that facilitates the communication between the nervous and muscular systems, allowing for the voluntary control of skeletal muscle movement.
Neuromuscular junction (NMJ): The neuromuscular junction is a specialized synapse between a motor neuron and a skeletal muscle fiber, facilitating the transmission of electrical signals that result in muscle contraction. It plays a pivotal role in converting neural commands into mechanical movement.
Nicotinic Acetylcholine Receptors: Nicotinic acetylcholine receptors are a type of ionotropic receptor that responds to the neurotransmitter acetylcholine, playing a crucial role in muscle contraction and neurotransmission in the nervous system. These receptors are found at the neuromuscular junction, where they mediate communication between motor neurons and muscle fibers, leading to muscle activation. Additionally, they are also present in the central and peripheral nervous systems, affecting various physiological processes, including the autonomic nervous system.
Oxygen debt: Oxygen debt is the amount of extra oxygen that muscles need to recover after strenuous exercise. It compensates for the lack of oxygen available during rapid muscle activity by repaying the oxygen used from reserves.
Power stroke: The power stroke is a phase during muscle contraction where the myosin head pulls the actin filament towards the center of the sarcomere, resulting in muscle shortening. This process is fueled by ATP hydrolysis, converting chemical energy into mechanical work.
Pyruvic acid: Pyruvic acid is a three-carbon compound that forms at the end of glycolysis during glucose metabolism, acting as a key intermediate in several metabolic pathways, including aerobic and anaerobic respiration. In the context of muscle fiber contraction and relaxation, it is crucial for generating ATP, which muscles use as energy.
Recovery Stroke: The recovery stroke is a critical phase in the muscle contraction and relaxation cycle, where the muscle fiber returns to its original resting state after a contraction. This process is essential for maintaining the muscle's ability to generate force and power during subsequent contractions.
Rigor Mortis: Rigor mortis is a postmortem change that occurs in the body, characterized by the stiffening of the muscles due to the depletion of adenosine triphosphate (ATP) and the formation of permanent cross-bridges between actin and myosin filaments in the muscle fibers. This process is a key indicator of the progression of death and is closely related to the topics of muscle fiber contraction and relaxation.
Ryanodine Receptors: Ryanodine receptors are a type of calcium release channel found primarily in muscle cells, playing a crucial role in muscle contraction. These receptors are located on the sarcoplasmic reticulum and allow calcium ions to flow into the cytoplasm when activated, which is essential for the process of excitation-contraction coupling in both skeletal and cardiac muscle tissues. They interact closely with other proteins to ensure efficient muscle function and are integral to understanding how muscles contract and relax.
Sarcomere: A sarcomere is the basic contractile unit of muscle fiber in skeletal muscle, made up of long protein filaments including actin and myosin that slide past each other to produce a muscle contraction. It is bounded by Z lines to which the actin filaments are attached.
Sarcomere: The sarcomere is the fundamental contractile unit of skeletal, cardiac, and smooth muscle fibers. It is the basic structural and functional unit responsible for the contraction and relaxation of muscle tissue, and is a key component in the overall process of muscle motion and activity.
Sarcoplasmic reticulum: The sarcoplasmic reticulum is a specialized form of endoplasmic reticulum found in muscle cells that functions primarily to store and release calcium ions (Ca²+) during muscle contraction and relaxation. This organelle plays a critical role in regulating calcium levels, which are essential for muscle fiber contraction, allowing muscles to contract and relax in response to stimulation.
SERCA: SERCA, or Sarcoplasmic Reticulum Calcium ATPase, is a critical protein that pumps calcium ions back into the sarcoplasmic reticulum of muscle cells after contraction. This process is essential for muscle relaxation and helps maintain calcium homeostasis within the muscle fibers. By actively transporting calcium against its concentration gradient, SERCA plays a vital role in ensuring that muscle fibers can efficiently contract and relax during repeated cycles of excitation and contraction.
Sliding Filament Theory: The sliding filament theory is a model that explains the mechanism of muscle contraction, describing how the thin and thick filaments in muscle fibers interact to generate force and shorten the muscle. This theory is central to understanding how skeletal muscles produce movement in the body.
T-tubules: T-tubules, or transverse tubules, are invaginations of the plasma membrane that extend deep into the interior of skeletal muscle fibers. They play a crucial role in the process of muscle contraction and relaxation, as well as in the overall functioning of skeletal muscle tissues.
Terminal cisternae: Terminal cisternae are enlarged regions of the sarcoplasmic reticulum in muscle fibers that store calcium ions. These structures play a crucial role in muscle contraction by releasing calcium into the cytosol when stimulated, triggering the interaction between actin and myosin filaments, which leads to muscle shortening and force generation.
Tropomyosin: Tropomyosin is a protein that wraps around actin filaments in muscle cells, playing a crucial role in regulating muscle contraction. It acts as a gatekeeper, blocking the binding sites for myosin on the actin molecules until calcium ions trigger contraction.
Tropomyosin: Tropomyosin is a thin, rod-like protein that runs along the actin filaments in muscle fibers. It plays a crucial role in the regulation of muscle contraction and relaxation by controlling the interaction between actin and myosin, the two key proteins involved in muscle movement.
Troponin: Troponin is a complex of three regulatory proteins (troponin C, I, and T) that is integral to muscle contraction in skeletal and cardiac muscles by controlling the calcium-mediated interaction between actin and myosin. It binds to calcium ions to initiate conformational changes that allow myosin heads to bind to actin filaments.
Troponin: Troponin is a complex of three regulatory proteins (troponin C, troponin I, and troponin T) that is integral to the contraction of striated muscle, including skeletal and cardiac muscle. It plays a crucial role in the regulation of muscle fiber contraction and relaxation.
Troponin C: Troponin C is a calcium-binding protein that is a crucial component of the troponin complex, which regulates the contraction and relaxation of skeletal and cardiac muscle fibers. It plays a central role in the process of muscle fiber contraction and relaxation by responding to changes in intracellular calcium levels.
Troponin I: Troponin I is a regulatory protein found in skeletal and cardiac muscle that plays a crucial role in the contraction and relaxation of muscle fibers. It is a component of the troponin complex, which is essential for the calcium-mediated control of muscle contraction.
Troponin T: Troponin T is a protein that plays a crucial role in the contraction of skeletal and cardiac muscle fibers by binding to tropomyosin and forming a complex that regulates muscle contraction. It is one of three subunits of the troponin complex, along with troponin C and troponin I, which work together to control the interaction between actin and myosin filaments in muscle cells during contraction and relaxation.