3.3 Muscular tissue
3 min read•august 14, 2024
Muscular tissue is the body's powerhouse, turning chemical energy into movement. It's made of long cells called that work together to make us move, keep us upright, and even generate heat when we're cold.
There are three types of muscle: skeletal, smooth, and cardiac. Each has its own special job in the body. moves bones, works in organs, and pumps blood through the heart.
Muscular Tissue Characteristics
Composition and Specialization
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- Muscular tissue is composed of elongated cells called muscle fibers specialized for contraction
- Muscle fibers generate force through the conversion of chemical energy (ATP) into mechanical energy
- Muscle fibers are multinucleated, with nuclei located peripherally beneath the cell membrane (sarcolemma)
Functions and Regulation
- The primary functions of muscular tissue are to produce movement, maintain posture, stabilize joints, and generate heat
- Muscle fibers contain , composed of myofilaments ( and ) that slide past each other during contraction
- is triggered by nerve impulses or hormones and regulated by calcium ions released from the sarcoplasmic reticulum
- Muscular tissue is highly vascularized to meet its high energy demands and remove metabolic waste products (lactic acid)
Skeletal vs Smooth vs Cardiac Muscle
Skeletal Muscle
- Skeletal muscle is voluntary, striated, and typically attached to bones via tendons
- Responsible for conscious body movements and under the control of the somatic nervous system
- Skeletal muscle fibers are long, cylindrical, and multinucleated
- Contracts and relaxes quickly (biceps brachii, quadriceps femoris)
Smooth Muscle
- Smooth muscle is involuntary, non-striated, and found in the walls of hollow organs and blood vessels
- Responsible for unconscious movements and controlled by the autonomic nervous system
- Smooth muscle fibers are spindle-shaped and uninucleated
- Contracts and relaxes slowly and can maintain prolonged contractions (stomach, intestines, urinary bladder)
Cardiac Muscle
- Cardiac muscle is involuntary, striated, and found only in the heart
- Responsible for pumping blood and is self-excitable, with its own conduction system
- Cardiac muscle fibers are branched, striated, and typically uninucleated
- Has a unique contraction pattern, with a long refractory period to prevent tetanic contractions (atria, ventricles)
Muscle Fiber Structure and Components
Myofibrils and Sarcomeres
- Muscle fibers are composed of myofibrils, long, cylindrical structures that run parallel to the length of the fiber
- Myofibrils are composed of repeating units called , the basic functional units of muscle contraction
- Sarcomeres contain thick filaments (myosin) and thin filaments (actin, troponin, and ) that slide past each other during contraction, shortening the sarcomere length
- The arrangement of light and dark bands within a sarcomere gives skeletal and cardiac muscle their striated appearance
Organelles and Membrane Systems
- The sarcoplasmic reticulum surrounds each myofibril and stores and releases calcium ions, which are essential for muscle contraction
- Transverse tubules (T-tubules) are invaginations of the sarcolemma that penetrate the muscle fiber and allow for the rapid transmission of action potentials into the interior of the fiber
- Mitochondria are abundant in muscle fibers and provide ATP for muscle contraction through cellular respiration
Muscular Tissue Locations and Roles
Skeletal Muscle
- Attached to bones and responsible for voluntary movements, maintaining posture, and stabilizing joints (biceps brachii, quadriceps femoris, external oblique muscles)
- Found in the walls of the pharynx, upper esophagus, and external anal sphincter, where it plays a role in swallowing and maintaining continence
Smooth Muscle
- Found in the walls of hollow organs, such as the stomach, intestines, urinary bladder, and uterus, where it is responsible for peristalsis, segmentation, and expulsion of contents
- Found in the walls of blood vessels, where it regulates blood flow and pressure
- The iris and ciliary body of the eye contain smooth muscle, which is responsible for controlling pupil size and accommodation (focusing) of the lens
Cardiac Muscle
- Found only in the heart and responsible for pumping blood through the circulatory system
- The atria and ventricles of the heart are composed of cardiac muscle
Key Terms to Review (23)
Acetylcholine: Acetylcholine is a neurotransmitter that plays a critical role in transmitting signals between nerve cells and muscles, as well as in various brain functions. It is essential for muscle contraction, modulating heart rate, and influencing learning and memory processes. The function of acetylcholine links it to muscle tissue, nerve signaling, the autonomic nervous system, digestive functions, and the physiology of smooth and cardiac muscles.
Actin: Actin is a globular protein that forms microfilaments, which are essential components of the cytoskeleton and muscle contraction. It plays a critical role in muscle movement, cellular shape, and motility by forming filaments that interact with myosin to generate force. Actin is found in all eukaryotic cells, with different forms facilitating diverse cellular functions including contraction in muscle tissue and maintaining cell structure.
Aerobic respiration: Aerobic respiration is a biological process in which cells convert glucose and oxygen into energy, carbon dioxide, and water. This process is essential for producing ATP, the energy currency of cells, especially during physical activity. It takes place primarily in the mitochondria and involves several key metabolic pathways, making it vital for muscle function and overall energy metabolism.
Anaerobic glycolysis: Anaerobic glycolysis is a metabolic process that breaks down glucose for energy in the absence of oxygen, producing lactate as a byproduct. This pathway is crucial for supplying energy during high-intensity exercise or when oxygen availability is limited, particularly in muscle tissues that require rapid energy production. It plays an essential role in various types of muscle fibers and supports activities such as sprinting or heavy lifting where immediate energy demand exceeds the aerobic capacity.
Cardiac muscle: Cardiac muscle is a specialized form of striated muscle found only in the heart, responsible for pumping blood throughout the body. Its unique structure allows for rhythmic contractions that are essential for maintaining a consistent heartbeat, connecting it closely to the overall functioning of muscular tissue, as well as the physiological roles of smooth and cardiac muscles. This type of muscle is involuntary, meaning it operates without conscious control, differentiating it from skeletal muscle and highlighting its importance in the circulatory system.
Endomysium: Endomysium is a delicate layer of connective tissue that surrounds each individual muscle fiber within a muscle. This thin layer plays a crucial role in providing support and nourishment to the muscle cells, facilitating the connection between muscle fibers and the overall muscle structure. It is essential for maintaining the integrity of the muscle and is involved in the distribution of blood vessels and nerves to each fiber.
Epimysium: Epimysium is a layer of connective tissue that surrounds the entire muscle, providing structural support and protection. It is crucial for the organization of muscle fibers into functional units, ensuring that muscles can contract effectively. This outer layer also helps in transmitting forces generated by muscle contractions to the tendons and bones.
Excitation-contraction coupling: Excitation-contraction coupling is the physiological process by which an electrical stimulus (action potential) triggers muscle contraction. This intricate sequence involves the conversion of the electrical signal into a mechanical response, highlighting how muscle fibers respond to nervous stimulation and contract through a series of biochemical events.
Fascicle: A fascicle is a bundle of skeletal muscle fibers surrounded by a connective tissue sheath called perimysium. This structure allows for efficient force transmission during muscle contraction and contributes to the overall organization and functionality of skeletal muscle tissue. Fascicles play a crucial role in the arrangement of muscle fibers, influencing the strength and direction of muscle contractions.
Motor unit: A motor unit is a functional entity composed of a single motor neuron and all the muscle fibers it innervates. This concept is crucial in understanding how muscle contractions are coordinated, as each motor unit can activate a specific number of muscle fibers to generate force. The size and number of motor units recruited determine the strength and precision of movements, linking the muscular system with the neural pathways that control them.
Muscle contraction: Muscle contraction is the process by which muscle fibers shorten and generate force, allowing for movement and stability in the body. This fundamental action is essential for various physiological functions, including locomotion, posture maintenance, and circulation. Muscle contraction occurs through a complex interplay of biochemical reactions, electrical signals, and mechanical interactions between muscle proteins.
Muscle fatigue: Muscle fatigue is the decline in the ability of a muscle to generate force or power, often occurring after prolonged or intense physical activity. This phenomenon is linked to a variety of factors, including depletion of energy sources, accumulation of metabolic byproducts, and alterations in the nervous system's ability to stimulate muscle fibers. Understanding muscle fatigue is crucial for athletes and individuals engaged in physical activities, as it impacts performance and recovery.
Muscle fibers: Muscle fibers are the individual cells that make up muscle tissue, responsible for contraction and movement in the body. These specialized cells can be categorized into different types, which vary in structure and function, allowing for a range of physical activities from fine motor skills to powerful, sustained contractions. The characteristics of muscle fibers, such as their metabolic properties and contraction speed, play a crucial role in determining an individual's strength, endurance, and overall physical capabilities.
Muscle tone: Muscle tone refers to the continuous and passive partial contraction of the muscles, which helps maintain posture and provides a state of readiness for action. This tension arises from the combination of neural activation and muscle fiber recruitment, allowing muscles to remain firm even when at rest. Muscle tone plays a crucial role in overall body stability and coordination, affecting movement efficiency and balance.
Muscular Dystrophy: Muscular dystrophy is a group of inherited disorders characterized by progressive muscle weakness and degeneration. This condition primarily affects skeletal muscles, which are responsible for movement, leading to challenges in mobility and overall physical function. It is linked to genetic mutations that disrupt the production of proteins needed for healthy muscle structure and function.
Myofibrils: Myofibrils are long, thread-like structures found within muscle cells that are essential for muscle contraction. They are composed of repeating units called sarcomeres, which contain the proteins actin and myosin. These proteins interact during contraction to shorten the muscle, facilitating movement and force generation.
Myopathy: Myopathy refers to a disease or disorder that affects the muscles, causing muscle weakness or dysfunction. This condition can result from various causes, including genetic mutations, autoimmune diseases, infections, or toxins, and can significantly impact a person's ability to perform everyday activities. Understanding myopathy is crucial because it relates to muscular tissue health and overall bodily movement.
Myosin: Myosin is a type of motor protein that plays a crucial role in muscle contraction and cellular movement. It interacts with actin filaments to facilitate muscle contractions, making it essential for various types of muscular tissues, including skeletal, smooth, and cardiac muscle. This protein not only contributes to the structural integrity of muscle cells but also supports numerous cellular functions, linking it to various physiological processes.
Neuromuscular transmission: Neuromuscular transmission is the process by which nerve impulses are transmitted from motor neurons to skeletal muscle fibers, resulting in muscle contraction. This process involves the release of neurotransmitters at the neuromuscular junction, where the nerve and muscle meet, and is crucial for voluntary movement and coordination of muscle activity.
Sarcomeres: Sarcomeres are the fundamental contractile units of muscle fibers, responsible for muscle contraction and relaxation. These structures are composed of actin and myosin filaments, organized in a highly structured manner that allows for the sliding filament mechanism, which is crucial for muscle movement. Sarcomeres are repeated along the length of myofibrils, forming a striated appearance in skeletal and cardiac muscle tissue.
Skeletal muscle: Skeletal muscle is a type of striated muscle tissue that is primarily responsible for voluntary movements of the body, attached to bones by tendons. It plays a crucial role in locomotion and maintaining posture, as well as generating heat through muscle contractions. This type of muscle is characterized by its long, cylindrical fibers that contain multiple nuclei and exhibit a striped appearance due to the arrangement of contractile proteins.
Smooth muscle: Smooth muscle is a type of involuntary, non-striated muscle tissue found in various internal organs, such as the gastrointestinal tract, blood vessels, and respiratory system. It is essential for controlling the movements within these organs and is characterized by its ability to contract slowly and sustain prolonged contractions without fatigue. Unlike skeletal muscle, smooth muscle does not have a striated appearance and is controlled by the autonomic nervous system, connecting it to both muscular tissue and cardiac muscle physiology.
Tropomyosin: Tropomyosin is a regulatory protein that plays a crucial role in muscle contraction by binding to actin filaments within muscle fibers. It helps to control the interaction between actin and myosin, the two primary proteins involved in muscle contraction. When tropomyosin is in its resting state, it blocks the binding sites on actin, preventing myosin from attaching and initiating contraction.