3.1 Structure and function of skeletal muscle
4 min read•august 16, 2024
Skeletal muscles are the powerhouses of movement in our bodies. They're made up of fibers bundled together, with each fiber containing tiny contractile units called sarcomeres. These structures work together to create the force needed for everything from lifting weights to blinking.
Understanding how muscles work is key to grasping exercise physiology. From the cellular level to the whole muscle, every part plays a role in generating movement. We'll explore how signals from nerves trigger muscle action and how different muscle types affect performance.
Skeletal Muscle Structure and Function
Muscle Fiber Organization
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- Skeletal muscle comprises muscle fibers (cells) organized into fascicles bundled together to form the whole muscle
- Muscle fibers contain myofibrils composed of sarcomeres functioning as the basic units of
- Sarcomeres consist of thick filaments (myosin) and thin filaments (actin) arranged in a specific pattern allowing for muscle contraction
- Myosin filaments form the A-band of the
- Actin filaments extend from the Z-lines and form the I-band
- Connective tissue components provide structural support and force transmission
- Endomysium surrounds individual muscle fibers
- Perimysium encases fascicles (bundles of muscle fibers)
- Epimysium covers the entire muscle
Cellular Components and Membranes
- Sarcolemma functions as the cell membrane of the muscle fiber containing specialized structures called T-tubules for transmitting electrical signals
- T-tubules form a network throughout the muscle fiber allowing rapid signal propagation
- Sarcoplasmic reticulum surrounds the myofibrils and stores calcium ions necessary for muscle contraction
- Terminal cisternae of the sarcoplasmic reticulum form triads with T-tubules
- Mitochondria provide energy for muscle contraction through (oxidative phosphorylation)
- Glycogen granules serve as a readily available energy source for
Excitation-Contraction Coupling in Muscle
Neuromuscular Junction and Action Potential Generation
- Excitation-contraction coupling converts an electrical stimulus into a mechanical response in skeletal muscle
- Process begins with an action potential arriving at the triggering acetylcholine release
- Acetylcholine binds to nicotinic receptors on the sarcolemma causing depolarization and generating an action potential in the muscle fiber
- Sodium ions flow into the muscle fiber through voltage-gated sodium channels
- Potassium ions flow out of the muscle fiber to repolarize the membrane
- Action potential propagates along the sarcolemma and into the T-tubules activating voltage-gated calcium channels
- Dihydropyridine receptors (DHPRs) in T-tubules sense the change in membrane potential
Calcium Release and Muscle Contraction
- Activated DHPRs trigger the opening of ryanodine receptors (RyRs) in the sarcoplasmic reticulum
- Calcium release from the sarcoplasmic reticulum through RyRs triggers the sliding filament mechanism
- Calcium binds to troponin C on the thin filaments
- Tropomyosin shifts position exposing myosin binding sites on actin
- Myosin heads attach to actin filaments forming cross-bridges and generating force through the power stroke
- ATP hydrolysis provides energy for the power stroke and cross-bridge cycling
- Process concludes with active reuptake of calcium into the sarcoplasmic reticulum by calcium ATPase pumps leading to muscle relaxation
Motor Units and Muscle Contraction
Motor Unit Structure and Recruitment
- Motor unit consists of a single motor neuron and all the muscle fibers it innervates
- Motor units vary in size with smaller units controlling fine movements (eye muscles) and larger units responsible for gross movements (leg muscles)
- Size principle of motor unit recruitment states that smaller motor units activate first followed by progressively larger units as force demands increase
- Ensures smooth and efficient
- Motor unit recruitment and firing rate modulation serve as primary mechanisms for controlling muscle force production
- Recruitment adds more motor units to increase force
- Rate coding increases the firing frequency of active motor units
Motor Unit Types and Properties
- Asynchronous firing of motor units allows for smooth sustained muscle contractions and prevents fatigue
- Different types of motor units have distinct physiological and metabolic properties
- Slow-twitch (Type I) motor units: fatigue-resistant with high oxidative capacity (marathon runners)
- Fast-twitch oxidative (Type IIa) motor units: intermediate fatigue resistance and force production (middle-distance runners)
- Fast-twitch glycolytic (Type IIx) motor units: high force production but low fatigue resistance (sprinters)
- Motor unit type composition varies between muscles and can be altered through training
- Endurance training increases the proportion of slow-twitch fibers
- Resistance training can increase the size and force production of fast-twitch fibers
Muscle Fiber Arrangement vs Force Production
Muscle Architecture and Force Generation
- Muscle fiber arrangement (muscle architecture) refers to the orientation of muscle fibers relative to the line of force generation
- Parallel fiber arrangement allows for greater range of motion but produces less force compared to pennate arrangements (biceps brachii)
- Pennate muscle arrangements increase force production due to increased physiological cross-sectional area
- Unipennate: fibers arranged at an angle to one side of the tendon (gastrocnemius)
- Bipennate: fibers arranged at angles on both sides of the tendon (rectus femoris)
- Multipennate: fibers arranged at multiple angles (deltoid)
- Angle of pennation affects force transmitted to the tendon with larger angles resulting in reduced force transmission but increased muscle packing
Muscle Fiber Characteristics and Force-Velocity Relationships
- Muscle fiber length influences the velocity of contraction with longer fibers capable of greater shortening velocities
- Sartorius muscle has long fibers for rapid knee flexion and hip flexion/rotation
- Force-length relationship of muscle fibers affects overall muscle force production with optimal force generated at resting sarcomere lengths
- Descending limb: force decreases as muscle shortens beyond optimal length
- Ascending limb: force decreases as muscle lengthens beyond optimal length
- Muscle fiber type composition influences the force-velocity characteristics and fatigue resistance of the muscle
- Fast-twitch fibers have higher maximum shortening velocities but fatigue more quickly
- Slow-twitch fibers have lower maximum shortening velocities but greater fatigue resistance
Key Terms to Review (18)
A.V. Hill: A.V. Hill was a pioneering British physiologist known for his significant contributions to exercise physiology, particularly in understanding the dynamics of gas exchange and oxygen uptake during physical activity. His research laid the foundation for studying how muscles utilize oxygen and energy during exercise, influencing our comprehension of metabolic and neuromuscular adaptations in athletes and the role of skeletal muscle function in performance.
Aerobic metabolism: Aerobic metabolism is the process by which the body converts carbohydrates, fats, and proteins into energy in the presence of oxygen. This energy production is vital for sustained physical activity and is linked to the efficiency of muscle fibers, the structure of skeletal muscle, and the body's overall energy systems.
Anaerobic metabolism: Anaerobic metabolism is a biochemical process that occurs in the absence of oxygen, primarily producing energy through the breakdown of glucose. This process is crucial during high-intensity activities when the demand for energy exceeds the capacity of aerobic pathways, leading to the production of ATP quickly but less efficiently, often resulting in byproducts like lactic acid. Understanding anaerobic metabolism helps clarify its relationship with heat production, muscle structure and function, energy systems, and substrate utilization during exercise.
Archibald V. Hill: Archibald V. Hill was a British physiologist and Nobel Prize winner known for his research on muscle physiology and bioenergetics. He made significant contributions to the understanding of how skeletal muscles generate energy during contraction, which has critical implications for exercise physiology and performance. His work laid the foundation for further studies on muscle function, the role of oxygen in metabolism, and how muscles adapt to various types of training.
DOMS: DOMS, or Delayed Onset Muscle Soreness, refers to the pain and stiffness that typically occurs in muscles after intense exercise or physical activity. This soreness usually peaks between 24 to 72 hours post-exercise and is often associated with eccentric muscle contractions, where the muscle lengthens while under tension. Understanding DOMS is crucial for athletes and individuals engaging in new or intense training, as it can inform recovery strategies and exercise programming.
Force production: Force production refers to the ability of muscles to generate tension and exert force, which is essential for movement and physical activities. This capability is influenced by the structural characteristics of skeletal muscle, including muscle fiber composition and architecture, as well as neuromuscular adaptations that occur with exercise. Understanding force production helps to reveal how different training regimens can enhance strength, power, and overall athletic performance.
Hyperplasia: Hyperplasia is an increase in the number of cells in a tissue or organ, which can result in its enlargement. This process often occurs in response to a stimulus, such as increased demand or hormonal signals, and is crucial for growth and repair in skeletal muscle. Understanding hyperplasia helps clarify how muscle tissues adapt to different types of training and physical stressors.
Hypertrophy: Hypertrophy refers to the increase in the size of skeletal muscle fibers, which occurs as a result of consistent resistance training or exercise. This process is crucial for improving muscle strength and endurance, and it significantly impacts overall athletic performance. As muscles undergo hypertrophy, changes in blood flow distribution and skeletal muscle structure enhance the body's ability to sustain physical activity and adapt to training demands.
Muscle contraction: Muscle contraction is the process by which muscle fibers generate force and shorten, leading to movement. This physiological phenomenon is crucial for a variety of functions, including locomotion, posture maintenance, and overall body movement, and involves intricate interactions between various structures within skeletal muscle.
Muscle strain: A muscle strain is an injury that occurs when muscle fibers are overstretched or torn, often resulting from excessive force or overuse. This condition can affect the muscle's ability to function properly, leading to pain, swelling, and reduced mobility. Understanding muscle strains is important as they can impact athletic performance and everyday activities.
Myofibril: A myofibril is a long, cylindrical structure within a muscle fiber that plays a crucial role in muscle contraction. Composed of repeating units called sarcomeres, myofibrils contain the proteins actin and myosin, which interact to produce the force necessary for muscle movement. This structural organization allows for efficient contraction and relaxation of skeletal muscles during physical activity.
Neuromuscular junction: The neuromuscular junction is a specialized synapse where a motor neuron communicates with a skeletal muscle fiber to initiate muscle contraction. This connection is crucial for translating the electrical impulses from the nervous system into mechanical movement, ensuring that muscles respond appropriately to signals from the brain.
Power output: Power output refers to the rate at which work is performed or energy is expended in a given timeframe, often measured in watts (W). This concept is crucial for understanding how effectively muscles can generate force during various types of physical activity, and it plays a significant role in the performance capabilities of different muscle fiber types, the efficiency of energy systems, and the impact of training adaptations.
Progressive Overload: Progressive overload is the gradual increase of stress placed on the body during exercise to stimulate physiological adaptations and improve performance. This principle is essential for enhancing strength, endurance, and overall fitness, ensuring that the body continues to adapt and grow stronger over time.
Sarcomere: A sarcomere is the basic structural and functional unit of skeletal muscle, responsible for muscle contraction. It is defined as the segment between two neighboring Z-discs, containing overlapping thick (myosin) and thin (actin) filaments that slide past one another to produce muscle shortening during contraction. This process is crucial for converting chemical energy into mechanical work, highlighting its central role in muscle function.
Specificity: Specificity refers to the principle that training adaptations are directly related to the type of exercise performed. This means that if you want to improve a particular aspect of fitness, such as strength, endurance, or flexibility, you need to engage in exercises that specifically target those areas. Understanding specificity helps in designing effective training programs and making informed decisions about exercise selection.
Type I fibers: Type I fibers, also known as slow-twitch fibers, are a type of muscle fiber that is highly resistant to fatigue and is primarily used for endurance activities. These fibers are rich in mitochondria, myoglobin, and capillaries, making them well-suited for aerobic metabolism. Their structure allows for sustained contractions over extended periods, which is essential in activities such as long-distance running or cycling.
Type II fibers: Type II fibers, also known as fast-twitch fibers, are a category of skeletal muscle fibers that are designed for quick, powerful contractions and fatigue rapidly. These fibers are essential for high-intensity activities like sprinting and weightlifting, showcasing a greater force output compared to their slow-twitch counterparts. They have a lower density of mitochondria and rely on anaerobic metabolism, making them less efficient for prolonged exercise but ideal for explosive movements.