10.2 Skeletal Muscle

Skeletal muscles are the powerhouses of movement in our bodies. They're made up of layers of connective tissue and fibers that work together to create force. Understanding their structure is key to grasping how we move and function.

The muscle-tendon interaction is crucial for turning muscle contractions into actual movement. Tendons connect muscles to bones, transmitting force and allowing for efficient energy storage and release during activities like running or jumping.

Skeletal Muscle Structure and Function

Connective tissue layers of muscle

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• Endomysium envelops each individual muscle fiber providing support and allowing for the exchange of nutrients and waste products
  • Composed of a thin layer of reticular fibers (collagen type III) forming a delicate network around the muscle fiber
• Perimysium surrounds bundles of muscle fibers called fascicles facilitating the organization and coordination of muscle fiber groups
  • Composed of thicker collagen fibers (collagen type I) that provide strength and support to the fascicles
• Epimysium is a dense irregular connective tissue layer that encases the entire skeletal muscle protecting it from friction and damage
  • Continuous with the perimysium and serves as an attachment point for tendons and aponeuroses
• Fascia is a layer of connective tissue that surrounds groups of muscles allowing them to slide smoothly against each other during movement
  • Reduces friction between adjacent muscles and compartmentalizes muscle groups (deep fascia, superficial fascia)

Muscle-tendon interaction for movement

• Skeletal muscles attach to bones via tendons which are composed of dense regular connective tissue rich in collagen fibers
  • Tendons are continuous with the epimysium of the muscle allowing for the efficient transfer of force from the muscle to the bone
  • Tendon attachment sites on bones are called entheses (fibrous entheses, fibrocartilaginous entheses)
• Muscle contraction generates tension that is transmitted through the tendon to the attached bone resulting in movement at the joint
  • Tendon elasticity allows for the storage and release of elastic energy during movement enhancing efficiency and power output
  • Tendon stiffness influences the rate of force development and the speed of movement
• Muscle and tendon function together as a muscle-tendon unit with the muscle providing active tension and the tendon providing passive tension
  • The interaction between muscle and tendon properties determines the overall function and performance of the muscle-tendon unit
  • Muscle-tendon units are adapted to specific functional demands (force production, speed, range of motion)

Key components of muscle fibers

• Sarcolemma is the plasma membrane of the muscle fiber with invaginations called transverse tubules (T-tubules) that allow for rapid signal propagation
  • T-tubules are continuous with the extracellular space and contain voltage-gated ion channels essential for excitation-contraction coupling
  • Sarcolemma contains specialized proteins (dystrophin, integrins) that link the cytoskeleton to the extracellular matrix
• Sarcoplasm is the cytoplasm of the muscle fiber containing glycogen for energy storage, mitochondria for ATP production, and myoglobin for oxygen storage
  • Sarcoplasmic enzymes (creatine kinase, glycolytic enzymes) support energy metabolism and muscle function
  • Sarcoplasmic inclusions (lipid droplets, iron deposits) can accumulate in certain conditions (obesity, iron overload)
• Myofibrils are the contractile units of the muscle fiber composed of myofilaments (actin and myosin) organized into sarcomeres
  • Myofibrils are arranged in parallel, giving the muscle fiber its striated appearance
  • Myofibrillar proteins (titin, nebulin) contribute to the structural integrity and elasticity of the sarcomere
• Sarcoplasmic reticulum (SR) is a specialized endoplasmic reticulum that surrounds each myofibril and plays a crucial role in calcium handling
  • SR contains calcium ATPase pumps (SERCA) that actively transport Ca2+ into the SR lumen during muscle relaxation
  • SR has calcium release channels (ryanodine receptors) that open in response to T-tubule depolarization, releasing Ca2+ into the sarcoplasm
• Sarcomere is the basic functional unit of a myofibril, composed of thick (myosin) and thin (actin) filaments arranged in a repeating pattern
  • Sarcomere is bounded by Z-lines that anchor the thin filaments and transmit force between adjacent sarcomeres
  • Sarcomere length changes during muscle contraction and relaxation (sliding filament theory)

Excitation-contraction coupling process

1. Motor neuron action potential reaches the neuromuscular junction and triggers the release of acetylcholine (ACh) into the synaptic cleft
   • ACh binds to nicotinic acetylcholine receptors on the sarcolemma, causing a localized depolarization called an end-plate potential
   • The neuromuscular junction is the specialized synapse between a motor neuron and a muscle fiber, crucial for initiating muscle contraction
2. The end-plate potential triggers the opening of voltage-gated sodium channels, generating an action potential that propagates along the sarcolemma and into the T-tubules
   • The action potential spreads quickly throughout the muscle fiber due to the extensive T-tubule network
3. The depolarization of the T-tubules activates voltage-gated calcium channels (dihydropyridine receptors) in the SR membrane, which in turn open the ryanodine receptors
   • This process is called calcium-induced calcium release (CICR) and results in a rapid increase in sarcoplasmic Ca2+ concentration
4. The released Ca2+ binds to troponin C on the thin filaments, causing a conformational change in the troponin-tropomyosin complex
   • Tropomyosin moves away from the myosin-binding sites on actin, allowing for the formation of cross-bridges between myosin heads and actin filaments
5. Myosin heads bind to actin (cross-bridge formation) and undergo a power stroke, pulling the thin filaments towards the center of the sarcomere
   • The cyclic attachment and detachment of myosin heads (cross-bridge cycling) results in sarcomere shortening and muscle contraction
   • ATP hydrolysis by myosin ATPase provides the energy for cross-bridge cycling and muscle contraction
6. To initiate muscle relaxation, Ca2+ is actively pumped back into the SR by SERCA, lowering the sarcoplasmic Ca2+ concentration
   • As Ca2+ dissociates from troponin C, tropomyosin returns to its original position, blocking the myosin-binding sites on actin
   • Cross-bridges detach, and the muscle fiber returns to its resting length

Muscle Fiber Types and Motor Units

• Skeletal muscles are composed of different fiber types with varying contractile and metabolic properties
  • Type I (slow-twitch) fibers: Slow contraction, high oxidative capacity, fatigue-resistant
  • Type IIa (fast-twitch oxidative) fibers: Fast contraction, moderate oxidative capacity, moderately fatigue-resistant
  • Type IIx (fast-twitch glycolytic) fibers: Fastest contraction, low oxidative capacity, easily fatigued
• Motor units consist of a single motor neuron and all the muscle fibers it innervates
  • Motor units are recruited in a size-dependent manner (size principle) during muscle activation
  • The number and type of motor units activated determine the force output and fatigue resistance of the muscle
• Muscle metabolism varies depending on fiber type and activity level
  • Aerobic metabolism: Predominant in Type I fibers, relies on oxidative phosphorylation for ATP production
  • Anaerobic metabolism: More prominent in Type II fibers, utilizes glycolysis and phosphocreatine for rapid ATP production