Skeletal Muscle

• Muscle relaxation process:

  • Work is done & the nerve stops stimulating Ach
  • Axon terminal stops releasing acetylcholine
  • Acetylcholinesterase breaks down the Ach in the synapse
  • The sarcoplasmic reticulum reabsorbs Ca²+, taken back into storage by active-transport pumps (requires ATP)
  • Troponin-tropomyosin complex shifts back into resting position, blocking myosin from binding to actin
• During muscle contraction:

  • Muscle fibers develop tension
  • Each myosin head binds an ATP molecule & hydrolyzes it into ADP + phosphate (PP) group, causing the head to move from a flexed to an extended high-energy position
  • Myosin head binds to an active site on the thin filament, forming a crossbridge
  • Myosin releases ADP & P, flexes back to its original position, tugging the thin filament along
  • Cycle repeats at a rate of around 5 strokes per second
• Signal transmission in muscle excitation:

  • Nerve signal stimulates synaptic vesicles to release acetylcholine (ACh) into the synaptic cleft
  • ACh binds to ACh receptors in the sarcolemma, opening gated channels for sodium ions (Na) to enter and potassium ions (K) to exit
  • Ion movements excite the sarcolemma, initiating a wave of electrical changes spreading away from the neuromuscular junction, exciting the muscle fiber
• Contraction is initiated when calcium ions released from the sarcoplasmic reticulum bind to troponin, which then causes a structural change in tropomyosin leading to exposure of actin-binding sites on myosin.
• Muscles are composed of myofibrils that contain actin and myosin filaments arranged in sarcomeres.
• The muscle contraction involves a shortening of the muscle tissue through the sliding of actin filaments over myosin filaments.
• Muscle contractions are generated by forces resulting from interactions between actin and myosin filaments in muscle tissue.
• Muscle Fatigue is the progressive weakness from prolonged use of the muscle
• There are only 2 ways to produce ATP: anaerobic fermentation and aerobic respiration
• Aerobic Respiration requires oxygen
• Anaerobic Fermentation

  Glucose is converted to lactate. For each glucose consumed, this pathway produces a net yield of 2 ATP
• Anaerobic Fermentation does not use oxygen and can produce ATP when demand is high
• ATP


  The only energy source that muscle contraction depends on
• Aerobic Respiration


  An intermediate product of glucose metabolism called pyruvate is oxidized in the Mitochondria to carbon dioxide and water. It produces 30 ATP molecules
• Fast-twitch fibers have quick responses and fatigue easily
• Slow twitch fibers have long, slow twitches and don't fatigue easily
• Endurance is the tolerance of prolonged exercise
• EPOC stands for Excess Post-Exercise Oxygen Consumption
• Recovery Period is necessary for muscle fibers
• Whole-Muscle Contraction

  Muscle twitch to tetanus
• A muscle twitch is the minimum contraction exhibited by a muscle and lasts as little as 100 milliseconds
• Summation is a phenomenon where multiple muscle twitches occur closely together so the muscle fiber cannot relax
• In complete Tetanus, the muscle goes through quick cycles of contraction with short relaxation periods
• Types of Muscle Contractions

  • Isometric
  • Isotonic
  • Concentric
  • Eccentric
• Isotonic Contraction


  Muscle shortens while maintaining tension
• Isometric Contraction


  A form of contraction in which a muscle develops increasing tension but does not shorten due to an overriding external resistance
• Concentric Contraction


  Muscle shortens as it maintains tension
• Eccentric Contraction


  A muscle maintains tension as it lengthens
• Functions of Muscle
• Nerve-Muscle Relationship

  • Skeletal Muscles cannot contract unless stimulated by a nerve
  • The nerve cells that stimulate Skeletal muscles are called Motor Neurons
  • Motor Neurons are located in the brain & spinal cord
  • Each branch of the neuron ends in a bulbous swelling called an Axon Terminal
  • The axon terminal contains sacs called Synaptic vesicles that contain a chemical called acetylcholine
  • The axon terminal and muscle fiber are separated by a gap called the Synaptic Cleft
• Muscle Contraction

  1. Understanding muscle contraction requires a closer look at the Myofibrils that consist of Myofilaments & Striations

  2. Each Myofibril is a bundle of parallel protein microfilaments called myofilaments

  3. Thick Filaments are made of several hundred molecules of a protein called Myosin, shaped like a golf club with 2 polypeptides intertwined to form a shaft-like tail

  4. Thin Filaments, half as wide as thick filaments, are composed mainly of intertwined strands of a protein called Actin, also containing Tropomyosin & Troponin
• Skeletal Muscle Fibers

  • Defined as voluntary striated muscle usually attached to one or more bones
  • Striated because it has alternating light & dark transverse bands/striations
• Functions of Muscle

  • Movement
  • Stability
  • Control of Body openings
  • Generation
  • Glycemic Control: Regulation of blood glucose
• Structure of Muscle Fiber

  • Each fiber has multiple nuclei pressed against the inside of the cell, forming thick bundles
  • Bundles are called myofibrils
  • The plasma membrane is called the sarcolemma
  • Has tunnel-like infoldings called transverse tubules that penetrate through the fiber and emerge on the other side
  • The Smooth ER is called the Sarcoplasmic reticulum in muscle cells