MUSCULAR

• Muscular System


  Muscle tissues, Tendons

  
• Components of the Muscular System


  • Generates internal and external body movement
  • Produces body heat
  • Gives body shape
  • Stabilizes joint
  • Maintains posture

  
• Types of Muscle

  • Contractile units within a muscle
  • Muscle fibers/Myocytes
  • Fascicle

  
• Muscle Fiber Composition


  • Basic rod-like unit of a muscle cell
  • Filaments of myofibrils, constructed from proteins, principally myosin or actin
  • Membrane covering the muscle fiber
  • Inner material surrounding fibers (like cytoplasm)
  • Stores calcium ions in muscle
  • Extensions of the cell membrane that penetrate into the centre of muscle cells

  
• Myofibril


  • Contains myofilaments - ACTIN (thin) and MYOSIN (thick)
  • A band = dark (thick, myosin)
  • I band = light (thin, actin)
  • These filaments overlap to form dark and light bands on the muscle fiber
  • In the middle of each I band are Z lines. A sarcomere is on Z line to the other

  
• Sarcomere


  • The functional contractile unit of a skeletal muscle fiber
  • Defined as the distance from one Z disc to the next adjacent Z disc
  • Myofibrils contain multiple Z discs

  
• Muscle tissue layers


  • Endomysium
  • Perimysium
  • Epimysium

  
• Deep Fascia


  An expansive sheet of dense irregular connective tissue that separates individual muscles, binds together muscles with similar functions, forms sheaths to help distribute nerves, blood vessels, and lymphatic vessels, fill spaces between muscles

  
• Action Potential


  A nerve signal. Neurons generate and conduct this signal along their processes in order to transmit them to the target tissues. Upon stimulation, they will either be stimulated, inhibited, or modulated. Only neurons and muscle cells are capable of generating an action potential; that property is called the excitability. It is caused by a stimulus with certain value. Subthreshold stimulus is a stimulus that cannot cause an action potential. Threshold stimuli is a stimulus having enough energy or potential to produce an action potential (nerve impulse). Supra-threshold stimuli produce an action potential, but their strength is higher than the threshold stimuli.
• Synapse


  Neuronal junction, site of transmission of electric nerve impulses between two nerve cells (neurons) or between a neuron and a gland or muscle cell (effector). Neuromuscular junction (or myoneural junction) is a chemical synapse between a motor neuron and a muscle fiber, allows the motor neuron to transmit a signal to the muscle fiber, causing muscle contraction.
• Contraction of a Muscle Fiber

  1. Acetylcholine (Ach) is a neurotransmitter released from the motor neuron innervating a muscle fiber

  2. Depolarization of the T-tubules triggers sarcoplasmic reticulum to release Calcium ions

  3. ATP Hydrolysis

  4. Cross bridge

  5. Power stroke

  6. Detachment of myosin from actin
• As long as Ca++ ions remain in the sarcoplasm to bind to troponin, and as long as ATP is available, the muscle fiber will continue to shorten.
• Contraction Cycle


  1. Ca++ bind to troponin

  2. Tropomyosin slide away from the binding sites on actin

  3. Myosin heads bind to binding sites -form cross-bridges

  4. Thin filaments are then pulled by the myosin heads to slide past the thick filaments toward the center of the sarcomere

  5. During power stroke, ADP and Pi are released
• Cross Bridges


  • Myosin is powered by the hydrolysis of ATP to ADP and Pi
  • Cross bridge is formed between actin and the myosin heads triggering contraction
  • During cross-bridge formation, ADP and Pi are still bound to myosin
• Power Stroke


  • Thin filaments are pulled which slide past the thick filaments within the fiber's sarcomeres
  • Cocked myosin head => myosin is in a high-energy configuration
  • End of the power stroke => myosin head is in a low-energy position
  • When ATP is available, it readily attaches to myosin, the cross-bridge cycle can recur, and muscle contraction can continue
• The Sliding Filament Model of Contraction

  • Sarcomere shortens => Z-discs move closer together=> I band becomes smaller, but A band stays the same width
  • At full contraction, the thin and thick filaments have the most amount of overlap
• Relaxation of a Muscle Fiber

  1. Ca++ ions are pumped back into the SR, which causes the tropomyosin to re-shield the binding sites on the actin strands

  2. A muscle may also stop contracting when it runs out of ATP and becomes fatigued
• Muscle Attachments


  Tendon - a connective tissue which attaches the muscle to bone, skin, or another muscle
• Muscle Origin and Insertion


  • Origin - Less movable attachment of a muscle
  • Insertion - More movable attachment of the muscle
  • Insertion is pulled toward the origin
  • Origin typically lies proximal to the insertion
• Hypertrophy - An increase in muscle fiber size. Muscle size may be improved by exercising
• Atrophy - Reduction in muscle size, tone, and power. Due to reduced stimulation, it loses both mass and tone
• What is rigor mortis?
• Label Practice
• The motor unit is the smallest functional unit of skeletal muscle.
• A single alpha motor neuron innervates all the muscle fibers within its territory, which are called a motor unit.
• Motor units can be classified as either slow-twitch or fast-twitch based on their contraction characteristics.
• A single alpha motor neuron innervates all of the muscle fibers within its territory (motor unit).
• Muscles that require fine control have smaller motor units with fewer muscle fibers per motor unit.
• Slow twitch (Type I) motor units have small muscle fibers that contract slowly but sustainably over long periods of time.
• Motor units can vary greatly in size depending on their function.
• Each motor unit has one or more muscle fibers that are connected together electrically through gap junctions.
• Motor units can vary from as few as one or two muscle fibers up to several hundred depending on the function of the muscle.
• Larger muscles like those used for running or jumping have larger motor units with more muscle fibers per motor unit.
• Fast twitch (Type IIa) motor units have medium-sized muscle fibers that contract quickly and generate more force than Type I fibers.
• Fast twitch (Type IIb) motor units have large muscle fibers that contract rapidly and generate high amounts of force.
• Slow-twitch motor units have small diameter axons that branch into many terminal branches to form a large number of synaptic endings with individual muscle fibers.
• Slow-twitch motor units contract slowly but sustainably over long periods of time.
• Fast-twitch motor units have larger diameter axons that branch less frequently than those of slow-twitch motor units.
• Fast twitch (Type IIb) motor units have very large muscle fibers that contract rapidly and generate high amounts of force.
• The number of motor units is determined by the amount of nerve impulses received from the brain.