Muscular System

• Muscular System


  System responsible for movement, posture, respiration, production of body heat, communication, constriction of organs and vessels, and contraction of heart
• Functions of the Muscular System

  • Movement
  • Maintain posture
  • Respiration
  • Production of body heat
  • Communication
  • Constriction of organs and vessels
  • Contraction of heart
• Types of muscle


  • Skeletal (attached to bones, striated, voluntarily controlled)
  • Cardiac (in the heart, striated, involuntarily controlled)
  • Smooth (in blood vessels and hollow organs, non-striated, involuntarily controlled)
• General Properties of Muscle Tissue

  • Contractility - ability of muscle to shorten forcefully, or contract
  • Excitability - capacity of muscle to respond to stimulus
  • Extensibility - ability to be stretched beyond normal resting length and still be able to contract
  • Elasticity - ability to recoil to original resting length after being stretched
• Skeletal muscle


  Constitutes approximately 40% of body weight
• Muscle fiber


  Each muscle cell
• Skeletal muscle is so named because many of the muscles are attached to the skeletal system, while some attach to skin or connective tissue sheets
• Connective tissue coverings of skeletal muscle

  • Epimysium - connective tissue sheath that surrounds each skeletal muscle
  • Perimysium - connective tissue covering that surrounds each fascicle
  • Fascicles - subdivision of groups of muscle cells
  • Endomysium - connective tissue covering that surrounds each muscle fiber
• Tendons


  Formed as most muscle merges into one another that attaches muscle to bone
• Motor neuron


  Specialized nerve cell responsible for stimulating muscle contraction
• Muscle fiber


  • Large cell, with several hundred nuclei located at its periphery
  • Muscle fibers range in length 1 mm to 30 cm
  • Alternating light and dark bands give muscle fibers a striated appearance
• The number of muscle fibers remains constant after birth so enlargement of muscles results from an increase muscle fibers size, not an increase in fiber number
• Muscle fiber components that respond to and transmit electrical signals

  • Sarcolemma - cell membrane of muscle fiber
  • Transverse tubules (T tubules) - tubelike inward fold of sarcolemma that carry electrical impulses into the center of muscle fiber
  • Sarcoplasmic reticulum - specialized smooth ER in muscle fibers that stores high levels of CA2 that induce muscle contraction
• Structures in muscle fibers that form the general muscle property of contractility

  • Myofibrils - bundles of protein filaments that every muscle fiber has
  • Actin myofilaments - thin filaments
  • Myosin myofilaments - thick filaments
• Sarcomere


  Basic structural and functional unit of skeletal muscle that join end to end to create myofibrils
• Components of actin myofilaments

  • Globular G actin
  • Tropomyosin
  • Troponin
• Myosin myofilaments


  Composed of many elongated myosin molecules shaped like golf clubs, with a rod portion and 2 myosin heads that bind to actin to form cross-bridges and contract the muscle
• Neuromuscular junction


  Point of contact of motor neuron axon branches with muscle fiber
• The neuromuscular junction consists of a group of enlarged axon terminals that rests in an invagination of the sarcolemma
• Components of the neuromuscular junction

  • Presynaptic terminal - each axon terminal
  • Motor neuron - nerve cell that stimulates muscle cells
  • Synaptic cleft - space between presynaptic terminal and muscle fiber
  • Motor end-plate (postsynaptic membrane) - muscle plasma membrane in area of junction
  • Synaptic vesicles - small, spherical sacs in each presynaptic terminal
  • Neurotransmitter - molecule that allows a neuron to communicate with its target
  • Ligand-gated ion channels - where neurotransmitter binds to stimulate or inhibit production of action potential
• Sliding Filament Model


  The interaction that causes muscle contractions, where the actin and myosin myofilaments in the sarcomere slide past one another and shorten the sarcomere
• Resting Membrane Potential


  The electrical charge difference across cell membrane of unstimulated cell, where the cell is more like a sprinter in starting blocks that it is ready to respond at a moment's notice
• Action Potential


  Reverses resting membrane potential to let inside the cell become positive and negative on the outside, lasting for about 1-3 milliseconds
• Phases of Action Potential

  • Depolarization phase - brief period that further depolarization occurs
  • Repolarization phase - return of membrane potential to its resting value
• Function of Neuromuscular Junction

  Contact between axon terminal and sarcolemma results in action potential that stimulate fibers to contract, triggered by the release of acetylcholine from the motor neuron
• Muscle Contraction


  1. Action potential travels down motor neuron to presynaptic terminal

  2. Action potential causes Ca2 to enter terminal

  3. Ca2 causes synaptic vesicles to release ACH to synaptic cleft

  4. ACH opens Na+ channels in sarcolemma that causes action potential

  5. Action potential travels along the entire sarcolemma

  6. Action potential moves down T tubules

  7. Action potentials open gated Ca2 channels in the sarcoplasmic reticulum which releases stored calcium

  8. Ca2 binds to troponin which is attached to actin causing tropomyosin to move exposing attachment sites for myosin

  9. Myosin heads bind to actin, and the heads of the myosin myofilaments bend, causing the actin to slide past the myosin
• Cross Bridge Movement


  Mechanical component for muscle contraction, causes sarcomeres to shorten and muscle to contract
• Muscle Relaxation


  1. ACH is no longer released at the neuromuscular junction

  2. Action potentials to sarcoplasmic reticulum stops

  3. Ca2 is actively transported back to sarcoplasmic reticulum using energy supplied by ATP

  4. Ca2 diffuses away form troponin molecules, and tropomyosin blocks again the attachment sites on actin molecules

  5. Cross-bridge cycle stops and muscle relaxes
• Muscle Twitch


  Single contraction of muscle fiber in response to stimuli
• Phases of Muscle Twitch

  • Lag phase (latent) - time between application of stimulus and beginning of contraction
  • Contraction phase - time that the muscle contracts
  • Relaxation phase - time that the muscle relaxes
• Types of Muscle Contraction

  • Isometric contraction: increase muscle tension without change in length
  • Isotonic contraction - increase tension and decreases length
• Summation


  Amount of force in an individual muscle fiber
• Recruitment


  Amount of force in a whole muscle
• Motor Unit


  Consist of single motor neuron and all the muscle fibers it innervates
• Small muscles have very few fibers per motor, while large and less precise muscles have fewer, large motor units
• Individual muscle fibers can generate different amounts of force, and the strength of muscle contraction varies from weak to strong
• Muscles respond to stimuli
• Muscular System


  • 16
• Phases of muscle twitch

  1. Lag phase (latent) - time between application of stimulus and beginning of contraction

  2. Contraction phase - time that the muscle contracts

  3. Relaxation phase - time that the muscle relaxes
• Types of contraction


  • Isometric contraction: increase muscle tension w/o change in length
  • Isotonic contraction - increase tension and decreases length