Exam 3

• Skeletal muscle


  Responsible for supporting and moving the skeleton

  • Voluntary control
  • Striated
• Cardiac muscle


  Muscle of the heart

  • Involuntary control
  • Striated
• Smooth muscle


  Muscle surrounding hollow organs and other tissues

  • Involuntary control
  • not striated
• Because skeletal and cardiac muscle have a striped pattern perpendicular to their long axis caused by myofibrils, they are called striated muscles
• Skeletal muscle fibers contain multiple nuclei and are made by fusion myoblasts (single nuclei cells) and they are extremely large
• Each skeletal muscle fiber is filled with many myofibrils (give muscle fibers striated appearance)
• The region between two z-lines on myofibrils is called a sarcomere
• Sarcomeres are the contractile unit of the muscle fiber
• The sarcomere is composed of thick and thin filaments:


  Thick filaments are composed of parallel bundles of myosin and are lined with cross-bridges
• The sarcomere is composed of thick and thin filaments:


  Thin filaments are composed of a double strand of actin and contain tropomyosin and troponin
• In the sarcomere the cross-bridges of the thick filament reach out and contact the thin filaments. This is required for muscle contraction
• The thick filament cross bridges are composed of myosin proteins. They contact the thin filaments and use ATP to pull on the thin filaments, contracting the muscle.
• Tropomyosin is a protein that wraps around the actin filament and blocks binding of the cross-bridges.
• Troponin is a small protein the controls placement of tropomyosin
• When Ca2+ in the cell is low, troponin keeps the tropomyosin in a position that blocks cross-bridge actin binding.


  Muscle is relaxed
• When Ca2+ in the cell is high, it binds to troponin which allows tropomyosin to move away from the cross-bridge binding sites, allowing cross-bridges to form.


  Muscle is contracted
• Contraction does not necessarily mean “shortening” of muscle. It refers to the activation of cross-bridges to create tension between the thick and thin filaments
• Shortening of muscle fibers proceeds by the sliding filament mechanism
• The sliding filament mechanism is when the thin filaments are pulled in closer around the thick filaments, resulting in shortening of muscle
• Motor neurons are mylinated and their bodies are in the brain stem and spinal cord. They have a large diameter that allows them to rapidly transmit signals from the CNS to the muscles
• A motor unit is defined as one motor neuron and the skeletal muscle fibers it innervates.
• Within a whole muscle there are many muscle units
• One motor neuron innervates many muscle fibers but a muscle fiber is innervated by only one motor neuron.
• When an action potential occurs in a motor neuron, all of its associated muscle fibers contract
• Every action potential in a neruomuscular junction produces an action potential in each muscle fiber. This differs from neuron-neuron synapses, where multiple excitatory postsynaptic potentials are required to reach threshold
• All neuromuscular junctions are excitatory. Inhibitory potentials do not occur in skeletal muscle
• Excitation-contraction coupling refers to the series of events that occur between an action potential in the muscle fiber membrane and muscle contraction
• Transverse tubules are continuous with the plasma membrane and run across the diameter of the muscle fiber. They propagate action potentials
• Sarcoplasmic reticulum contains. lateral sacs called the terminal cisternae that contain Ca2+ ions that are released to trigger muscle contraction
• T-tubules contain voltage sensors called DHP receptors that associate with calcium channels in the lateral sacs of the sarcoplasmic reticulum.
• Contraction ends because Ca2+ is pumped back into the sarcoplasmic reticulum by a Ca2+ ATPase, causing Ca2+ to be removed from the troponin so tropomyosin can block the cross-bridges
• Curare is a competitive antagonist for ACh at nicotinic receptors
• Succinylcholine acts as an agonist for ACh receptors and keeps the motor end plate in a continued state of depolarization, preventing action potentials from occuring
• The use of paralytic agents in surgery reduces the required dose of general anesthetic
• Clostridium botulinum blocks ACh release from nerve terminals
• Action potentials travel down t-tubules to activate DHP and ryanodine receptors resulting in transient release of CA2+ ions from the SR, which triggers cross-bridge formation and muscle contraction. Muscle contraction is then ended by movement of Ca2+ back into the SR.
• Load is the force exerted on muscle by an object (usually weight)
• Muscle tension and load are opposing forces.


  You must create tension in a muscle to hold or move a load.
• Isometric contractions: when muscle develops tension but muscle length stays the same (e.g. holding a weight)
• Isotonic contraction: when the load is constant but the length of the muscle changes (e.g. bicep curls)