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      •Anatomy of a Muscle Fiber
–Muscle fiber = muscle cell
–Transverse (T) Tubules
–Sarcoplasmic Reticulum
    •Muscle fiber = muscle cell
  –Generally these run the length of the muscle and they have a thread-like shape
  Note: Sarco means????
  flesh, this is a common prefix for muscle tissue
  –Cells have a plasma membrane, this seperates cell contents from ECF 
  –Muscle cells plasma membrane is called the sarcolemma
  –Cells have a cytoplasm, this is fluid within the cell plus organelles, except the nucleus.  In other words ‘cell stuff’

  –The cytoplasm of the muscle cells is called sarcoplasm
  –Unlike other cells which only have one nucleus, muscle cells have many nuclei

  –Embryologically they come from many cells that fuse together
  Transverse (T) Tubules
  –Extension of the sarcolemma that ‘tunnels’ (this is called invagination) into the center of the muscle fiber

  –They allow the electric signal to travel deeper into the muscle fiber
  Sarcoplasmic Reticulum
  –Network of tubules or sacs that are wrapped around the T Tubules

  –Stores, releases and reabsorbs Ca++ 
  –Calcium is necessary for a muscle contraction
  •Review structures from large to small
–Entire muscle (biceps brachii for instance)
    •What fascial layer wraps the    muscle? 
–This muscle is made up of bundles of individual muscle fibers (hundreds to thousands bundled together
    •What are these bundles called?   
      –Muscle fasicles
    •What fascial layer wraps these fasicles?
–Muscle fibers also have a fascial wrapping called?
  –Muscle fibers are composed bundles of very fine fibers called myofibrils

  –A muscle fiber contains thousands of myofibrils

  –Myofibrils extend lengthwise along the entire skeletal muscle fiber
  –The basic contractile unit of the muscle fiber

  –Sarcomeres make up repeating sections of the myofilaments

  –Sarcomeres are made up of myofilaments
  –Made up of proteins and arranged in sarcomeres –Composed of thick filaments and thin filaments •Thick filaments –Made up of the protein myosin •Thin filaments –Made up of three proteins (actin, tropomyosin, troponin)
  What is the basic contractile unit of the muscle (fibers)
  •What protein makes up thick filaments?
  What is the primary protein that makes up thin filaments?
  •Compare contrast muscle fibers/myofibrils/myofilaments
  Which is the same as a muscle cell
  •Muscle fibers
  Which runs the length of the muscle fibers, it is composed of repeating sequences of sarcomeres?
  Which is made up of thick and thin filaments?
  –Which combine to form the sarcomere?
  Function of Skeletal Muscle
•Motor Unit
•Muscle Tone
  •Motor Unit
  –Defined as a motor neuron and the muscle fibers it attaches to
     •If it attaches to a few muscle fibers, movement will be more precise 
      –Example) eye muscles (motor neurons attach to 10-20 fibers)
     •If attaches to many muscle fibers, movement will be less precise
      –Example) Arm or Leg muscles (motor neuron will attach to 2000-3000 fibers
  –Smooth, sustained contractions resulting from coordinated contractions of different motor units within the muscle
  –This is a typical skeletal muscle contraction
  •Muscle Tone
  –A continual, partial contraction of a muscle. –Low level of continuous contraction characteristic of the muscles of normal individuals during waking hours
Action of a Muscle
Muscle belly contraction
 –Combined contraction of multiple muscle fibers
 -Recruiting – groups of muscle fibers take turns contracting
    •More strength requires more muscle fiber recruitment
    •Less strength requires less muscle fiber recruitment
    •Fibers do not have weaker or stronger contraction, the muscle creates more force by recruiting more fibers
  Muscle Contraction

  –Nerve cells (neurons) that are sending a motor signal from CNS (Central Nervous System) to effectors (today we are discussing skeletal muscles as the effector) 

   –Motor neurons come close to muscle fibers

   –There is a extremely small space between the terminal ends of the neuron and the muscle fibers

   –This region where the motor neurons communicate with the muscle fibers is called the neuromuscular junction 
  Muscle Contraction

•  Neurotransmitter
  –Chemicals Motor Neuron that nerves use to cross this small space (called the synaptic cleft) and continue the nerve impulse.  They are released from the neuron and bind to receptors on the cell membrane of the next cell

–Acetylcholine (Ach) is used at the neuromuscular junction
  Muscle Contraction

•  Excitation
  –Electrical signal going to the next cell
  Muscle Contraction

•  Contraction
  –Ach binds to receptor on muscle cell membrane which continues the signal

–The signal travels along the sarcolemma and down the T Tubules

–Ca++ is released from the sarcoplasmic reticulum

–Ca binds to troponin, changes shape of troponin which moves tropomyosin from binding sites on the actin

–Myosin heads grab actin at binding site, pulls the z discs closer together and shortens the sarcomere (and thus muscle)
  Muscle Contraction

•  Relaxation
  –SR pumps Ca++ back into its sacs, shutting down contraction process
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