Structure of Skeletal Muscle

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Structure of Skeletal Muscle We will begin
our look at the structure of muscle starting
with the largest structures and working our
way down to the smallest microscopic features
found within the cell. A whole muscle, like
the biceps muscle of the upper arm, is
composed of groups of fasciculi surrounded by
a white connective tissue called perimysium.
Each fasciculus, in turn, is made up of
bundles of muscle cells (also called muscle
fibers). Within each cell there are
cylindrical bundles of myofibrils. These
myofibrils are composed of two types of
myofilaments, which are the actual contractile
elements of the cell. Let's have a closer look
at a muscle cell . . .
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• Structure of Skeletal Muscle
• Muscle cells (or fibers) are one of the few
  cells in the body with more than one nucleus.
• They are surrounded by the sarcolemmathe muscle
  cell membraneover which the action potential is
  transmitted.
• The sarcolemma has small tube-like projections
  called transverse (T) tubules that extend down
  into the cell.

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• Structure of skeletal muscle
• These T tubules conduct the action potential
  deep into the cell where the contractile proteins
  are located.
• Within the muscle cell are long cylindrical
  myofibrils that contain the contractile proteins
  of the muscle.

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• Structure of skeletal muscle
• The myofibrils are surrounded by the
  sarcoplasmic reticulum (SR).
• This is a mesh-like network of tubes containing
  Ca, which are essential for contraction.
• At either end of and continuous with the SR is
  the terminal cisterna, which is close to the T
  tubule where the action potential travels.
• Lets now have a look at the contractile
  proteins found in the myofibrils.

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• Thin Myofilaments
• The myofibrils contain two types of
  myofilaments.
• The thin myofilaments are composed predominantly
  of the globular protein actin.
• Each actin molecule contains a special binding
  site for the other contractile protein myosin.
• Many actin molecules are strung together like
  the beads on a necklace and then twisted to form
  the backbone of the thin myofilaments.

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• Thin myofilaments
• Also found on the thin myofilaments are long
  protein strands called tropomyosin. When the
  muscle is at rest, the proteins cover the binding
  sites for the head of myosin.
• A third regulatory protein, called troponin, is
  made up of three subunitstroponin , which binds
  with Ca which binds to the tropomyosin which
  then exposes the binding site on actin. At rest,
  the troponin holds the tropomyosin over the
  myosin binding sites.
• As we will see later, when Ca bind to the
  troponin unit, the tropomyosin is pulled off the
  myosin binding
• sites by the troponin.

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• Thick Myofilaments
• The second type of myofilamentthe thick
• myofilamentis made up of the protein
• myosin. This protein has a long, bendable tail
• and two heads that can each attach to the
• myosin binding sites on actin (as mentioned
• on the previous page).
• The heads also have a site that can bind and
  split adenosine triphosphate (ATP).
• As we will see, it is the splitting of ATP that
  releases energy to the myosin that powers the
  contraction of the muscle.

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• Thick Myofilaments
• The second type of myofilaments the thick
  myofilament- is made of the protein myosin. This
  protein has a long bendable tail and two heads
  that can each attach to the myosin binding sites
  on actin.
• The heads also have a site that can bind and
  split adenosine triphosphate. As we will see, it
  is the splitting of ATP that releases energy to
  the myosin that powers the contraction of the
  muscle. Many myosin molecules are arranged to
  form one thick filament. Under a microscope, the
  arrangement of the thin and thick myofilaments
  gives the myofibril and the muscle cell a banded
  appearance.
• This is why skeletal muscle is called striated
  muscle.

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• Actin / Myosin Relationship
• Groups of thin (actin) myofilaments and
• groups of thick (myosin) myofilaments are
• arranged in a repeating pattern (thick, thin,
• thick, thin, and so on) along the length of
  the
• myofibril from one end of the cell to the
  other.
• Each group of thin myofilaments extends
• outward in opposite directions from a central
  Z
• line, where they are anchored.

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• Actin/myosin relationship
• Similarly, groups of thick myofilaments extend
  outward from a central M line, where they are
  attached. Each myofilament is parallel to the
  length of the myofibril and the muscle cell.
• The region from one Z line to another is called
  a sarcomere. This is the smallest functional
  contractile unit of the muscle cell.

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Excitation-contraction

• Excitation-contraction this is the process by
  which an action potential in the sarcolemma
  excites the muscle cell to produce a muscle
  contraction.
• The AP that was generated at the neuromuscular
  junction will spread out over the sarcolemma and
  down the T-tubules into the core of the muscle.
  This AP reaches the SR and increases CA, which
  will bind to troponin causing tropomyosin to move
  exposing the binding sites on actin.
• Myosin will now be able to attach to the actin
  and a power stroke will occur.

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• Relaxation of Muscle
• Once action potentials stop, Ca will no
• longer diffuse out of the sarcoplasmic
• reticulum (SR). Special calcium pumps rapidly
• pump Ca back into the SR, up its
• concentration gradient this requires ATP.
• Without the calcium present in the cytoplasm
• of the muscle cell, the tropomyosin will cover
• the myosin binding sites once again. Myosin
• will be unable to bind to actin and power
• strokes will not occur. The muscle will relax.

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• Actin-Myosin and ATP Cycle
• Here are the steps summarized
• 1. Myosin, which has been energized by the
• splitting of ATP to adenosine diphosphate
• (ADP) and inorganic phosphate (Pi), attaches
• to actin and forms a crossbridge.
• 2. A power stroke is initiated while ADP and
  Pi
• are expelled from the myosin head.
• 3. Actin and myosin slide past one another.

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• Actin-Myosin
• 4. Actin and myosin are bound together until a
  new molecule of ATP attaches to myosin the
  crossbridge is broken.
• 5. ATP is split to form ADP and Pi,energizing
  the myosin molecule.
• 6. The cycle repeats as long as actin and myosin
  can interact.

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• The Motor Unit
• A motor unit is a motor neuron and all of the
• muscle cells/fibers it contacts. In almost
  all
• situations, one motor neuron will contact
  (or
• innervate) several muscle cells, but each
• muscle cell is innervated by only one motor
• neuron. The number of muscle cells
• innervated by a motor neuron varies. A large
• motor unit has a motor nerve in contact with
  a
• large number of muscle cells, while a small
• motor unit is one in which the motor neuron
• contacts a few muscle cells.

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• The Structure of the Neuromuscular
• Junction
• The neuron that contacts a muscle cell is
• sometimes called a motor nerve fiber. The
• membrane of the axon terminal contains
• Ca voltage-gated channels. These channels
  open in response to a nerve impulse.
• The axon terminal of the motor cell/fiber also
  contains synaptic vesicles that contain the
  neurotransmitter acetylcholine(ACh).

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Neuromuscular -JCT The basement membrane of the
axon terminal contains the enzyme
acetylcholinesterase (AChE). The muscle cell
membrane (also called the sarcolemma) directly
under the axon terminal is thrown into folds.
This region is called the end plate. The end
plate contains receptors for ACh, which are
associated with gated ion channels. The gap
between the motor fiber and muscle cell is
called the synaptic cleft.
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• Events at the Neuromuscular
• 1.The action potential on the motor nerve fiber
  triggers Ca voltage-gate channels to open. Ca
  flow into the cell, down their concentration
  gradient.
• 2.Ca trigger the fusing of synaptic vesicles
  to the membrane and the release of ACh into the
  synaptic cleft by exocytosis.
• 3.ACh diffuses across the synaptic cleft and
  attaches to receptors on the muscle cell
  membrane/sarcolemma.
• 4.The ACh is broken down by the enzyme AChE and
  is taken back into the axon terminal to be
  recycled.

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