Skeletal Muscle

1
Skeletal Muscle

• Microscopic Anatomy and Contraction

2
Gross Anatomy

• Each skeletal muscle is a discrete organ
• Muscle fibers
• Blood vessels
• Nerve fibers
• Connective tissue

3
Connective Tissue

• Individual muscle fibers are wrapped and held
  together by connective tissue
• Epimysium overcoat, dense irregular connective
  tissue
• Perimysium and fascicles
• Fascicle group of muscle fibers bundled together
• Perimysium fibrous connective tissue that
  surrounds each fascicle
• Endomysium fine sheath of connective tissue
  that surrounds each muscle fiber

4
Nerve and Blood Supply

• In general, each muscle is served by one nerve,
  an artery and one or more veins
• Each skeletal muscle fiber is supplied with a
  nerve ending that controls its activity
• This nerve-muscle junction is very important
• Muscles use great amounts of energy, so required
  continuous supply of oxygen and nutrients
• Muscles also give off a lot of metabolic waste

5
Muscle Attachments

• Span joints and attach to bones at at least two
  places
• Direct epimysium of muscle is fused to
  periosteum of bone or perichondrium of cartilage
• Indirect muscles connective tissue wrappings
  extend beyond muscle as a tendon which anchors
  the muscle to the connective tissue covering of
  bone or cartilage
• When muscle contracts, the movable bone
  (insertion) moves toward less movable bone
  (origin)

6
Microscopic Anatomy

• Each skeletal muscle fiber (cell) has multiple
  oval nuclei arranged just below the sarcolemma
• Syncytium fusion of hundreds of embryonic cells
• Sarcoplasm
• Glycosomes
• Myoglobin
• Myofibrils
• T tubules

7
Myofibrils

• Rodlike structures that run entire length of the
  cell
• Contractile elements of skeletal muscle cells

8
Sarcomere

• Functional Unit
• See Diagram!!!

9
Thick Filament

• Composed mainly of myosin
• Tail
• Two globular heads
• cross bridges
• Actin binding
• ATP binding

10
Thin Filaments

• Composed mainly of actin
• Forms chains
• Coils back on itself
• Tropomyosin
• Rod-shaped protein
• 2 strands spiral around the actin helical
  structure
• Block actins myosin binding site
• Troponin
• 3 polypeptide complex
• Inhibitory subunit (TnI) binds to actin
• TnT binds to tropopmyosin
• TnC binds Calcium ions

11
Elastic Filament

• Composed mainly of titin
• Holds thick filament in place in sarcomere,
  maintaining organization
• Part of the titn that spans the I band is
  extensible
• Contributes to muscles ability to recoil after
  being stretched

12
Sacroplasmic Reticulum

• Interconnecting tubules surround each myofibril
• Most run longitundinally along myofibril
• Terminal cisternae perpendicular cross channels
• Regulates intracellular levels of calcium

13
T Tubules

• At each A I band junction, the sarcolemma
  penetrates into cell to form transverse tubule
• Runs between the paired terminal cisternae of the
  SR
• Forms triad
• Muscle contraction controlled by nerve impulses
  that travel along the sarcolemma of the cell
• T tubules allow that impulse to reach every
  sarcomere

14
Triad Relationships

• Signal for contraction
• Integral proteins protrude into intermembrane
  space
• Act as voltage sensors on T tubules
• Act as Calcium gates on SR

15
Sliding Filament Theory of Contraction

• 1954, Hugh Huxley
• Thin filaments slide past the thick ones so that
  the actin and myosin filaments overlap to a
  greater degree

16
Crossbridge Attachments

• Each cross bridge attaches and detaches several
  times during contraction, acting like a ratchet
  to generate tension and propel the thin filaments
  toward the center of the sarcomere reducing the
  distance between Z lines
• As this occurs simultaneously to every muscle
  cell, the entire muscle shortens

17
Contraction Mechanism

• When intracellular Calcium levels are low, muscle
  is relaxed
• Active myosin binding sites on actin are
  physically blocked by tropomyosin
• As calcium levels rise, they bind to sites on
  troponin
• Troponin changes shape
• Tropomyosin moves deeper into groove of actin
  helix
• blockade is removed
• Myosin can then bind to actin

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1. Cross Bridge Attachment

• Activated myosin head is attracted to exposed
  binding sites on actin and cross bridge
  attachment occurs

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2. Power Stroke

• As myosin head binds, pulling on the thin
  filament and sliding the myosin toward the center
  of the sarcomere
• ADP and Pi are released

20
3. Crossbridge Detachment

• As new ATP binds to myosin head, myosins hold on
  actin loosens and the cross bridge detaches from
  actin

21
4. Cocking of the myosin head

• Hydrolysis of ATP to ADP provides the energy
  needed to return the myosin to its high energy
  cocked position.
• Provides the potential energy for its next
  sequence of attachment and power stroke