Muscle Tissue

• 3 Main Types of muscle:

  1. Skeletal (musculoskeletal)
  2. Cardiac
  3. Smooth
• Skeletal Muscle Structure (most variable):

  • Long
  • Cylindrical
  • Multinucleate (more than 1 nuclei)
  • Obvious striations = consequence of thick & thin filaments
• Skeletal Muscle Functions:

  • Voluntary movement/control
  • Locomotion (walking)
  • Manipulation of the environment (grasping)
  • Facial expression
  • Posture (keep you upright)
• Skeletal Muscle attaches to bone or occasionally skin to allow movement of body
• Gross Anatomy of Skeletal Muscle: (outer - inner)

  • Epimysium - CT sheath (wraps whole muscle)
  • Endomysium - protect/wrap individual muscle fibres
  • Perimysium - sheath bundles of muscle fibres
  • Fascicles - bundles of muscle fibres
• Muscle fibres sheathed by endomysium, bundled together & wrapped by perimysium to make a fascicle; fascicles together to make a bundle, covered by epimysium
• Muscle fibres:

  • Sarcolemma = cell membrane of muscle fibre; transfer info
  • Transverse (T)-Tubules = invagination of sarcolemma; info from outside into muscle fibre
  • Sarcoplasmic reticulum (SR) = store & release Ca, used to initiate contraction
• Muscle Myofibril - Sarcomere:

  • Z line = outside of myofibril
  • M line = mid line
  • Stretching from Z line towards M line are thin filaments
  • Stretching from M line outwards to Z line = thick filaments

  Bind of thick & thin filaments allows for contraction
• 3 Thin Filament Proteins:

  1. Actin - contain binding site for thick filament, (cylindrical)
  2. Tropomyosin - protein that covers binding site during relaxed state
  3. Troponin
• 1 Main Thick Filament Protein:

  1. Myosin - elongated with myosin head; head binds & 'walks' along thin filament
• Sarcomere Zones:

  • I band = only thin filament
  • A band = where overlap (thick & thin)
  • H zone = only thick filament
• AP --> Neuromuscular junction:

  1. Signal to initiate skeletal muscle contraction
  2. Acetylcholine (ACh) release into synaptic cleft binds to ion channels - depolarisation of membrane
  3. AP along sarcolemma of muscle fibre
  4. AP into T-Tubules connect to SR, where Ca is stored & released (in response of AP)
  5. Ca interact with myofibril for contraction
• Relaxed Sarcomere:

  • Binding sites covered by tropomyosin
  • So thick filaments can't bind
• Excited Sarcomere:

  • Ca binds to troponin causing movement of tropomyosin
  • Exposing binding site
  • Myosin (thick) binds to actin (thin)
• Sarcomere Contraction:

  1. Binding
  2. Power Stroke
  3. Detachment
  4. Binding
• Sarcomere Contraction:

  • Myosin head changes shape & pulls actin to M line, Z line pulled towards centre (increase overlap)
  • ATP (if present) binds to myosin & energy utilised to detach myosin = reverting shape
  • Myosin head binds to another actin molecule further towards the Z line
• Binding of ATP to Myosin:

  • AP arrives at Neuromuscular junction making free Ca available
  • Myosin head binds to actin
  • Power stroke, ADP release; myosin head changes position; filaments slide past one another
  • ATP binds to myosin causing it to release actin
  • ATP is split & myosin head returns to resting position
  • Resting myosin fibril with ADP & P, bound to head
• ATP NOT utilised for binding & contraction but for what?

  Releasing myosin head
• ATP cause relacation by releasing myosin head from actin
• What happens to sarcomere zones during contraction =

  • I band = decreases
  • A band = stays the same
  • H zone = decreases
  • Distance bw/ Z lines = decreases
• Sarcomere Contraction:

  • Pull Z lines towards centre (M line)
  • Sarcomere decreased in size
  • More overlap bw/ filaments (as consequence)
  • I band = narrowed
  • H zone = narrowed in size
  • A band = not changed
• • Ca from SR when excited binds to troponin
  • Binding enables troponin protein complex to pull tropomyosin aside - so no longer covers binding site on actin
  • Myosin head bind to now-exposed sites on actin & pulls thin filament inwards
  • Head detached & shape reverted by ATP
• Rigor Mortis = muscle undergo partial contraction & can't release as no ATP for relaxation so are stiff
• Nermaline myopathy - disease of muscle:

  • Muscle weakness, swallowing dysfunction, impaired speech
  • Mutations in at least 10 genes
  • NEB (Nebulin) = ~50% of cases
  • ACTA1 (Actin isoform) = 15-25%
• Cardiac Muscle:

  • Continuous rhythmic activity
  • Modulated by external autonomic & hormonal stimuli (involuntary - external stimuli = stress or exercise)
  • Structurally bw/ skeletal & smooth muscle
• Cardiac Muscle Structure:

  • Branching (different)
  • Striated (same as skeletal)
  • Generally uninucleate cell (1 nuclei)
  • Interdigitate at specialised junction = intercalated discs
• Cardiac Muscle Function:

  • Contracts to propel blood into circulation
  • To pump blood
  • Involuntary control
• Cardiac Muscle can be found in the walls of the heart
• 3 Structures of Intercalated discs:

  1. Desmosomes = anchors cell to each other via cytoskeleton
  2. Fascia adherins = anchor actin & transmit contractile tissue - important in allowing contraction; where 2 Z lines meet
  3. Gap junctions = communication to contract rhythmically; transmit contraction stimulus
• Arrrhythmogenic = irregular heart beat

  • Leads to heart attacks
  • Caused by mutations to genes for desmosome proteins (fewer desmosomes or fragmented)
  • Intracellular gap widened
  • Cells die & replaced by fatty infiltration
• Smooth Muscle:

  • Involuntary control - under inherent autonomic & hormonal control
  • Continuous contractions of slow force
  • Whole muscle contract in wave-like fashion to propel substances
• Smooth Muscle Structure:

  • Spindle shaped cells
  • 1 central nuclei
  • No striations (difference)
  • Cells arranged closely to form sheets
• Smooth Muscle Function:

  • To propel substances or objects along internal passageways
  • Involuntary control
• Smooth muscle is mostly in the walls of hollow organs (eg blood vessels, gastrointestinal tract, uterus, bladder)
• Smooth Muscle Contraction:

  • No sarcomeres, instead contractile filaments - criss-cross the cell = allow whole cell to contract
  • Anchored to cell at focal densities in cytoplasm & focal adhesion densities on cell membrane
• Smooth Muscle Contraction:

  • Focal densities are functionally & structurally similar to Z lines
• Smooth Muscle - Focal densities:

  • Anchor actin filaments (still have thick & thin)
  • Actin filaments link to both sides of focal densities
  • Myosin filaments partially overlap actin like in skeletal
  • Intermediate filaments provide cytoskeleton structure bw/ densities
• Smooth Muscle Contraction still sliding & increasing overlap (just not laid out as sarcomeres)

  • Works on sliding filament theory
  • Actin slide over myosin
  • Takes on globular shape
• Increase tissue mass by:

  • Hypertrophy - cell size
  • Hyperplasia - cell number
  • or both
• Adult skeletal muscle can't produce new cells