Muscle structure & composition Flashcards
What are the 3 muscle types in the animal body? what type of control do they have?
Skeletal - voluntary
Smooth - involuntary
Cardiac - involuntary
What is the implication of skeletal muscle’s structure?
Key in meat science, striated and multi-nucleated with nuclei at the periphery
What is the sequence of muscle fiber development during prenatal myogenesis?
Stem cells → myoblasts → myotubes → myofibers
What happens to satellite cells (stem cells) postnatally?
Activated by stress or injury, they divide asymmetrically
-can produce new myofibers or add nuclei to existing fibers
how do primary and secondary myofibers differ?
primary myofibers are larger in structure for organization, alignments and fusion into secondary fibers
Why are muscle fibers multinucleated?
- Fusion of many mononucleated myoblasts
- Long cells require multiple nuclei
- High energy demand necessitates multiple mitochondria
- High protein synthesis requires distributed nuclei
What is the hierarchy of muscle structure?
- Muscle → Muscle fiber bundle (fascicle) → fiber → myofibril → myofilament
what 3 kinds of tissue does a muscle have? what are they responsible for?
1) muscle tissue
-muscle fibers/myofibers
2) adipose tissue
-marbling
3) connective tissue (epimysium, perimysium, endomysium)
-protection + contraction of muscle fibers
What are the layers of the muscle? what do they do?
epimysium: surrounds entire muscle
perimysium: surrounds buncles of fiber
endomysium: surrounds individual muscle fibers
what strutcure of the muscle cell contains muscle fibers? what is contained inside of this?
muscle fascicles contain muscle fibers
-myofibrils are contained inside muscle fibers
What is the contractile unit of muscle? where is this located? how do each of the bands change during contraction?
Sarcomere
-located inside myofibrils
A-band: Myosin length (unchanged during contraction)
I-band & H-zone: Shrink during contraction
M-line, Z-line, titin: structural anchors.
what filaments are sarcomeres made of?
actin (thin) and myosin (thick) filaments
what proteins are in actin?
nebulin, troponin, tropomyosin, tropomodulin
where is the sarcoplasmic retiiculum located? what is its function?
it surrounds myofibrils and acts to sequester ca2+ through Ca ATPase pump
What is the function of T-tubules? where are they located?
they are extensions of the sarcolemma and they regulate action potentials by opening Ryr gate to release Ca2+
What triggers muscle contraction at the molecular level?
Ca²⁺ binds troponin C (TnC) → tropomyosin moves → myosin binds
What are the three solubility categories of muscle proteins? provide examples of each protein.
1) Myofibrillar (Salt-soluble)
-contractile (actin/myosin), regulatory (troponin/tropomyosin), structural (titin,nebulin,desmin)
2) Sarcoplasmic (Water-soluble)
-enzymes, myoglobin, cytochromes
3) Connective tissue/ stromal (Acid/alkaline soluble)
-collagen, elastin, proteoglycans
what is myosin? what is the function?
thick filament muscle protein responsible for hydrolyzing ATP -> releasing ADP + Pi + energy
- needed to contract muscles
what is the thin filament? what is its function? what is it composed of?
Thin muscle protein filament that is comprised of:
1) actin: globular regulatory ptotein
2) tropomyosin: rod like molecule forming helix
3) Troponin: protein containing Troponin C (binds Ca), Troponin I (inhibits tropomyosin), Troponin T (attaches tropomysin)
Forms the track for myosin heads to “walk” on during contraction
What is collagen? What is the structure of collagen?
The main connective tissue in meat
-Composed of repeating units of proline and hydroxyproline
What effect does increased collagen have on meat?
Tougher meat
What is the impact of cross-linking in collagen maturation?
Increases toughness, especially in older animals
-collagen has no strength if its not cross linked
-trivalent cross links =mature
-divalent cross links = immature
Why is young animal meat more tender?
Due to less cross-linking
Less cross-linking in collagen contributes to the tenderness of meat.
what part of an animal do you expect to be the most tender? and least?
more collagen = tougher meat
-legs, chest, rump have higher amounts of collagen (less tender)
-rib, loin and sirloin have less collagen = more tender
Enzymatic processes break down proteins, making meat more tender over time.
What are action potentials? How are they generated? where does this occur?
Electrical impulses that propagate along the muscle fiber’s membrane
1) local depolarization
2) generation + propogation of AP
-occurs along sarcolemma and T-tubules
3) Repolarization
What happens at the neuromuscular junction?
Action potential triggers release of ACh from movement of Ca2+, which binds to receptors on the sarcolemma
Binding of ACh regulates movement of Na+/K+
ACh effects terminated by acetylcholinesterase
What is excitation-contraction coupling? what overall steps are involved?
The process linking the neural signal and muscle contraction through Ca²⁺ release in the sarcolemma
1) AP propogation down T-tubules
-interaction with voltage-sensitive DHPR senses propogation and activates ryanodine receptor
2) Ca2+ release through ryanodine receptor
3) Ca binding to troponin C removing tropomyosin
4) myosin binds to actin forming cross bridge
5) power stroke / muscle contraction
What role does Ca²⁺ play in muscle contraction?
Ca²⁺ binds to troponin C, causing tropomyosin to shift and expose binding sites on actin
What is the power stroke? How does this relate to rigor mortis?
The pivoting of myosin heads that pulls actin filaments inward
ATP needed to detach myosin head from actin, without it RIGOR MORTIS (muscle stiffening) occurs in post mortem muscle
How does muscle relaxation of skeletal muscle occur?
Reduction of ACh release causes repolarization, closing Ca2+ gates (less Ca2+ release)
-Ca2+ pumped back into SR through ATP pumps
List the three major muscle fiber types and how they differ.
- Type I (Slow Oxidative)
-tender, slower cooked red meats (thin fibre) - Type IIA (Fast Oxidative/Glycolytic)
-pink/red meats - Type IIB (Fast Glycolytic)
-less tender, more connective tissue
From when an animal dies to meat consumption, what are the steps involved?
1) removal of blood + O2
2) Complete deplation of ATP and creatine phosphate
3) Anaerobic glycolysis + lactate accumulation
4) Protein denaturation due to pH drop
5) structural modifications
6) enzymatic proteolysis on myofibril + connective tissues
When is muscle considered meat? what is this?
when ‘rigor’ has been resolved
rigor = stiffening + loss of extensibility and it occurs when ATP is gone
What are the stages of rigor mortis?
- Delay: ATP present, muscle flexible
- Onset: ATP low, cross-bridges form but can’t detach
- Completion: ATP gone, full stiffness
what occurs during the delay phase of rigor mortis? what timefram does this typically occur?
ATP is either still available or generated through:
1) anaerobic glycolysis
2) Creatine phosphate
The muscle is still extensible and rigor is delayed due to production of ATP (better for animal)
-less than an hour (0-55min)
what occurs during the onset of rigor mortis? what timeframe does this typically occur?
Creatine phosphate is exhausted and anaerobic glycolysis becomes the main source energy
-cross bridges form but dont detach due to ATP depletion
-muscle begins to stiffen (1-3hr)
What occurs during the rigor completion phase? what timeframe does this typically occur?
all energy in the muscle is depleted and no regeneration occurs (complete stiffness)
-3+ hours
what physical-chemical changes occur post mortem? what does this due to the meat? when does this occur?
decreases in pH impacts:
1) light scattering
2) WHC
3) protein denaturation
what is light scattering? why does it occur?
pH drops -> protein denaturation
-> spaces form between structures and more light is scattered
This leads to a pale appearance in the meat
How does pH affect WHC?
pH lowers closer to isoelectric point, which (around 5.0) reduces water retention due to protein denaturation
-loss of water retention during processing and application of external forces
How does tenderness of meat increase when storage time is increased?
myofibrillar and cytoskeletal denaturation from proteolytic enzymes
what are the 4 major proteolytic enzymes involved in post mortem tenderization?
1) calpains *
-Ca2+ dependent proteases which break down myofibrillar proteins (loosen structure without destroying)
2) lysosomal system
3) Ubiquitin-dependent proteasome
4) matric metalloproteinases
What is the role of cathepsin in postmortem aging?
lysosomal protease that works in the later stages of aging
-breaks connective tissue to contribute to tenderization
What are matrix metalloproteinases (MMPs)?
Ca/Zn dependet enzymes that remodel collagen and connective tissue, aiding in meat tenderness
-work in later stages of aging
what major factor has made poultry/pigs more susceptible to ante-mortem stressors?
genetic selection for rapid growth + increased muscle mass
What are the effects of stress on meat quality?
Increases risk of PSE and DFD meat, impacts animal welfare and final meat quality
What does PSE stand for and what are its characteristics?
Pale, Soft, Exudative
Caused by short-term stress leading to rapid pH decline.
What does DFD stand for and what are its characteristics?
Dark, Firm, Dry
Caused by long-term stress resulting in low glycogen and high pH.
What triggers the biochemical process leading to pH decline in meat?
Stress → adrenaline → cAMP cascade → glycogen breakdown
What is the ideal pH decline for meat quality?
Approximately 5.5
Crucial for meat texture and water holding capacity (WHC).
What mutation is associated with PSE meat?
RyR mutation (RYR1 gene)
How does a RyR mutation contribute to PSE?
Ca²⁺ leaks stimulate glycolysis, leading to a fast pH drop
What is a major consequence of PSE on meat texture?
Soft, exudative texture with decreased WHC
How can PSE be prevented?
Rapid chilling, gentle handling, genetic screening
What is the cause of DFD meat?
High pH (>6.0) due to less lactic acid from depleted glycogen
What are the characteristics of DFD meat?
Dark, firm, dry, high WHC due to charge repulsion
What is the downside of DFD meat?
Poor shelf life and increased microbial risk
What is cold shortening?
Pre-rigor meat chilled <10°C leading to uncontrolled contraction
What is thaw rigor?
Freezing before rigor causing calcium flooding and contraction on thaw
How can cold shortening and thaw rigor be prevented?
Electrical stimulation, delay chilling or freezing
What pigment is key to fresh meat color?
Myoglobin
What are the iron states in myoglobin and their colors?
Fe²⁺ (bright red, oxymyoglobin), Fe³⁺ (brown, metmyoglobin)
What effect does pH have on meat color stability?
Affects pigment stability and light reflection
What is the relationship between pH, WHC, and meat color?
↓ pH → ↑ protein denaturation → ↑ light scattering
↑ pH (DFD) → darker color, ↑ WHC
What factors affect meat texture?
Age, breed, gender, muscle function
What role does marbling play in meat quality?
Enhances tenderness, juiciness, and flavor
What are flavor precursors in meat?
Amino acids, sugars, fats, nucleotides
What reactions are key to cooked meat flavor?
Maillard reactions + lipid oxidation
What causes boar taint?
Androstenone & skatole in uncastrated males
How can boar taint be minimized?
Vaccination (Improvest®)
What are common poultry myopathies?
White striping, woody breast, spaghetti meat
What causes deep pectoral myopathy?
Ischemic necrosis due to muscle compression
What are key exam focus areas from this lecture?
Causes, mechanisms, and outcomes of PSE and DFD meat
Fill in the blank: Proper _______ decline is crucial for meat texture.
pH
True or False: High pH in meat is associated with PSE.
False
What is the effect of age on meat tenderness?
Older animals typically have tougher meat
