Meat Science Exam 2 (Muscle Structure and Contraction)

Skeletal muscles

Over 600 in the body; responsible for movement and contraction in living animals.

Tendon vs Ligament

Tendon connects muscle to bone; ligament connects bone to bone.

Connective tissue layers

Epimysium (muscle), Perimysium (muscle bundle), Endomysium (muscle fiber).

Muscle organization hierarchy

Muscle → Muscle bundle → Muscle fiber (myofiber) → Myofibril → Myofilament (actin/myosin).

Myofibril structure

Contains repeating units called sarcomeres; made of actin (thin) and myosin (thick) filaments.

Sarcomere structure

Smallest functional unit of a muscle fiber; bounded by Z-lines and contains A-band (myosin) and I-band (actin).

Actin

Thin filament protein; interacts with myosin during muscle contraction.

Myosin

Thick filament protein; most abundant muscle protein; forms crossbridges with actin.

Z-line

Defines the boundaries of a sarcomere; where actin filaments anchor.

M-line

Center of sarcomere; connects thick filaments together.

Titin

Elastic protein that stabilizes myosin and contributes to elasticity of muscle.

Desmin

Intermediate filament linking Z-lines; maintains structural integrity.

Sarcoplasmic reticulum (SR)

Organelle regulating calcium ion release and reuptake during contraction and relaxation.

Sarcolemma

Plasma membrane surrounding the muscle fiber; involved in electrical excitation.

T-tubules

Invaginations of sarcolemma that transmit action potentials deep into the muscle fiber.

Motor neurons

Axons that control skeletal muscle contraction.

Action potential

Rapid rise and fall in electrical membrane potential that triggers muscle excitation.

Acetylcholine (ACh)

Neurotransmitter released at neuromuscular junction; binds to receptors on sarcolemma to trigger depolarization.

Resting membrane potential

Approx. -70 mV; maintained by Na+/K+ ATPase pump (3 Na+ out, 2 K+ in).

Depolarization

Voltage-gated Na+ channels open, Na+ enters cell, making inside more positive.

Repolarization

Voltage-gated K+ channels open, K+ exits cell, restoring negative potential.

Hyperpolarization

Membrane potential briefly more negative than resting potential (~-90 mV).

Actomyosin

Complex of actin and myosin formed during crossbridge cycle; responsible for contraction.

Excitation-contraction coupling

Link between action potential and muscle contraction; involves Ca2+ release from SR.

Calcium role

Ca2+ binds to troponin, causing tropomyosin to shift and expose binding sites on actin for myosin attachment.

ATP in contraction

ATP binds myosin head to release crossbridge; hydrolysis provides energy for power stroke.

Rigor mortis

Postmortem process where muscles stiffen due to ATP depletion and permanent actomyosin crossbridges.

Rigor mortis stages

Delay (ATP present), Onset (stiffening), Completion (actomyosin fixed), Resolution (proteolytic degradation).

Muscle fiber types

Red (Type I), White (Type IIB), and Intermediate (Type IIA) fibers with distinct functions and metabolism.

Type I fibers

Slow-twitch, oxidative, high myoglobin, small diameter, many mitochondria, high endurance.

Type IIA fibers

Fast oxidative-glycolytic, intermediate diameter and color, balanced metabolism.

Type IIB fibers

Fast-twitch, glycolytic, low myoglobin, large diameter, low endurance, rapid contraction.

Color differences in fibers

Red = more myoglobin and mitochondria; White = less myoglobin, anaerobic metabolism.

Red muscle function

Slow, sustained contractions (posture muscles); fatigue-resistant.

White muscle function

Fast, powerful contractions (locomotion); easily fatigued.

Myoglobin function

Oxygen storage and transport protein in muscle; influences color.

Comparative examples

Marathon runner = more red fibers; sprinter = more white fibers.

Animal examples

Duck breast = red (sustained flight); Chicken breast = white (short bursts).

Chewing vs eye muscles

Chewing muscles = red, slow, sustained; Eye muscles = white, fast, short bursts.

Fiber type and species

Rabbit, Pig, Ox show varying fiber distributions (pig muscles have red fibers grouped centrally).

White fiber prevalence

Most muscles contain majority white fibers, even in red muscles.

Structure-function relationship

Red fibers have more mitochondria, capillaries, lipids, and smaller diameters for heat/waste dissipation.

White fiber structure

Larger diameter, more myofibrils, high glycogen, adapted for anaerobic metabolism and rapid contraction.

Feel the burn

During intense activity, limited oxygen leads to lactic acid accumulation and burning sensation.

Effect of training

Endurance training increases oxidative capacity; sprint training enhances glycolytic fibers.

Rigor mortis and meat texture

Permanent actomyosin crossbridges increase toughness until proteolysis softens tissue.

Protein degradation postmortem

Desmin, titin, and myosin heavy chain degrade over time, aiding tenderness.

Cooking and carcinogens

High-temperature cooking produces HCAs, PAHs, and nitrosamines linked to cancer risk.

HCAs (Heterocyclic amines)

Formed from Maillard reaction between creatine, amino acids, and sugars at high heat; carcinogenic.

PAHs (Polycyclic aromatic hydrocarbons)

Formed from incomplete fat combustion (grilling/broiling); carcinogenic potential.

Nitrosamines

Formed when proteins react with nitrite/nitric oxide at high temperatures or acidity; found in cured meats.

Factors affecting HCA formation

Cooking temperature/time, method, moisture, lipid content, and meat type.

Cooking methods and HCAs

High: grilling, broiling, pan-frying; Low: roasting, baking, boiling.

Inhibiting HCA formation

Lower cooking temps, microwave precooking, antioxidants (BHA, BHT, polyphenols), phosphates.

Cancer risk chemicals in cooked meat

HCAs (burned), PAHs (incomplete combustion), nitrosamines (cured meat).