A&P: lecture exam 3 pt 1

Muscle Tissue

Overview of Muscle Tissue

Muscle tissue is a primary tissue type in the human body, specialized for contraction and movement. It is divided into three main types: skeletal muscle, cardiac muscle, and smooth muscle. Each type has unique structural and functional characteristics.

• Skeletal muscle: Attached to the skeleton, responsible for voluntary movements.

• Cardiac muscle: Found only in the heart, responsible for pumping blood.

• Smooth muscle: Found in walls of hollow organs, responsible for involuntary movements.

Skeletal Muscle

Functions of Skeletal Muscle

• Produce skeletal movement: Muscles contract to move bones.

• Maintain body position: Continuous muscle contractions stabilize joints and posture.

• Support soft tissues: Muscles protect internal organs.

• Guard body openings: Sphincter muscles control entry and exit of materials.

• Maintain body temperature: Muscle activity generates heat.

Muscle Attachments and Movement

• Origin: The fixed, non-moving attachment of a muscle.

• Insertion: The moving attachment, typically on the bone that moves.

• Tendons: Connect muscle to bone.

• Antagonistic muscles: Muscles that oppose each other's actions.

• Synergists: Muscles that assist in the same action.

Structural Organization of Skeletal Muscle

• Muscle (organ level)

• Muscle fascicle: Bundle of muscle fibers.

• Muscle fiber: Single muscle cell.

• Myofibril: Subunit within muscle fiber, composed of myofilaments.

• Myofilaments: Protein filaments (actin and myosin) responsible for contraction.

Connective Tissue Organization

• Epimysium: Outer collagen layer, separates muscle from surrounding tissues.

• Perimysium: Surrounds fascicles, contains blood vessels and nerves.

• Endomysium: Surrounds individual muscle fibers, contains capillaries, nerves, and satellite cells (for repair).

• All three layers converge to form tendons (bundles) or aponeuroses (sheets) for attachment to bone.

Formation and Structure of Skeletal Muscle Fibers

• Develop from fusion of myoblasts (mesodermal cells).

• Are multinucleated and very long.

• Contain specialized structures for contraction.

Organization of Skeletal Muscle Fibers

• Sarcolemma: Muscle cell membrane; initiates contraction via changes in membrane potential.

• Transverse (T) tubules: Invaginations of sarcolemma that transmit action potentials deep into the cell.

• Myofibrils: Contain repeating units called sarcomeres, the functional units of contraction.

• Sarcoplasmic reticulum (SR): Specialized endoplasmic reticulum that stores and releases Ca2+.

• Triad: Structure formed by one T tubule and two terminal cisternae of the SR.

Sarcomere Structure

• A bands: Dark, thick filaments (myosin).

• I bands: Light, thin filaments (actin).

• M line: Center of A band.

• Z lines: Boundaries of sarcomere, center of I bands.

• Zone of overlap: Area where thick and thin filaments overlap.

• H zone: Area around M line with only thick filaments.

• Titin: Protein that stabilizes thick filaments and connects them to Z line.

Muscle Contraction: Molecular Mechanisms

• Initiated by release of Ca2+ from SR.

• Involves interaction between thick (myosin) and thin (actin) filaments.

Thin Filament Proteins

• F actin: Two twisted rows of globular G actin; G actin has active sites for myosin binding.

• Nebulin: Holds F actin strands together.

• Tropomyosin: Covers active sites on actin, preventing myosin binding.

• Troponin: Binds tropomyosin to actin; regulated by Ca2+.

Thick Filament Proteins

• Myosin: Has a tail (binds other myosin) and head (binds actin).

• Titin: Provides elasticity and stabilization.

Sliding Filament Theory

• Thin filaments slide toward the M line, shortening the sarcomere.

• The width of the A band remains constant; Z lines move closer together.

Neural Control of Skeletal Muscle Contraction

• Action potential travels along motor neuron to neuromuscular junction (NMJ).

• Acetylcholine (ACh): Neurotransmitter released into synaptic cleft, binds to receptors on sarcolemma.

• Binding causes Na+ influx, generating action potential in muscle fiber.

• Action potential travels along T tubules, triggers Ca2+ release from SR.

Excitation–Contraction Coupling

• Action potential reaches triad (T tubule + 2 terminal cisternae).

• Ca2+ released, binds to troponin, exposing actin active sites.

• Myosin heads bind to actin, initiating contraction cycle.

Contraction Cycle Steps

1. Exposure of active sites on actin.

2. Formation of cross-bridges (myosin binds actin).

3. Pivoting of myosin heads (power stroke).

4. Detachment of cross-bridges.

5. Reactivation of myosin heads (ATP hydrolysis).

Contraction Duration and Relaxation

• Depends on neural stimulus, Ca2+ availability, and ATP supply.

• Relaxation occurs when Ca2+ is pumped back into SR, active sites are covered by tropomyosin.

Rigor Mortis

• Post-mortem stiffening due to lack of ATP and Ca2+ accumulation.

Types of Muscle Contractions

• Isotonic contraction: Muscle changes length (shortens or lengthens) to produce movement.

• Isometric contraction: Muscle develops tension but does not change length.

Resistance and Speed of Contraction

• Heavier resistance slows contraction and reduces shortening.

Muscle Relaxation Mechanisms

• Elastic forces (tendons, ligaments), opposing muscle contractions, and gravity help return muscle to resting length.

ATP and Muscle Contraction

• ATP is required for contraction and relaxation.

• Muscles store ATP and creatine phosphate (CP) for rapid energy.

• ATP is regenerated by:

  • Aerobic metabolism: In mitochondria, uses fatty acids, yields 34 ATP/glucose.

  • Anaerobic glycolysis: In cytoplasm, uses glucose, yields 2 ATP/glucose.

Energy Use During Muscle Activity

• At peak activity, oxygen is limited; glycolysis predominates, leading to lactic acid buildup.

Muscle Fatigue and Recovery

• Muscle fatigue: Inability to maintain contraction due to metabolic depletion, damage, low pH, or exhaustion.

• Recovery period: Time needed to restore normal conditions; includes oxygen replenishment and lactic acid removal.

• Cori cycle: Lactic acid is transported to liver, converted to glucose, and returned to muscles.

• Oxygen debt: Extra oxygen required post-exercise to restore metabolic balance.

Types of Skeletal Muscle Fibers

• Fast fibers: Large, powerful, fatigue quickly, few mitochondria, low myoglobin.

• Slow fibers: Small, contract slowly, resist fatigue, many mitochondria, high myoglobin.

• Intermediate fibers: Characteristics between fast and slow fibers.

Fiber Type	Contraction Speed	Fatigue Resistance	Color
Fast	Fast	Low	White
Slow	Slow	High	Red
Intermediate	Intermediate	Intermediate	Pink

Muscle Hypertrophy and Atrophy

• Hypertrophy: Increase in muscle size due to training (more myofibrils, mitochondria, glycogen).

• Atrophy: Decrease in muscle size due to inactivity.

Aerobic vs. Anaerobic Endurance

• Anaerobic endurance: Short, intense activities; relies on fast fibers and glycolysis.

• Aerobic endurance: Prolonged activities; relies on slow fibers and aerobic metabolism.

Cardiac Muscle Tissue

Structure and Characteristics

• Striated, found only in the heart.

• Cells (cardiocytes) are small, single-nucleated, branched.

• Short, wide T tubules; no triads; SR lacks terminal cisternae.

• High in myoglobin and mitochondria (aerobic).

• Connected by intercalated discs (gap junctions, desmosomes).

Functions of Intercalated Discs

• Maintain structural integrity.

• Facilitate electrical and molecular communication.

• Allow the heart to function as a single, coordinated unit.

Functional Properties of Cardiac Muscle

• Automaticity: Can contract without neural input (pacemaker cells).

• Variable contraction tension: Modulated by nervous system.

• Extended contraction time: Longer than skeletal muscle.

• No tetanic contractions: Prevented by cell membrane properties.

Smooth Muscle Tissue

Locations and Functions

• Found in walls of blood vessels, digestive, urinary, reproductive, and glandular systems, and skin (arrector pili).

• Regulates blood flow, moves materials, forms sphincters, and causes piloerection (goosebumps).

Structure of Smooth Muscle

• Nonstriated, spindle-shaped cells with single central nucleus.

• No T tubules, myofibrils, or sarcomeres.

• Scattered myosin fibers with more heads per thick filament.

• Thin filaments attached to dense bodies (anchor points).

• Contractions transmitted from cell to cell via dense bodies.

Functional Characteristics of Smooth Muscle

• Excitation–contraction coupling: Ca2+ binds to calmodulin, activating myosin light chain kinase, which initiates contraction.

• Length–tension relationship: Can contract over a wide range of lengths (plasticity).

• Control of contractions: Multiunit (connected to motor neurons) and visceral (not connected, controlled by pacesetter cells).

• Smooth muscle tone: Maintains baseline activity, modulated by neural, hormonal, or chemical factors.

Comparison of Muscle Types

Feature	Skeletal Muscle	Cardiac Muscle	Smooth Muscle
Striations	Yes	Yes	No
Nuclei per cell	Multiple	Single	Single
Control	Voluntary	Involuntary	Involuntary
Location	Skeletal system	Heart	Hollow organs
Regeneration	Limited	None	Good

Example: The biceps brachii muscle in the arm is a skeletal muscle responsible for flexing the elbow. The heart's myocardium is composed of cardiac muscle, while the walls of the intestines contain smooth muscle to propel food.