General Characteristics, Functions, and Physiology of Muscle Tissue

Types of Muscle Tissue

Muscle tissue is classified into three main types: skeletal, cardiac, and smooth. Each type has distinct structural and functional properties.

Characteristics of Muscle Tissue

• Excitability: Ability to receive and respond to stimuli.

• Contractility: Ability to shorten forcibly when stimulated.

• Extensibility: Ability to be stretched or extended.

• Elasticity: Ability to recoil and resume resting length after stretching.

Functions of Muscle Tissue

• Movement: Locomotion, manipulation, heart beat, propulsion, squeezing, etc.

• Posture: Maintains posture and body position.

• Stabilization: Stabilizes joints by muscle tone.

• Heat Generation: Muscles produce up to 40% of body mass heat; important for maintaining body temperature.

Skeletal Muscle Structure

Gross Anatomy of Skeletal Muscle

• Muscle fibers: Skeletal muscle is made of long, cylindrical cells called fibers.

• Fascicle: A group of muscle cells.

• Muscle: A group of fascicles.

Nerve and Blood Supply

• Each muscle fiber is supplied by one or more motor neurons (voluntary control).

• Muscles have a rich blood supply to deliver oxygen and nutrients and remove wastes.

Muscle Attachments

• Direct attachment: Epimysium fuses to periosteum of bone or perichondrium of cartilage.

• Indirect attachment: Muscle is connected to bone via tendons or aponeuroses.

Connective Tissue Wrappings

• Endomysium: Surrounds each muscle fiber.

• Perimysium: Surrounds fascicles.

• Epimysium: Surrounds entire muscle.

• Tendon: Dense regular connective tissue connecting muscle to bone.

• Aponeurosis: Broad sheet-like tendon.

Microscopic Anatomy of Skeletal Muscle

Muscle Fiber Structure

• Sarcolemma: Plasma membrane of muscle fiber.

• Sarcoplasm: Cytoplasm of muscle fiber.

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

• Mitochondria: Energy source for contraction.

• Myofibrils: Rod-like contractile elements; multinucleated.

• Transverse (T) tubules: Invaginations of sarcolemma that conduct action potentials deep into muscle fiber.

Structure of a Sarcomere

• H zone: Middle of A band (where actin and myosin do not overlap).

• A band: Includes myosin, but also includes actin.

• I band: Includes actin only.

• Z disc: Anchor for actin fibers at ends of sarcomere.

• M line: Center of H zone; anchor for myosin fibers.

Myofilaments

• Actin: Thin filament; light bands of sarcomere; looks like twisted double strand of pearls.

• Myosin: Thick filament; dark bands; has globular heads that bind to actin; contains ATPase enzymes for energy.

• Tropomyosin: Rod-shaped protein; blocks myosin binding sites on actin.

• Troponin: Complex protein; binds Ca2+ to move tropomyosin and expose binding sites.

• Elastic filaments: Composed of titin; allow for stretch and recoil.

Physiology of Skeletal Muscle Fiber

The Neuromuscular Junction (NMJ)

• Motor nerve: Bundle of motor neurons.

• Axon terminal: End of motor neuron axon; forms NMJ with muscle fiber.

• Synaptic vesicles: Contain neurotransmitter acetylcholine (ACh).

• Synaptic cleft: Space between axon terminal and muscle fiber.

• Motor end plate: Specialized region of muscle membrane with ACh receptors.

Generating an Action Potential

1. Voltage-regulated Ca2+ channels open at axon terminal.

2. Ca2+ causes synaptic vesicles to release ACh by exocytosis.

3. ACh diffuses across synaptic cleft and binds to receptors on sarcolemma.

4. Na+ channels open; Na+ rushes into cell, depolarizing membrane.

5. Depolarization spreads across sarcolemma and down T-tubules.

6. SR releases Ca2+ into sarcoplasm.

7. Repolarization restores resting membrane potential.

Sliding Filament Model of Contraction

• Excitation-Contraction Coupling: Action potential leads to release of Ca2+ from SR; Ca2+ binds to troponin, moving tropomyosin and exposing actin binding sites.

• Cross Bridge Attachment: Myosin heads bind to actin.

• Power Stroke: Myosin heads pivot, pulling actin filaments inward.

• Detachment: ATP binds to myosin, causing it to release actin.

• Cocking of Myosin Head: ATP hydrolysis re-energizes myosin head for next cycle.

Muscle Relaxation

• Ca2+ is pumped back into SR by ATP-dependent pumps.

• ACh is broken down by acetylcholinesterase.

• Troponin and tropomyosin return to resting positions, blocking actin binding sites.

Motor Units and Muscle Contraction

Motor Units

• Motor unit: A single motor neuron and all the muscle fibers it innervates.

• Large motor units: Many muscle fibers per unit (e.g., leg muscles).

• Small motor units: Few muscle fibers per unit (e.g., eye muscles).

Muscle Twitch and Graded Response

• Latent period: Time between stimulus and contraction.

• Contraction period: Muscle shortens.

• Relaxation period: Muscle returns to resting length.

Graded Responses

• Wave/Temporal Summation: Second stimulus before muscle relaxes increases contraction.

• Tetanus: Sustained contraction due to rapid stimulation.

• Multiple Motor Unit Summation: Recruitment of more motor units increases force.

• Muscle Tone: Slight contraction of muscles at rest.

Types of Muscle Contraction

• Isotonic: Muscle shortens and generates force (e.g., lifting a weight).

• Eccentric: Muscle lengthens while generating force (e.g., lowering a weight).

• Isometric: Muscle does not shorten; force generated but no movement (e.g., pushing against a door jamb).

Muscle Metabolism

ATP and Muscle Contraction

• ATP is necessary for contraction (cross bridge movement), calcium pump, and relaxation.

ATP Generation in Muscle

1. Creatine Phosphate: Stores high-energy phosphate to quickly replenish ATP.

2. Anaerobic Glycolysis: Glucose is broken down to produce ATP without oxygen; produces lactic acid.

3. Aerobic Respiration: Uses oxygen to produce ATP; most efficient, produces 36 ATP per glucose.

Summary Table: Muscle Metabolism Pathways

Key Terms and Definitions

• Sarcomere: The contractile unit of muscle fiber.

• Myofibril: Rod-like structure within muscle fiber containing sarcomeres.

• Actin: Thin filament protein in muscle contraction.

• Myosin: Thick filament protein in muscle contraction.

• Troponin: Regulatory protein that binds Ca2+ and moves tropomyosin.

• Tropomyosin: Blocks myosin binding sites on actin.

• Motor Unit: A motor neuron and all the muscle fibers it innervates.

• ATP: Adenosine triphosphate, energy source for muscle contraction.

Important Equations

• ATP hydrolysis:

• Creatine phosphate reaction:

• Aerobic respiration:

• Anaerobic glycolysis:

Example: During intense exercise, skeletal muscle relies first on creatine phosphate and anaerobic glycolysis for ATP, then switches to aerobic respiration as oxygen becomes available.

Additional info: Some details, such as the exact sequence of excitation-contraction coupling and the role of specific ions, have been expanded for clarity and completeness.