Muscle Tissue Types and Characteristics

3 Types of Muscle Tissues

Muscle tissue is specialized for contraction and is essential for movement, posture, and vital organ function. There are three main types:

• Skeletal Muscle: Attaches to bones, enables voluntary movement, has striations, and is under conscious control. Makes up about 40% of body weight.

• Cardiac Muscle: Found only in the heart, striated, and involuntary.

• Smooth Muscle: Located in walls of hollow organs (e.g., stomach, bladder, blood vessels), non-striated, and involuntary.

Special Characteristics of Muscle Tissue

• Excitability (Responsiveness): Ability to receive and respond to stimuli (usually via nerve impulses).

• Contractility: Ability to shorten forcibly when stimulated.

• Extensibility: Ability to be stretched or extended.

• Elasticity: Ability to recoil to original length after stretching.

Functions of Muscle Tissues

• Movement: Moves bones (skeletal), blood (cardiac), and substances (smooth).

• Posture and Body Position: Maintains posture and stabilizes joints.

• Support: Stabilizes joints and supports soft tissues.

• Heat Generation: Muscle contractions produce heat, helping maintain body temperature.

Gross Anatomy of Skeletal Muscle

Structure and Connective Tissue Components

A skeletal muscle is composed of muscle fibers, nerves, blood vessels, and connective tissues. These components are organized at several structural levels:

• Epimysium: Dense irregular connective tissue surrounding the entire muscle.

• Perimysium & Fascicles: Perimysium surrounds bundles of muscle fibers called fascicles.

• Endomysium: Wispy sheath of connective tissue surrounding each individual muscle fiber.

Attachments:

• Origin: Less movable bone.

• Insertion: Movable bone.

• Direct Attachment: Epimysium fused to periosteum or cartilage.

• Indirect Attachment: Connective tissue wrappings extend as tendon or aponeurosis.

Microscopic Anatomy of Skeletal Muscle

Muscle Fiber Structure

• Long, cylindrical cells (muscle fibers) with multiple nuclei.

• Sarcolemma: Plasma membrane of muscle fiber.

• Sarcoplasm: Cytoplasm containing glycosomes (glycogen storage) and myoglobin (oxygen-binding protein).

Specialized Structures

• Myofibrils: Account for ~80% of cell volume; contain contractile elements.

• Sarcoplasmic Reticulum (SR): Smooth ER that stores and releases calcium ions.

• T Tubules: Invaginations of sarcolemma that conduct electrical impulses into the cell.

Molecular Composition of Myofibrils

• Striations: Alternating dark (A bands) and light (I bands) due to arrangement of myofilaments.

• Sarcomere: Smallest contractile unit; extends from Z disc to Z disc.

• Myofilaments:

  • Thick Filaments: Composed of myosin.

  • Thin Filaments: Composed of actin, tropomyosin, and troponin.

  • Elastic Filaments: Composed of titin, providing elasticity.

Other Proteins: Nebulin, myomesin, and C proteins help maintain alignment; dystrophin links thin filaments to sarcolemma.

Physiology of Muscle Contraction

Sliding Filament Model of Contraction

During contraction, thin filaments slide past thick filaments, causing actin and myosin to overlap more. This shortens the sarcomere and thus the muscle fiber.

• Contraction = generation of force.

• Myosin heads bind to actin, detach, and re-cock, pulling actin filaments toward the M line (center of sarcomere).

Neuromuscular Activation

• Motor Neurons: Stimulate muscle fibers to contract via action potentials.

• Acetylcholine (ACh): Neurotransmitter released at neuromuscular junction (NMJ) to trigger muscle contraction.

• Action Potential: Electrical signal that travels along sarcolemma and T tubules, leading to calcium release from SR.

Excitation-Contraction Coupling

• Action potential triggers Ca2+ release from SR.

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

• Myosin heads bind to actin, initiating contraction (cross-bridge cycling).

Cross Bridge Cycling

• Myosin binds to actin, pivots, releases ADP and Pi, and pulls actin (power stroke).

• ATP binds to myosin, causing detachment from actin.

• ATP hydrolysis re-cocks myosin head for another cycle.

Properties of Whole Muscle Contraction

Temporal Summation and Motor Unit Recruitment

• Temporal Summation: Increased frequency of stimulation increases muscle tension (can lead to tetanus if no relaxation occurs).

• Motor Unit Recruitment: More motor units activated increases force; follows the "size principle" (smaller units recruited first).

Types of Contraction

• Isotonic Contraction: Muscle changes length while moving a load.

  • Concentric: Muscle shortens (e.g., lifting a weight).

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

• Isometric Contraction: Muscle generates tension but does not change length (load does not move).

ATP and Muscle Contraction

ATP Production

• ATP is required for cross-bridge cycling, Ca2+ reuptake, and Na+/K+ pump function.

• Muscle fibers store limited ATP (enough for 4-6 seconds of contraction).

3 Ways ATP is Regenerated:

1. Phosphagen System: Creatine phosphate donates phosphate to ADP (fast, short-term energy).

2. Glycolysis: Glucose broken down to pyruvic acid (anaerobic, produces lactic acid).

3. Aerobic Respiration: Pyruvic acid enters mitochondria, producing large amounts of ATP (requires oxygen).

Anaerobic vs. Aerobic ATP Production

• Anaerobic: Fast, less efficient, produces lactic acid, used during intense, short-duration activity.

• Aerobic: Slower, more efficient, sustainable, uses fatty acids and glucose, used during prolonged activity.

Muscle Fatigue and Recovery

• Muscle Fatigue: Inability to contract despite stimulation, due to ionic imbalances, lactic acid accumulation, or ATP depletion.

• Excess Postexercise Oxygen Consumption (EPOC): Increased oxygen intake after exercise to restore metabolic conditions (replenish ATP, remove lactic acid).

Factors Influencing Muscle Contraction

• Number of muscle fibers recruited (more fibers = greater force).

• Size of muscle fibers (larger fibers = greater force).

• Frequency of stimulation (higher frequency = greater force).

• Degree of muscle stretch (optimal length = maximal force).

Smooth Muscle

Structure and Function

• Found in walls of hollow organs (e.g., blood vessels, digestive tract).

• Non-striated, involuntary, spindle-shaped cells.

• Contracts to move substances through organs (peristalsis).

Types of Smooth Muscle

• Single-unit (visceral): Most common, cells connected by gap junctions, contract as a unit.

• Multi-unit: Fewer gap junctions, cells act independently (e.g., iris of eye, arrector pili muscles).

Regulation of Contraction

• Regulated by autonomic nervous system, hormones, and local factors.

• Can contract in response to stretch, hormones, or chemical signals.

• Slow, sustained contractions; can maintain tension for long periods with low energy use.

Comparison Table: Skeletal vs. Cardiac vs. Smooth Muscle