Muscle Tissue Overview

Types and Functions of Muscle Tissue

Muscle tissue is specialized for contraction, allowing movement and force generation in the body. There are three main types of muscle tissue, each with distinct characteristics and functions.

• Muscles convert chemical energy (ATP) into mechanical energy to exert force.

Comparison of Muscle Tissue Types

Main Functions of Skeletal Muscle

• Movement: Produces voluntary movements of the body.

• Posture: Maintains body posture and position.

• Support: Supports soft tissues and organs.

• Guarding Entrances/Exits: Controls openings of digestive and urinary tracts.

• Heat Production: Generates heat to maintain body temperature.

• Storage of Nutrients: Stores glycogen and proteins.

Common Properties of All 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 return to original length after stretching.

Connective Tissue Support

Muscle Connective Tissue Layers

Muscle fibers are organized and protected by three layers of connective tissue:

• Epimysium: Surrounds the entire muscle; dense connective tissue.

• Perimysium: Surrounds bundles of muscle fibers (fascicles).

• Endomysium: Surrounds individual muscle fibers.

Anatomy of a Skeletal Muscle Fiber

Structure and Components

• Muscle fiber: A single muscle cell, multinucleated and elongated.

• Sarcolemma: Plasma membrane of the muscle fiber.

• Sarcoplasm: Cytoplasm of the muscle fiber, containing organelles and myofibrils.

• Sarcoplasmic reticulum: Specialized endoplasmic reticulum that stores calcium ions.

• Myofibrils: Rod-like units within muscle fibers, composed of sarcomeres.

• Myoglobin: Oxygen-binding protein in muscle cells.

• T-tubules (Transverse tubules): Invaginations of the sarcolemma that help transmit action potentials.

Myofilaments

• Actin: Thin filament, involved in contraction.

• Myosin: Thick filament, has heads that bind to actin during contraction.

Functional unit of contraction: The sarcomere, the segment between two Z-lines in a myofibril.

Muscle Contraction

Steps Initiating Muscle Contraction

1. Action potential arrives at the axon terminal of a motor neuron.

2. Voltage-gated calcium channels open and calcium ions rush into the axon terminal.

3. Acetylcholine (ACh) is released into the synaptic cleft.

4. ACh binds to receptors on the sarcolemma of the muscle fiber.

5. Sodium channels open and sodium ions rush into the muscle fiber, initiating an action potential.

Role of Acetylcholine, Calcium, and Sodium Ions at the Neuromuscular Junction

• Acetylcholine (ACh): Neurotransmitter that transmits the nerve signal to the muscle fiber.

• Calcium ions (Ca2+): Released from the sarcoplasmic reticulum, they enable actin-myosin binding.

• Sodium ions (Na+): Influx generates the action potential in the muscle fiber.

Excitation-Contraction Coupling

• Action potentials sweep along the sarcolemma and down T-tubules.

• This triggers release of calcium ions from the sarcoplasmic reticulum.

• Calcium binds to troponin (on actin), causing tropomyosin to move and expose active sites for myosin binding.

Steps that End a Muscle Contraction

1. Acetylcholine in the synaptic cleft is broken down by acetylcholinesterase.

2. Calcium ions are reabsorbed into the sarcoplasmic reticulum.

3. Without calcium, tropomyosin returns to its position, covering myosin-binding sites on actin.

4. Cross-bridges cannot form, and muscle relaxation occurs.

Muscle Twitch Phases

Phases of a Muscle Twitch

• Latent period: Time between stimulus and start of contraction; action potential spreads, Ca2+ released.

• Contraction phase: Cross-bridge cycling occurs, muscle tension increases.

• Relaxation phase: Ca2+ reabsorbed, muscle tension decreases.

Motor Units and Recruitment

Motor Unit Structure and Function

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

• Recruitment: Increasing the number of active motor units to increase force.

• Recruitment proceeds from smaller to larger motor units.

Factors Affecting Tension

• Frequency of stimulation

• Number of motor units recruited

• Degree of muscle stretch

• Muscle fiber size

Effects of Repeated Stimulation

• Summation: Increased force due to rapid, repeated stimulation.

• Incomplete tetanus: Muscle partially relaxes between stimuli.

• Complete tetanus: No relaxation; sustained maximal contraction.

Muscle Tone

• Continuous, passive partial contraction of muscles, important for posture and readiness.

Energy for Contraction

ATP Generation in Muscle

• Immediate: Creatine phosphate pathway

• Short-term: Anaerobic glycolysis

• Long-term: Aerobic respiration

Fatigue occurs when ATP demand exceeds supply, or metabolic byproducts accumulate.

Muscle Adaptation & Physical Activity

Hypertrophy and Atrophy

• Hypertrophy: Increase in muscle size due to increased use.

• Atrophy: Decrease in muscle size due to disuse.

Comparison of Fast-Twitch and Slow-Twitch Muscle Fibers

Mechanics of Movement

Types of Muscle Contractions

• Isotonic: Muscle changes length to move a load.

  • Concentric: Muscle shortens.

  • Eccentric: Muscle lengthens.

• Isometric: Muscle length does not change; tension increases.

Cardiac Muscle

Structure and Function

• Intercalated discs: Specialized connections between cardiac muscle cells that allow rapid transmission of electrical impulses and synchronized contraction.

• Found only in the heart.

• Calcium for contraction comes from both extracellular fluid and sarcoplasmic reticulum.

• Contracts about 10x longer than skeletal muscle, cannot undergo tetanus, and relies solely on aerobic metabolism.

Smooth Muscle

Structure and Function

• Lacks striations (non-striated).

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

• Calcium comes from extracellular fluid and some from sarcoplasmic reticulum.

• Contracts slowly and can sustain contractions for long periods.