Muscle System Overview

Introduction to the Muscular System

The muscular system comprises nearly half of the body's mass and is essential for movement, posture, joint stabilization, and heat generation. Muscles transform chemical energy (ATP) produced via cellular respiration into mechanical energy for bodily functions.

• Movement: Muscles move bones or fluids within the body.

• Posture: Muscles assist in maintaining body posture and position.

• Joint Stabilization: Muscles stabilize joints during movement.

• Heat Generation: Muscle activity produces heat as a byproduct.

Muscle Structure and Organization

Levels of Muscle Organization

• Myofibrils: Protein filaments within muscle fibers responsible for contraction.

• Sarcolemma: The plasma membrane of a muscle fiber.

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

• Muscle Fiber: A single muscle cell containing myofibrils.

• Muscle Tissue Types: Three main types—skeletal, cardiac, and smooth.

Types of Muscle Tissue

Muscle Connective Tissue Coverings

• Fascia: Fibrous connective tissue covering muscles, extending into tendons and aponeuroses for attachment.

• Epimysium: Dense connective tissue surrounding the entire muscle.

• Perimysium: Connective tissue surrounding fascicles (bundles of muscle fibers).

• Endomysium: Fine connective tissue surrounding each muscle fiber.

Additional info: These layers provide support, protection, and pathways for nerves and blood vessels.

Internal Structure of Skeletal Muscle

• Muscle: Composed of bundles called fascicles.

• Fascicle: Bundle of muscle fibers wrapped in perimysium.

• Muscle Fiber: Individual muscle cell wrapped in endomysium.

• Myofibril: Contractile element within the muscle fiber.

Microscopic Anatomy of Muscle Fibers

Key Structures

• Sarcolemma: Plasma membrane of the muscle fiber.

• Sarcoplasm: Cytoplasm of the muscle fiber.

• Mitochondria: Organelles providing ATP for contraction.

• Myofibrils: Cylindrical structures containing contractile proteins.

Myofibril Structure

• Sarcomere: The functional contractile unit of a myofibril, defined by Z discs.

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

• Thick Filaments: Composed of myosin.

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

• Titin: Elastic protein connecting myosin to Z disc, providing elasticity.

Specialized Structures

• Sarcoplasmic Reticulum (SR): Stores and releases calcium ions for contraction.

• Transverse Tubules (T-tubules): Invaginations of the sarcolemma that conduct electrical impulses.

• Triad: A T-tubule flanked by two terminal cisternae of the SR, crucial for excitation-contraction coupling.

Sarcomere Structure and Function

Sarcomere Organization

• Extends from one Z disc to the next.

• A band: Contains thick filaments (myosin) and overlapping thin filaments (actin).

• I band: Contains only thin filaments (actin).

• H zone: Central region of A band with only thick filaments.

• M line: Center of the sarcomere, supporting proteins.

Contractile and Regulatory Proteins

• Myosin: Thick filament protein with heads that bind actin and hydrolyze ATP.

• Actin: Thin filament protein with binding sites for myosin heads.

• Troponin and Tropomyosin: Regulatory proteins that inhibit or permit myosin binding to actin.

Muscle Contraction Physiology

Neuromuscular Junction and Excitation-Contraction Coupling

1. Motor neuron stimulates muscle fiber at the neuromuscular junction.

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

3. ACh binds to receptors on the sarcolemma, increasing membrane permeability to sodium ions and generating an action potential.

4. Action potential triggers calcium release from the sarcoplasmic reticulum.

5. Calcium binds to troponin, causing tropomyosin to move and expose myosin-binding sites on actin.

6. Myosin heads bind to actin, forming cross-bridges and initiating contraction.

7. Stimulation ends as sodium/potassium pumps restore ion balance.

Sliding Filament Model

• Muscle contraction occurs as thick (myosin) and thin (actin) filaments slide past each other, shortening the sarcomere.

• ATP is required for cross-bridge cycling and detachment.

Key Equations:

• ATP hydrolysis:

Rigor Mortis

• After death, ATP production ceases, preventing cross-bridge detachment and causing muscle stiffness.

• Begins 3–4 hours post-mortem, peaks at 12 hours.

Muscle Energy Sources

ATP and Creatine Phosphate

• ATP: Immediate but short-lived energy source for contraction.

• Creatine Phosphate: High-energy phosphate storage, replenishes ATP for brief periods (up to 10 seconds).

Glycogen and Cellular Respiration

• Aerobic Respiration: Uses oxygen to generate ATP from glucose in mitochondria; supports sustained activity.

• Anaerobic Respiration: Occurs without oxygen, converts glucose to lactic acid; supports short, intense activity.

• With prolonged exercise, muscle increases mitochondrial content for aerobic capacity.

Muscle Fatigue, Cramps, and Soreness

Muscle Fatigue

Muscle fatigue is the inability to contract after prolonged activity, caused by:

• Ion imbalances

• Psychological factors

• Lactic acid accumulation

• Decreased blood flow

Muscle Cramps

• Uncontrolled contractions, often due to dehydration or electrolyte imbalance (potassium, calcium).

Muscle Soreness (DOMS)

• Delayed onset muscle soreness caused by microtearing and inflammation after exercise.

Motor Units and Muscle Contraction Types

Motor Unit

• Consists of a motor neuron and all the muscle fibers it innervates.

• Smaller motor units allow for fine control; larger units for gross movements.

Types of Muscle Contraction

• Isotonic Contraction: Muscle shortens as it moves a load; actin filaments slide in the sarcomere.

• Isometric Contraction: Muscle tension increases without shortening; no movement of actin filaments.

• Muscle Twitch: Single contraction-relaxation cycle in a muscle fiber.

Phases of a Muscle Twitch

• Latent Period: Events of excitation-contraction coupling; no tension.

• Contraction Period: Cross-bridge formation; tension increases.

• Relaxation Period: Calcium reuptake; tension decreases.

Summation and Tetany

• Summation: Increased force of contraction due to increased frequency or strength of stimulation.

• Incomplete Tetany: Partial relaxation between contractions.

• Complete Tetany: No relaxation; sustained contraction.

• Treppe: Gradual increase in contraction strength with repeated stimulation.

Muscle Disorders

Common Muscle Disorders

• Muscular Dystrophy: Genetic disorder causing progressive muscle degeneration.

• Myasthenia Gravis: Autoimmune disease targeting acetylcholine receptors, leading to muscle weakness.

• Myoma: Tumor of muscle tissue.