The Muscular System: Skeletal Muscle Tissue and Organization

The Muscular System: Skeletal Muscle Tissue and Organization

Introduction

The muscular system is essential for numerous physiological processes and dynamic interactions with the environment. Muscles enable movement, maintain posture, and support various bodily functions.

• Muscle tissue types:

  • Skeletal muscle: Attaches to bones, enables voluntary movement.

  • Cardiac muscle: Found in the heart, responsible for pumping blood with rhythmic contractions.

  • Smooth muscle: Located in walls of hollow organs (e.g., digestive tract), controls involuntary movements.

• Basic properties of muscle tissue:

  • Excitability: Ability to respond to stimuli.

  • Contractility: Ability to shorten and generate force.

  • Extensibility: Ability to stretch without being damaged.

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

Functions of Skeletal Muscles

Skeletal muscles perform several vital functions in the body:

• Produce skeletal movement: Pull on tendons to move bones.

• Maintain posture and body position: Stabilize joints and maintain posture.

• Support soft tissues: Support the weight of visceral organs.

• Regulate entry and exit of material: Voluntary control over swallowing, defecation, and urination.

• Maintain body temperature: Muscle contraction generates heat.

Muscle Anatomy

Gross Anatomy

• Epimysium: Dense connective tissue surrounding the entire muscle.

• Perimysium: Dense tissue dividing the muscle into fascicles (bundles of muscle fibers).

• Endomysium: Connective tissue surrounding individual muscle fibers.

• Tendons: Connect muscle to bone.

• Aponeuroses: Connect muscle to muscle.

• Nerves: Innervate muscles, enabling chemical communication at the neuromuscular junction.

• Blood vessels: Supply nutrients and oxygen, remove waste products.

Microscopic Anatomy

• Sarcolemma: Plasma membrane of a muscle cell.

• Sarcoplasm: Cytoplasm of a muscle cell.

• Muscle fiber: Single muscle cell, multinucleated, can be 30–40 cm long.

• Myofibrils: Cylindrical structures within muscle fibers responsible for contraction.

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

• Myofilaments: Protein filaments (actin and myosin) that make up myofibrils.

Myofibrils and Myofilaments

• Myofibrils: Responsible for muscle contraction, composed of repeating units called sarcomeres.

• Myofilaments: Include thick filaments (myosin) and thin filaments (actin).

Sarcomere Structure

The sarcomere is the basic functional unit of muscle fibers, responsible for the striated appearance of skeletal muscle.

• Each myofibril contains approximately 10,000 sarcomeres.

• Sarcomere components:

  • Z line (Z disc): Boundary between sarcomeres.

  • I band: Region containing only thin filaments.

  • A band: Region containing thick filaments (may overlap with thin filaments).

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

  • M line: Center of the sarcomere, holds thick filaments together.

Actin and Myosin

• Actin: Twisted filament of G actin molecules, each with an active site for myosin binding. Associated proteins include tropomyosin (covers binding sites) and troponin (holds tropomyosin in place).

• Myosin: Thick filament with elongated tail and globular head (cross-bridge). Myosin heads bind to actin during contraction. Myosin is anchored by proteins at the M line and titin connecting to Z lines.

Muscle Contraction

Muscle contraction involves the shortening of muscle fibers due to interactions between actin and myosin within the sarcomere. This process is explained by the sliding filament theory.

• Contraction is triggered by calcium ions and requires ATP.

• Upon contraction:

  • H band and I band decrease in size.

  • Zone of overlap increases.

  • Z lines move closer together.

  • A band width remains constant.

• Events leading to contraction:

  1. Nerve impulse travels down axon to neuromuscular junction.

  2. Acetylcholine is released, triggering sarcoplasmic reticulum to release calcium ions.

  3. Calcium binds to troponin, causing tropomyosin to shift and expose actin binding sites.

  4. Myosin heads bind to actin, forming cross-bridges.

  5. Cross-bridges pivot, pulling actin filaments toward the center of the sarcomere.

  6. Muscle fiber shortens as actin and myosin filaments slide past each other.

Motor Units and Muscle Control

A motor unit consists of a motor neuron and all the muscle fibers it controls. The size of a motor unit determines the precision of muscle control.

• Precise control: Motor neuron controls few muscle fibers (e.g., eye muscles).

• Less precise control: Motor neuron controls many muscle fibers (e.g., leg muscles).

Organization of Skeletal Muscle Fibers

Muscle fibers can be classified by their arrangement and shape, which affects their function and range of motion.

• Parallel muscle fibers: Fascicles run parallel to the long axis (e.g., biceps brachii, rectus abdominis).

• Convergent muscle fibers: Fibers spread out over a broad area and converge at a common point (e.g., pectoralis major).

• Pennate muscle fibers: Fibers form an oblique angle to the tendon.

  • Unipennate: Fibers on one side of the tendon (e.g., extensor digitorum).

  • Bipennate: Fibers on both sides of the tendon (e.g., rectus femoris).

  • Multipennate: Tendon branches within the muscle (e.g., deltoid muscle).

• Circular muscle fibers: Fibers form concentric rings (sphincter muscles).

Levers and Pulleys: A Systems Design for Movement

Muscles produce movement by acting on bones, which function as levers. Joints serve as fulcrums, and muscles apply force to move the levers.

• There are three classes of levers:

  1. First class: Fulcrum is between applied force and resistance (e.g., tilting the head forward and backward).

  2. Second class: Resistance is between fulcrum and applied force.

  3. Third class: Applied force is between fulcrum and resistance.

Aging and the Muscular System

With aging, skeletal muscle fibers decrease in size and number, leading to reduced strength and endurance.

• Decrease in myofibrils, glycogen reserves, and myoglobin content.

• Increase in fibrous connective tissue (fibrosis).

• Reduced ability to recover from muscular injuries.

Summary Table: Muscle Tissue Types

Muscle Type	Location	Control	Function
Skeletal	Attached to bones	Voluntary	Movement, posture, heat production
Cardiac	Heart	Involuntary	Pumping blood
Smooth	Walls of hollow organs	Involuntary	Movement of substances (e.g., food, urine)

Key Equations

• Force generated by muscle contraction:

$\text{Force} = \text{Cross-sectional area} \times \text{Tension per unit area}$

• Sliding filament theory (conceptual):

$\text{Muscle shortening} = \text{Sum of sarcomere shortening}$