Chapter 9: The Muscular System – Structure and Function of Skeletal Muscles

Chapter 9: The Muscular System

Introduction

The muscular system is essential for movement, posture, and heat production in the human body. This chapter focuses on the structure and function of skeletal muscles, their organization, naming conventions, and their role in body movements.

Structure of Skeletal Muscles

Anatomy of Skeletal Muscle

• Muscle Fibers: Skeletal muscle cells are called muscle fibers due to their elongated shape. Each fiber is surrounded by a layer of extracellular matrix known as the endomysium.

• Fascicle: Groups of 10–100 muscle fibers are bundled together into a fascicle, which is encased by connective tissue called perimysium.

• Epimysium: Multiple fascicles are grouped together and surrounded by another connective tissue layer called epimysium.

• Fascia: The epimysium is continuous with the superficial connective tissue known as the fascia.

• Tendon: Tendons attach muscle to bone, allowing for movement.

Example: The biceps brachii muscle is composed of muscle fibers grouped into fascicles, surrounded by connective tissue layers, and attached to bones via tendons.

Organization and Muscle Shapes

Muscle Shapes

• Parallel Muscles: Fibers are evenly spaced and run parallel to a tendon. Example: Sartorius.

• Convergent Muscles: Broad at one end and tapers to a single tendon. Example: Pectoralis major.

• Circular Muscles (Sphincters): Encircle an opening and constrict to close it. Example: Orbicularis oculi.

• Fusiform Muscles: Thicker in the belly and tapered at the ends. Example: Biceps brachii.

• Pennate Muscles: Fascicles attach to a tendon at an angle, resembling a feather. Types include:

  • Unipennate: Fascicles feather out from one side of a tendon. Example: Flexor pollicis longus.

  • Bipennate: Fascicles feather out from both sides of a tendon. Example: Rectus femoris.

  • Multipennate: Multiple tendons with fascicles arranged at multiple angles. Example: Deltoid.

Naming Muscles

Muscle Naming Conventions

• Size: Terms such as major, minor, longus, brevis, and vastus indicate relative size.

• Location: Directional terms like superior, inferior, medial, and lateral specify anatomical position. Example: Pectoralis minor.

• Attachment: Muscles may be named for their attachment points. Example: Sternocleidomastoid (attaches to sternum, clavicle, and mastoid process).

• Function: Names may reflect function, such as flexors, extensors, levators, adductors, and abductors.

• Depth: Terms like profundus (deep) and superficialis (superficial) indicate relative depth.

Functions of Skeletal Muscles

Muscle Tension and Movement

• Muscle Tension: The primary function of skeletal muscles is to contract and generate muscle tension, which produces movement.

• Heat Production: Muscle contraction converts chemical energy (ATP) into mechanical energy, producing heat as a byproduct. This explains shivering when cold.

Roles of Muscles in Movement

• Agonist (Prime Mover): Produces most of the force during movement.

• Antagonist: Opposes and slows the action of the agonist for controlled movement.

• Synergist: Works with the agonist to guide movement and ensure smooth action.

• Fixator: Stabilizes the bone or joint to make movement more efficient and reduce injury risk.

Origin and Insertion

• Origin: The attachment point of a muscle to the bone that remains stationary during movement.

• Insertion: The attachment point to the bone that moves during contraction.

• Example: The biceps brachii originates on the scapula and inserts on the radius. During elbow flexion, the radius moves while the scapula remains stationary.

• Note: In some movements, the origin and insertion can switch depending on which bone is fixed.

Lever Systems in Body Movements

Components of a Lever System

• Lever: The bone itself.

• Load: The object or body part being moved.

• Force: Muscle tension applied to the lever.

• Fulcrum: The joint or hinge point around which the lever moves.

Types of Levers

Lever Type	Arrangement	Example
First-Class	Fulcrum in the middle, load and force on opposite sides	Atlanto-occipital joint (neck)
Second-Class	Fulcrum at one end, load in the middle, force at the other end	Metatarsophalangeal joints (foot)
Third-Class	Load at one end, force in the middle, fulcrum at the other end	Elbow joint (biceps brachii)

Mechanical Advantage

• Mechanical Advantage: A lever system that allows a small force to move a large load.

• Mechanical Disadvantage: A lever system that requires a greater force to move a load, but allows for greater speed and range of motion.

Muscle Injuries

Muscle Strain

• Definition: A muscle strain occurs when a muscle is subjected to more stretching or contractile force than it can handle, resulting in torn fibers or rupture.

• Symptoms: Pain, limited movement, swelling, and bruising.

• Treatment: The ICE method is commonly used:

  • Protect: Prevent further injury.

  • Rest: Allow time for healing.

  • Ice and Compression: Reduce inflammation.

  • Elevation: Drain fluid away from the injury site.

• Additional treatments may include anti-inflammatory medications, physical therapy, or surgical repair in severe cases.

Additional info: Muscle strains are common in athletes and can vary in severity from mild overstretching to complete muscle rupture. Early intervention and proper rehabilitation are crucial for recovery.