The Muscular System

Introduction

The muscular system is a fundamental component of human anatomy and physiology, responsible for movement, posture, and various essential bodily functions. This guide explores the structure, types, and organization of muscle tissue, with a focus on skeletal muscle.

Muscular Tissue

Types of Muscle Tissue

There are three main types of muscle tissue in the human body, each with distinct structural and functional characteristics.

• Smooth muscle is found in the walls of hollow organs and is responsible for involuntary movements such as peristalsis.

• Cardiac muscle makes up the heart and contracts involuntarily to pump blood.

• Skeletal muscle attaches to bones and is responsible for voluntary movements.

Shared Characteristics of Muscle Tissue

Properties of Muscle Cells

• Excitability: Ability to respond to stimuli, such as nerve signals.

• Contractility: Ability to shorten forcefully when stimulated.

• Extensibility: Ability to be stretched without being damaged.

• Elasticity: Ability to return to original length after being stretched or contracted.

Functions of Skeletal Muscles

Major Roles

• Support: Maintain posture and stabilize joints.

• Movement: Produce voluntary movements and resist unwanted movement (tension).

• Thermoregulation: Generate heat as a byproduct of contraction (accounts for ~75% of body heat).

• Circulation Assistance: Aid movement of blood in veins and lymph in lymphatic vessels.

• Protection: Shield internal organs from injury.

Skeletal Muscle Structure and Attachments

Basic Anatomy

• Most skeletal muscles attach to bones via tendons.

• They usually cross at least one joint.

• Origin: The stationary attachment point (anchor).

• Insertion: The attachment that moves toward the origin during contraction.

• Muscles can only pull (shorten), not push.

• Humans have over 600 skeletal muscles.

Muscle Action

• Prime mover (agonist): Main muscle responsible for a specific movement.

• Synergists: Muscles that assist the prime mover.

• Antagonists: Muscles that oppose the action of the agonist, acting as a brake.

• Example: Biceps brachii (flexor) and Triceps brachii (extensor) in the arm.

Attachment Example

• When the biceps brachii contracts, the forearm flexes (bends) at the elbow; when the triceps brachii contracts, the forearm extends (straightens).

Structure of Muscle

Levels of Organization

• Whole muscle: Group of muscle cells with the same origin, insertion, and function.

• Fascicle: Bundle of muscle fibers (cells) within the muscle.

• Muscle fiber: A single muscle cell, containing many myofibrils.

• Each fascicle is wrapped in connective tissue.

Connective Tissue Wrappings

• Epimysium: Tough outer covering of the whole muscle.

• Perimysium: Fibrous connective tissue surrounding each fascicle.

• Endomysium: Delicate areolar connective tissue surrounding each muscle fiber.

• At the ends of muscles, all connective tissue sheaths merge to form the tendon.

Organization of Muscle Tissue

Hierarchical Structure

• Muscle: Bundle of fascicles.

• Fascicle: Bundle of muscle fibers (cells).

• Muscle fiber: Contains many myofibrils.

• Myofibril: Bundle of protein filaments (myofilaments).

• Myofilaments: Two main types—actin (thin) and myosin (thick).

Additional info: This organization allows for efficient force generation and transmission from the cellular to the whole-muscle level.

Muscle Fiber Anatomy

Key Structures

• Sarcolemma: Plasma membrane of the muscle fiber.

• Sarcoplasm: Cytoplasm of the muscle fiber.

• Sarcoplasmic reticulum: Specialized smooth endoplasmic reticulum that stores and releases calcium ions.

• T (transverse) tubules: Extensions of the sarcolemma that penetrate into the cell, allowing rapid transmission of action potentials.

• Myofibrils: Bundles of protein myofilaments within the muscle cell; make up about 80% of cell volume.

Striations

• Skeletal muscle fibers appear striated (striped) due to the regular arrangement of actin and myosin filaments within myofibrils.