Muscles and Muscle Tissue: Structure, Types, and Organization

Muscles and Muscle Tissue

Introduction

Muscle tissue is essential for movement, posture, and many physiological processes in the human body. There are three main types of muscle tissue, each with distinct structural and functional characteristics.

Types of Muscle Tissue

Skeletal Muscle

• Definition: Skeletal muscle is a type of voluntary muscle attached to bones and responsible for body movement.

• Structure: Composed of long, cylindrical, multinucleate cells with obvious striations.

• Function: Enables locomotion, posture, and facial expressions.

• Example: Biceps brachii, quadriceps femoris.

Cardiac Muscle

• Definition: Cardiac muscle is found only in the walls of the heart and is responsible for pumping blood throughout the body.

• Structure: Branching chains of cells; uni- or binucleate; striated; cells are connected by intercalated discs.

• Function: Involuntary contraction to propel blood.

• Example: Myocardium of the heart.

Smooth Muscle

• Definition: Smooth muscle is found in the walls of hollow organs (e.g., stomach, intestines, blood vessels) and is responsible for involuntary movements.

• Structure: Single, spindle-shaped, uninucleate cells; no striations.

• Function: Moves substances through internal body channels by contracting and relaxing.

• Example: Muscular layer of the stomach, arteries.

Comparison of Skeletal, Cardiac, and Smooth Muscle

Structural and Functional Differences

The following table summarizes the main characteristics of the three muscle types:

Characteristic	Skeletal	Cardiac	Smooth
Location	Attached to bones or skin	Walls of the heart	Walls of hollow organs, blood vessels
Cell Shape & Appearance	Long, cylindrical, multinucleate, striated	Branching chains, uni/binucleate, striated	Spindle-shaped, uninucleate, non-striated
Control	Voluntary	Involuntary	Involuntary
Function	Movement, posture	Pumping blood	Propelling substances

Connective Tissue Sheaths of Skeletal Muscle

Organization and Structure

Skeletal muscle is organized into bundles and surrounded by connective tissue sheaths that provide support, protection, and pathways for nerves and blood vessels.

• Epimysium: Dense irregular connective tissue that surrounds the entire muscle.

• Perimysium: Connective tissue that surrounds groups of muscle fibers, forming fascicles.

• Endomysium: Fine areolar connective tissue that surrounds each individual muscle fiber.

Hierarchical Organization

1. Muscle (organ): Surrounded by epimysium.

2. Fascicle (bundle of fibers): Surrounded by perimysium.

3. Muscle fiber (cell): Surrounded by endomysium.

Example

In the biceps brachii, the epimysium covers the entire muscle, perimysium wraps each fascicle, and endomysium surrounds each muscle fiber, allowing for coordinated contraction and force generation.

Additional info: The connective tissue sheaths also play a role in transmitting force generated by muscle fibers to tendons and bones, facilitating movement.